Weinreb Amide Based Building Blocks for Convenient Access to Various Synthetic Targets

Indian Journal of Chemistry Vol. 48B, December 2009, pp. 1749-1756

Weinreb amide based building blocks for convenient access to various synthetic targets

B Sivaraman, B N Manjunath, A Senthilmurugan, K Harikrishna & Aidhen Indrapal Singh* Department of Chemistry, Indian Institute of Technology, Madras, Chennai 600 036, India E-mail: [email protected] Received and accepted 16 August 2009

N-Methoxy-N-methylamide, popularly known as the Weinreb amide (WA), has served as an excellent acylating agent for organolithium and/or organomagnesium reagents and a robust equivalent for an aldehyde group. The stability of the WA functionality, its ease of preparation, the scalability of its reactions and its predictable reactivity are the key features responsible for its prominent use in several synthetic endeavors by the chemists world-wide. The development of WA-based building blocks and synthetic equivalents for interesting synthons has been a long drawn pursuit initiated in nineties and through this mini-review accomplishments in this direction, with particular emphasis on the building blocks, developed recently in the last three years are summarized.

Keywords: Weinreb amide, building blocks, synthetic equivalent, FTY 720, olefination, sulfone, α-diketones, 4-aryl- tetrahydroisoquinoline

N-Methoxy-N-methylamide 1, popularly addressed as in late nineties has come a long way3a. In this mini- the Weinreb amide, has served as an excellent review, our accomplishments in this direction, with acylating agent for organolithium or organomagnesium particular emphasis on the building blocks, developed reagents and as a robust equivalent for an aldehyde recently in last three years have been summarized3. group1. These two aspects have been exploited Although the initial developments of these synthetic exhaustively in various synthetic endeavors2. equivalents were driven by specific objectivity, their Successful acylation by a variety of organolithium and use is left to one’s imagination and for any synthetic organomagnesium reagents or reductions by lithium endeavors. The interest in deoxy-sugars and acyl aluminum hydride or diisobutylaluminum hydride to anion chemistry lead to the idea of synthesizing two aldehyde is due to the putative and stable tetrahedral WA-based building blocks 4 and 5 (Figure 2). The intermediate 2 or 3 formed upon addition of the first former was aimed with the purpose of two-carbon equivalent of the organometallic species or reducing homologation of acyclic sugar derivatives and the agent (Figure 1). This stability precludes the collapse later for the synthesis of 1,4 diketones through the to a ketone or aldehyde under the reaction conditions acyl anion chemistry. The synthetic equivalent 4, and thus prevents the formation and subsequent N-methoxy-N-methyl-2-phenylsulfonyl-acetamide, possibility of addition to the ketone or aldehyde. The developed for two carