molecules

Review HPLC Separation of : Chiral Molecular Tools Useful for the Preparation of Enantiopure Compounds and Simultaneous Determination of Their Absolute Configurations

Nobuyuki Harada Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577, Japan; [email protected]

Academic Editor: Yoshio Okamoto Received: 20 August 2016; Accepted: 26 September 2016; Published: 4 October 2016

Abstract: To obtain enantiopure compounds, the so-called chiral high performance liquid chromatography (HPLC) method, i.e., HPLC using a chiral stationary phase, is very useful, as reviewed in the present Special Issue. On the other hand, normal HPLC (on silica gel) separation of diastereomers is also useful for the preparation of enantiopure compounds and also for the simultaneous determination of their absolute configurations (ACs). The author and coworkers have developed some chiral molecular tools, e.g., camphorsultam dichlorophthalic acid (CSDP acid), 2-methoxy-2-(1-naphthyl)propionic acid (MαNP acid), and others suitable for this purpose. For example, a racemic is esterified with (S)-(+)-MαNP acid, yielding diastereomeric , which are easily separable by HPLC on silica gel. The ACs of the obtained enantiopure MαNP esters can be determined by the 1H-NMR diamagnetic anisotropy method. In addition, MαNP or CSDP esters have a high probability of giving single suitable for X-ray crystallography. From the X-ray Oak Ridge thermal ellipsoid plot (ORTEP) drawing, the AC of the alcohol part can be unambiguously determined because the AC of the acid part is already known. The of MαNP or CSDP esters yields enantiopure with the established ACs. The mechanism and application examples of these methods are explained.

Keywords: chiral molecule; absolute configuration; diastereomers; HPLC separation on silica gel; 1H-NMR diamagnetic anisotropy; X-ray crystallography; internal reference of absolute configuration

1. Introduction It is well known that the so-called chiral high performance liquid chromatography (HPLC) method, i.e., HPLC with a chiral stationary phase, is very useful for the separation of , i.e., the preparation of enantiopure compounds [1–5], as will be explained in other review articles and/or research papers reported in this Special Issue. On the other hand, there is another HPLC method for the preparation of enantiopure compounds, where diastereomers are separated by normal HPLC on silica gel [6–11], or by reversed-phase HPLC [12]. For example, racemic 4-octanol (±)-2 was esterified with (S)-(+)-2-methoxy-2-(1-naphthyl)propionic acid 1 (MαNP acid) yielding a diastereomeric mixture of esters 3a and 3b (Figure1)[ 13]. It was surprising to find that diastereomeric esters 3a and 3b (40 mg sample) could be completely separated by HPLC on silica gel (separation factor α = 1.25; resolution factor Rs = 1.03), regardless of the very small of the 4-octanol moiety, which is generated by the difference between propyl (C3) and butyl (C4) groups; especially since both are normal chain alkyl groups. Please note that in this review article, the diastereomers are designated by compound numbers., e.g., 3a and 3b, where small letter a indicates the first-eluted fraction in HPLC, and b is the second-eluted one.

Molecules 2016, 21, 1328; doi:10.3390/molecules21101328 www.mdpi.com/journal/molecules MoleculesMolecules2016 2016, 21, ,21 1328, 1328 2 of2 37 of 37 Molecules 2016, 21, 1328 2 of 37

Figure 1. Preparation of diastereomeric 4-octanol (S)-(+)-2-methoxy-2-(1-naphthyl)propionic acid FigureFigure 1. 1.Preparation Preparation of of diastereomeric diastereomeric 4-octanol4-octanol ((SS)-(+)-2-methoxy-2-(1-naphthyl)propionic)-(+)-2-methoxy-2-(1-naphthyl)propionic acid acid (MαNP) esters 3a and 3b (a) and separation by high performance liquid chromatography (HPLC) on (M(MαNP)αNP) esters esters3a 3aand and3b 3b( a()a) and and separation separation byby high performance liquid liquid chromatography chromatography (HPLC) (HPLC) on on silica gel (b). Redrawn with permission from [13]. silicasilica gel gel (b ().b). Redrawn Redrawn with with permission permission fromfrom [[13].13]. In general, it is very difficult to separate enantiomers or diastereomers composed of C, H, and InIn general, general, it it is is very very difficult difficult to to separate separate enantiomers enantiomers or or diastereomers diastereomers composedcomposed of C, H, and O O atoms by HPLC or gas chromatography (GC), especially in the case of aliphatic chain compounds. atoms by HPLC or gas chromatography (GC), especially in the case of aliphatic chain compounds. For O atomsFor example, by HPLC Figure or gas 2 shows chromatography the separation (GC), results especially of 4-octanol in the or case 4-nonanol, of aliphatic reported chain to compounds.date: (a) a example,Forracemic example, Figure 4-octanol Figure2 shows 3,5-DNPU 2 shows the separation the(3,5-dinitrophenylurethane) separation results results of 4-octanol of 4-octanol derivative or 4-nonanol, or 4-nonanol,was subjected reported reported to to chiral date: to HPLC (a) date: a racemic at(a) a ◦ 4-octanolracemic−20 °C, 3,5-DNPU4-octanol but the separation 3,5-DNPU (3,5-dinitrophenylurethane) factor (3,5-dinitrophenylurethane) α remained low as derivative α = 1.06 derivative [14]; was (b) subjected separation was subjected to of chiral racemic to HPLCchiral 4-nonanol HPLC at −20 at C, but−20 theacetate °C, separation but was the attempted separation factor αby remained factorchiral GC,α remained lowbut this as αwas low= 1.06not as αsuccessful, [ 14= 1.06]; (b) [14]; separation α = (b) 1.0~1.01 separationof [15]; racemic (c) of racemic racemic 4-nonanol 4-octanol 4-nonanol acetate wasacetate attemptedwas esterifiedwas attempted by with chiral a by chiral GC, chiral but acid GC, this yielding but was this notdiaste was successful, reomers,not successful, whichα = 1.0~1.01 αwere = 1.0~1.01 separated [15]; [15]; (c) by racemic(c) HPLC racemic 4-octanol(reversed 4-octanol was esterifiedwasphase) esterified with at −40 a with chiral°C, but a acidchiral α remained yielding acid yielding low diastereomers, as α diaste= 1.04 reomers,[16]. which which were were separated separated by HPLC by HPLC (reversed (reversed phase) at phase)−40 ◦C, at but−40 α°C,remained but α remained low as αlow= 1.04as α [=16 1.04]. [16].

Figure 2. HPLC or gas chromatography (GC) separation of alkyl chain alcohol derivatives (a)–(c).

FigureFigureIn contrast, 2. 2.HPLC HPLC as or orshown gas gas chromatography chromatography in Figure 1, M (GC)(GC)αNP separation esters 3a ofand of alkyl alkyl 3b chain chainwere alcohol more alcohol effectivelyderivatives derivatives separated,(a)–( (a)–(c). c). indicating that MαNP acid is especially useful for such separation. Therefore, our first purpose is to developInIn contrast, contrast, chiral as molecularas shown shown intoolsin FigureFigure such 1as ,M1, MMααNPNP acid esters for effective3a3a andand 3bdiastereomers3b werewere more more separationeffectively effectively by separated, HPLC separated, indicatingindicatingon silica that gel,that M and MαNPα thenNP acid acid to obtain is is especially especially enantiopure usefuluseful comp forounds such sepafrom separation.ration. the separated Therefore, Therefore, diastereomers, our our first first purpose as purpose will be is to is to developdevelopexplained chiral chiral in molecular thismolecular review tools article.tools such such as as M MαNPαNP acid acid for for effective effective diastereomers diastereomers separation separation byby HPLCHPLC on silicaon silica gel, andgel, and then then to obtain to obtain enantiopure enantiopure compounds compounds from from thethe separated diastereomers, diastereomers, as as will will be be explainedexplained in in this this review review article. article. There is another resolution method where diastereomeric ionic crystals, e.g., acid/, or inclusion complex crystals are fractionally recrystallized to separate diastereomers. If good crystals Molecules 2016, 21, 1328 3 of 37

MoleculesThere2016 ,is21 ,another 1328 resolution method where diastereomeric ionic crystals, e.g., acid/amine,3 of or 37 inclusion complex crystals are fractionally recrystallized to separate diastereomers. If good crystals are obtained, the method is useful for separation on a large scale. However, the separated crystals are are obtained, the method is useful for separation on a large scale. However, the separated crystals not always diastereomerically pure, despite many . If so, it is difficult to obtain are not always diastereomerically pure, despite many crystallizations. If so, it is difficult to obtain enantiopure target compounds. This is the reason why we have not used the ionic crystals or enantiopure target compounds. This is the reason why we have not used the ionic crystals or inclusion inclusion crystals, but selected covalently bonded diastereomers such as esters or , to which crystals, but selected covalently bonded diastereomers such as esters or amides, to which HPLC on HPLC on silica gel is applicable for separation and further purification. silica gel is applicable for separation and further purification. Our second purpose is to determine the (AC). It is well known that the Our second purpose is to determine the absolute configuration (AC). It is well known that X-ray Bijvoet method [17] using the heavy atom effect, the (CD) exciton chirality the X-ray Bijvoet method [17] using the heavy atom effect, the circular dichroism (CD) exciton method [18], and the recent density functional theory (DFT) molecular orbital calculation [19] are all chirality method [18], and the recent density functional theory (DFT) molecular orbital calculation [19] very useful as non-empirical methods of AC determination. There is another category of AC are all very useful as non-empirical methods of AC determination. There is another category of determination where the relative configuration against the internal reference of AC could be AC determination where the relative configuration against the internal reference of AC could be determined by X-ray crystallography and/or 1H-NMR diamagnetic anisotropy methods [6–11]. If determined by X-ray crystallography and/or 1H-NMR diamagnetic anisotropy methods [6–11]. single crystals suitable for X-ray diffraction are available and the final Oak Ridge thermal ellipsoid If single crystals suitable for X-ray diffraction are available and the final Oak Ridge thermal ellipsoid plot (ORTEP) drawing is obtained, it is very easy to determine the AC of the part in question, based plot (ORTEP) drawing is obtained, it is very easy to determine the AC of the part in question, based on the AC of the internal reference. This X-ray internal reference method is the most straightforward on the AC of the internal reference. This X-ray internal reference method is the most straightforward and reliable. and reliable. 1 Another relative method is the 1H-NMRH-NMR diamagneticdiamagnetic anisotropy method, that is explained in Figure3 3[ [13].13]. The first-elutedfirst-eluted MMααNPNP esterester ((−−)-3a and the second-eluted esterester ((−−)-)-3b3b taketake the the preferred preferred conformations asas shownshown inin FigureFigure3 3a.a. In In ester ( (−−)-)-3a3a,, the the propyl propyl group group is is located above the naphthyl group plane, and hence the protons of the propyl gr groupoup feel the diamagnetic diamagnetic anisotropy effect leading to a high- high-fieldfield shift. shift. On On the the other other hand, hand, in in ester ester (− ()-−3b)-,3b the, the butyl butyl group group protons protons are areplaced placed above above the thenaphthalene naphthalene ring, ring, leading leading to a to high-field a high-field shift. shift. The The diamagnetic diamagnetic anisotropy anisotropy effect effect (∆δ ()∆ isδ) defined is defined as asshown shown in Figure in Figure 3b,3 whereb, where X isX theis AC the of AC the of first-eluted the first-eluted ester ester(−)-3a (− to)- be3a determined.to be determined. The ∆δ The values∆δ 1 valueswere calculated were calculated from the from observed the observed H-NMR1H-NMR spectra spectra as shown as shownin Figure in Figure3c, where3c, wherethe propyl the propyl group groupshowing showing positive positive ∆δ values∆δ values is placed is placed on the on theright right side, side, while while the the butyl butyl group group giving giving negative negative ∆∆δ values is placed on the left side. By applying th thee present sector rule, the AC of the first-elutedfirst-eluted MαNP ester (−)-)-3a3a waswas determined determined to to be be ( (RR))[ [13].13].

Figure 3. DeterminationDetermination of of absolute absolute configurations configurations of of 4-octanol 4-octanol M MαNPαNP esters esters (−)- (−3a)- and3a and (−)- (3b− )-by3b 1H-by 1 NMRH-NMR diamagnetic diamagnetic anisotropy: anisotropy: (a) Preferred (a) Preferred conformations. conformations; (b) Definition (b) Definition of Δδ of value.∆δ value; (c) Sector (c) Sector rule. rule.Redrawn Redrawn with withpermission permission from from[13]. [13].

Molecules 2016, 21, 1328 4 of 37

MoleculesMolecules 20162016,, 2121,, 1328 1328 44 of of 37 37 The first-eluted ester (−)-3a was hydrolyzed with KOH/MeOH yielding enantiopure 4-octanol 35 (R)-(−The)-2 (yieldfirst-eluted 70%), ester whose (−)- specific3a was hydrolyzedrotation value with was KOH/MeOH negative: [ yieldingα]D = −enantiopure0.50 (ρ = 0.819, 4-octanol neat) The first-eluted ester (−)-3a was hydrolyzed with KOH/MeOH yielding35 enantiopure 4-octanol ((FigureR)-(−)-2 4) (yield [13]. To70%), compare whose with specific the reportedrotation valuedata, wewas checked negative: the [ αliterature,]D = −0.50 and ( ρfound = 0.819, only neat) one (R)-(−)-2 (yield 70%), whose specific rotation value was negative: [α] 35 = −0.50 (ρ = 0.819, neat) (Figurepaper, reported 4) [13]. To in compare1936, where with (R the)-AC reported was assigned data, we to checked4-octanol the (+)- literature,2 D(Figure and4) [20]. found Namely, only onethe (Figure4)[ 13]. To compare with the reported data, we checked the literature, and found only one paper,AC assignment reported wasin 1936, opposite where to ( Rours.)-AC Itwas was assigned a time beforeto 4-octanol the discovery (+)-2 (Figure of the 4) unambiguous[20]. Namely, ACthe paper, reported in 1936, where (R)-AC was assigned to 4-octanol (+)-2 (Figure4)[ 20]. Namely, ACdetermination assignment by was the oppositeX-ray Bijvoet to ours. method It was [17] a usin timeg thebefore anomalous the discovery scattering of the effect unambiguous of heavy atoms, AC the AC assignment was opposite to ours. It was a time before the discovery of the unambiguous determinationand hence the by AC the assignment X-ray Bijvoet in method1936 would [17] usin be gunreliable. the anomalous Thus, scattering we have effectfirst unambiguouslyof heavy atoms, AC determination by the X-ray Bijvoet method [17] using the anomalous scattering effect of heavy anddetermined hence the the AC(R)-AC assignment of 4-octanol in 1936 (−)-2 .would be unreliable. Thus, we have first unambiguously atoms, and hence the AC assignment in 1936 would be unreliable. Thus, we have first unambiguously determined the (R)-AC of 4-octanol (−)-2. determined the (R)-AC of 4-octanol (−)-2.

Figure 4. Recovery of enantiopure 4-octanol (R)-(−)-2 and its data. The literature data: 25 25 − 25 Figure[FigureM]D 4. 4. value RecoveryRecovery from of of[20]; enantiopure enantiopure [α]D value 4-octanol estimated ( R)-( from−)-)-22 and and[M ]itsD its optical optical by us. rotation rotation data. data. The The literature literature data: data: 25 25 25 [M]D25value from [20]; [α]D 25value estimated from [M]D 25by us. [M]D value from [20]; [α]D value estimated from [M]D by us. We have developed powerful chiral molecular tools suitable for the HPLC separation of We have developed powerful chiral molecular tools suitable for the HPLC separation of diastereomersWe have anddeveloped also for powerfulthe determination chiral molecular of ACs. The tools development suitable for and the application HPLC separation examples of diastereomers and also for the determination of ACs. The development and application examples of diastereomersthe methods using and alsothese for chiral the determinationmolecular tools of will AC s.be The explained development in the following and application sections. examples of the methods using these chiral molecular tools will be explained in the following sections. the methods using these chiral molecular tools will be explained in the following sections. 2. Use of (−)-Camphorsultam for Carboxylic Acids 2. Use of (−)-Camphorsultam for Carboxylic Acids 2. Use of (−)-Camphorsultam for Carboxylic Acids 2.1. Application Application to to Spiro[3.3]Heptane-Dicarboxylic Acids 2.1. ApplicationFecht acid to4 isSpiro[3.3]Heptane-Dicarboxylic a unique dicarboxylic acid withAcids spir o[3.3]heptane skeleton (Figure 5). However, Fecht acid 4 is a unique dicarboxylic acid with spiro[3.3]heptane skeleton (Figure5). However, it has protons at the α-positions of groups, and hence it may be unstable under basic it hasFecht protons acid 4 atis a the uniqueα-positions dicarboxylic of carboxylic acid with spir acido[3.3]heptane groups, and skeleton hence (Figure it may 5). be However, unstable conditions. To prevent such a possibility, we designed a Fecht acid analog, 2,6-dimethyl- itunder has protons basic conditions.at the α-positions To of prevent carboxylic such acid a possibility,groups, and wehence designed it may be a unstable Fecht acid under analog, basic spiro[3.3]heptane-2,6-dicarboxylic acid (5) in which the α-positions are blocked by methyl groups conditions.2,6-dimethyl-spiro[3.3]heptane-2,6-dicarboxylic To prevent such a possibility, acidwe (designed5) in which a theFechtα-positions acid analog, are blocked 2,6-dimethyl- by methyl (Figure 5) [21]. spiro[3.3]heptane-2,6-dicarboxylicgroups (Figure5)[21]. acid (5) in which the α-positions are blocked by methyl groups (Figure 5) [21].

Figure 5. Absolute configurations configurations of Fecht acid (+)-4 and analog (+)- 55.. Figure 5. Absolute configurations of Fecht acid (+)-4 and analog (+)-5. To make make the the chiral resolution of acid of 5, acidwe have5, wetried have the HPLC tried separation the HPLC of diastereomeric separation of − α amidesdiastereomericTo makeformed the from amides chiral chiral resolution formed , of from suchacid chiral5as, we (S )-(have amines,−)-α tried-phenylethylamine, the such HPLC as separation (S)-( (S)-)-(−-phenylethylamine,)- ofα-naphthylethyl- diastereomeric − α amidesamine,(S)-( )- andformed-naphthylethyl-amine, (+)-dehydroabietylamine, from chiral amines, and (+)-dehydroabietylamine,butsuch all as attempts (S)-(−)- wereα-phenylethylamine, unsuccessful. but all attempts Finally, (S)-(− we were)-α have-naphthylethyl- unsuccessful. found that − amine,(Finally,−)-camphorsultam and we (+)-dehydroabietylamine, have found 6 was that useful ( )-camphorsultam as shown but all in attempts Figure6 was6. were Racemic useful unsuccessful. acid as shown (±)-5 wasFinally, in Figureconverted we6 have. Racemic into found the acid acidthat ± − (chloride,(−)-camphorsultam)-5 was which converted was 6 into wasthen theuseful treated acid as chloride, shownwith camphorsultam in which Figure was 6. Racemic then (− treated)-6 acid/NaH. with(±)- The5 camphorsultamwas obtained converted diastereomeric into ( )- the6/NaH. acid chloride,mixtureThe obtained ( 7awhich/7b diastereomeric) was was well then separated treated mixture bywith (HPLC7a /camphorsultam7b on) was silica well gel separated (resolution (−)-6/NaH. by factor HPLCThe Rsobtained on = 1.79) silica (Figurediastereomeric gel (resolution 6b) [21]. mixtureSincefactor theRs (7a =sample/ 1.79)7b) was (Figure (120 well mg)6 separatedb) [was21]. separable Since by HPLC the samplein on one silica (120ru n,gel mg)this (resolution wasmethod separable factoris good inRs one=for 1.79) run,the (Figure preparation this method 6b) [21]. of is Sinceenantiopuregood forthe thesample preparationtarget (120 compounds mg) of enantiopurewas separableon a laboratory target in one compounds scale.run, thisWe on methodhave a laboratory a isquestion good scale. for why Wethe camphorsultam havepreparation a question of enantiopureamideswhy camphorsultam 7a and target 7b are compounds amidesmore easily7a onand separable a 7blaboratoryare by more HPLC scale. easily on We separablesilica have gel a than byquestion HPLC the other onwhy silicaamides camphorsultam gel described than the amidesabove.other amides The 7a and author described 7b areconsiders more above. easily that The the separable author polar considersSO by2 moietyHPLC that onstrongly the silica polar interactsgel SO than2 moiety thewith other stronglysilica amides gel. interacts described with silica gel. above. The author considers that the polar SO2 moiety strongly interacts with silica gel.

Molecules 2016, 21, 1328 5 of 37 Molecules 2016, 21, 1328 5 of 37 MoleculesThe second-eluted 2016, 21, 1328 (−)-7b was recrystallized from EtOAc giving prisms suitable for5 of X-ray 37 analysis. The diffraction measurements were performed with Cu Kα X-ray giving the ORTEP TheThe second-eluted second-eluted amide amide ( −(−)-7b was recrystallized recrystallized from from EtOAc EtOAc giving giving prisms prisms suitable suitable for for X-ray X-ray drawing as shown in Figure 7. analysis.analysis. The The diffraction diffraction measurements measurements were were performed performed with with Cu KCuα X-rayKα X-ray giving giving the ORTEP the ORTEP drawing asdrawing shown in as Figure shown7 .in Figure 7.

FigureFigure 6. 6.Preparation Preparation of of diastereomeric diastereomeric amidesamides 7a7a andandand 7b7b7b ( (a(aa),),), separation separation separation by by by HPLC HPLC HPLC on on on silica silica silica gel gelgel (b ),( ( bb),), andand recovery recovery of of acid acid (S ()-(+)-S)-(+)-55 ( c(c).). HPLC, HPLC,HPLC, redrawnredrawn fromfrom from [22].[22]. [22].

Figure 7. X-ray ORTEP drawing of amide (R)-(−)-7b. Reprinted from [21]. FigureFigure 7.7. X-ray ORTEP drawingdrawing ofof amideamide ((RR)-()-(−−)-)-7b7b.. Reprinted Reprinted from from [21]. [21]. The X-ray final R value (resudual factor) was 0.0581 for the (R)-AC shown, while that of the The X-ray final final R value (resudual factor) was 0.0581 for the (R)-AC shown, while that of the mirror image was 0.0650. Therefore, the (R)-AC was assigned to amide (−)-7b by the X-ray heavy mirror image was 0.0650. Therefore, the (R)-AC was assignedassigned toto amideamide ((−−)-)-7b7b byby the X-ray heavy atom effect of the sulfur atom contained in the camphorsultam unit. In addition, the (R)-AC of the atom effect of the sulfur atom contained in the camphorsultam unit. In addition, the (R)-AC of the atom2,6-dimethyl-spiro[3.3]heptane-2,6-dicarboxylic effect of the sulfur atom contained in the camphorsultamacid unit was confirmed unit. In addition, by using the the ( Rcamphor-)-AC of the 2,6-dimethyl-spiro[3.3]heptane-2,6-dicarboxylicsultam groups as the internal reference of AC. Thus, acid acid unitthe unit AC was wasof confirmed the confirmed dicarboxylic by using by acid using the moiety - the in camphor- amide sultam sultamgroups(−)-7b groups as was the internal doublyas the internal referencedetermined reference of AC.by Thus,X-rayof AC. the crystalloThus, AC the ofgraphy; the AC dicarboxylic of thethis dicarboxylic is a acidgreat moiety acidadvantage moiety in amide ofin (amide−the)- 7b was(−)-camphorsultam7b doubly was determineddoubly method determined by[21]. X-ray crystallography;by X-ray crystallo thisgraphy; is a great this advantage is a great of theadvantage camphorsultam of the camphorsultammethodTo [21 cleave]. methodthe amide [21]. C–N bond, the first-eluted amide (S)-(−)-7a was reduced with LiAlH4 yieldingTo cleavecleave a bis(primary thethe amideamide alcohol), C–NC–N bond,bond, which thethe first-elutedwasfirst-eluted then oxidized amideamide ((SwithS)-()-(−− )-the7a wasJones reduced reagent, with affording LiAlH4LiAlH 4 yieldingdicarboxylic aa bis(primary bis(primary acid (S)-(+)- alcohol), alcohol),5 (Figures which which5 and was 6c). thenwas The oxidizedthen camphorsultam oxidized with the with Jonesmethod the reagent, is Jonesthus affording very reagent, useful dicarboxylic affordingfor the dicarboxylicacidpreparation (S)-(+)-5 acid(Figures of enantiopure (S)-(+)-5 and5 (Figures6c). carboxylic The 5 camphorsultam and acids 6c). and The also camphorsultam method for determining is thus method very their useful ACs is thus for[21]. thevery preparation useful for the of preparationenantiopureThe related of carboxylic enantiopure diastereomers acids carboxylic and 8a also and foracids 8b determining with and benzyloxy also for their determining groups ACs [ 21were]. their largely ACs separated [21]. by HPLC on Thesilica related gel (Figure diastereomersdiastereomers 8). It should8a 8a beand and emphasized8b 8bwith with benzyloxy benzyloxythat these groups bis-amides groups were were appeared largely largely separated as separated clearly byseparated by HPLC HPLC on onsilicatwo silica gel spots gel (Figure even(Figure8 on). It a8). should5 Itcm should thin be layer emphasized be emphasized chromatography that that these (TLC)these bis-amides bis-amidesplate of appeared silica appeared gel. asThe clearly first-elutedas clearly separated separated amide two twospots(− )-spots8a even was even on recrystallized a 5on cm a 5 thin cm layer thinfrom layer chromatography EtOAc chromatography giving prisms, (TLC) (TLC)one plate of of platewhich silica of was gel.silica subjected The gel. first-eluted The to first-eluted X-ray amide analysis (amide−)- 8a was(−)- 8a recrystallized was recrystallized from EtOAc from givingEtOAc prisms, giving oneprisms, of which one of was which subjected was subjected to X-ray analysis to X-ray [23 analysis]. From

Molecules 2016, 21, 1328 6 of 37 Molecules 2016, 21, 1328 6 of 37 [23].Molecules From 2016 the, 21 ,ORTEP 1328 drawing, its (S) absolute configuration was established as shown. Amide6 (ofS )-37 (−)-8a was then converted to dicarboxylic acid (S)-(+)-5 (Figure 5). the[23]. ORTEP From the drawing, ORTEP its drawing, (S) absolute its (S configuration) absolute configuration was established was established as shown. Amideas shown. (S)-( Amide−)-8a was (S)- then(−)-8a converted was then toconverted dicarboxylic to dicarboxylic acid (S)-(+)- acid5 (Figure (S)-(+)-5).5 (Figure 5).

Figure 8. HPLC separation of camphorsultam amides 8a and 8b, and an X-ray ORTEP drawing of the first-eluted amide (S)-(−)-8a. HPLC, redrawn from [24]. X-ray ORTEP drawing, reprinted with Figure 8. HPLC separation of camphorsultam amides 8a and 8b, and an X-ray ORTEP drawing of permissionFigure 8. HPLC from separation[23]. of camphorsultam amides 8a and 8b, and an X-ray ORTEP drawing of the thefirst-eluted first-eluted amide amide (S)-( (S−)-()-8a−)-. 8aHPLC,. HPLC, redrawn redrawn from from [24]. [24 ].X-ray X-ray ORTEP ORTEP drawing, drawing, reprinted withwith 2.2. Applicationpermissionpermission tofromfrom Cyclophane-Carboxylic [[23].23]. Acids

2.2.2.2. ApplicationThe camphorsultam toto Cyclophane-CarboxylicCyclophane-Carboxylic method was AcidsAcids useful for the preparation of enantiopure cyclophane compounds with a plane chirality, and also for the direct and clear-cut determination of their ACs. TheThe camphorsultamcamphorsultam method method was was useful useful for the for preparation the preparation of enantiopure of enantiopure cyclophane compoundscyclophane In 1970, Schloegl and coworkers reported the AC of [2.2]paracyclophane-4-carboxylic acid 9 [25]. withcompounds a plane with chirality, a plane and chirality, also for and the also direct for andthe direct clear-cut and determination clear-cut determination of their ACs. of their In 1970,ACs. Acid 9 was enantiomerically enriched by the of its anhydride with (−)-phenyl- SchloeglIn 1970, andSchloegl coworkers and coworkers reported thereported AC of [2.2]paracyclophane-4-carboxylicthe AC of [2.2]paracyclophane-4-carboxylic acid 9 [25]. Acidacid 99 [25].was ethylamine, where (S)-AC was empirically assigned to acid (+)-9. Furthermore, the (R)-AC of acid enantiomericallyAcid 9 was enantiomerically enriched by enriched the kinetic by resolution the kinetic of resolution its anhydride of its with anhydride (−)-phenyl-ethylamine, with (−)-phenyl- (−)-9 was also determined by applying the empirical Horeau’s rule to related compounds. However, whereethylamine, (S)-AC where was empirically (S)-AC was assigned empirically to acid assigned (+)-9. Furthermore,to acid (+)-9. theFurthermore, (R)-AC of acidthe ( R−)-AC)-9 was of alsoacid the AC of acid 9 was later involved in a controversy in relation to the AC determination of other determined(−)-9 was also by determined applying the by empirical applying Horeau’s the empirical rule toHoreau’s related compounds.rule to related However, compounds. the AC However, of acid paracyclophane compounds. Therefore, it was necessary to determine its AC in a non-empirical 9thewas AC later of acid involved 9 was in later a controversy involved in in a relation controversy to the in AC relation determination to the AC of determination other paracyclophane of other manner (Figure 9) [23]. compounds.paracyclophane Therefore, compounds. it was necessaryTherefore, to it determine was necessary its AC into adetermine non-empirical its AC manner in a (Figurenon-empirical9)[ 23]. manner (Figure 9) [23].

Figure 9. Preparation of diastereomeric amides 10a and 10b (a), separation by HPLC on silica gel (b), Figure 9. Preparation of diastereomeric amides 10a and 10b (a), separation by HPLC on silica gel (b), and preparation of ester (S)-(+)-11 (c). HPLC, redrawn from [24]. and preparation of ester (S)-(+)-11 (c). HPLC, redrawn from [24]. Figure 9. Preparation of diastereomeric amides 10a and 10b (a), separation by HPLC on silica gel (b), and preparation of ester (S)-(+)-11 (c). HPLC, redrawn from [24].

MoleculesMolecules 20162016,, 2121,, 13281328 77 ofof 3737

Racemic acid (±)-9 was converted to the acid chloride, which was then treated with camphorsultamRacemic acid (−)- (6±/NaH)-9 was (Figure converted 9) [23]. The to theobtained acid chloride,diastereomeric which mixture was thenof amides treated 10a with and camphorsultam10b was separated (− by)-6 HPLC/NaH on (Figure silica9 gel)[ as23 ].shown The in obtained Figure 9b. diastereomeric The second-eluted mixture amide of amides(−)-10b had10a anda good10b crystallinity,was separated and by HPLCso it crystallized on silica gel before as shown HPLC. in Figure Thus,9 b.by The combining second-eluted recrystallization amide ( −)- 10band hadHPLC a good separation, crystallinity, diastereomeric and so it crystallized amides 10a beforeand 10b HPLC. were Thus,completely by combining separated. recrystallization and HPLCThe separation, second-eluted diastereomeric amide 10b amides was recrystallized10a and 10b fromwere EtOAc completely yielding separated. prisms suitable for X-ray. BasedThe on second-elutedthe AC of the camphorsultam amide 10b was recrystallizedpart in the ORTEP from drawing, EtOAc yielding the AC prismsof the cyclophane suitable for moiety X-ray. Basedin (−)- on10b the was AC unambiguously of the camphorsultam determined part in to the be ORTEP (S) (Figure drawing, 10), thewhich AC was of the corroborated cyclophane moietyby the inanomalous (−)-10b was scattering unambiguously effect of the determined sulfur atom to (real be (image,S) (Figure R = 0.0299;10), which mirror was image, corroborated R = 0.0348) by [23]. the anomalousThe first-eluted scattering amide effect (−)-10a of thewas sulfur recrystallized atom (real from image, MeOHR = giving 0.0299; prisms; mirror X-ray image, analysisR = 0.0348) revealed [23]. Thethat first-elutedone asymmetric amide (unit−)-10a containedwas recrystallized two independent from MeOH molecules, giving prisms;and hence X-ray the analysis final revealedR value thatremained one asymmetric large. Therefore, unit contained it was difficult two independent to determine molecules, the AC andby the hence heavy the atom final Reffect,value because remained of large.the small Therefore, difference it was between difficult final to R determine values (R the= 0.1226 AC by for the the heavy real image atom and effect, R = because 0.1239 for of theits mirror small differenceimage). However, between by final usingR values the camphorsultam (R = 0.1226 for un theit as real an image internal and referenceR = 0.1239 of AC, for itsthe mirror (R)-AC image). of (−)- However,10a was clearly by using determined the camphorsultam as shown in unit Figure as an 10. internal Finally,reference amide (S)-( of− AC,)-10b the was (R )-ACconverted of (− )-to10a methylwas clearlyester ( determinedS)-(+)-11 (Figure as shown 9c). inThe Figure ACs 10 .of Finally, [2.2]paracyclophane-4-carboxylic amide (S)-(−)-10b was converted acid to9 methyland related ester (compoundsS)-(+)-11 (Figure were9 thusc). The unambiguously ACs of [2.2]paracyclophane-4-carboxylic determined [23]. acid 9 and related compounds were thus unambiguously determined [23].

− − FigureFigure 10.10. X-rayX-ray ORTEPORTEP drawings drawings of of camphorsultam camphorsultam amides amides (R ()-(R)-(−)-)-10a10aand and ( S(S)-()-(−)-)-10b10b.. ReprintedReprinted withwith permissionpermission fromfrom [[23].23].

ThereThere hadhad beenbeen conflicting conflicting reports reports concerning concerning the th ACse ACs of of [8]paracyclophane-10-carboxylic [8]paracyclophane-10-carboxylic acid acid12, [10]paracyclophane-12-carboxylic12, [10]paracyclophane-12-carboxylic acid 14 acid, and 14 a, related and a compound related compound15 (Figure 1115). (Figure In 1972, 11). Schloegl In 1972, and aSchloegl coworker and determined a coworker the determined AC of [10]paracyclophane-12-carboxylic the AC of [10]paracyclophane-12-carboxylic acid 14 as follows acid [ 2614]. as As follows in the case[26]. ofAs [2.2]paracyclophane-4-carboxylic in the case of [2.2]paracyclophane-4-carboxylic acid 9, acid 14 wasacid enantiomerically9, acid 14 was enantiomerically enriched by the enriched kinetic resolutionby the kinetic of its resolution anhydride of withits anhydride (−)-phenylethylamine, with (−)-phenylethylamine, where (S)-AC where was empirically(S)-AC was assignedempirically to acidassigned (−)-14 to. acid Furthermore, (−)-14. Furthermore, the (S)-AC ofthe acid (S)-AC (−)- of14 acidwas ( also−)-14 determined was also determined by applying by the applying empirical the Horeau’sempirical ruleHoreau’s to related rule compounds.to related compounds. OnOn the other other hand, hand, in in1974–1977, 1974–1977, Nakazaki Nakazaki and andcoworkers coworkers reported reported the synthesis the synthesis of a unique of a uniquecompound, compound, (+)-[8]bridged (+)-[8]bridged [2.2]paracyclophane [2.2]paracyclophane 13, and 13related, and compounds related compounds starting startingfrom [8]para- from [8]para-cyclophane-10-carboxyliccyclophane-10-carboxylic acid (+)-12 acid (Figure (+)- 1211) (Figure[27]. The 11 ACs)[27 of]. th Theese compounds ACs of these were compounds determined wereby chemical determined correlation by chemical with correlation[2.2]paracyclophane-4-carboxylic with [2.2]paracyclophane-4-carboxylic acid (S)-(+)-9, where acid (chemicalS)-(+)-9, wherereactions chemical of many reactions steps were of performed. many steps So, were the (S performed.)-AC was assigned So, the to (S acid)-AC (+)- was12. assignedThe CD spectrum to acid (+)-of acid12. The(S)-(+)-[CD(+)248]- CD spectrum12 of was acid almost (S)-(+)-[CD(+)248]- a mirror image12 of wasthat of almost 15-methyl-[10]paracyclophane-12- a mirror image of that of 15-methyl-[10]paracyclophane-12-carboxyliccarboxylic acid (R)-(−)-[CD(−)245]-15 [27]. On acid the ( Rother)-(−)-[CD( hand,− the)245]- CD15 of[27 [10]paracyclophane-12-]. On the other hand, thecarboxylic CD of [10]paracyclophane-12-carboxylicacid (S)-(−)-[CD(−)239]-14 reported acid by Schloegl (S)-(−)-[CD( was similar−)239]- including14 reported a sign by Schloeglto that of wasacid similar(−)-15 despite including their a opposite sign to thatACs of(Figure acid (11)−)- [26].15 despite Thus, these their AC opposite assignments ACs (Figure were clearly 11)[26 in]. conflict Thus,

Molecules 2016, 21, 1328 8 of 37 Molecules 2016, 21, 1328 8 of 37 Molecules 2016, 21, 1328 8 of 37 thesewith AC each assignments other [28,29]. were To clearlysolve these in conflict problems, with we each have other applied [28,29 a]. more To solve unambiguous these problems, method, we i.e., have with each other [28,29]. To solve these problems, we have applied a more unambiguous method, i.e., the camphorsultam method, as explained below [30]. appliedthe camphorsultam a more unambiguous method, as method, explained i.e., below the camphorsultam [30]. method, as explained below [30].

Figure 11. Absolute configurations (ACs) of chiral paracyclophane-carboxylic acids assigned by FigureFigure 11. 11.Absolute Absolute configurationsconfigurations (ACs) of chiral paracyclophane-carboxylic paracyclophane-carboxylic acids acids assigned assigned by by chemical correlation and comparison of circular dichroism (CD) spectra, where the ACs designated chemicalchemical correlation correlation and and comparison comparison ofof circularcircular dichroism (CD) (CD) spectra, spectra, where where the the ACs ACs designated designated with an asterisk * were reversed, as will be explained below. The AC of compound (+)-13 has remained withwith an an asterisk asterisk * were* were reversed, reversed, as aswill will bebe explained below. The The AC AC of of compound compound (+)- (+)-1313 hashas remained remained unclear. Redrawn with permission from [30]. unclear.unclear. Redrawn Redrawn with with permission permission fromfrom [[30].30]. Racemic [10]paracyclophane-12-carboxylic acid, (±)-14 was converted to the acid chloride which RacemicRacemic [10]paracyclophane-12-carboxylic [10]paracyclophane-12-carboxylic acid,acid, ((±)-±)-1414 waswas converted converted to to the the acid acid chloride chloride which which was then treated with camphorsultam (−)-6/NaH (Figure 12). The obtained diastereomeric mixture of waswas then then treated treated with with camphorsultam camphorsultam (−)-)-66/NaH/NaH (Figure (Figure 12). 12 The). The obtained obtained diastereomeric diastereomeric mixture mixture of amides 16a and 16b was separated by HPLC on silica gel as shown in Figure 12b, where two peaks ofamides amides 16a16a andand 16b16b waswas separated separated by HPLC by HPLC on silica on silicagel asgel shown as shown in Figure in 12b, Figure where 12b, two where peaks two partially overlap with each other. So, HPLC separation was repeated twice to obtain pure peakspartially partially overlap overlap with witheach each other. other. So, So,HPLC HPLC separation separation was was repeated repeated twice twice to toobtain obtain pure pure diastereomers [30]. diastereomersdiastereomers [30 [30].].

Figure 12. Preparation of diastereomeric amides 16a and 16b (a), separation by HPLC on silica gel (b), FigureFigure 12. 12.Preparation Preparation of of diastereomeric diastereomeric amidesamides 16a andand 16b16b (a(a),), separation separation by by HPLC HPLC on on silica silica gel gel (b), ( b), and preparation of methyl ester (S)-(−)-17 (c). HPLC, redrawn from [24]. andand preparation preparation of of methyl methyl ester ester ( S(S)-()-(−−)-17 ((cc).). HPLC, HPLC, redrawn redrawn from from [24]. [24 ]. The second-eluted amide 16b had a good crystallinity, and it was recrystallized from MeOH TheThe second-eluted second-eluted amide amide 16b16b hadhad aa goodgood crystallinity, and and it it was was recrystallized recrystallized from from MeOH MeOH giving prisms. However, X-ray experiments indicated that the asymmetrical unit contained three giving prisms. However, X-ray experiments indicated that the asymmetrical unit contained three givingindependent prisms. molecules, However, and X-ray hence experiments it was difficult indicated to continue that the the asymmetrical X-ray analysis. unit The contained first-eluted three independent molecules, and hence it was difficult to continue the X-ray analysis. The first-eluted independentamide 16a was molecules, similarly andrecrystallized hence it wasfrom difficult MeOH givi to continueng plate crystals. the X-ray Alth analysis.ough the Theasymmetrical first-eluted amideamide16a 16awas was similarly similarly recrystallized recrystallized fromfrom MeOHMeOH givi givingng plate plate crystals. crystals. Alth Althoughough the the asymmetrical asymmetrical

Molecules 2016, 21, 1328 9 of 37 Molecules 2016, 21, 1328 9 of 37

unitMolecules contained 2016, 21 two, 1328 independent molecules, it was possible to obtain the structure as9 shownof 37 unit contained two independent molecules, it was possible to obtain the crystal structure as shown in in Figure 13 [30]. Figureunit contained13[30]. two independent molecules, it was possible to obtain the crystal structure as shown in Figure 13 [30].

Figure 13.13. X-ray ORTEP drawingdrawing ofof amideamide ((SS)-)-16a16a.. ReprintedReprinted withwith permissionpermission fromfrom [[30].30]. Figure 13. X-ray ORTEP drawing of amide (S)-16a. Reprinted with permission from [30]. AsAs seenseen inin thethe ORTEPORTEP drawing,drawing, thethe methylenemethylene chainchain showedshowed largelarge thermalthermalvibration vibration and/orand/or disorder,disorder,As andandseen sosoin thethethe finalfinalORTEP RR value valuedrawing, remainedremained the methylene largelarge ((RR chain== 0.11190.1119 showed forfor thethe large realreal thermal imageimage andvibrationand RR == 0.11220.1122and/or forfor mirrormirrordisorder, image).image). and Thus,Thus,so the itit final was R impossible value remained toto determinedetermine large (R thethe = 0.1119 ACAC byby for thethe the anomalous real image scatteringscattering and R = 0.1122 effecteffect offorof thethe sulfursulfurmirror atom.atom. image). However, Thus, it itit was waswas impossible easyeasy toto assignassign to determine thethe ((SS)-AC)-AC the AC ofof the theby the paracyclophaneparacyclophane anomalous scattering partpart fromfrom effect thethe of ORTEPORTEP the sulfur atom. However, it was easy to assign the (S)-AC of the paracyclophane part from the ORTEP drawing,drawing, becausebecause thethe ACAC ofof thethe camphorsultamcamphorsultam unitunit waswas alreadyalready known.known. TheThe first-elutedfirst-eluted amideamide drawing, because the AC of the camphorsultam unit was already known. The first-eluted amide ((SS)-)-16a16a waswas convertedconvertedto tomethyl methyl ester ester ( S(S)-()-(−−)-[CD()-[CD(−−)238.2]-)238.2]-1717 [30],[30], the the CD CD data data of which were similar (S)-16a was converted to methyl ester (S)-(−)-[CD(−)238.2]-17 [30], the CD data of which were similar toto thosethose ofof acidacid ((−−)-)-14 [26].[26]. This This X-ray result was consistentconsistent withwith thethe ((SS)-AC)-AC ofof acidacid ((−−)-14 previously to those of acid (−)-14 [26]. This X-ray result was consistent with the (S)-AC of acid (−)-14 previously assignedassigned byby thethe empiricalempirical methodsmethods [[26].26]. assigned by the empirical methods [26]. Next, the camphorsultam method was applied to [8]paracyclophane-10-carboxylic acid 12. Next,Next, the the camphorsultam camphorsultam methodmethod was applied to to [8]paracyclophane-10-carboxylic [8]paracyclophane-10-carboxylic acid acid 12. . ± RacemicRacemicRacemic acidacid acid ((±)- (±)-)-121212 was waswas converted converted toto diastereomeric amidesamides 18A 18A and and 18B 18B (Figure (Figure(Figure 14), 1414), which), whichwhich were werewere subjectedsubjectedsubjected toto to HPLC,HPLC, HPLC, but butbut unfortunately unfortunatelyunfortunately thethe amidesamidesamides couldcould not notnot be bebe separated. separated.separated. (Please (Please note note that thatthat in inin compoundscompoundscompounds18A 18A 18A andand and 18B18B 18B,, , thethe the capitalcapital capital letterslettersletters A andand B B do do not not indicate indicate HPLC HPLC elution elution order, order, just just meaning meaning twotwotwo diastereomers).diastereomers). diastereomers). So,So, So, thethe the fractionalfractional fractional recrystallizationrecrystallization from fromfrom MeOH MeOHMeOH was waswas applied, applied,applied, giving givinggiving amide amideamide 18B 18B18B as asas plateplateplate singlesingle single crystals crystals crystals suitable suitablesuitable for for X-rayfor X-rayX-ray crystallography. crystallography. The ORTEPTheThe ORTEP ORTEP drawing drawing drawing of amide of ofamide18B amidewas 18B obtained18B was was asobtainedobtained shown inas as Figureshown shown 14 in inb. Figure TheFigure (S )-AC14b.14b. The of 18B (S)-AC)-ACwas determinedofof 18B18B waswas bydetermined determined the heavy by atomby the the effectheavy heavy (Ratom =atom 0.0441 effect effectfor the(R( R= real =0.0441 0.0441 image for for and the theR real=real 0.0538 imageimage for andand its mirrorR = 0.05380.0538 image). forfor Thisitsits mirrormirror assignment image). image). was This This also assignment assignment corroborated was was also by also the internalcorroboratedcorroborated reference by by the methodthe internal internal of camphorsultamrefere referencence method [30 ofof]. camphorsultamcamphorsultam [30]. [30].

FigureFigure 14. 14.Preparation Preparation of of amides amides 18A18A andand 18B (a), and and the the X-ray X-ray ORTEP ORTEP drawing drawing of of camphorsultam camphorsultam amideFigureamide (S14. ()-S18B )-Preparation18B(b ().b). Reprinted Reprinted of amides with with 18A permissionpermission and 18B from (a), and[30]. [30]. the X-ray ORTEP drawing of camphorsultam amide (S)-18B (b). Reprinted with permission from [30]. TheThe amount amount ofof amideamide 18B18B obtainedobtained by by recrystallization recrystallization was was limited, limited, and so and we so adopted we adopted the themethod methodThe amountof camphorsultam-phthalic of camphorsultam-phthalic of amide 18B obtained acid (CSP acidby recrystallizationacid) (CSP (− acid))-22 (see (− Figure)- was22 (see limited,16), Figure which and had16 ),so been whichwe developedadopted had been the developedmethodby us offor camphorsultam-phthalic bythe us chiral for theresolution chiral resolutionof acid alcohols, (CSP of acid)as alcohols, will (−)- be22 as(seediscussed will Figure be in discussed16), the which next inhadsection. the been next Racemicdeveloped section.

by us for the chiral resolution of alcohols, as will be discussed in the next section. Racemic

Molecules 2016, 21, 1328 10 of 37 Molecules 2016, 21, 1328 10 of 37

[8]paracyclophane-10-methanolRacemic [8]paracyclophane-10-methanol (±)-19 was (esteri±)-19fiedwas with esterified (−)-CSP withacid 22 (−,)-CSP yielding acid diastereomeric22, yielding estersdiastereomeric 20a and 20b esters (Figure20a 15a),and 20bwhich(Figure were not15a), base-line which wereseparated not base-linein HPLC, separatedas shown in in Figure HPLC, 15b as [30].shown However, in Figure by 15 repeatingb [30]. However, HPLC, byit was repeating possible HPLC, to separate it was possiblethem completely. to separate Next, them we completely. tried the recrystallizationNext, we tried the of recrystallization the obtained esters of the from obtained various esters solvents, from various but in solvents, all cases but both in all esters cases were both obtainedesters were as fine obtained needles, as which fine needles, were unsuitable which were for unsuitableX-ray analysis. for X-raySo, to determine analysis. So,the toACs determine of these compounds,the ACs of these the compounds,second-eluted the ester second-eluted 20b was converted ester 20b was to convertedcamphorsultam to camphorsultam amide, which amide, was identicalwhich was to identicalamide (S to)-18B amide (Figure (S)-18B 15c).(Figure The ( 15S)-ACc). The of 18B (S)-AC was of clearly18B was determined clearly determined by X-ray byanalysis X-ray asanalysis shown as in shown Figure in Figure14. So, 14the. So, ACs the of ACs esters of esters 20a and20a and20b20b werewere determined determined to tobe be (R (R) )and and ( (SS),), respectively [30]. [30]. The first-eluted first-eluted ester (R)-20a was convertedconverted toto [8]paracyclophane-10-carboxylic[8]paracyclophane-10-carboxylic acid (R)-(+)-12 and then to the methyl methyl ester ( R)-(+)-[CD(+)247.7]-2121 (Figure(Figure 15d).15d). The The second-eluted second-eluted ester ( S)-)-20b was similarly convertedconverted to to methyl methyl ester ester (S)-[CD( (S)-[CD(−)247.2]-−)247.2]-21.21 These. These results results clearly clearly indicated indicated that the that (S)-AC the (ofS)-AC [8]paracyclophane-10-carboxylic of [8]paracyclophane-10-carboxylic acid (+)- acid12 previously (+)-12 previously assigned assigned by chemical by chemical correlations correlations requiring requiringmany steps many was steps wrong was and wrong should and be should reversed be reversed (Figure 11(Figure)[30]. 11) In addition,[30]. In addition, the AC the of acidAC of (− acid)-15 (was−)-15 also was reversed also reversed (Figure (Figure 11). 11). Thus, Thus, it should it should be be emphasized emphasized that that to to determine determine the the ACsACs ofof chiral compounds, it is necessary to select more straightforwardstraightforward and reliable methodsmethods asas exemplifiedexemplified here.here.

Figure 15. PreparationPreparation ( aa)) and and HPLC HPLC separation separation ( (b)) of of esters esters 20a20a andand 20b20b,, which which were were then then converted converted toto methyl esters esters ( (R)-(+)-)-(+)-2121 ((dd)) and and ( (SS)-)-2121 ((ee),), respectively. respectively. Ester Ester 20b20b waswas converted converted to to amide amide 18B18B ((cc),), by which the ACs of these compounds were established. HPLC, redrawn from [[24].24].

3. Use Use of of Camphorsultam-Phthalic Camphorsultam-Phthalic Acid Acid (CSP (CSP Acid) Acid) for for Alcohols—The Alcohols—The CSP CSP Acid Acid Method Method

Application to Alcohols with an Aromatic Group ((−)-Camphorsultam)-Camphorsultam 6 6waswas useful useful for for the the chiral chiral resolution resolution of racemic of racemic carboxylic carboxylic acids acids and andAC determinationAC determination by X-ray by X-ray crystallography, crystallography, as explained as explained in Section in Section 2. The2. development The development of novel of novelchiral molecularchiral molecular tools applicable tools applicable to racemic to racemicalcohols alcoholswas desired. was For desired. this purpose, For this we purpose, have developed we have noveldeveloped chiral novelmolecular chiral tools, molecular camphorsultam-phthalic tools, camphorsultam-phthalic acid (CSP acid) acid (−)-22 (CSP and acid) camphorsultam- (−)-22 and dichlorophthaliccamphorsultam-dichlorophthalic acid (CSDP acid) acid (−)-23 (CSDP (Figure acid) 16). (− )-23 (Figure 16).

Molecules 2016, 21, 1328 11 of 37

Molecules 2016,, 21,, 1328 1328 1111 of of 37 37

Figure 16. Chiral (−)-CSP and (−)-CSDP acids useful for alcohols. Figure 16. Chiral (−)-CSP and (−)-CSDP acids useful for alcohols. Camphorsultam-phthalicFigure 16. Chiralacid ( −(−)-)-CSP22 was and easily (−)-CSDP prepared acids useful by treating for alcohols. phthalic anhydride with the camphorsultam anion. The CSP acid method was first applied to a simple alcohol, as shown in Camphorsultam-phthalic acid acid ( (−−)-)-2222 waswas easily easily prepared prepared by by treating treating phthalic anhydride with Figure 17 [31]. Racemic 1-phenylethanol (±)-24 was esterified with CSP acid (−)-24 yielding thethe camphorsultam camphorsultam anion. anion. The The CSP CSP acid acid method method was was first first applied applied to toa simple a simple alcohol, alcohol, as asshown shown in diastereomeric esters 25a and 25b, which were separated well by HPLC on silica gel (α = 1.1, Figurein Figure 17 17[31].[ 31 ].Racemic Racemic 1-phenylethanol 1-phenylethanol (±)- (±24)- 24waswas esteri esterifiedfied with with CSP CSP acid acid (−)-)-2424 yielding Rs = 1.3) (Figure 17b). The reason why we selected the phthalic acid as the connector between diastereomeric estersesters25a 25aand and25b 25b, which, which were were separated separated well well by HPLC by HPLC on silica on gelsilica (α =gel 1.1, (αRs == 1.1, 1.3) camphorsultam and acid moieties is that these two groups are close to each other, and hence the Rs(Figure = 1.3) 17 (Figureb). The reason17b). The why reason we selected why thewe phthalicselected acidthe asphthalic the connector acid as betweenthe connector camphorsultam between diastereomers are more different in and polarity, which would lead to larger camphorsultamand acid moieties and is thatacid thesemoieties two is groups that these are close two togroups each other,are close and to hence each theother, diastereomers and hence the are separation in HPLC. diastereomersmore different inare stereochemistry more different and in polarity,stereochemis whichtry would and leadpolarity, to larger which separation would lead in HPLC. to larger The first-eluted ester 25a was recrystallized from MeOH giving prisms suitable for X-ray separationThe first-eluted in HPLC. ester 25a was recrystallized from MeOH giving prisms suitable for X-ray analysis. analysis. From the ORTEP drawing shown in Figure 17c, the AC of the 1-phenylethanol moiety was FromThe the first-eluted ORTEP drawing ester shown25a was in recrystallized Figure 17c, the from AC ofMeOH the 1-phenylethanol giving prisms moietysuitable was for clearlyX-ray clearly determined as (R), because the AC of the CSP acid part was already known. The first-eluted analysis.determined From as the (R), ORTEP because drawing the AC shown of the CSPin Figure acid part17c, wasthe AC already of the known. 1-phenylethanol The first-eluted moiety esterwas ester (R)-25a was then converted to 1-phenylethanol (R)-(+)-24. The AC of the 1-phenylethanol clearly(R)-25a determinedwas then converted as (R), because to 1-phenylethanol the AC of the (R CSP)-(+)- acid24. Thepart AC was of already the 1-phenylethanol known. The first-eluted previously previously assigned was thus corroborated by X-ray crystallography [31]. esterassigned (R)- was25a thuswas corroboratedthen converted by X-rayto 1-phenylethanol crystallography (R [)-(+)-31]. 24. The AC of the 1-phenylethanol previously assigned was thus corroborated by X-ray crystallography [31].

Figure 17. Preparation ( a)) and and HPLC HPLC separation separation ( (b)) of of esters esters 25a and 25b,, and and the the AC AC determination of ester ( R)-25a by X-ray analysis ( c). From ester ( R)-25a, alcohol (R)-(+)-2424 waswas obtained ( (dd).). HPLC, HPLC, Figure 17. Preparation (a) and HPLC separation (b) of esters 25a and 25b, and the AC determination redrawn from [32]. [32]. ORTEP draw drawing,ing, reprinted from [[31].31]. of ester (R)-25a by X-ray analysis (c). From ester (R)-25a, alcohol (R)-(+)-24 was obtained (d). HPLC, redrawnThe CSP from acid [32].method method ORTEP was was draw next nexting, app applied reprintedlied to to 1-(4-bromophenyl) 1-(4-bromophenyl)ethanol from [31]. 2626 asas shown shown in in Figure Figure 1818 [[31].31].

The CSP acid method was next applied to 1-(4-bromophenyl)ethanol 26 as shown in Figure 18 [31].

Molecules 2016, 21, 1328 12 of 37 Molecules 2016, 21, 1328 12 of 37 Molecules 2016, 21, 1328 12 of 37

Figure 18. Preparation and HPLC separation of esters 27a and 27b, and the AC determination of ester Figure(S)-27b 18. 18.by Preparation PreparationX-ray analysis. and and HPLCFrom HPLC separationester separation (S)-27b of esters, of alcohol esters 27a 27a (andS)-(and −27b)-26,27b andwas, andthe obtained. AC the determination AC ORTEP determination drawing, of ester of (reprintedesterS)-27b (S )-by27b from X-rayby [31]. X-ray analysis. analysis. From From ester ester (S)- (S27b)-27b, alcohol, alcohol (S ()-(S)-(−)-−26)- 26waswas obtained. obtained. ORTEP ORTEP drawing, drawing, reprinted from [31]. [31]. Racemic alcohol (±)-26 was esterified with CSP acid (−)-24 yielding diastereomeric esters 27a and 27b, Racemicwhich were alcohol similarly ((±)-±)-2626 separated waswas esteri esterified byfied HPLC with with onCSP CSP silica acid gel ( −)- )-(24α24 =yielding yielding1.1, Rs = diastereomeric diastereomeric1.3). Both esters esters esters 27a and27a andand27b 27bwere,, which whichrecrystallized were similarly from MeOH separated giving byby prisms. HPLCHPLC ononA single silicasilica gelgelcrystal ((αα == of 1.1, 27b Rs was = 1.3). subjected Both Both esters esters to X-ray 27a andanalysis 27b weregiving recrystallized the ORTEP drawing from MeOH as shown giving in prisms. Figure 18, A single where crystal the (S )-ACof 27b was was unambiguously subjected to X-ray X-ray determined analysis analysis givingby the the heavy ORTEP atom drawing effects of as S shown and Br in atoms Figure Figure (real 18,18, where whereimage, the the R =( (S S0.0352;)-AC)-AC was was mirror unambiguously unambiguously image, R = 0.0469).determined determined The by(S)-AC the the heavy heavywas also atom atom confirmed effects effects of ofby S Sandthe and internalBr Br atoms atoms refere (real (realnce image, image,method R =R 0.0352;of= AC 0.0352; usingmirror mirror the image, camphorsultam image, R = 0.0469).R = 0.0469). unit. The (TheFromS)-AC (S ester)-AC was also was27b, confirmed alsoalcohol confirmed (S by)-(− the)-[CD(+)260.2]- by internal the internal refere26 referencencewas method recovered. method of AC The of using AC AC using theof camphorsultamalcohol the camphorsultam 26 was unit.thus Fromestablishedunit. Fromester esterby27b X-ray, 27balcohol ,crystallography alcohol (S)-( (S−)-[CD(+)260.2]-)-(−)-[CD(+)260.2]- [31]. 26 was26 wasrecovered. recovered. The The AC AC of ofalcohol alcohol 2626 waswas thus establishedThe enzymatic by X-ray method crystallography is also useful [31]. [31]. for the chiral resolution of racemates. For example, alcohol 28 wasThe resolved enzymatic by method treating is its also racemic useful acetat for the e withchiral the resolution lipase PS of yieldingracemates. chiral For example, example,alcohol 28 alcohol , from 28which was a resolvedresolved chiral synthon by by treating treating 29 itswas its racemic racemicprepared. acetate acetat However, withe with the thethe lipase enantiomlipase PS yielding PS ericyielding excess chiral chiral alcohol(ee) andalcohol28 ,AC from 28of, which thesefrom whichcompoundsa chiral a synthonchiral remained synthon29 was undetermined. prepared.29 was prepared. However, So, the However, the CSP enantiomeric acid the method enantiom excess was ericapplied (ee) excess and to AC alcohol (ee) of theseand 29 ACas compounds shown of these in compoundsFigureremained 19 undetermined.[31]. remained undetermined. So, the CSP acid So, the method CSP wasacid appliedmethod to was alcohol applied29 as to shownalcohol in 29 Figure as shown 19[ 31 in]. Figure 19 [31].

Figure 19. PreparationPreparation ( a,,b) and and HPLC HPLC separation separation ( c) of esters esters 30a and 30b,, and and the the AC AC determination determination Figureof ester 19. ( S)- Preparation30b by X-ray (a analysis, b) and HPLC ( d).). From From separation ester ester ( (SS ()-)-c30b)30b of ,esters, alcohol alcohol 30a ( (S Sand)-[CD(+)282.3]-)-[CD(+)282.3]- 30b, and the2929 AC waswas determination obtained obtained ( (ee).). ofHPLC, ester redrawn (S)-30b by from X-ray [[32].32 analysis]. ORTEP (d drawing,). From ester reprinted (S)-30b from, alcohol [[31].31]. ( S)-[CD(+)282.3]-29 was obtained (e). HPLC, redrawn from [32]. ORTEP drawing, reprinted from [31].

Molecules 2016, 21, 1328 13 of 37

Molecules 2016, 21, 1328 13 of 37 The CSP esters 30a and 30b were almost baseline separated as shown in Figure 19c (α = 1.1, Rs = 1.6The). The CSP X-ray esters analysis 30a and of 30b ester were30b almostgave baseline the ORTEP separated drawing as shown (Figure in 19 Figured), where 19c ( theα = final1.1, R valueRs =remained 1.6). The X-ray as high analysis as R = of 0.146 ester because 30b gave of the the ORTEP poor crystallinity drawing (Figure of ester 19d),30b where. Therefore, the final its R AC value remained as high as R = 0.146 because of the poor crystallinity of ester 30b. Therefore, its AC could not be determined by the heavy atom effect, but easily determined by the internal reference could not be determined by the heavy atom effect, but easily determined by the internal reference method of camphorsultam to be (S). Finally, ester (S)-30b was converted to alcohol (S)-[CD(+)282.3]-29, method of camphorsultam to be (S). Finally, ester (S)-30b was converted to alcohol (S)-[CD(+)282.3]- where [CD(+)282.3] indicates the showing a positive CD Cotton effect at 282.3 nm [31]. 29, where [CD(+)282.3] indicates the enantiomer showing a positive CD Cotton effect at 282.3 nm [31]. To specify an enantiomer, CD data are useful because the CD measurements need a smaller amount of To specify an enantiomer, CD data are useful because the CD measurements need a smaller amount sample than that for the [α] measurement. of sample than that for theD [α]D measurement. 4. Camphorsultam-Dichlorophthalic Acid (CSDP Acid) for Alcohols—The CSDP Acid Method 4. Camphorsultam-Dichlorophthalic Acid (CSDP Acid) for Alcohols—The CSDP Acid Method As exemplified above, the CSP acid method is useful for the chiral resolution of racemic alcohols As exemplified above, the CSP acid method is useful for the chiral resolution of racemic alcohols and the determination of ACs by X-ray crystallography. However, HPLC separation of diastereomeric and the determination of ACs by X-ray crystallography. However, HPLC separation of CSP esters was not always effective, and in some cases, CSP esters crystallized as fine needles, diastereomeric CSP esters was not always effective, and in some cases, CSP esters crystallized as fine whichneedles, were which not suitable were not for suitable X-ray for analysis. X-ray analysis. So, to improve So, to improve the performance the performance of the CSPof the acid CSP method, acid wemethod, have explored we have the explored structure the of chiralstructure acids, of andchiral have acids, found and that have the found camphorsultam that the camphorsultam dichlorophthalic aciddichlorophthalic (CSDP acid) ( −acid)-23 (CSDP(Figure acid) 16) ( was−)-23 much (Figure more 16) was useful much than more CSP useful acid( than−)-22 CSP. acid (−)-22.

4.1.4.1. Synthesis Synthesis of of Chiral Chiral Molecular Molecular MotorMotor MolecularMolecular machines machines areare veryvery interestinginteresting and a attractivettractive as as future future mechanical mechanical machines machines of of molecularmolecular scale. scale. We We have have developed developed the light–powered the light–powered molecular molecular motor 31a–31d motor 31a–31d[33–36] as[ 33shown–36] as shownin Figure in Figure 20 [37], 20 where[ 37], wheretrans-olefintrans -olefin31a isomerizes31a isomerizes to cis-olefin to cis 31b-olefin under31b photo-irradiation.under photo-irradiation. In this Instep, this step,the left the naphthalene left naphthalene moiety moietyrotates counte rotatesr-clockwise counter-clockwise against the against right naphthalene the right naphthalene moiety. moiety.Furthermore, Furthermore, thermally thermally unstable unstable cis-olefincis 31b-olefin is converted31b is converted to thermally to thermally stable cis-olefin stable cis31c-olefin, where31c , wherethe left the naphthalene left naphthalene group group again againrotates rotates counter-cl counter-clockwiseockwise against against the right the naphthalene right naphthalene group. This group. Thisthermal thermal step step is irreversible, is irreversible, although although the the photo-isomerization photo-isomerization step step is reversible, is reversible, and and hence hence in the in the steps,steps,trans trans-olefin-olefin31a 31a→ →cis cis-olefin-olefin 31b31b →→ cis-olefin-olefin 31c31c, ,the the rotation rotation occurs occurs in in a acounter- counter- clockwise clockwise mannermanner [36 [36,37].,37].

FigureFigure 20. 20.Light-powered Light-poweredmolecular molecular motor 31 and rotation rotation mechanism, mechanism, where where 31a31a–31d–31d areare rotation rotation .isomers. Redrawn Redrawn with with permission permission fromfrom [[37].37].

Similarly,Similarly,cis cis-olefin-olefin31c 31cisomerizes isomerizes toto thermallythermally unstable unstable transtrans-olefin-olefin 31d31d underunder photo-irradiation photo-irradiation (Figure(Figure 20 20).). In In this this step, step, the the left left naphthalenenaphthalene moie moietyty again again rotates rotates counter-clockwise counter-clockwise against against the the right right naphthalene moiety. Furthermore, trans-olefin 31d is converted to thermally more stable trans-olefin naphthalene moiety. Furthermore, trans-olefin 31d is converted to thermally more stable trans-olefin 31a, where the left naphthalene group again rotates counter-clockwise against the right naphthalene

Molecules 2016, 21, 1328 14 of 37

31a, where the left naphthalene group again rotates counter-clockwise against the right naphthalene group.Molecules This 2016 thermal, 21, 1328step is again irreversible, although the photo-isomerization step is14 reversible, of 37 and hence the rotation occurs in a counter-clockwise manner in the steps, cis-olefin 31c → trans-olefin group. This thermal step is again irreversible, although the photo-isomerization step is reversible, 31d → trans-olefin 31a. The rotation occurs only in a counter-clockwise manner [36→,37]. and hence the rotation occurs in a counter-clockwise manner in the steps, cis-olefin 31c → trans-olefin → 31dIt should → trans be-olefin emphasized 31a. The rotation that molecular occurs only motor in a counter-clockwise31 undergoes the manner photo [36,37]. and thermal reactions 31a → 31bIt should→ 31c →be emphasized31d → 31a, that where molecular the left motor naphthalene 31 undergoes group the rotates photo counter-clockwise and thermal reactions against → → → → the right31a → naphthalene 31b → 31c → group. 31d → Therefore, 31a, where by the repeating left naphthalene these reactions, group rotates the molecular counter-clockwise motor continuously against rotatesthe one-way right naphthalene [36,37]. It isgroup. obvious Therefore, that the directionby repeating of motor these rotation reactions, is governedthe molecular by the motor molecular chirality,continuously and hence rotates it isone-way important [36,37]. to It synthesize is obvious that the the enantiopure direction of molecularmotor rotation motor is governed31 and by also to the molecular chirality, and hence it is important to synthesize the enantiopure molecular motor 31 determinethe molecular its absolute chirality, configuration and hence it in is an important unambiguous to synthesize manner. the enantiopure molecular motor 31 and also to determine its absolute configuration in an unambiguous manner. andThe also molecular to determine motor its absolute31a was configuration synthesized in an in unambiguous an enantiopure manner. form, starting from alcohol The molecular motor 31a was synthesized in an enantiopure form, starting from alcohol (3R,4R)- (3R,4R)-(+)-The32 molecularas shown motor in Figure31a was 21synthesized[ 33]. To in obtain an enantiopur enantiopuree form, alcoholstarting from32 and alcohol to determine (3R,4R)- its (+)-32 as shown in Figure 21 [33]. To obtain enantiopure alcohol 32 and to determine its AC, racemic AC, racemic cis-alcohol (±)-32 was esterified with CSP acid (−)-22, and the obtained diastereomeric cis-alcohol (±)-32 was esterified with CSP acid (−)-22, and the obtained diastereomeric CSP esters were α CSP estersseparated were by separatedHPLC on silica by HPLC gel (α on= 1.10). silica However, gel ( = both 1.10). CSP However, esters were both obtained CSP esters as fine were needles obtained as fineor needlesan amorphous or an solid. amorphous Hence their solid. ACs Hence could their not be ACs determined. could not So, be we determined. applied the new So, weCSDP applied acid the newmethod, CSDP acid as shown method, in Figure as shown 22 [33,38]. in Figure 22[33,38].

FigureFigure 21. Synthesis 21. Synthesis of aofchiral a chiral light-powered light-powered molecular motor motor (3 (3R,3’R,3R0)-(R)-(P,PP)-(,PE)-()-(E−)-()-31a−)- starting31a starting from from alcoholalcohol (3R,4 (3RR)-(+)-,4R)-(+)-32.32.

FigureFigure 22. Preparation 22. Preparation (a )(a and) and HPLC HPLC separation separation ( (b)) of of esters esters 34a34a andand 34b34b, and, and the theAC ACdetermination determination of ester 34b by X-ray analysis (c). From ester (3R,4R)-34a, alcohol (3R,4R)-(+)-32 was obtained (d). of esterof ester34b 34bby X-rayby X-ray analysis analysis (c ).(c). From From esterester (3R,4,4RR)-)-34a34a, ,alcohol alcohol (3R (3,4RR),4-(+)-R)-(+)-32 was32 wasobtained obtained (d). (d). Redrawn from [38]. RedrawnRedrawn from from [38 ].[38]. Racemic cis-alcoho l (±)-32 was esterified with CSDP acid (−)-23, yielding diastereomeric CSDP RacemicRacemiccis-alcoho cis-alcoho l (± l (±)-)-3232was was esterified esterified with CSDP CSDP acid acid (− ()-−23,)-23 yielding, yielding diastereomeric diastereomeric CSDP CSDP esters, which were separated well by HPLC on silica gel (α = 1.18, Rs = 1.06) [38]. Thus, the HPLC esters, which were separated well by HPLC on silica gel (α = 1.18, Rs = 1.06) [38]. Thus, the HPLC separation of CSDP esters was better than that of CSP esters. Furthermore, the second-eluted CSDP separationester 34b of was CSDP obtained esters as was colorless better prisms than thatby recrysta of CSPllizing esters. from Furthermore, EtOAc, and hence the second-eluted the AC of CSDP CSDP ester 34b was obtained as colorless prisms by recrystallizing from EtOAc, and hence the AC of CSDP MoleculesMolecules 20162016,, 2121,, 1328 1328 1515 of of 37 37 ester 34b could be determined to be (3S,4S) by X-ray crystallography using the heavy atom effects of ester 34b could be determined to be (3S,4S) by X-ray crystallography using the heavy atom effects S and the two Cl atoms (real image, R = 0.0287; mirror image, R = 0.0448). The (3S,4S) AC was also of S and the two Cl atoms (real image, R = 0.0287; mirror image, R = 0.0448). The (3S,4S) AC was confirmed by the internal reference method of AC using the CSDP acid unit. Treatment of the first- also confirmed by the internal reference method of AC using the CSDP acid unit. Treatment of the eluted ester (3R,4R)-34a with LiAlH4 yielded the enantiopure alcohol (3R,4R)-(+)-32, from which first-eluted ester (3R,4R)-34a with LiAlH4 yielded the enantiopure alcohol (3R,4R)-(+)-32, from which molecular motor (−)-31a was synthesized as shown in Figures 21 and 22 [33]. molecular motor (−)-31a was synthesized as shown in Figures 21 and 22[33]. Motor compound (−)-31a was obtained as prisms by recrystallizing from MeOH, and a single Motor compound (−)-31a was obtained as prisms by recrystallizing from MeOH, and a single crystal was subjected to X-ray crystallography. It was easy to determine the helicity of the molecular crystal was subjected to X-ray crystallography. It was easy to determine the helicity of the molecular skeleton of (−)-31a to be (P,P) based on the AC of the methyl group position. Thus, the relative and skeleton of (−)-31a to be (P,P) based on the AC of the methyl group position. Thus, the relative absolute configurations of molecular motor (−)-31a were unambiguously determined to be (3R,3’R)- and absolute configurations of molecular motor (−)-31a were unambiguously determined to be (P,P)-(E) [33]. (3R,30R)-(P,P)-(E)[33].

4.2.4.2. Application Application to to Diphenyl Diphenyl Methanols Methanols TheThe so-called so-called asymmetric asymmetric synthesis synthesis is is very very useful useful for for the the preparation preparation of of chiral chiral compounds, compounds, as asit isit iswell well known. known. However, However, there there are are some some drawbacks drawbacks in in the the asymmetric asymmetric synthesis: synthesis: (i) (i) the the products products were not always enantiopure; (ii) (ii) the the ACs ACs were were sometimes sometimes assigned assigned in in an an empirical empirical manner, manner, e.g., e.g., comparisoncomparison with with the the previous previous data data of similar co compounds,mpounds, or or assignment due due to to the asymmetric reactionreaction mechanism.mechanism. The The next next example example emphasizes emphas thatizes the that AC ofthe products AC of should products be unambiguously should be unambiguouslydetermined by X-ray determined crystallography. by X-ray crystallography. TheThe AC AC of chiral alcohol 35, (2-methylphenyl)phenylmethanol had had been been a a source source of of much much confusionconfusion (Figure (Figure 23).23). In In 1967, 1967, Cervinka Cervinka and and coworkers coworkers reported reported it it to to be (R)-(−)) based based on on the the asymmetricasymmetric reductions reductions [39]. [39]. However, However, in in 1985 1985,, Seebach Seebach and and coworkers coworkers assigned assigned it it to to be be ( R)-(+) by by catalyticcatalytic asymmetric asymmetric reactions reactions [40]. [40]. Which Which assignment assignment is is correct? correct? To To solve solve this this problem, problem, we we have appliedapplied the the CSDP acid method as follows.

FigureFigure 23. 23. ACsACs of of chiral chiral oo-substituted-substituted diphenylmethanols. diphenylmethanols.

RacemicRacemic alcohol (±)- (±)-3535 waswas esteri esterifiedfied with with CSDP acid ((−)-)-23,23, yielding yielding diastereomeric diastereomeric CSDP CSDP esters.esters. However, However, the diastereomeric diastereomeric esters appeared as a single peak in HPLC on silica silica gel; gel; they they could could notnot be separated at all. So, So, we we have have tested some synthetic precursors precursors of of alcohol alcohol 3535,, but but all all attempts attempts were unsuccessful. Finally, Finally, we have found that diol 36 (Figure(Figure 23)23) could could be be separated separated as as CSDP CSDP esters esters asas shown shown in Figure 24 [[41].41]. DiolDiol (±)- (±)-3636 waswas esterified esterified with with CSDP CSDP acid acid (–)- (–)-23,23, yielding yielding diastereomeric diastereomeric CSDP CSDP esters esters 38a38a and 38b38b,, where where the the primary primary OH OH group group was was esterified. esterified. We We had had worried worried that that the the HPLC separation separation would would becomebecome difficult, difficult, because because in in these these esters esters the the chiral chiral groups groups in in the CSDP acid moiety and alcohol part areare remote remote from from each each other. other. However, However, the diastereom diastereomericeric esters could be separated well by HPLC HPLC on on silicasilica gel, gel, as as shown shown in Figure 24b24b ( α = 1.14, 1.14, RsRs == 0.91) 0.91) [41]. [41]. TheThe first-eluted first-eluted ester, (−)-)-38a,38a ,was was recrystallized recrystallized from from EtOH, EtOH, giving giving prisms, prisms, one one of of which which was was subjectedsubjected to to X-ray X-ray analysis. analysis. However, However, it it was was foun foundd that that the the crystal crystal was was a a twin, twin, and and hence hence it it was unsuitableunsuitable for for X-ray crystallography.crystallography. The second-eluted ester ((−)-)-38b38b waswas similarly recrystallized fromfrom EtOH, EtOH, giving giving single single crystals, crystals, and and the the ORTEP ORTEP drawing drawing was was obtained obtained as as shown shown in in Figure Figure 24c,24c, where the ( S)-AC was unambiguously determined determined by by th thee Bijvoet Bijvoet method method using using one one S S and and two two Cl Cl atomsatoms (real image, R = 0.0324, 0.0324, RwRw (weighted(weighted R-value)-value) = 0.0415; 0.0415; mirror mirror image, image, R = 0.0470, 0.0470, RwRw == 0.0627). 0.0627). The ( (SS)-AC)-AC was was also also corroborated corroborated by by the the internal internal refe referencerence method method of AC of using AC using the CSDP the CSDP acid unit. acid Startingunit. Starting from fromthe thefirst-eluted first-eluted ester, ester, (R ()-(R)-(−)-−38a)-38a, the, the desired desired alcohol alcohol ( R)-[CD(+)225.4]-(−−)-)-3535 waswas synthesized (Figure 24d). Thus, the (R)-AC of alcohol (−)-35 has first been unambiguously determined by X-ray crystallography and chemical correlation [41]. It is again emphasized that the ACs of the

Molecules 2016, 21, 1328 16 of 37

synthesizedMolecules 2016 (Figure, 21, 1328 24 d). Thus, the (R)-AC of alcohol (−)-35 has first been unambiguously determined16 of 37 by X-ray crystallography and chemical correlation [41]. It is again emphasized that the ACs of the asymmetricasymmetric reaction reaction productsproducts should be be determined determined by by anan independent independent physical physical method method such suchas X- as X-rayray crystallography.

FigureFigure 24. 24.Preparation Preparation ( a(a)) and and HPLCHPLC separationseparation ( (bb)) of of esters esters 38a38a andand 38b38b, and, and the the AC AC determination determination ofof ester ester ( S(S)-(–)-)-(–)-38b38b byby X-ray crystallography crystallography (c). (c ).Ester Ester (R)-(–)- (R)-(–)-38a was38a convertedwas converted to alcohol to alcohol(R)- (R[CD(+)225.4]-()-[CD(+)225.4]-(−)-35− ()-d35). HPLC,(d). HPLC, redrawn redrawn from [42]. from ORTEP [42]. drawing, ORTEP reprinted drawing, with reprinted permission with from permission [41]. from [41]. The next data provide the very interesting results and precautions for AC determination of similarThe and next closely data providerelated compounds the very interesting [43]. Chiral resultsalcohol 37 and, (2,6-dimethylphenyl)phenyl- precautions for AC determination methanol of similar(Figure and 23), closely is similar related in compoundsstructure to [ 43(2-methylphenyl)phenylmethanol]. Chiral alcohol 37, (2,6-dimethylphenyl)phenyl- 35; the difference methanolis one (Figuremethyl 23 or), istwo similar methyl in structuregroups in to the (2-methylphenyl)phenylmethanol o-position. Therefore, we thought35 that; the the difference CSDP acid is one method methyl orwould two methyl not be groups applicable in the to oalcohol-position. 37 directly, Therefore, because we thought CSDP esters that the of CSDPalcohol acid 35 could method not would be separated. However, it was surprising to find that CSDP esters 39a and 39b of alcohol 37 were not be applicable to alcohol 37 directly, because CSDP esters of alcohol 35 could not be separated. base-line separated by HPLC on silica gel as shown in Figure 25b (α = 1.25, Rs = 1.94) [43]. However, it was surprising to find that CSDP esters 39a and 39b of alcohol 37 were base-line separated The first-eluted CSDP ester (−)-39a was converted to alcohol (+)-37, whose CD and UV spectra by HPLC on silica gel as shown in Figure 25b (α = 1.25, Rs = 1.94) [43]. are shown in Figure 26b. It is interesting to see that the CD spectra of (2-methylphenyl)- The first-eluted CSDP ester (−)-39a was converted to alcohol (+)-37, whose CD and UV spectra are phenylmethanol (R)-(−)-35 and (2,6-dimethylphenyl)phenylmethanol (+)-37 are similar in shape to shown in Figure 26b. It is interesting to see that the CD spectra of (2-methylphenyl)-phenylmethanol each other, but opposite in sign (Figure 26). Therefore, we had once naturally thought that alcohol − (R)-((+)-37)- 35wouldand (2,6-dimethylphenyl)phenylmethanolhave the opposite AC, i.e., (S)-AC. However, (+)-37 are it similarwas surprising in shape to to find each that other, the but oppositecomparison in sign of (FigureCD spectra 26). leads Therefore, to an erroneous we had once AC assignment naturally thought in this case that [43], alcohol as will (+)- be37 explainedwould have thebelow. opposite AC, i.e., (S)-AC. However, it was surprising to find that the comparison of CD spectra leads toLater, an erroneous we could ACobtain assignment single crys intals this of case the [43first-eluted], as will beester explained (−)-39a by below. recrystallization from EtOH,Later, and we a couldcrystal obtain was subjected single crystals to X-ray of crystallography. the first-eluted esterThe ORTEP (−)-39a drawingby recrystallization is illustrated fromin EtOH,Figure and 25c, a crystalwhere was(R)-AC subjected was determined to X-raycrystallography. by the heavy atom The effect ORTEP (real drawing image, is R illustrated = 0.0358, in FigureRw = 25 0.0488;c, where mirror (R)-AC image, was determinedR = 0.0395, Rw by the= 0.0536) heavy [43]. atom The effect (R (real)-AC image,was alsoR = confirmed 0.0358, Rw by= 0.0488;the mirrorinternal image, referenceR = 0.0395, methodRw of= AC 0.0536) using [ 43the]. TheCSDP (R acid)-AC unit. was Since also confirmedalcohol (+)- by37 thewas internal obtained reference from methodthe first-eluted of AC using CSDP the ester CSDP (R acid)-(−)- unit.39a, ( SinceR)-AC alcohol was assigned (+)-37 was to alcohol obtained (+)- from37. Thus, the first-eluted we could find CSDP esterthat ( Rthe)-( −AC)-39a assignment,(R)-AC wasby comparison assigned to of alcohol CD spectr (+)-a37 led. Thus, to erroneous we could conclusions find that thein this AC case assignment [43]. byX-ray comparison crystallography of CD spectra using ledan tointernal erroneous referenc conclusionse of AC inis thisthuscase very [43 powerful]. X-ray crystallographyfor clear AC usingdetermination. an internal reference of AC is thus very powerful for clear AC determination.

Molecules 2016, 21, 1328 17 of 37 MoleculesMolecules 20162016,, 2121,, 13281328 1717 ofof 3737

FigureFigureFigure 25. 25.25.Preparation PreparationPreparation ( (a(aa))) and andand HPLC HPLCHPLC separation separation (( (bbb)) ) ofof of estersesters esters 39a39a39a andandand 39b39b39b,, andand, and thethe the ACAC AC determinationdetermination determination ofofof ester esterester (R (()-(RR)-()-(−−)-−)-)-39a39a39a by byby X-ray X-rayX-ray crystallographycrystallography ( (cc).). AlcoholAlcohol ((R (RR)-[CD()-[CD()-[CD(−−)227.0]-(+)-−)227.0]-(+)-)227.0]-(+)-3737 37waswaswas recoveredrecovered recovered fromfrom from ester (R)-(−)-39a (d). HPLC, redrawn from [44]. ORTEP drawing, reprinted from [43]. esterester (R ()-(R)-(−−)-)-39a39a( d(d).). HPLC,HPLC, redrawn from from [44]. [44]. ORTE ORTEPP drawing, drawing, reprinted reprinted from from [43]. [43 ].

Figure 26. CD and UV spectra of o-substituted diphenylmethanols. (a) (2-methylphenyl)phenyl- methanolFigureFigure 26.26. (R )-[CD(+)225.4]-(CDCD andand UVUV spectraspectra−)-35 ofof. Reprintedoo-substituted-substituted with diphenylmethanols.diphenylmethanols. permission from [((a41a)) ];(2-methylphenyl)phenyl-(2-methylphenyl)phenyl- (b) (2,6-dimethylphenyl)- methanol (R)-[CD(+)225.4]-(−)-35. Reprinted with permission from [41]; (b) (2,6-dimethylphenyl)- phenylmethanolmethanol (R)-[CD(+)225.4]-( (R)-[CD(−)227.0]-(+)-−)-35. Reprinted37. Reprinted with permission from [43]. from [41]; (b) (2,6-dimethylphenyl)- phenylmethanolphenylmethanol ((RR)-[CD()-[CD(−−)227.0]-(+)-)227.0]-(+)-3737.. ReprintedReprinted fromfrom [43].[43].

Molecules 2016, 21, 1328 18 of 37

4.3. Synthesis of a Cryptochiral Hydrocarbon 4-Ethyl-4-methyloctane 40 can exist as a basic and simple chiral hydrocarbon, where methyl, ethyl,Molecules propyl, 2016, 21 ,and 1328 butyl groups are connected to a quaternary chiral center (Figure 27). Since18 ofthe 37 opticalMolecules rotation2016, 21, 1328 value of 40 is very small as [α]D = +0.19 (neat), it may be called a cryptochiral18 of 37 4.3. Synthesis of a Cryptochiral Hydrocarbon hydrocarbon [45]. In4-Ethyl-4-methyloctane 1980, H. Wynberg and 40 a coworkercan exist fiasrst a synthesizedbasic and simple chiral chiralhydrocarbon hydrocarbon, (−)-40 (95% where ee), methyl, but its ACethyl,4.3. could Synthesis propyl, not of andbe a Cryptochiraldetermined butyl groups Hydrocarbon[46]. are In connected 1988, L. Latordicci a quaternary and coworkers chiral center reported (Figure the synthesis27). Since andthe determinationoptical4-Ethyl-4-methyloctane rotation of value the AC of of40 (+)-40 is can40very, where exist small as they a as basic applied[α] andD = simple +0.19the CD (neat), chiral excito hydrocarbon,itn maychirality be calledmethod where a methyl, cryptochiralto acetylene ethyl, tertiaryhydrocarbonpropyl, alcohol and butyl[45]. benzoate groups for are determining connected to AC, a quaternary but its observed chiral centerCD was (Figure very weak27). Since (λext the= 239 optical nm,

∆rotationε = +0.8)In 1980, value [47]. H. ofTherefore, Wynberg40 is very itand small is difficulta coworker as [α]D to= say +0.19first that synthesized (neat), the AC it may was chiral be unambiguously called hydrocarbon a cryptochiral determined.(−)-40 hydrocarbon (95% ee), but [45 its]. AC could not be determined [46]. In 1988, L. Lardicci and coworkers reported the synthesis and determination of the AC of (+)-40, where they applied the CD exciton chirality method to acetylene tertiary alcohol benzoate for determining AC, but its observed CD was very weak (λext = 239 nm, ∆ε = +0.8) [47]. Therefore, it is difficult to say that the AC was unambiguously determined.

FigureFigure 27. Cryptochiral hydrocarbonhydrocarbon ( R(R)-[VCD()-[VCD(−−)984]-(+)-)984]-(+)-4040and and synthetic synthetic precursor precursor (1 S(1,2SS,2)-(+)-S)-(+)-ciscis-41-. 41. In 1980, H. Wynberg and a coworker first synthesized chiral hydrocarbon (−)-40 (95% ee), but its AC couldWe selected not be the determined alcohol cis [-4641]. as In a 1988,synthetic L. Lardicci precursor and of coworkershydrocarbon reported 40 (Figure the 27). synthesis To obtain and enantiopuredeterminationFigure 27. alcohol Cryptochiral of the cis AC-41 of andhydrocarbon (+)- to40 determine, where (R)-[VCD( they its appliedAC,−)984]-(+)- the CSDP the40 CD and acid exciton synthetic method chirality precursor was applied, method (1S,2S )-(+)-as to shown acetylenecis- in Figuretertiary41 . 28 alcohol [45]. benzoateRacemic foralcohol determining (±)-cis- AC,41 was but itsesteri observedfied with CD wasCSDP very acid weak (− ()-λ23,ext =yielding 239 nm, diastereomeric∆ε = +0.8) [47]. esters Therefore, 42a and it is 42b difficult, which to were say that baseline the AC separated was unambiguously by HPLC on determined.silica gel as shown in FigureWe 28b selected [45]. Thethe alcohol second-eluted cis-41 as ester a synthetic 42b was precursor recrystallized of hydrocarbon from EtOH/CH 40 (Figure(Figure2Cl2 27).27giving). To To obtain single crystals,enantiopure one alcoholof which cis was-41 andsubjected to determine to X-ray its analysis. AC, the Figure CSDP 28cacid shows method the was ORTEP applied, drawing as shown of 42b in , whereFigure 28AC28[ 45 was[45].]. Racemic determined Racemic alcohol alcoholby ( ±the)-cis heavy(±)--41 ciswas atom-41 esterified was effect esteri with(realfi CSDPedimage, with acid R (=CSDP− 0.0598,)-23, yieldingacid Rw =(− 0.0740;)- diastereomeric23, yielding mirror image,diastereomericesters 42a R =and 0.0719,42b esters, Rw which 42a = 0.0899).and were 42b baseline The, which (1R separated were,2R)-AC baseline of by ester HPLC separated 42b on was silica by also HPLC gel asestablished shownon silica in gelby Figure asthe shown internal28b [45 in]. referenceFigureThe second-eluted 28b method [45]. The ofester AC second-eluted using42b was the CSDP recrystallized ester acid 42b unit was from [45]. recrystallized EtOH/CH 2fromCl2 giving EtOH/CH single2Cl crystals,2 giving onesingle of crystals,which was one subjected of which to was X-ray subjected analysis. to X-ray Figure analysis. 28c shows Figure the ORTEP28c shows drawing the ORTEP of 42b drawing, where AC of 42b was, wheredetermined AC was by thedetermined heavy atom by effectthe heavy (real atom image, effectR = 0.0598,(real image,Rw = 0.0740;R = 0.0598, mirror Rw image, = 0.0740;R = mirror 0.0719, image,Rw = 0.0899). R = 0.0719, The (1 RwR,2 R=)-AC 0.0899). of ester The 42b(1R,2wasR)-AC also of established ester 42b by was the also internal established reference by method the internal of AC referenceusing the method CSDP acid of AC unit using [45]. the CSDP acid unit [45].

Figure 28. Preparation (a) and HPLC separation (b) of CSDP esters 42a and 42b, and the AC determination of ester (1R,2R)-(+)-cis-42b by X-ray crystallography (c). HPLC and ORTEP, reprinted with permission from [45].

Figure 28. PreparationPreparation (a (a) )and and HPLC HPLC separation separation (b (b) )of of CSDP CSDP esters esters 42a42a andand 42b42b,, and and the the AC AC determination of of ester (1 R,2,2R)-(+)-)-(+)-ciscis--42b42b byby X-ray X-ray crystallography crystallography ( (cc).). HPLC HPLC and and ORTEP, ORTEP, reprinted reprinted with permission from [[45].45].

Molecules 2016, 21, 1328 19 of 37 Molecules 2016, 21, 1328 19 of 37

Starting from the first-eluted ester (1S,2S)-(−)-cis-42a, enantiopure cryptochiral hydrocarbon (R)- Starting from the first-eluted ester (1S,2S)-(−)-cis-42a, enantiopure cryptochiral hydrocarbon [VCD(−)984]-(+)-40 was synthesized as shown in Figure 29, where [VCD(−)984] indicates an (R)-[VCD(−)984]-(+)-40 was synthesized as shown in Figure 29, where [VCD(−)984] indicates an enantiomer showing a negative vibrational circular dichroism (VCD) band at 984 cm–1 [45]. It should enantiomerenantiomer showing showing a negative negative vibrational circular dichroism (VCD) band at 984 cm−1 [45].[45]. It It should should be noted that the (R)-AC of [VCD(−)984]-40 was also determined by the quantum mechanical bebe noted noted that the (R)-AC of [VCD([VCD(−)984]-)984]-4040 waswas also also determined determined by by the the quantum mechanical calculation of VCD and IR, where the theoretically calculated VCD and IR spectra were compared calculationcalculation of VCDVCD andand IR,IR, where where the the theoretically theoreticall calculatedy calculated VCD VCD and and IR spectraIR spectra were were compared compared with with the observed spectra [48]. The theoretical AC determination was thus established in an withthe observed the observed spectra spectra [48]. The [48]. theoretical The theoretical AC determination AC determination was thus establishedwas thus inestablished an experimental in an experimental manner, as explained here. experimentalmanner, as explained manner, here.as explained here.

FigureFigure 29. Synthesis of cryptochiral hydrocarbon (R)-[VCD()-[VCD(−)984]-(+)-)984]-(+)-4040 startingstarting from from CSDP CSDP ester ester (1(1SS,2,2SS)-()-(−−)-)-ciscis-42a-42a. .

4.4.4.4. Application Application to to 2-(1-Naphthyl)Propane-1,2-Diol 2-(1-Naphthyl)Propane-1,2-Diol ItIt was was previously previously reported reported that that 1-iso 1--propylnaphthaleneiso-propylnaphthalene was was biotrans biotransformedformed in rabbits in rabbits to 2-(1- to naphthyl)propane-1,2-diol2-(1-naphthyl)propane-1,2-diol (S)-(−)- (S45)-( (Figure−)-45 (Figure 30) [49]. 30 However,)[49]. However, its enantiomeric its was excess very low was (ca.very 20% low ee) (ca. and 20% its ee)AC and was its determined AC was determined by chemical by correlation chemical correlationto a compound, to a compound, the AC of which the AC had of beenwhich determined had been determined by an empirical by an rule. empirical Therefor rule.e, it Therefore, was desired it was to obtain desired enantiopure to obtain enantiopure diol 45 and diol to determine45 and to determineits AC in an its unambiguous AC in an unambiguous manner. manner.

FigureFigure 30. 30. BiotransformationBiotransformation of of 1- 1-isoiso-propylnaphthalene-propylnaphthalene to to 2-(1-naphthyl)propane-1,2-diol 2-(1-naphthyl)propane-1,2-diol 4545..

WeWe have applied applied the the CSDP CSDP acid acid method method to to racemic racemic diol diol 45,45 ,as as illustrated illustrated in in Figure Figure 3131 [[50].50]. AlthoughAlthough these these are are CSDP CSDP es estersters of of primary primary alcohols, alcohols, where where the the chiral chiral moiety moiety of the of thealcohol alcohol part part is far is fromfar from that thatof the of theCSDP CSDP acid acid part, part, these these diastereom diastereomersers were were largely largely separated separated by HPLC by HPLC on silica on silica gel (separationgel (separation factor factor α = α1.27)= 1.27) [50]. [50 This]. This large large α valueα value indicates indicates that that esters esters 46a46a andand 46b46b werewere more more efficientlyefficiently separated separated than than the the other other CSDP CSDP esters esters disc discussedussed above. above. It It may may be be due due to to the the free free OH OH group. group. ToTo obtain single crystals suitable for for X-ray X-ray analysis, analysis, we we tried tried to to recrystallize recrystallize these these esters esters from from variousvarious solvents, solvents, but but all all attempts attempts were were unsuccessful. unsuccessful. So, So, we we have have adopted adopted the thefollowing following strategy, strategy, as shownas shown in Figure in Figure 32 [50].32[50 ].

MoleculesMolecules 20162016,,21 21,, 13281328 2020 ofof 3737 Molecules 2016, 21, 1328 20 of 37

Figure 31. Preparation (a) and HPLC separation (b) of CSDP esters 46a and 46b. HPLC, reprinted FigureFigure 31. PreparationPreparation ( (aa)) and and HPLC HPLC separation separation ( (bb)) of of CSDP esters 46a and 46b. HPLC, HPLC, reprinted reprinted from [42]. fromfrom [42]. [42].

FigureFigure 32.32. PreparationPreparation ((aa)) ofofp p-bromobenzoate-bromobenzoate ((SS)-()-(−−)-47 and its X-ray analysis (b).). ORTEP drawing,drawing, Figure 32. Preparation (a) of p-bromobenzoate (S)-(−)-47 and its X-ray analysis (b). ORTEP drawing, reprintedreprinted fromfrom [[50].50]. reprinted from [50].

TheThe reductionreduction ofof thethe first-elutedfirst-eluted esterester 46a46a withwith LiAlH4LiAlH4 andand successivesuccessive benzoylationbenzoylation withwith The reduction of the first-eluted ester 46a with LiAlH4 and successive benzoylation with pp-Br-BzCl-Br-BzCl furnishedfurnished pp-bromobenzoate-bromobenzoate ((−−)-)-4747,, which which was recrystallizedrecrystallized fromfrom EtOH,EtOH, givinggiving goodgood p-Br-BzCl furnished p-bromobenzoate (−)-47, which was recrystallized from EtOH, giving good singlesingle crystalscrystals suitable suitable for for X-ray X-ray crystallography. crystallography. The ORTEP The ORTEP drawing drawing of (−)-47 ofis shown(−)-47 inis Figureshown 32 inb, single crystals suitable for X-ray crystallography. The ORTEP drawing of (−)-47 is shown in whereFigure the32b, (S )-ACwhere was the unambiguously (S)-AC was unambiguously determined by determined the original by Bijvoet the original method. Bijvoet Namely, method. the 18 Figure 32b, where the (S)-AC was unambiguously determined by the original Bijvoet method. BijvoetNamely, pairs the 18 were Bijvoet observed, pairs were and observed, their observed and thei intensityr observed ratios intensity agreed ratios well agreed with the well calculated with the Namely, the 18 Bijvoet pairs were observed, and their observed intensity ratios agreed well with the ratioscalculated [50]. Inratios addition, [50]. In the addition, final R-value the final also R indicated-value also the indicated (S)-AC (real the image,(S)-AC R(real= 0.0249, image,Rw R = 0.0249, 0.0342; calculated ratios [50]. In addition, the final R-value also indicated the (S)-AC (real image, R = 0.0249, mirrorRw = image,0.0342;R mirror= 0.0358, image,Rw = 0.0520).R = 0.0358, From Rw these = results,0.0520). the From (S)-AC these of 2-(1-naphthyl)propane-1,2-diol results, the (S)-AC of 2-(1- Rw = 0.0342; mirror image, R = 0.0358, Rw = 0.0520). From these results, the (S)-AC of 2-(1- (naphthyl)propane-1,2-diol−)-45 was established. (−)-45 was established. naphthyl)propane-1,2-diol (−)-45 was established. 5. Use of 2-Methoxy-2-(1-Naphthyl)Propionic Acid (MαNP Acid) for Alcohols—The MαNP 5. Use of 2-Methoxy-2-(1-Naphthyl)Propionic Acid (MαNP Acid) for Alcohols—The MαNP 5.Acid Use Method of 2-Methoxy-2-(1-Naphthyl)Propionic Acid (MαNP Acid) for Alcohols—The MαNP Acid Method Acid Method 5.1. Synthesis of Enantiopure MαNP Acid, HPLC Separation, and AC Determination by 1H-NMR Diamagnetic5.1. Synthesis Anisotropy of Enantiopure MαNP Acid, HPLC Separation, and AC Determination by 1H-NMR 5.1. Synthesis of Enantiopure MαNP Acid, HPLC Separation, and AC Determination by 1H-NMR Diamagnetic Anisotropy DiamagneticDuring Anisotropy the AC determination of diol (−)-45, we realized that it was possible to synthesize enantiopureDuring 2-methoxy-2-(1-naphthyl)propionicthe AC determination of diol (−)-45 acid, we1 realized, and the that conversion it was possible was actually to synthesize carried During the AC determination of diol (−)-45, we realized that it was possible to synthesize outenantiopure as shown 2-methoxy-2-(1-naphthyl)propionic in Figure 33[ 51]. Starting from enantiopure acid 1, and diol the (conversionS)-(−)-45, enantiopurewas actually 2-methoxy-carried out enantiopure 2-methoxy-2-(1-naphthyl)propionic acid 1, and the conversion was actually carried out 2-(1-naphthyl)propionicas shown in Figure 33 [51]. acid Starting (S)-(+)- from1 was enantiopure obtained, diol indicating (S)-(−)-45 that, enantiopure the (S)-AC 2-methoxy- of acid (+)-2-(1-1 as shown in Figure 33 [51]. Starting from enantiopure diol (S)-(−)-45, enantiopure 2-methoxy- 2-(1- wasnaphthyl)propionic established. acid (S)-(+)-1 was obtained, indicating that the (S)-AC of acid (+)-1 was naphthyl)propionic acid (S)-(+)-1 was obtained, indicating that the (S)-AC of acid (+)-1 was established. established.

Molecules 2016, 21, 1328 21 of 37 Molecules 2016, 21, 1328 21 of 37

Figure 33. Conversion of diol (S)-(−)-45 to 2-methoxy-2-(1-naphthyl)propionic acid (S)-(+)-1. FigureFigure 33.33. Conversion ofof dioldiol ((SS)-()-(−−)-)-4545 toto 2-methoxy-2-(1-naphthyl)propionic 2-methoxy-2-(1-naphthyl)propionic acid acid ( (S)-(+)-)-(+)-11.. Chiral acid 1 is similar in structure to the Mosher’s α-methoxy-α-(trifluoromethyl)phenylacetic Chiral acid 1 is similar in structure to the Mosher’s α-methoxy-α-(trifluoromethyl)phenylacetic acid (MTPAChiral acid acid) 1 51is similar [52,53] inand structure also to αto-methoxyphenylacetic the Mosher’s -methoxy- acid (MPA-(trifluoromethyl)phenylacetic acid) 52 [54] (Figure 34). acid (MTPA acid) 51 [52,53] and also to α-methoxyphenylacetic acid (MPA acid) 52 [54] (Figure 34). Theacid NMR(MTPA methods acid) 51 using [52,53] these and chiral also toacids -methoxyphenylacetic have been widely employed acid (MPA for acid)determining 52 [54] (Figure the ACs 34). of The NMR methods using these chiral acids have been widely employed for determining the ACs The NMR methods using these chiral acids have been widely employed for determining1 the ACs of various chiral secondary alcohols including natural products by using 1H-NMR diamagnetic variousof various chiral chiral secondary secondary alcohols alcohols including including natural natural products products by by using using 1H-NMR diamagnetic anisotropy shift data, although these methods are are empirical rules. So, So, we we had had expected expected that that acid acid 11 anisotropy shift data, although these methods are empirical rules. So, we had1 expected that acid 1 was also useful for determining the ACs of chiral secondary alcohols by the 1H-NMR diamagnetic was also useful for determining the ACs of chiral secondary alcohols by the 1H-NMR diamagnetic anisotropy method. anisotropy method.

1 FigureFigure 34. 34. ChiralChiral acids acids useful useful for for the the 1H-NMR diamagnetic anisotropy method. Figure 34. Chiral acids useful for the 1H-NMR diamagnetic anisotropy method. To employemploy asas aa chiral chiral shift shift reagent, reagent, it wasit was necessary necessary to prepareto prepare enantiopure enantiopure acid acid1 on 1 a on large a large scale. scale.So, weTo So, hademploy we looked had as looked a for chiral a for simpler shift a simpler reagent, method method it to was enantioresolve to necessaryenantioresolve to racemic prepare racemic acid enantiopure acid (±)- (±)-1. It1. was Itacid was surprising 1 surprisingon a large to toscale.find find that So, that naturalwe natural had menthollooked for (− (a−)- )-simpler5353was was very methodveryuseful useful to enantioresolve forfor thethe chiralchiral resolution racemic acid of racemic (±)-1. It acid acidwas 11surprising.. Namely, Namely, racemictoracemic find that M α naturalNP acid menthol ((±)-±)-11 waswas (− reacted)- reacted53 was with with very mentholmenthol useful for ( (−− )-the)-5353 chiralyieldingyielding resolution diastereomeric diastereomeric of racemic esters esters acid 54a54a 1. andNamely,and 54b54b,, whichracemic were Mα NPlargely acid separated(±)-1 was reacted by HPLC with on menthol silica gel ( (−α)- 53= 1.83, yielding Rs = diastereomeric2.26) 2.26) (Figure (Figure 35)35 esters)[ [51].51]. 54aSuch Such and a a large large 54b, separationwhich were factor factor largely αα hasseparated never bybeen HPLC observed on silica in thegel previous(α = 1.83, CSPRs = and2.26) CSDP (Figure esters. 35) [51]. The Such first-eluted first-eluted a large separation factor α has never been observed in the previous CSP and CSDP esters. The first-eluted ester ( −)-)-54a54a waswas treated treated with with NaOCH NaOCH33 inin MeOH MeOH and and then then with with water water yielding yielding M MααNPNP acid acid (+)- 11,, ester (−)-54a was treated with NaOCH in MeOH and then with water yielding MαNP acid (+)-1, which was identical to acid (S)-(+)-11 shownshown3 in in Figure 33.33. Therefore, Therefore, the the AC AC of of the the first-eluted first-eluted ester ester (which−)-)-54a54a waswaswas identicaldetermined determined to acidto to be be ( S( (S)-(+)-S))[ [51].511]. shown in Figure 33. Therefore, the AC of the first-eluted ester (−)-54aFigure was determined36 shows the to ACbe (S determination) [51]. of a secondary alcohol by the 1H-NMR diamagnetic Figure 36 shows the AC determination of a secondary alcohol by the 1H-NMR diamagnetic anisotropy method using (R)- and (S)-MαNP acids [55]. The (R)-acid ester1 55 takes a preferred anisotropyFigure method36 shows using the AC(R)- determinationand (S)-MαNP of acids a secondary [55]. The alcohol (R)-acid by esterthe H-NMR55 takes diamagnetica preferred anisotropyconformation method as shown using in Figure(R)- and 36a, (S where)-MαNP the acids substituent [55]. The R2 is ( locatedR)-acid aboveester 55 the takes naphthalene a preferred ring, conformation as shown in Figure 36a, where the substituent R2 is located above the naphthalene ring, and hence it shows a high field shift. 2 andconformation hence it shows as shown a high in Figurefield shift. 36a, where the substituent R is located above the naphthalene ring, On the other hand, the (S)-acid ester 55 takes a similar preferred conformation, as shown in and henceOn the it othershows hand, a high the field (S )-acidshift. ester 55 takes a similar preferred conformation, as shown in Figure 36a. However, the substituent R1 shows a high field shift because it is located above the On the other hand, the (S)-acid ester1 55 takes a similar preferred conformation, as shown in Figure 36a. However, the substituent R showsδ a high field shiftδ becauseδ it −is δlocated above the Figurenaphthalene 36a. However, ring. The chemicalthe substituent shift difference R1 shows∆ ais high defined field as shift∆ = because(R,X)-55 it is located(S,X)-55 above, where theX naphthalenedenotes the AC ring. of theThe alcohol chemical part shift to be difference determined. ∆δ is From defined the observed as ∆δ = δ1(H-NMRR,X)-55 spectra,− δ(S,X)-∆55δ ,values where are X naphthalene ring. The chemical shift difference ∆δ is defined as ∆δ = δ(R1,X)-55 − δ(S,X)-55, where X denotescalculated the for AC each of the proton, alcohol and part are plottedto be determined. in the sector From rule shownthe observed in Figure H-NMR 36c, where spectra, the substituent ∆δ values denotes the AC of the alcohol part to be determined. From the observed 1H-NMR spectra, ∆δ values areR1 showingcalculated positive for each∆ δproton,values and is placed are plotted on the in right the side.sector On rule the shown other in hand, Figure the 36c, substituent where the R2 are calculated for each proton, and are plotted in the sector rule shown in Figure 36c, where the substituentshowing negative R1 showing∆δ values positive is placed ∆δ values on the leftis placed side. Based on the on right these data,side. theOn ACthe (Xother) of thehand, alcohol the substituent R1 showing positive ∆δ values is placed on the right side. On the other hand, the part can be determined [55].

Molecules 2016, 21, 1328 22 of 37

substituent R2 showing negative ∆δ values is placed on the left side. Based on these data, the AC (X) Moleculesof the 2016alcohol, 21, 1328part can be determined [55]. 22 of 37

FigureFigure 35. 35.Preparation Preparation ( a(a)) and and HPLC HPLC separationseparation ( b)) of of menthol menthol M MααNPNP esters esters 54a54a andand 54b54b. (c.() Recoveryc) Recovery ofof M αMNPαNP acid acid (S ()-(+)-S)-(+)-11from from the the first first elutedeluted esterester (−)-)-54a54a. .HPLC, HPLC, reprinted reprinted with with permission permission from from [51]. [51 ].

FigureFigure 36. 36.AC AC determination determinationand and mechanismmechanism of the 11H-NMRH-NMR diamagnetic diamagnetic anisotropy anisotropy method method using using (R)-(R and)- and (S ()-MS)-MαNPαNP acids. acids. ( a(a)) Preferred Preferred conformationconformation of of M MααNPNP esters. esters; (b ()b Definition) Definition of ofΔδ∆. (δc;() Sectorc) Sector rulerule for for determining determining the the absolute absoluteconfiguration. configuration. Redrawn with with permission permission from from [55]. [55 ].

Figure 37a shows the distribution of ∆δ values of menthol MαNP esters (R)-(−)-54b and (S)-(−)- FigureFigure 37 37aa showsshows the the distribution distribution of of∆δ ∆valuesδ values of menthol of menthol MαNP M estersαNP (R esters)-(−)-54b (R)-( and−)- (S54b)-(−)-and 54a, where the isopropyl group showing negative ∆δ values is placed on the left side, while the (S)-(−)-54a, where the isopropyl group showing negative ∆δ values is placed on the left side, while methyl group showing positive ∆δ value is placed on the right side. Based on these data, the AC of the methyl group showing positive ∆δ value is placed on the right side. Based on these data, the AC of (−)-menthol was determined as shown in Figure 35[ 51]. Of course, this AC agreed with the previously established AC of (−)-menthol. Molecules 2016, 21, 1328 23 of 37 Molecules 2016, 21, 1328 23 of 37 (−)-menthol was determined as shown in Figure 35 [51]. Of course, this AC agreed with the previously (established−)-menthol wasAC ofdetermined (−)-menthol. as shown in Figure 35 [51]. Of course, this AC agreed with the previously Molecules 2016, 21, 1328 23 of 37 established AC of (−)-menthol.

Figure 37. Distribution of ∆δ values in ppm and AC determination. (a) Menthol MαNP esters and (b) FigureFigurementhol 37. 37. MTPA DistributionDistribution esters. ofReprinted of ∆δ∆ δvaluesvalues with in in ppm permission ppm and and AC ACfrom determination. determination. [51]. (a) (Menthola) Menthol Mα MNPα NPesters esters and and(b) menthol(b) menthol MTPA MTPA esters. esters. Reprinted Reprinted with with permission permission from from [51]. [51 ]. Figure 37b shows the distribution of ∆δ values of menthol MTPA esters. It should be noted that ∆δ isFigureFigure defined 37b37 asb showsshows ∆δ = δ the the(S,X distribution distribution) − δ(R,X). From of ∆δ∆δ the valuesvalues distribution of of menthol menthol of ∆δMTPA MTPA values, esters. the It ItAC should should of (−)-menthol be be noted noted that thatwas ∆δ∆assignedδ isis defined defined as shown. as ∆δ∆δ = However, δ((S,,X)) − δ δcompared((RR,X,X).). From From with the the the distribution distribution data of Figure of of ∆δ∆δ 37a,b, values, the the ∆δ AC values of ((− −of)-menthol)-menthol MαNP esters was was assignedassignedare much as as larger shown. shown. than However, However, those of comparedMTPA esters; with ca. the four data times of Figure larger 37a,b, 37[51].a,b, This the ∆δ∆isδ a valuesvalues great advantageof of M MααNPNP esters estersof the areareMα much muchNP ester larger larger method. than those of MTPA esters; ca. four times larger [51]. [51]. This This is is a a great great advantage advantage of of the the MMααNPNPIt esterwas ester difficult method. method. to obtain single crystals of menthol MαNP esters 54a and 54b, although we had triedItIt many was was difficult difficultrecrystallizations to to obtain obtain single singlefrom variouscrystals crystals solvents.of of menthol menthol Finally, M MααNPNP we esters esters could 54a54a obtain andand 54bsingle54b,, although although crystals we weof esterhad had triedtried(−)-54b many many suitable recrystallizations recrystallizations for X-ray analysis from from various variousby recrystallizing solvents. solvents. Finally, Finally,from a wemixed we could could solvent obtain obtain (Et single single2O/MeOH). crystals crystals From of of ester ester the − ((ORTEP−)-)-54b54b suitable drawing,suitable for for the X-ray X-ray (R)-AC analysis analysis of the by byM αrecrystallizing recrystallizingNP acid moiety from from was a established mixed solvent (Figure (Et(Et2 38)O/MeOH). [56]. The From Fromchemical the the ORTEP drawing, the (R)-AC of the MαNP acid moiety was established (Figure 38)[56]. The chemical ORTEPcorrelation, drawing, 1H-NMR the ( Rspectra,)-AC of and the MX-rayαNP analysis acid moiety are thus was consistent established with (Figure each 38) other. [56]. The chemical correlation, 1H-NMR spectra, and X-ray analysis are thus consistent with each other. correlation, 1H-NMR spectra, and X-ray analysis are thus consistent with each other.

FigureFigure 38. X-rayX-ray ORTEP ORTEP drawing drawing of of menthol menthol M MαNPαNP ester ester (R)-( (R−)-()-54b−)-54b. Reprinted. Reprinted with withpermission permission from Figurefrom[56]. [56 38.]. X-ray ORTEP drawing of menthol MαNP ester (R)-(−)-54b. Reprinted with permission from [56]. 5.2.5.2. Application of thethe MMααNPNP AcidAcid MethodMethod toto AromaticAromatic AlcoholAlcohol 5.2. Application of the MαNP Acid Method to Aromatic Alcohol AsAs explainedexplained inin FigureFigure 3535,, oneone of the advantages of the MαNP acid method is that diastereomeric estersestersAs are explained largely inseparated Figure 35, by by one HP HPLC ofLC the on on advantages silica silica gel. gel. Thisof This the was M wasα alsoNP also acid proved proved method by by the is the that example example diastereomeric shown shown in estersinFigure Figure are 39. largely39 Alcohol. Alcohol separated (±)- (41±,)- a 41by synthetic, aHP syntheticLC onprecursor silica precursor gel. of Thisa ofcryptochiral awas cryptochiral also proved hydrocarbon hydrocarbon by the 40example (Figure40 (Figure shown 27), was27 in), Figurewasesterified esterified 39. withAlcohol with Mα MNP(±)-αNP 41acid, acida (syntheticS)-(+)- (S)-(+)-1, yielding1 precursor, yielding diastereomeric diastereomericof a cryptochiral esters esters hydrocarbon 56a56a andand 56b56b ,40 ,which which (Figure were were 27), largely was esterifiedseparatedseparated with byby HPLCHPLC MαNP onon acid silicasilica (S)-(+)- gelgel ((α1,α =yielding= 1.81,1.81, RsRs diastereomeric == 5.97)5.97) [[45].45]. ThisThis esters isis veryvery 56a remarkable,andremarkable, 56b, which whenwhen were comparedcompared largely separatedwithwith thethe casecase by ofHPLC of CSDP CSDP on acid acidsilica esters esters gel shown( αshown = 1.81, in in FigureRs Figure = 5.97) 28 28,, where[45]. where Thisα =α 1.17,is= 1.17,very and andremarkable,Rs Rs= 1.51.= 1.51. The when The (1R (1 ,2comparedRR,2)-ACR)-AC of with(of− )-(−cis )-thecis-56b case-56bwas ofwas alsoCSDP also confirmed acidconfirmed esters by shownby X-ray X-ray crystallographyin crystallography Figure 28, where as as shown α shown = 1.17, in in Figureand Figure Rs 39 = c39c1.51. [57 [57].]. The (1R,2R)-AC of (−)-cis-56b was also confirmed by X-ray crystallography as shown in Figure 39c [57].

Molecules 2016, 21, 1328 24 of 37 Molecules 2016, 21, 1328 24 of 37

Figure 39. Preparation ( (aa)) and and HPLC separation ( (b)) of M αNP esters 56a and 56b. HPLC, reprinted with permission from [[45].45]. ( c) X-rayX-ray ORTEPORTEP drawingdrawing ofof MMααNPNP esterester (1(1RR,2,2RR)-()-(−−)-)-ciscis--56b56b,, reprinted with permission from [[57].57].

5.3. Application Application of of Aliphatic Aliphatic Chain Chain Alcohols Another advantage of the M αNP acid acid method method is is that aliphatic aliphatic chain chain alcohols alcohols can can be be resolved resolved as MαNPNP esters, as exemplified exemplified in Figure 40 [[58].58]. Fo Forr example, racemic 2-buta 2-butanolnol was esterified esterified with (S)-(+)-MααNPNP acid acid yielding yielding diastereomeric diastereomeric esters, esters, which which were were almost almost baseline baseline separated separated as asshown shown in Figurein Figure 40a 40 (aα (=α 1.15,= 1.15, RsRs = =1.18). 1.18). In In this this case, case, the the chirality chirality of of 2-butanol, 2-butanol, i.e., i.e., th thee difference difference between methyl and ethyl groups, was recognized well by the MαNP acid/HPLC. InIn the case of 2-pentanol, the the chirality is is ma madede by the difference between methyl and propyl groups. The The difference difference is is larger larger than than the differen differencece between methyl and ethyl groups. Therefore, Therefore, 2-pentanol M MααNPNP esters esters are are more more largely separated by by HPLC HPLC on on silica silica gel gel as shown in Figure 40b40b (α = 1.25, 1.25, RsRs == 2.02) 2.02) [58]. [58]. Thus, Thus, the the HPLC HPLC separation separation reflects reflects the the difference difference of of chain chain length. length. This tendency becomes remarkable, when when extendin extendingg to to longer longer chain chain alcohols alcohols [58], [58], as as seen in Figure 4040;; (c)(c) 2-hexanol, 2-hexanol, difference difference between between methyl methyl and butyland groups:butyl groups:α = 1.54, αRs = =1.54, 2.66; Rs (d) = 2-heptanol, 2.66; (d) 2-heptanol,difference between difference methyl between and methyl pentyl and groups: pentylα =groups: 1.61, Rs α == 1.61, 2.66; Rs (e) = 2-octanol, 2.66; (e) 2-octanol, difference difference between methyl and hexyl groups: α = 1.69, Rs = 4.10; (f) 2-hexadecanol, difference between CH and between methyl and hexyl groups: α = 1.69, Rs = 4.10; (f) 2-hexadecanol, difference between CH33 and CH3(CH2)13 groups: α = 1.93, Rs = 3.68. Thus, HPLC separation is very sensitive to the difference of CH3(CH2)13 groups: α = 1.93, Rs = 3.68. Thus, HPLC separation is very sensitive to the difference of chain length. chain length. The case of 1-octyn-3-ol is also unique, and diastereomers 63a/63b were largely separated The case of 1-octyn-3-ol is also unique, and diastereomers 63a/63b were largely separated (α = 1.74, Rs = 4.53) (Figure 40g) [58]. Thus, the separation factor α of esters 63a/63b is larger than (α = 1.74, Rs = 4.53) (Figure 40g) [58]. Thus, the separation factor α of esters 63a/63b is larger than that that of 2-heptanol esters 60a/60b, implying that the acetylene group is effective for HPLC separation. of 2-heptanol esters 60a/60b, implying that the acetylene group is effective for HPLC separation. Such Such a large α value enabled HPLC separation on a large scale as shown in Figure 41, where an 850 mg a large α value enabled HPLC separation on a large scale as shown in Figure 41, where an 850 mg sample was injected [59]. There is still enough space between the two bands, and hence it would be sample was injected [59]. There is still enough space between the two bands, and hence it would be possible to load a larger amount of the sample. Thus, the MαNP acid method is very useful for the possible to load a larger amount of the sample. Thus, the MαNP acid method is very useful for the synthesis of enantiopure 1-octyn-3-ol on a large scale, which would be useful as a chiral synthon. synthesis of enantiopure 1-octyn-3-ol on a large scale, which would be useful as a chiral synthon.

Molecules 2016, 21, 1328 25 of 37 Molecules 2016, 21, 1328 25 of 37

FigureFigure 40.40. HPLCHPLC separationseparation of diastereomeric M αNPNP esters esters of of aliphatic aliphatic linear linear alcohols alcohols and and acetylene acetylene alcoholalcohol ((aa–)–(g).g Reprinted). Reprinted with with permission permission from from [58 [58].].

FigureFigure 41.41. PreparativePreparative HPLC separation of 1-octyn-3-ol M αNPNP esters 63a63a//63b63b: :silica silica gel gel column, column, theoreticaltheoretical plateplate number,number, nn == 9500–11,600;9,500–11,600; samplesample (850 mg) was in injected.jected. Reprinted Reprinted with with permission permission fromfrom [ [59].59].

ToTo determinedetermine thethe ACsACs ofof thesethese chainchain alcohol M ααNPNP esters, esters, 11H-NMRH-NMR spectra spectra were were measured, measured, andand their their∆ δ∆δvalues values were were calculated calculated as shownas shown in Figure in Figure 42[ 58 42]. In[58]. the In case the of case Mα NPof M esters,αNP theesters, chemical the shiftchemical difference shift difference∆δ is originally ∆δ is definedoriginally as defined∆δ = δ(R as;X ∆) δ− =δ δ(S(R;X;X),) where− δ(S;XR),and whereS indicate R and S the indicate ACs of the the MACsαNP of acid the M part,αNP while acid Xpart,indicates while X the indicates AC of the the alcohol AC of the moiety alcohol to be moiety determined. to be determined.

Molecules 20162016,, 2121,, 13281328 26 of 37

Figure 42. 1H-NMR ∆δ values in ppm and ACs of the first-eluted MαNP esters. Reprinted with permissionFigure 42. 1 fromH-NMR [58]. ∆δ values in ppm and ACs of the first-eluted MαNP esters. Reprinted with permission from [58]. In Figure1, Figure 39, and Figure 40, racemic alcohols were esterified with ( S)-(+)-MαNP acid 1. In suchIn Figures a case, 1, the 39, formula and 40, ofracemic∆δ is transformed alcohols were as esterified follows. For with example, (S)-(+)-M inα theNP caseacid of1. 2-butanol,In such a thecase, first-eluted the formula ester of is∆δ defined is transformed as (S;X)- 57aas ,follows. where XFordenotes example, the ACin the of thecase alcohol of 2-butanol, part in the first-elutedfirst-eluted MesterαNP is ester.defined In addition,as (S;X)-57a the, where second-eluted X denotes ester the can ACbe of definedthe alcohol as ( Spart;−X in)- 57bthe ,first-eluted where −X denotesMαNP ester. the opposite In addition, AC oftheX second. It should-eluted be notedester can that be esters defined (S;− asX ()S and;−X)- (R57b;X), where are enantiomers, −X denotes and the therefore,opposite AC their of chemicalX. It should shift be data noted are that equal esters to each (S;− other.X) and So, (R;∆Xδ) =areδ( Renantiomers,;X) − δ(S;X) and = δ( Stherefore,;−X) − δ (theirS;X) =chemicalδ(second-eluted shift data ester) are eq−ualδ(first-eluted to each other. ester) So, ∆ (seeδ = Figureδ(R;X) 3−). δ Based(S;X) = on δ(S this;−X) definition, − δ(S;X) = δ∆(second-elutedδ values were calculatedester) − δ(fi [rst-eluted58]. ester) (see Figure 3). Based on this definition, ∆δ values were calculated [58]. InIn thethe casecase ofof 2-butanol2-butanol estersesters 57a57a//57b,, the the methyl methyl group showed a positive ∆δ value, and hence waswas placedplaced onon thethe right side, while the ethyl group showedshowed negativenegative ∆∆δ values, and hence was placed on thethe leftleft side. side. Therefore, Therefore, the the AC AC of of the the first-eluted first-eluted ester ester was was determined determined to be to (R be) (Figure (R) (Figure 42). In 42). other In estersother esters58–62 ,58∆δ–62values, ∆δ values are reasonably are reasonably distributed distributed indicating indicating (R)-ACs (R (Figure)-ACs (Figure 42). Thus 42). in Thus the case in the of allcase M ofαNP all estersMαNP57 esters–62, the57– (62S;R, the)-diastereomers (S;R)-diastereomers were eluted were first eluted [58 ].first [58]. InIn thethe case case of of 1-octyn-3-ol 1-octyn-3-ol M αMNPαNP esters esters63a /63a63b/63b, the, the acetylene acetylene proton proton showed showed a positive a positive∆δ value, ∆δ whilevalue, thewhile pentyl the pentyl group showedgroup showed negative negative∆δ values, ∆δ values, leading leading to the( Sto)-AC the ( (FigureS)-AC (Figure 42)[58 ].42) [58].

5.4. Application of Acetylene Chain Alcohols The MMααNPNP acidacid methodmethod has has been been applied applied to to other other acetylene acetylene alcohols alcohols as shownas shown in Figurein Figure 43. In43. the In casethe case of 3-butyn-2-ol, of 3-butyn-2-ol, the diastereomers the diastereomers64a/64b 64awere/64b separatedwere separated well as well shown as inshown figure in 42a figure (α = 1.20,42a Rs(α = 2.09)1.20, [Rs60 ].= 2.09) The result [60]. The indicated result that indicated acetylene that and acetylene methyl and groups methyl are recognized groups are well recognized by HPLC well on silicaby HPLC gel. Inon the silica case gel. of 5-methyl-1-hexyn-3-ol,In the case of 5-methyl-1-hexyn-3-ol, the iso-butyl group the iso is-butyl longer group than theis longer methyl than group, the andmethyl hence group, esters and65a hence/65b esterswere much65a/65b more were largely much separated more largely (α = separated 1.54, Rs = 2.97)(α = 1.54, [60]. Rs = 2.97) [60].

Molecules 2016, 21, 1328 27 of 37 MoleculesMolecules 2016 2016, ,21 21, ,1328 1328 2727 of of 37 37 Molecules 2016, 21, 1328 27 of 37

Figure 43. HPLC separation of diastereomeric MαNP esters of acetylene alcohols: (a) 3-butyn-2-ol, FigureFigure 43. HPLCHPLC separation separation of of diastereomeric M MααNPNP esters esters of of acetylene alcohols: ( (aa)) 3-butyn-2-ol, 3-butyn-2-ol, hexane/EtOAc 20:1, reprinted with permission from [60]; (b) 5-methyl-1-hexyn-3-ol, hexane/EtOAc hexane/EtOAchexane/EtOAc 20:1, 20:1, reprinted reprinted wi withth permission permission from [[60];60]; ((b)) 5-methyl-1-hexyn-3-ol,5-methyl-1-hexyn-3-ol, hexane/EtOAchexane/EtOAc 10:1, reprinted from [61]. 10:1, reprinted from [61]. [61]. It was easy to determine the ACs of these esters by applying the MαNP ester diamagnetic ItIt was was easy easy to determine the the ACs of these esters by applying the M αNP ester diamagnetic anisotropy method as shown Figure 44 [60]. In both cases, acetylene protons showed positive ∆∆δ anisotropyanisotropy method method as as shown Figure 44 [[60].60]. In In both cases, acetylene protons showed positive ∆δδ values, while methyl and iso-butyl groups showed negative ∆∆δ values. Therefore, the ACs of the values, while methyl and iso-butyl groups showed negative ∆δ values. Therefore, Therefore, the the ACs ACs of of the first-eluted esters were unambiguously determined to be (S). first-elutedfirst-eluted esters were unambiguously determined to be (S).

FigureFigure 44. 44. 1 1H-NMRH-NMR ∆ ∆δδ values values in in ppm ppm and and ACs ACs of of the the first-eluted first-eluted M MααNPNP esters esters 64a 64a ( (aa)) and and 65a 65a ( (bb).). FigureFigure 44. 11H-NMRH-NMR ∆∆δδ valuesvalues in in ppm ppm and and ACs ACs of of the the first-eluted first-eluted M MααNPNP esters esters 64a64a ((aa)) and and 65a65a ((bb).). ReprintedReprinted with with permission permission from from [60]. [60]. ReprintedReprinted with permission from [60]. [60].

ItIt shouldshould bebe notednoted thatthat thethe ACsACs ofof thesethese MMααNPNP estersesters couldcould bebe confirmedconfirmed byby X-rayX-ray ItIt should should be be noted noted that that the the ACs ACs of of these M αNPNP esters could be be confirmed confirmed by by X-ray crystallography.crystallography. We We were were lucky lucky to to obtain obtain single single crystals crystals of of M MααNPNP esters esters ( (SS;R;R)-(+)-)-(+)-64b64b and and ( (SS;S;S)-()-(−−)-)- crystallography.crystallography. WeWe were were lucky lucky to to obtain obtain single single crystals crystals of Mofα MNPαNP esters esters (S;R ()-(+)-S;R)-(+)-64b64band and (S;S )-((S−;S)-)-(65a−)-, 65a65a, ,when when recrystallizing recrystallizing from from MeOH. MeOH. The The ORTEP ORTEP drawings drawings are are illustrated illustrated in in Figure Figure 45, 45, where where the the 65awhen, when recrystallizing recrystallizing from from MeOH. MeOH. The ORTEPThe ORTEP drawings drawings are illustrated are illustrated in Figure in Figure 45, where 45, where the ACs the ACsACs ofof thethe alcoholalcohol partsparts werewere clearlyclearly determineddetermined byby usingusing thethe MMααNPNP acidacid partpart asas anan internalinternal ACsof the of alcohol the alcohol parts wereparts clearly were determinedclearly determined by using by the using MαNP the acid Mα partNP asacid an internalpart as referencean internal of reference of AC [56]. The ACs determined by the1 1H-NMR diamagnetic anisotropy method were thus referenceAC [56]. The of AC ACs [56]. determined The ACs determined by the 1H-NMR by the diamagnetic 1H-NMR diamagnetic anisotropy method anisotropy were method thus corroborated were thus corroborated by X-ray crystallography. corroboratedby X-ray crystallography. by X-ray crystallography.

α − FigureFigure 45. X-rayX-ray ORTEP ORTEP drawings drawings of M ofαNP M estersNP esters(S;R)-(+)- (S;64bR)-(+)- (a) 64band ((aS);S)-( and−)-65a (S;S (b).)-( Reprinted)-65a (b). Figure 45. X-ray ORTEP drawings of MαNP esters (S;R)-(+)-64b (a) and (S;S)-(−)-65a (b). Reprinted Reprintedwith permission with permission from [56]. from [56]. with permission from [56].

The1 1H-NMR diamagnetic anisotropy method using MαNP acid has been originally developed The 1H-NMR diamagnetic anisotropy method using MαNP acid has been originally developed as an empirical rule. However, we have never encountered any exception, where the AC determined as an empirical rule. However, we have never encountered any exception, where the AC determined

Molecules 2016, 21, 1328 28 of 37

MoleculesThe 20161H-NMR, 21, 1328 diamagnetic anisotropy method using MαNP acid has been originally developed28 of 37 as an empirical rule. However, we have never encountered any exception, where the AC determined 1 byMolecules the 1H-NMR 2016, 21, 1328 Mα NP ester method disagreed with that by X-ray crystallography. This fact 28 ofis 37very important for making the 1H-NMR MαNP ester method moremore reliable.reliable. by the 1H-NMR MαNP ester method disagreed with that by X-ray crystallography. This fact is very The M MααNPNP acid acid method method is isalso also applicable applicable to tointernal internal acetylene acetylene alcohols alcohols with with long longchains, chains, and important for making the 1H-NMR MαNP ester method more reliable. andsome some enantiopure enantiopure alcohols alcohols with with established established ACs ACs were were synthesized synthesized as as exemplified exemplified in in Figure Figure 46 46 [[62].62]. The MαNP acid method is also applicable to internal acetylene alcohols with long chains, and Racemic acetylene alcohol ((±)-±)-6666 waswas esterified esterified with with ( (SS)-(+)-M)-(+)-MααNPNP acid acid yielding yielding diastereomeric diastereomeric esters some enantiopure alcohols with established ACs were synthesized as exemplified in Figure 46 [62]. 68a/68b68b, ,which which were were largely largely separated separated by by HPLC HPLC on on silica silica gel gel (α ( α= =1.61, 1.61, RsRs = 1.93)= 1.93) (Figure (Figure 47a) 47 a)[62]. [62 It]. Racemic acetylene alcohol (±)-66 was esterified with (S)-(+)-MαNP acid yielding diastereomeric esters Itwas was surprising surprising to tofind find that that although although the thetwo two side side chains chains are aresimilar similar in length, in length, i.e., i.e.,C8 and C andC9, the C , 68a/68b, which were largely separated by HPLC on silica gel (α = 1.61, Rs = 1.93) (Figure 47a) 8[62]. It 9 thediastereomers diastereomers were were well well separate separated.d. This This fact fact implies implies that that the the acetylene acetylene group group makes makes a a dominant was surprising to find that although the two side chains are similar in length, i.e., C8 and C9, the contribution for HPLC separation. diastereomers were well separated. This fact implies that the acetylene group makes a dominant contribution for HPLC separation.

Figure 46. Enantiopure acetylene alcohols with establ establishedished ACs, which were prepared by the MαNP Figure 46. Enantiopure acetylene alcohols with established ACs, which were prepared by the MαNP acid method. acid method. The HPLC data of esters 69a/69b are also interesting, because the separation factor α became TheThe HPLC HPLC data data of of esters esters 69a69a//69b areare also also interesting, interesting, because because the the separation separation factor factor α becameα became larger ((αα == 1.78,1.78, RsRs= =4.10) 4.10) (Figure (Figure 47 47b)b) [62 [62].]. The The two two side side chains chains consist consist of Cof Cand18 and C andC19 henceand hence they larger (α = 1.78, Rs = 4.10) (Figure 47b) [62]. The two side chains consist of C1818 and 19C19 and hence they are more similar to each other than in the case of 68a/68b. However, the separation factor α is arethey more are similar more similar to each to other each thanotherin than the in case the of case68a of/68b 68a./68b However,. However, the separationthe separation factor factorα is α larger is thanlargerlarger that than than of that68a that /of of68b 68a 68a./ This68b/68b. . factThis This again factfact again indicates indicatesindicates that that thethat acetylenethe the acetylene acetylene group group group is ais keyisa keya key factor factor factor to to control controlto control the HPLCthethe HPLC HPLC separation separation separation on silicaon on silica silica gel. gel.gel.

FigureFigure 47. 47. HPLC HPLC separation separation ofof diastereomeric diastereomeric MM ααNPNP esters esters of of long long chain chain in internal internalternal acetylene acetylene alcohols: alcohols: alcohols: (a)(a 68a) 68a/68b68b/68b,, hexane/EtOAc, hexane/EtOAc hexane/EtOAc 20:1,20:1, reprintedreprinted reprinted fromfrom from [61]; [[61];61]; ( (b(b)) )69a 69a69a/69b//69b69b, hexane/EtOAc,, hexane/EtOAc hexane/EtOAc 50:1, 50:1, 50:1, reprinted reprinted reprinted with with permissionpermission from from [[62]. 62[62].].

To determine the ACs of these compounds, 1H-NMR ∆δ values were calculated as shown in To determinedetermine the ACs ofof thesethese compounds,compounds, 1H-NMR ∆δ values were calculated as shown in Figure 48 [62]. Figure 48[ [62].62].

Molecules 2016, 21, 1328 29 of 37

Molecules 2016,, 21,, 13281328 29 of 37

Figure 48. 1H-NMR ∆δ values in ppm and ACs of the first-eluted MαNP esters 68a and 69a. Reprinted with permission from [62].

In MαNP esters 68a/68b, the side chain containing the acetylene group showed positive ∆δ values, while the saturated side chain showed negative ∆δ values. Therefore, the AC of the first-eluted MαNP ester (−)-68a was determined to be (S). The same was found in MαNP esters 69a/69b, which led to the (S)-AC of the first eluted ester (−)-69a. FigureMαNP 48. esters1H-NMR 68a, ∆68bδ values, andin 69a ppm were and obtained ACs of the as first-eluted a syrup Morα amorphousNP esters 68a solidand 69a except. Reprinted ester 69b, Figure 48. 1H-NMR ∆δ values in ppm and ACs of the first-eluted MαNP esters 68a and 69a. Reprinted whichwith was permission recrystallized from [ 62from]. iso-PrOH, giving thin plate crystals. However, because their thickness with permission from [62]. was ca. 5 μm, a conventional X-ray machine could not be used. Instead, the strong X-ray of synchrotronIn MαNP radiation esters 68a in/68b the, theSPring-8 side chain in Hyogo, containing Japan the was acetylene used groupfor X-ray showed crystallography positive ∆δ values, (final In MαNP esters 68a/68b, the side chain containing the acetylene group showed positive ∆δ whileR = 0.0814). the saturated The ORTEP side chain drawing showed was negative obtained∆δ values. as shown Therefore, in Figure the AC 49a, of thewhere first-eluted (19R)-AC M αwasNP values, while the saturated side chain showed negative ∆δ values. Therefore, the AC of the esterunambiguously (−)-68a was determined determined by to using be (S ).the The Mα sameNP acid was part found as an in MinternalαNP esters reference69a/ [62].69b, Thus, which the led AC to first-eluted MαNP ester (−)-68a was determined to be (S). The same was found in MαNP esters the (S)-AC of the1 first eluted ester (−)-69a. 69aassigned/69b, which by the led H-NMR to the (diamagneticS)-AC of the anisotropyfirst eluted wasester confirmed (−)-69a. by X-ray crystallography. MαNP esters 68a, 68b, and 69a were obtained as a syrup or amorphous solid except ester 69b, MFigureαNP 49besters shows 68a, the68b ,crystal and 69a packing were obtainedof MαNP as ester a syrup molecules or amorphous 69b, where solid the except rectangle ester shows 69b, which was recrystallized from iso-PrOH, giving thin plate crystals. However, because their thickness whicha unit waslattice recrystallized containing fourfrom molecules iso-PrOH, [62].giving It thinis interesting plate crystals. that twoHowever, side chains because of theira molecule thickness are was ca. 5 µm, a conventional X-ray machine could not be used. Instead, the strong X-ray of synchrotron wasplaced ca. in 5 parallel,μm, a conventionaland that these X-ray two alkylmachine chains could form not a pair be withused. those Instead, of the the second strong molecule X-ray ofin radiation in the SPring-8 in Hyogo, Japan was used for X-ray crystallography (final R = 0.0814). synchrotronthe unit lattice. radiation These pairsin the are SPring-8 arranged in soHyogo, as to formJapan an was aliphatic used for bilayer X-ray in crystallography the crystal. The (finalthird The ORTEP drawing was obtained as shown in Figure 49a, where (19R)-AC was unambiguously Rand = 0.0814).fourth molecules The ORTEP similarly drawing form was an aliphaticobtained bilaas shownyer. It isin very Figure interesting 49a, where that (19theseR)-AC aliphatic was determined by using the MαNP acid part as an internal reference [62]. Thus, the AC assigned by the unambiguouslybilayer structures determined are similar by to usingthose theof cell Mα membranes.NP acid part as an internal reference [62]. Thus, the AC 1H-NMR diamagnetic anisotropy was confirmed by X-ray crystallography. assigned by the 1H-NMR diamagnetic anisotropy was confirmed by X-ray crystallography. Figure 49b shows the crystal packing of MαNP ester molecules 69b, where the rectangle shows a unit lattice containing four molecules [62]. It is interesting that two side chains of a molecule are placed in parallel, and that these two alkyl chains form a pair with those of the second molecule in the unit lattice. These pairs are arranged so as to form an aliphatic bilayer in the crystal. The third and fourth molecules similarly form an aliphatic bilayer. It is very interesting that these aliphatic bilayer structures are similar to those of cell membranes.

Figure 49. X-ray crystal structure of MαNP ester (S;19R)-(−)-69b.(a) ORTEP drawing; (b) Crystal Figure 49. X-ray crystal structure of MαNP ester (S;19R)-(−)-69b. (a) ORTEP drawing; (b) Crystal packing of (S;19R)-(−)-69b: view along the a axis; the rectangle indicates a unit lattice. Reprinted with packing of (S;19R)-(−)-69b: view along the a axis; the rectangle indicates a unit lattice. Reprinted with permission from [62]. permission from [62]. Figure 49b shows the crystal packing of MαNP ester molecules 69b, where the rectangle shows a unit lattice containing four molecules [62]. It is interesting that two side chains of a molecule are placed in parallel, and that these two alkyl chains form a pair with those of the second molecule in the unit lattice. These pairs are arranged so as to form an aliphatic bilayer in the crystal. The third and fourthFigure molecules 49. X-ray similarly crystal formstructure analiphatic of MαNP bilayer. ester (S;19 It isR)-( very−)-69b interesting. (a) ORTEP that drawing; these aliphatic (b) Crystal bilayer structurespacking are of similar (S;19R)-( to−)- those69b: view of cell along membranes. the a axis; the rectangle indicates a unit lattice. Reprinted with permission from [62].

Molecules 2016,, 21,, 1328 1328 3030 of of 37 37 Molecules 2016, 21, 1328 30 of 37

It was easy to recover enantiopure long chain acetylene alcohol (S)-(−)-67, as shown in Figure 50. ItIt was was easy easy to to recover recover enantiopur enantiopuree long chain acetylene alcohol ( S)-(−)-)-67,67 ,as as shown shown in in Figure Figure 50. 50 . The value of alcohol (S)-(−)-67 was small, but it was still measurable as listed in The specific specific rotation value of alcohol (S)-(−)-)-6767 waswas small, small, but but it it was was still still measurable measurable as as listed listed in in Figure 46, where the observed [α] was larger than the standard deviation σ [62]. Figure 46, where the observed [α]D was larger than the standard deviation σ [62]. Figure 46, where the observed [α]D was larger than the standard deviation σ [62].

Figure 50. Preparation of enantiopure acetylene alcohol (S)-(−)-67. Figure 50. Preparation of enantiopure acetylene alcohol (S)-(−)-)-6767. . 5.5. Synthesis of Enantiopure Saturated Long Chain Alcohol with Established AC 5.5.5.5. Synthesis Synthesis of of Enantiopure Enantiopure Saturated Saturated Long Chain Alcohol with Established AC Next we tried to synthesize enantiopure saturated long chain alcohols. However, it was already NextNext we we tried tried to to synthesize synthesize enantiopure enantiopure saturate saturatedd long long chain chain alcohols. alcohols. However, However, it it was was already already reported that the direct catalytic reduction of acetylene alcohol led to partial [63]. reportedreported that that the the direct direct catalytic catalytic reduction reduction of of acetylene acetylene alcohol alcohol led led to to partial partial racemization racemization [63]. [63]. Therefore, to prevent the racemization, acetylene alcohol MαNP ester was subjected to the catalytic Therefore, to prevent the racemization, acetylene alcohol M MααNPNP ester ester was was subjecte subjectedd to to the catalytic reduction, and the diastereomeric purity was checked as follows [13]. reduction,reduction, and and the the diastereomeric pu purityrity was checked as follows [[13].13]. As a model compound, 5-octyn-4-ol was selected, and it was easy to separate its diastereomeric AsAs a a model model compound, compound, 5-octyn-4-ol 5-octyn-4-ol was was selected, selected, and and it it was was easy easy to to separate separate its its diastereomeric diastereomeric MαNP esters as shown in Figure 51a [60]. The first-eluted MαNP ester (S;S)-(−)-70a was reduced with MMαNPNP esters esters as shown in Figure 51a51a [60]. [60]. The The first-eluted first-eluted MαNP ester (S;;S)-()-(−)-)-70a70a waswas reduced reduced with with H2/PtO2 in diethyl ether yielding 4-octanol MαNP ester (S;S)-(−)-3b [13]: see also Figure 1. HH22/PtO22 inin diethyl diethyl ether ether yielding yielding 4-octanol 4-octanol M MααNPNP ester ester ( (SS;S;S)-()-(−−)-)-3b3b [13]:[13 ]:see see also also Figure Figure 1.1 .

α Figure 51. HPLCHPLC separation separation of of diastereomeric diastereomeric 5-octyn-4-ol 5-octyn-4-ol M αNPNP esters, esters, and conversion conversion to 4-octanol 4-octanol Figureα 51. HPLC separation of diastereomeric 5-octyn-4-ol MαNP esters, and conversion to 4-octanol MαNPNP ester ester by by catalytic catalytic reduction: reduction: (a) ( a70a) 70a/70b/70b, hexane/EtOAc, hexane/EtOAc 20:1, 20:1,reprinted reprinted with permission with permission from MfromαNP [60 ester]; (b )by Reduction catalytic withreduction: H /PtO (a) 70ain diethyl/70b, hexane/EtOAc ether. 20:1, reprinted with permission from [60]; (b) Reduction with H2/PtO22 in diethyl2 ether. [60]; (b) Reduction with H2/PtO2 in diethyl ether. α − To checkcheck the the diastereomeric diastereomeric purity purity of M ofNP M esterαNP (Sester;S)-( (S)-;3bS)-(obtained−)-3b obtained by thecatalytic by the reductioncatalytic To check the diastereomericα purity− of MαNP ester (S;S)-(−)-3b obtained by the catalytic reductionof acetylene of acetylene alcohol MalcoholNP esterMαNP (S ester;S)-( (S)-;70aS)-(,−)- HPLC70a, HPLC comparison comparison was was performed performed as shownas shown in reduction of acetylene alcohol MαNP ester (S;S)-(−)-70a, HPLC comparison was performed as shown inFigure Figure 52 [5213 [13].]. in Figure 52 [13]. − − Figure 52a52a shows the HPLCHPLC ofof estersesters ((SS;R;R)-()-(−)-3a and ((SS;;S)-()-(−)-)-3b3b preparedprepared from from racemic Figure 52a shows the HPLC of esters (S;R)-(−)-3a and (S;S)-(−)-3b prepared from racemic 4-octanol (±)- (±)-22; ;the the part part (b) (b) shows shows the the HPLC HPLC of of ester (S;S)-(−)-)-3b3b obtainedobtained by by reduction reduction of of ester 4-octanol (±)-2; the part (b) shows the HPLC of ester (S;S)-(−)-3b obtained by reduction of ester (S;;S)-()-(−−)-)-70a70a; ;the the part part (c) (c) shows shows the the coinjection coinjection of of two two samples samples used used in in (a) (a) and and (b). (b). The The HPLC HPLC (b) (b) (S;S)-(−)-70a; the part (c) shows the coinjection of two samples used in (a) and (b). The HPLC (b) shows only only one band indicating that the produc productt was diastereomerically pure, pure, and and hence hence it it was shows only one band indicating that the product was diastereomerically pure, and hence it αwas concluded thatthat nono racemization racemization occurred occurred during during the the catalytic catalytic reduction reduction of the of acetylene the acetylene alcohol alcohol M NP concluded that no racemization occurred during the catalytic reduction of the acetylene alcohol MesterαNP [13 ester]. [13]. MαNP ester [13].

Molecules 2016, 21, 1328 31 of 37 Molecules 2016,, 21,, 13281328 31 of 37

Figure 52. Diastereomeric purity check of MαNP ester (S;S)-(−)-3b obtained by the catalytic reduction ofFigure acetylene 52. Diastereomeric alcohol Mα NP purity ester check (S; Sof)-( M−α)-αNP70a . ester HPLC, ((SS;;SS )-()-(hexane/EtOAc−−)-)-3b3b obtainedobtained 20:1. by by thethe (a )catalytic catalytic (S;R)-(− reduction )-3a and (ofS; S acetyleneacetylene)-(−)-3b prepared alcoholalcohol MfromMααNPNP racemic esterester ( 4-octanolS(;S;)-(S)-(−−)-)-70a (±)-70a.2. ; HPLC, HPLC,(b) (S;S hexane/EtOAc)-(hexane/EtOAc−)-3b obtained 20:1.20:1. by the ((a)( )reduction (SS;;RR)-()-(−−)-)- of3a3a esterand ((S;;S)-()-(−−)-)-)-70a3b3b prepared;prepared (c) coinjection from racemicof two samples 4-octanol used ((±)-±)- in22;;( (abb,)b)( ().S ;Reprinted;S)-(−)-)-3b3b obtained obtainedwith permission by by the the reduction reduction from [13]. of ester (S;;S)-(−)-)-70a70a; ;((cc)) coinjection coinjection of of two two samples samples used used in in ( (aa,,bb).). Reprinted Reprinted with with permission permission from from [13]. [13]. Next, the ultimate cryptochiral alcohol, (R)-(−)-19-octatriacontanol 72, was synthesized (FigureNext, 53) [13]. the ultimateEnantiopultimate ure cryptochiralcryptochiral acetylene alcohol,alcoholalcohol, M ((RαRNP)-()-(−− ester)-19-octatriacontanol (S;S)-(−)-69a was 72subjected,, was synthesized synthesizedto catalytic reduction,(Figure 5353) )[ yielding[13].13]. EnantiopureEnantiop saturatedure acetylene acetylenealcohol M alcoholalcoholαNP ester MMααNPNP (S ester;esterR)-(− ()-(SS71;;SS.)-( )-(To−−)-)- recover69a69a was alcohol subjected 72 to, a catalytic drastic reactionreduction, condition yieldingyielding was saturated saturated necessary; alcohol alcohol ester Mα M(NPS;αRNP ester)-(− )-ester71 (S; Rwas )-((S−; Rtreated)-)-(71−.)- To71 with recover. To NaOCHrecover alcohol3 alcoholin72 iso, a-PrOH drastic 72, ayielding reactiondrastic enantiopureconditionreaction condition was alcohol necessary; was (R)-( necessary; ester−)-72 (: S[α;R])-( ester53− )-= 71 −(S0.038was;R)-( treated−(σ)- 710.56, was withc 1.04, treated NaOCH CHCl with3).in TheNaOCHiso-PrOH specific3 in yielding rotationiso-PrOH enantiopure value yielding was 53 D 3 alcohol (R)-(−)-72: [α] = −0.03853 (σ 0.56, c 1.04, CHCl3). The specific rotation value was thus very thusenantiopure very small, alcohol and (muchRD)-(−)- smaller72: [α]D than = the−0.038 standard (σ 0.56, deviation c 1.04, CHCl σ. Therefore,3). The specific the observed rotation [ αvalue]D value was small, and much smaller than the standard deviation σ. Therefore, the observed [α] value is not isthus not very reliable, small, but and there much is no smaller proper than physical the standard data to specifydeviation this σ enantiomer.. Therefore, the So, observedthe observedD [α]D minus value reliable, but there is no proper physical data to specify this enantiomer. So, the observed minus sign signis not was reliable, used buthere there [13]. is no proper physical data to specify this enantiomer. So, the observed minus was used here [13]. sign was used here [13].

Figure 53. Preparation of enantiopure long chain alcohol with ultimate cryptochiralitycryptochirality ((RR)-()-(−−)-)-7272.. Figure 53. Preparation of enantiopure long chain alcohol with ultimate cryptochirality (R)-(−)-72. The presentpresent assignment assignment of of the the minus minus sign sign to alcohol to alcohol (R)-72 is(R logically)-72 is logically reasonable, reasonable, when compared when comparedwithThe the present opticalwith the rotationassignment optical rotation data of ofthe data (R )-(minus of− ()-2-butanol,R )-(sign−)-2-butanol, to alcohol (R)-( (−R )-3-hexanol,)-((R−)-)-3-hexanol,72 is logically and and ( R reasonable,()-(R)-(−−)-4-octanol)-4-octanol when 2 (Figurecompared 5454).). with These These the are optical are chain chain rotation alcohol alcohol homologsdata ofhomologs (R with)-(−)-2-butanol, the with structure the (R of)-(structure CH−)-3-hexanol,3(CH 2of)nn -CH(OH)-(CH CHand3 (CH(R)-(2−)n)-4-octanoln-CH(OH)-2)n+1CH 32, where n = 0 for 2-butanol; n = 1 for 3-hexanol; n = 2 for 4-octanol 2; n = 17 for 19-octatriacontanol (CH(Figure2)n+1 54).CH 3,These where are n =chain 0 for alcohol2-butanol; homologs n = 1 for with 3-hexanol; the structure n = 2 for of 4-octanolCH3(CH 22);n n-CH(OH)- = 17 for 72. Therefore, these compounds should have the same relationship between AC and the optical 19-octatriacontanol(CH2)n+1CH3, where 72 . nTherefore, = 0 for 2-butanol; these compounds n = 1 for should 3-hexanol; have then = same 2 for relationship 4-octanol 2 between; n = 17 ACfor rotation sign. and19-octatriacontanol the optical rotation 72. Therefore,sign. these compounds should have the same relationship between AC and the optical rotation sign.

Molecules 2016 21 Molecules 2016,, 21,, 1328 1328 3232 ofof 3737 Molecules 2016, 21, 1328 32 of 37

Figure 54. ACs and optical rotation data of alcohol homologs with the structure of CH (CH ) - Figure 54. ACs and optical rotation data of alcohol homologs with the structure of CH 3(CH 2) n- Figure 54. ACs and optical rotation data of alcohol homologs with the structure of CH3 (CH2 )nn- CH(OH)-(CH2)n+1CH3: (R)-(−)-2-butanol [64], (R)-(−)-3-hexanol [65], and (R)-(−)-4-octanol 23 [13].2 CH(OH)-(CH2)n+1+1CHCH33:(: (RR)-()-(−−)-2-butanol)-2-butanol [64], [64], ( (R)-()-(−)-3-hexanol)-3-hexanol [65], [65], and and (R (R)-()-(−−)-4-octanol)-4-octanol 2 2[13].[13 ]. It is interesting to study whether the MαNP ester method is still useful for recognizing the It is interesting to study whether the MαNP ester method is still useful for recognizing the chiralityIt is interestingof saturated to long study chain whether alcohols the Morα NPnot. esterFigure method 55 shows is still the useful analytical for recognizing HPLC of two the chirality of saturated long chain alcohols or not. Figure 55 shows the analytical HPLC of two chiralityexamples, of saturated10-nonacosanol long chain esters alcohols 73a/73b or and not. 19-octatriacontanol Figure 55 shows the esters analytical 71 [13]. HPLC of two examples, examples,10-nonacosanol 10-nonacosanol esters 73a/ esters73b and 73a 19-octatriacontanol/73b and 19-octatriacontanol esters 71 [ 13esters]. 71 [13]. In the case of esters 73a/73b, CH3(CH2)88- and CH3(CH2)18- groups are compared. It was InIn the case ofof estersesters 73a73a//73b73b,, CH CH3(CH(CH2))88-- and CH 3(CH2)18- groupsgroups areare compared.compared. It It was surprising to find that MαNP esters (S3;R)-73a2 8 8and (S;S)-73b3 were2 18 clearly separated, although two surprising toto findfind thatthat M MααNPNP esters esters (S ;(RS;)-R73a)-73aand and (S ;S(S)-;73bS)-73bwere were clearly clearly separated, separated, although although two longtwo long alkyl chains CH3(CH2)8- and CH3(CH2)18- were compared (Figure 55a). The MαNP acid is thus longalkyl alkyl chains chains CH (CH CH3)(CH- and2)8- CH and(CH CH3)(CH- were2)18- comparedwere compared (Figure (Figure 55a). The 55a). M TheαNP M acidαNP is acid thus is useful thus useful for recognizing3 2 8 the chirality3 of2 18long alkyl chain alcohols, where two chains are different in usefulfor recognizing for recognizing the chirality the chirality of long of alkyl long chain alkyl alcohols, chain alcohols, where two where chains two are chains different are different in length in to length to some extent. lengthsome extent. to some extent.

FigureFigure 55. 55. AnalyticalAnalytical HPLC HPLC of saturated of saturated long chain long alcohols chain M alcoholsαNP esters: Mα silicaNP esters:gel column silica (10 gelφ × Figure 55. Analytical HPLC of saturated long chain alcohols MαNP esters: silica gel column (10 φ × column300 mm, ( 10n =φ 34,400);× 300 mmsample, n (0.1= 34,400); mg); detected sample by (0.1 UV. mg); (a) hexane/EtOAc detected by UV.80:1; ( a(b)) hexane/EtOAc hexane/EtOAc 150:1. 80:1; 300 mm, n = 34,400); sample (0.1 mg); detected by UV. (a) hexane/EtOAc 80:1; (b) hexane/EtOAc 150:1. (Redrawnb) hexane/EtOAc with permission 150:1. Redrawn from [13]. with permission from [13]. Redrawn with permission from [13]. Diastereomeric 19-octatriacontanol MαNP esters 71 were prepared from racemic 19-octa- Diastereomeric 19-octatriacontanol 19-octatriacontanol M MαNPNP esters esters 7171 werewere prepared prepared from from racemic racemic 19-octa- 19-octa- triacontanol (±)-± 72 and (S)-(+)-MααNP acid, and the mixture was subjected to analytical HPLC as triacontanoltriacontanol (±)- ( )-7272 andand ( (SS)-(+)-M)-(+)-MαNPNP acid, acid, and and the the mixture mixture was was subjected subjected to to analytical HPLC as shown in Figure 55b. However, it was not possible to separate the two esters and they were eluted as shown in Figure 5555b.b. However, itit waswas notnot possiblepossible totoseparate separate the the two two esters esters and and they they were were eluted eluted as as a singlea single peak peak [13]. [13]. It may It bemay reasonable, be reasonable, because because CH (CH CH) 3–(CH and2) CH17– (CHand CH) –3 chains(CH2)18 are– chains compared are a single peak [13]. It may be reasonable, because3 CH2 173(CH2)17– 3and 2CH183(CH2)18– chains are incompared the case ofin estersthe case71. Namely,of esters the 71 difference. Namely, isthe only difference a –CH2 moietyis only between a –CH2 the moiety CH3(CH between2)17– and the compared in the case of esters 71. Namely, the difference is only a –CH2 moiety between the CHCH33(CH(CH22)18)17–– groups. and CH This3(CH alcohol2)18– isgroups. thus one This of thealcohol ultimate is cryptochiralthus one of compounds,the ultimate and cryptochiral hence the CH3(CH2)17– and CH3(CH2)18– groups. This alcohol is thus one of the ultimate cryptochiral enantiopurecompounds, synthesis and hence of suchthe enantiopure a compound withsynthesis established of such AC a iscompound extremely with difficult. established AC is compounds, and hence the enantiopure synthesis of such a compound with established AC is extremelyAs shown difficult. in Figure 53, however, it was possible to synthesize enantiopure 19-octatriacontanol extremely difficult. (R)-(−As)-72 shownwith establishedin Figure 53, AC. however, The Mα NPit was acid possibl methode to is synthesize thus also very enantiopure powerful for19-octatriacontanol the enantiopure As shown in Figure 53, however, it was possible to synthesize enantiopure 19-octatriacontanol synthesis(R)-(−)-72 of with such established ultimate cryptochiral AC. The Mα compoundsNP acid method with is established thus also very ACs powerful [13]. for the enantiopure (R)-(−)-72 with established AC. The MαNP acid method is thus also very powerful for the enantiopure synthesis of such ultimate cryptochiral compounds with established ACs [13]. synthesis6. Conclusions of such ultimate cryptochiral compounds with established ACs [13]. 6. ConclusionsWe have developed some chiral auxiliaries, such as camphorsultam, CSP acid, CSDP acid, 6. Conclusions and MαNP acid, which are very useful for the synthesis of enantiopure compounds and simultaneous We have developed some chiral auxiliaries, such as camphorsultam, CSP acid, CSDP acid, and determinationWe have developed of their ACs. some As summarizedchiral auxiliaries, in Table such1, the as preparedcamphorsultam, diastereomers CSP acid, were CSDP separated acid, welland MαNP acid, which are very useful for the synthesis of enantiopure compounds and simultaneous Mbyα HPLCNP acid, on silicawhich gel; are the very separation useful for factor the syntheα for CSPsis of esters enantiopure (average αcompounds= 1.09), CSDP and esters simultaneous (average determination of their ACs. As summarized in Table 1, the prepared diastereomers were separated determination of their ACs. As summarized in Table 1, the prepared diastereomers were separated well by HPLC on silica gel; the separation factor α for CSP esters (average α = 1.09), CSDP esters well by HPLC on silica gel; the separation factor α for CSP esters (average α = 1.09), CSDP esters

Molecules 2016, 21, 1328 33 of 37

MoleculesMolecules 2016 20162016,, , ,21 2121,, , ,1328 1328 13281328 333333 of ofof 37 3737 α = 1.20), MαNP esters (average α = 1.58). So, in most cases, HPLC separation on a preparative scale was(average(average(average possible. αα ==== 1.20),1.20),1.20),1.20), MMMMααNPNP estersesters (average(average αα ==== 1.58).1.58).1.58).1.58). So,So,So,So, inininin mostmostmostmost cases,cases,cases,cases, HPLCHPLCHPLCHPLC separationseparationseparationseparation onononon aaaa preparativepreparative scalescale waswas possible.possible. Table 1. Summary of the preparation of diastereomers, HPLC separation, and AC determination by TableTable 1.1. SummarySummary SummarySummary ofof ofof thethe thethe preparationpreparation preparationpreparation ofof ofof diastereomers,diastereomers, diastereomers,diastereomers, HPLCHPLC HPLCHPLC separation,separation, separation,separation, andand andand ACAC ACAC determinationdetermination determinationdetermination byby byby X-ray crystallography and/or 1H-NMR diamagnetic anisotropy. X-rayX-ray crystallography crystallography and/or and/or 1 1H-NMRH-NMR diamagnetic diamagnetic anisotropy. anisotropy.

PreparationPreparationPreparation of of Diastereom DiastereomersDiastereomersers HPLCHPLC HPLC ACAC Determination Determination AC Determination

αα,, , ,not not notnotα, notdetermineddetermined determineddetermined determined RsRs = = =Rs= 0.7~2.87 0.7~2.87 0.7~2.870.7~2.87= 0.7~2.87 X-ray,X-ray,X-ray, 6 6 examples examples 6 examples av.av.av. 1.74 1.74 1.74av.1.74 1.74

αα = = == α1.05~1.1 1.05~1.1 1.05~1.11.05~1.1= 1.05~1.1 av.av.av. 1.09 1.09 1.09av.1.09 1.09 X-ray,X-ray,X-ray, 3 3 examples examples 3 examples RsRs = = =Rs= 1.3~1.6 1.3~1.6 1.3~1.61.3~1.6= 1.3~1.6 av.av.av. 1.4 1.4 1.4av.1.4 1.4

αα = = == α1.14~1.25 1.14~1.25 1.14~1.251.14~1.25= 1.14~1.25 av.av.av. 1.20 1.20 1.20av.1.20 1.20 X-ray,X-ray,X-ray, 4 4 examples examples 4 examples RsRs = = Rs== 0.91~1.94 0.91~1.94 0.91~1.940.91~1.94= 0.91~1.94 av.av.av. 1.31 1.31 1.31av.1.31 1.31

αα = = == α1.15~1.93 1.15~1.93 1.15~1.931.15~1.93= 1.15~1.93 111H-NMR,1H-NMR, av.av.av. 1.58 1.58 1.58av.1.58 1.58 1H-NMR, 13 examples 1313 examples examples RsRs = = Rs== 1.18~5.97 1.18~5.97 1.18~5.971.18~5.97= 1.18~5.97 X-ray, 5 examples X-ray,X-ray, 5 5 examples examples av.av.av. 2.98 av.2.98 2.982.982.98

αα,, , α,separation separation separationseparation, separation factor; factor; factor;factor; factor; Rs, Rs,Rs resolution resolution,resolution resolution factor; factor;factor; factor; R*, R*,R*, R*, chiral chiralchiral chiral substituent. substituent.substituent. substituent.

AnotherAnotherAnother advantage advantageadvantage of theofof the presentthe presentpresent methods methodsmethods is that isis thatthat thethatthat separated thethethethe separatedseparatedseparatedseparated derivatives derivativesderivativesderivativesderivatives have havehavehavehave a high aaaa probabilityhighhighhighhigh probabilityprobability to to form form single single crystals crystals suitable suitable for for X-ray X-ray crystallography. crystallography. Camphorsultam, Camphorsultam, CSP CSP acid, acid, and and to form single crystals suitable for X-ray crystallography. Camphorsultam, CSP acid, and CSDP acid CSDPCSDP acidacid containcontain heavyheavy atomsatoms suchsuch asas SS andand Cl,Cl, andand hencehence theirtheir ACsACs couldcould bebe determineddetermined byby thethe contain heavy atoms such as S and Cl, and hence their ACs could be determined by the X-ray X-rayX-ray anomalousanomalous scatteringscattering effecteffect ofof heavyheavy atoms.atoms. InIn addition,addition, thethe ACsACs ofof thesethese chiralchiral auxiliaries,auxiliaries, anomalousincludingincludingincludingincluding scattering M MMMααNPNP acid,acid, effect areare established ofestablished heavy atoms. andand hencehence In addition, theythey couldcould the bebe ACs usedused of asas these thethe internalinternal chiral reference auxiliaries,reference ofof AC.AC. including MαSo,NPSo, itit acid, waswas easy areeasy established toto determinedetermine and thethe ACsACs hence fromfrom they thethecould ORTEPORTEP be drawings.drawings. used as the internal reference of AC. So, it was easy to determineItItIt shouldshouldshould bebebe the notednotednoted ACs thatthatthat from thethethethe targettargettarget ORTEP compoundcompoundcompound drawings. andandand chiralchiralchiral auxiliaryauxiliaryauxiliary areareare connectedconnectedconnected bybyby aaa covalentcovalentcovalent bond,bond,It should notnot byby be anan noted ionicionic bond,bond, that the andand target hencehence compound thethe covalently-bondedcovalently-bonded and chiral diastereomersdiastereomers auxiliary are cancan connected bebe separatedseparated by a andand covalent bond,purifiedpurified not by by by anHPLC HPLC ionic on on bond,silica silica gel. gel. and On On hence the the other other the ha ha covalently-bondednd,nd, ionic-bonded ionic-bonded diastereom diastereom diastereomersersers are are usually usually can be separated separated separated and purifiedbyby fractionalfractional by HPLC ,crystallization, on silica gel. and Onand thehencehence other inin hand,somesome ionic-bondedcases,cases, itit waswas diastereomersdifficultdifficult toto obtainobtain are usuallyenantiopureenantiopure separated by fractionalcompoundscompounds crystallization, byby fractionalfractional crystallization.crystallization. and hence in some cases, it was difficult to obtain enantiopure compounds MMααNPNP acid acid is is very very unique, unique, because because it it can can be be applie appliedd to to aliphatic aliphatic alcohols alcohols in in addition addition to to aromatic aromatic by fractional crystallization. alcohols.alcohols. Diastereomeric Diastereomeric M MααNPNP esters esters are are largely largely separated separated by by HPLC HPLC on on silica silica gel, gel, as as shown shown in in Table Table MαNP acid is very unique, because it can be applied to aliphatic alcohols in addition to aromatic 1.1. InIn thisthis sense,sense, thethe MMααNPNP acidacid isis superiorsuperior toto thethe Mosher’sMosher’s MTPAMTPA acidacid andand MPAMPA acid.acid. TheThe ACsACs ofof alcohols. Diastereomeric MαNP esters are largely separated by HPLC on silica gel, as shown in Table1. MMααNPNP estersesters couldcould bebe determineddetermined byby 11H-NMRH-NMR diamagneticdiamagnetic anisotropyanisotropy wherewhere ∆δ∆δ valuesvalues valuesvalues areare areare ca.ca. ca.ca. fourfour fourfour α In thistimestimestimestimes sense, larger larger largerlarger thethan than thanthan M those those thosethoseNP of of ofof acid MTPA MTPA MTPAMTPA is esters. superioresters. esters.esters. In InIn some somesome to the cases, cases,cases, Mosher’s single singlesingle crystals crystalscrystals MTPA of ofof acid M MMααNPNP and esters esters MPA were were acid. obtained, obtained, The ACs of 1 MαandNPand ACs estersACs werewere could easilyeasily be determineddetermined from from by ORTEP ORTEPH-NMR draw draw diamagneticings.ings.ings.ings. WeWe WeWe havehave havehave anisotropy nevernever nevernever encountered encountered encounteredencountered where ∆ δ anyany anyanyvalues exception,exception, exception,exception, are ca. four timeswherewhere larger the the thanAC AC determined determined those of MTPA by by 1 1H-NMRH-NMR esters. disagreed disagreed In some with with cases, that that single by by X-ray X-ray crystals crystallography. crystallography. of MαNP estersThis This fact fact were is is very very obtained, andimportantimportantimportantimportant ACs were to to toto easilyevaluateevaluate evaluateevaluate determined the the thethe reliability reliability reliabilityreliability from of of ofof the the thethe ORTEP 1 1H-NMRH-NMR drawings. diamagnetic diamagnetic We anisotropy anisotropy have never method method encountered using using M Mαα anyNPNP acid. acid. exception, 1 where theTheThe AC authorauthor determined believesbelieves thatbythat H-NMRthethe diastereomerdiastereomer disagreed methodmethod with using thatusing by chiralchiral X-ray molecularmolecular crystallography. toolstools explainedexplained This facthere here is very importantwouldwould be be to applicable applicable evaluate theto to a a reliabilityvariety variety of of compounds, ofcompounds, the 1H-NMR and and hopes diamagnetichopes that that it it would would anisotropy be be useful useful method for for the the using progress progress Mα of NPof acid. molecularmolecularThe author chiralitychirality believes science.science. that the method using chiral molecular tools explained here would be applicable to a variety of compounds, and hopes that it would be useful for the progress of Acknowledgments:Acknowledgments: TheThe studiesstudies explainedexplained herehere werewere carriedcarried outout inin collaborationcollaboration withwith manymany coworkers,coworkers, molecularAcknowledgments: chirality science. The studies explained here were carried out in collaboration with many coworkers, especiallyespecially my my previous previous graduate graduate students, students, whose whose names names ar aree listed listed in in the the references. references. The The author author sincerely sincerely thanks thanks themthemthemthem forforforfor theirtheirtheirtheir effortseffortseffortsefforts andandandand contributicontributicontributicontributions.ons. TheThe authorauthor alsoalso thanksthanks Dr.Dr. GeGeorgeorge A.A. Ellestad,Ellestad, DepartmentDepartment ofof Acknowledgments: The studies explained here were carried out in collaboration with many coworkers, especially ,Chemistry, Columbia Columbia University, University, for for his his valuable valuable suggestions. suggestions. myChemistry, previousChemistry, graduate ColumbiaColumbia students, University,University, whose forfor hishis valuablenamesvaluable are suggestions.suggestions. listed in the references. The author sincerely thanks them for their efforts and contributions. The author also thanks Dr. George A. Ellestad, Department of Chemistry,

Columbia University, for his valuable suggestions. Molecules 2016, 21, 1328 34 of 37

Conflicts of Interest: The author declares no conflict of interest.

References

1. Okamoto, Y.; Ikai, T. Chiral HPLC for Efficient Resolution of Enantiomers. Chem. Soc. Rev. 2008, 37, 2593–2608. [CrossRef][PubMed] 2. Wolf, C.; Pirkle, W.H. Conformational Effects on the Enantioselective Recognition of 4-(3,5-Dinitro- benzamido)-1,2,3,4-tetrahydrophenanthrene Derivatives by a Naproxen-derived Chiral Stationary Phase. Tetrahedron 2002, 58, 3597–3603. [CrossRef] 3. Davankov, V.A. Separation of Enantiomeric Compounds Using Chiral HPLC Systems. A Brief Review of General Principles, Advances, and Development Trends. Chromatographia 1989, 27, 475–482. [CrossRef] 4. Allenmark, S.; Andersson, S. Optical Resolution of Some Biologically Active Compounds by Chiral Liquid Chromatography on BSA-silica (Resolvosil) Columns. Chirality 1989, 1, 154–160. [CrossRef] 5. Breitbach, A.S.; Lim, Y.; Xu, Q.-L.; Kurti, L.; Armstrong, D.W.; Breitbach, A.Z. Enantiomeric Separations of α-Aryl with Cyclofructan Chiral Stationary Phases via High Performance Liquid Chromatography and Supercritical Fluid Chromatography. J. Chromatogr. A 2016, 1427, 45–54. [CrossRef][PubMed] 6. Harada, N. Powerful Novel Chiral Acids for Enantioresolution, Determination of Absolute Configuration, and MS Spectral Determination of Enantiomeric Excess. TCI MAIL 2003, 117, 2–27. 7. Harada, N. Chiral Auxiliaries Powerful for Both Enantiomer Resolution and Determination of Absolute Configuration by X-Ray Crystallography. In Topics in Stereochemistry; Denmark, S.E., Siegel, J.S., Eds.; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2006; Volume 25, Chapter 6; pp. 177–203. 8. Harada, N. Powerful Chiral Molecular Tools for Preparation of Enantiopure Alcohols and Simultaneous Determination of Their Absolute Configurations by X-Ray Crystallography and/or 1H-NMR Anisotropy Methods. In Chirality in Drug Research; Francotte, E., Lindner, W., Eds.; Wiley-VCH: Weinheim, Germany, 2006; Chapter 9; pp. 283–321. 9. Harada, N. Determination of Absolute Configurations by X-Ray Crystallography and 1H-NMR Anisotropy. Chirality 2008, 20, 691–723. [CrossRef][PubMed] 10. Harada, N. X-Ray Crystallography and 1H-NMR Anisotropy Methods for Determination of Absolute Configurations. In Stereoselective Synthesis of Drugs and Natural Products; Andrushko, V., Andrushko, N., Eds.; Wiley-Blackwell: New York, NY, USA, 2013; Chapter 46; pp. 1629–1662. 11. Harada, N. Determination of Absolute Configurations by Electronic CD Exciton Chirality, Vibrational CD, 1H-NMR Anisotropy, and X-Ray Crystallography Methods—Principles, Practices, and Reliability. In Structure Elucidation in ; Cid, M.M., Bravo, J., Eds.; Wiley-VCH: Weinheim, Germany, 2015; Chapter 11, pp. 393–443. 12. Ohrui, H.; Terashima, H.; Imaizumi, K.; Akasaka, K. A Solution of the “Intrinsic Problem” of Diastereomer Method in Chiral Discrimination. Development of a Method for Highly Efficient and Sensitive Discrimination of Chiral Alcohols. Proc. Jpn. Acad. Ser. B 2002, 78, 69–72. [CrossRef] 13. Akagi, M.; Sekiguchi, S.; Taji, H.; Kasai, Y.; Kuwahara, S.; Watanabe, M.; Harada, N. A General Method for the Synthesis of Enantiopure Aliphatic Chain Alcohols with Established Absolute Configurations. Part 2, via Catalytic Reduction of Acetylene Alcohol MαNP Esters. Tetrahedron Asymmetry 2014, 25, 1466–1477. [CrossRef] 14. Takagi, T.; Aoyanagi, N.; Nishimura, K.; Ando, Y.; Ota, T. Enantiomer Separations of Secondary Alkanols with Little Asymmetry by High-performance Liquid Chromatography on Chiral Columns. J. Chromatogr. 1993, 629, 385–388. [CrossRef] 15. Nie, M.-Y.; Zhou, L.-M.; Liu, X.-L.; Wang, Q.-H.; Zhu, D.-Q. Gas Chromatographic Enantiomer Separation on Long-chain Alkylated β-Cyclodextrin Chiral Stationary Phases. Anal. Chim. Acta 2000, 408, 279–284. [CrossRef] 16. Ohtaki, T.; Akasaka, K.; Kabuto, C.; Ohrui, H. Chiral Discrimination of Secondary Alcohols by Both 1H-NMR and HPLC After Labeling with a Chiral Derivatization Reagent, 2-(2,3-Anthracenedicarbox- )cyclohexane Carboxylic Acid. Chirality 2005, 17, S171–S176. [CrossRef][PubMed] 17. Bijvoet, J.M.; Peerdeman, A.F.; van Bommel, A.J. Determination of the Absolute Configuration of Optically Active Compounds by Means of X-rays. Nature 1951, 168, 271–272. [CrossRef] Molecules 2016, 21, 1328 35 of 37

18. Harada, N.; Nakanishi, K. Circular Dichroic Spectroscopy—Exciton Coupling in Organic Stereochemistry; University Science Books: Mill Valley, CA, USA, 1983. 19. Berova, N.; Polavarapu, P.; Nakanishi, K.; Woody, R.W. Comprehensive Chiroptical Spectroscopy; John Wiley & Sons: Hoboken, NJ, USA, 2012; Volumes 1 and 2. 20. Levene, P.A.; Rothen, A. Optical Activity and Chemical Structure. J. Org. Chem. 1936, 1, 76–133. [CrossRef] 21. Murai, S.; Soutome, T.; Yoshida, N.; Osawa, S.; Harada, N. Synthesis, Circular Dichroism, and Absolute Stereochemistry of a Fecht Acid Analog and Related Compounds. Enantiomer 2000, 5, 197–202. [PubMed] 22. Murai, S. Synthesis of Chiral Spiro[3.3]heptane Compounds and Determination of their Absolute Configurations. Master’s Thesis, Tohoku University, Sendai, Japan, 1991. 23. Harada, N.; Soutome, T.; Murai, S.; Uda, H. A Chiral Probe Useful for Optical Resolution and X-Ray Crystallographic Determination of the Absolute Stereochemistry of Carboxylic Acids. Tetrahedron Asymmetry 1993, 4, 1755–1758. [CrossRef] 24. Soutome, T. Synthesis of Chiral Bridged Aromatic Compounds and Determination of their Absolute Configurations. Master’s Thesis, Tohoku University, Sendai, Japan, 1993. 25. Falk, H.; Reich-Rohrwig, P.; Schloegl, K. Absolute Configuration and Circular Dichroism of Optically Active [2.2]Paracyclophane Derivatives. Tetrahedron 1970, 26, 511–527. [CrossRef] 26. Eberhardt, H.; Schloegl, K. Stereochemistry of Planar-chiral Compounds. II. Preparation, Chiroptical Properties, and Absolute Configuration of [10]Paracyclophane Derivatives. Eur. J. Org. Chem. 1972, 760, 157–170. 27. Nakazaki, M.; Yamamoto, K.; Ito, M.; Tanaka, S. Preparations of Optically Active [8][8]- and [8][10]-Paracyclophanes with Known Absolute Configurations. J. Org. Chem. 1977, 42, 3468–3473. [CrossRef] 28. Schwartz, L.H.; Bathija, B.L. Absolute Configuration of an Ansa Compound: Gentisic Acid Nona-methylene Ether. J. Am. Chem. Soc. 1976, 98, 5344–5347. [CrossRef] 29. Schloegl adopted the configurations of (R)-(–)-12,(R)-(+)-14, and (R)-(–)-15: Schloegl, K. Planar Chiral Molecular Structures. Top. Curr. Chem. 1984, 125, 27–62. 30. Harada, N.; Soutome, T.; Nehira, T.; Uda, H.; Oi, S.; Okamura, A.; Miyano, S. Revision of the Absolute Configurations of [8]Paracyclophane-10-carboxylic and 15-Methyl[10]paracyclophane-12-carboxylic Acids. J. Am. Chem. Soc. 1993, 115, 7547–7548. [CrossRef] 31. Harada, N.; Nehira, T.; Soutome, T.; Hiyoshi, N.; Kido, F. Chiral Phthalic Acid Amide, a Useful for Enantiomer Resolution and X-Ray Crystallographic Determination of the Absolute Stereochemistry of Alcohols. Enantiomer 1996, 1, 35–39. 32. Nehira, T. Development of Novel Chiral Resolution Methods and Determination of the Absolute Configurations of Twisted π-Electron Compounds. Ph.D. Thesis, Tohoku University, Sendai, Japan, 1996. 33. Harada, N.; Koumura, N.; Feringa, B.L. Chemistry of Unique Chiral Olefins. 3. Synthesis and Absolute Stereochemistry of trans- and cis-1,10,2,20,3,30,4,40-Octahydro-3,30-dimethyl-4,40-biphenanthrylidenes. J. Am. Chem. Soc. 1997, 119, 7256–7264. [CrossRef] 34. Koumura, N.; Harada, N. X-Ray Crystallographic Determination of the Stereochemistry of a Unique Chiral Olefin, [CD(–)238.0]-(3R,30R)-(P,P)-(Z)-(+)-1,10,2,20,3,30,4,40-Octahydro-3,30-dimethyl-4,40-biphenan-thrylidene. Enantiomer 1998, 3, 251–253. 35. Koumura, N.; Harada, N. Photochemistry and Absolute Stereochemistry of Unique Chiral Olefins, trans- and cis-1,10,2,20,3,30,4,40-Octahydro-3,30-dimethyl-4,40-biphenanthrylidenes. Chem. Lett. 1998, 1998, 1151–1152. [CrossRef] 36. Koumura, N.; Zijlstra, R.W.J.; van Delden, R.A.; Harada, N.; Feringa, B.L. Light-driven Monodirectional Molecular Rotor. Nature 1999, 401, 152–155. [PubMed] 37. Fujita, T.; Kuwahara, S.; Harada, N. A New Model of Light Powered Chiral Molecular Motor with Higher Speed of Rotation (1). Synthesis and Absolute Stereostructure. Eur. J. Org. Chem. 2005, 2005, 4533–4543. [CrossRef] 38. Harada, N.; Koumura, N.; Robillard, M. Chiral Dichlorophthalic Acid Amide, an Improved Chiral Auxiliary Useful for Enantioresolution and X-Ray Crystallographic Determination of Absolute Stereo-chemistry. Enantiomer 1997, 2, 303–309. 39. Cervinka, O.; Belovsky, O.; Fabryova, A.; Dudek, V.; Grohman, K. Asymmetric Reactions. XII. A Limited Use of Asymmetric Meerwein-Ponndorf-Verley Type Reduction for Determination of Absolute Configuration of Alcohols. Collect. Czech. Chem. Commun. 1967, 32, 2618–2624. [CrossRef] Molecules 2016, 21, 1328 36 of 37

40. Seebach, D.; Beck, A.K.; Roggo, S.; Wonnacott, A. Enantioselective Addition of Aryl Groups to Aromatic using Chiral Aryltitanium binaphthol Derivatives. Eur. J. Inorg. Chem. 1985, 118, 3673–3682. [CrossRef] 41. Watanabe, M.; Kuwahara, S.; Harada, N.; Koizumi, M.; Ohkuma, T. Enantioresolution by the Chiral Phthalic Acid Method: Absolute Configurations of (2-Methylphenyl)phenylmethanol and Related Compounds. Tetrahedron Asymmetry 1999, 10, 2075–2078. [CrossRef] 42. Kuwahara, S. Chiral Resolution of Alcohols by the Chiral Phthalic Acid Method and Determination of their Absolute Configurations: New Developments. Master’s Thesis, Tohoku University, Sendai, Japan, 1999. 43. Kosaka, M.; Kuwahara, S.; Watanabe, M.; Harada, N.; Job, G.E.; Pirkle, W.H. Comparison of CD Spectra of (2-Methylphenyl)- and (2,6-Dimethylphenyl)-phenylmethanols Leads to Erroneous Absolute Configurations. Enantiomer 2002, 7, 213–217. [CrossRef][PubMed] 44. Kosaka, M. Chiral Resolution of Aromatic Alcohols by the Chiral Phthalic Acid Method and Determination of their Absolute Configurations: Applications in the Synthetic Studies of Chloromonilicin. Master’s Thesis, Tohoku University, Sendai, Japan, 2000. 45. Fujita, T.; Obata, K.; Kuwahara, S.; Nakahashi, A.; Monde, K.; Decatur, J.; Harada, N. (R)-(+)- [VCD(−)984]-4-Ethyl-4-methyloctane, a Cryptochiral Hydrocarbon with a Quaternary Chirality Center. (1) Synthesis of Enantiopure Compound and Unambiguous Determination of Absolute Configuration. Eur. J. Org. Chem. 2010, 2010, 6372–6384. [CrossRef] 46. Hoeve, W.T.; Wynberg, H. Chiral Tetraalkylmethanes. Two Syntheses of Optically Active Butylethyl- methylpropylmethane of Known and High Optical Purity. J. Org. Chem. 1980, 45, 2754–2763. [CrossRef] 47. Caporusso, A.M.; Consoloni, C.; Lardicci, L. Stereoselective Construction of Quaternary Carbon Centers via Reaction of Heterocuprates with Chiral Allenic Bromides: A Facile Synthesis of Butylethyl- methylpropylmethane in High Enantiomeric Purity. Gazz. Chim. Ital. 1988, 118, 857–859. 48. Kuwahara, S.; Obata, K.; Fujita, T.; Miura, N.; Nakahashi, A.; Monde, K.; Harada, N. (R)-(+)- [VCD(–)984]- 4-Ethyl-4-methyloctane: A Cryptochiral Hydrocarbon with a Quaternary Chiral Center. (2) Vibrational CD Spectra of Both Enantiomers and Absolute Configurational Assignment. Eur. J. Org. Chem. 2010, 2010, 6385–6392. [CrossRef] 49. Matsumoto, T.; Ishida, T.; Takeda, Y.; Soh, K.; Kubo, I.; Sakamoto, M. Enantioselective Biotransformation of 1-Isopropylnaphthalene in Rabbits. Chem. Pharm. Bull. 1995, 43, 216–222. [CrossRef][PubMed] 50. Kuwahara, S.; Fujita, K.; Watanabe, M.; Harada, N.; Ishida, T. Enantioresolution and Absolute Stereochemistry of 2-(1-Naphthyl)propane-1,2-diol and Related Compounds. Enantiomer 1999, 4, 141–145. 51. Harada, N.; Watanabe, M.; Kuwahara, S.; Sugio, A.; Kasai, Y.; Ichikawa, A. 2-Methoxy-2-(1-naphthyl)- propionic Acid, a Powerful Chiral Auxiliary for Enantioresolution of Alcohols and Determination of Their Absolute Configurations by the 1H-NMR Anisotropy Method. Tetrahedron: Asymmetry 2000, 11, 1249–1253. [CrossRef] 52. Dale, J.A.; Mosher, H.S. Nuclear Magnetic Resonance Enantiomer Reagents. Configurational Correlations via Nuclear Magnetic Resonance Chemical Shifts of Diastereomeric Mandelate, O-Methylmandelate, and α-Methoxy-α-trifluoromethyl-phenylacetate (MTPA) Esters. J. Am. Chem. Soc. 1973, 95, 512–519. [CrossRef] 53. Ohtani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H. High-field FT NMR Application of Mosher’s Method. The Absolute Configurations of Marine Terpenoids. J. Am. Chem. Soc. 1991, 113, 4092–4096. [CrossRef] 54. Trost, B.M.; Belletire, J.L.; Godleski, S.; McDougal, P.G.; Balkovec, J.M.; Baldwin, J.J.; Christy, M.E.; Ponticello, G.S.; Varga, S.L.; Springer, J.P. On the Use of the O-Methylmandelate Ester for Establishment of Absolute Configuration of Secondary Alcohols. J. Org. Chem. 1986, 51, 2370–2374. [CrossRef] 55. Kasai, Y.; Sugio, A.; Sekiguchi, S.; Kuwahara, S.; Matsumoto, T.; Watanabe, M.; Ichikawa, A.; Harada, N. Conformational Analysis of MαNP Esters, Powerful Chiral Resolution and 1H-NMR Anisotropy Tools—Aromatic Geometry and Solvent Effects on ∆δ Values. Eur. J. Org. Chem. 2007, 2007, 1811–1826. [CrossRef] 56. Kuwahara, S.; Naito, J.; Yamamoto, Y.; Kasai, Y.; Fujita, T.; Noro, K.; Shimanuki, K.; Akagi, M.; Watanabe, M.; Matsumoto, T.; et al. Crystalline State Conformational Analysis of MαNP Esters, Powerful Resolution and Chiral 1H-NMR Anisotropy Tools. Eur. J. Org. Chem. 2007, 2007, 1827–1840. [CrossRef] 57. Fujita, T.; Obata, K.; Kuwahara, S.; Miura, N.; Nakahashi, A.; Monde, K.; Decatur, J.; Harada, N. (R)-(+)-[VCD(+)945]-4-Ethyl-4-methyloctane, the Simplest Chiral Saturated Hydrocarbon with a Quaternary Stereogenic Center. Tetrahedron Lett. 2007, 48, 4219–4222. [CrossRef] Molecules 2016, 21, 1328 37 of 37

58. Taji, H.; Kasai, Y.; Sugio, A.; Kuwahara, S.; Watanabe, M.; Harada, N.; Ichikawa, A. Practical Enantioresolution of Alcohols with 2-Methoxy-2-(1-naphthyl)propionic Acid and Determination of Their Absolute Configurations by the 1H-NMR Anisotropy Method. Chirality 2002, 14, 81–84. [CrossRef][PubMed] 59. Kasai, Y.; Taji, H.; Fujita, T.; Yamamoto, Y.; Akagi, M.; Sugio, A.; Kuwahara, S.; Watanabe, M.; Harada, N.; Ichikawa, A.; et al. MαNP Acid, a Powerful Chiral Molecular Tool for Preparation of Enantio- pure Alcohols by Resolution and Determination of Their Absolute Configurations by the 1H-NMR Anisotropy Method. Chirality 2004, 16, 569–585. [CrossRef][PubMed] 60. Yamamoto, Y.; Akagi, M.; Shimanuki, K.; Kuwahara, S.; Watanabe, M.; Harada, N. A General Method for the Synthesis of Enantiopure Aliphatic Chain Alcohols with Established Absolute Configurations. Part 1, Application of the MαNP Acid Method to Acetylene Alcohols. Tetrahedron Asymmetry 2014, 25, 1456–1465. [CrossRef] 61. Akagi, M. Synthesis of Ultimate Cryptochiral Alcohols with Aliphatic Long Chains and Development of the Novel Methods for Determining their Absolute Configurations. Master’s Thesis, Tohoku University, Sendai, Japan, 2004. 62. Sekiguchi, S.; Akagi, M.; Naito, J.; Yamamoto, Y.; Taji, H.; Kuwahara, S.; Watanabe, M.; Ozawa, Y.; Toriumi, K.; Harada, N. Synthesis of Enantiopure Aliphatic Acetylene Alcohols and Determination of Their Absolute Configurations by 1H-NMR Anisotropy and/or X-Ray Crystallography. Eur. J. Org. Chem. 2008, 2008, 2313–2324. [CrossRef] 63. Mori, K.; Ohtaki, T.; Ohrui, H.; Berkebile, D.R.; Carlson, D.A. Synthesis of the Four Stereoisomers of 6-Acetoxy-19-methylnonacosane, the Most Potent Component of the Female Sex Pheromone of the New World Screwworm Fly, with Special Emphasis on Partial Racemization in the Course of Catalytic Hydrogenation. Eur. J. Org. Chem. 2004, 2004, 1089–1096. [CrossRef] 64. TCI Catalog. Research Chemicals, 2016. Available online: http://www.tcichemicals.com/ (accessed on 3 October 2016). 65. Coke, J.L.; Shue, R.S. Nucleophilic Ring Opening of Optically Pure (R)-(+)-1,2-Epoxybutane. Synthesis of New (R)-2-Butanol Derivatives. J. Org. Chem. 1973, 38, 2210–2211. [CrossRef]

© 2016 by the author; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).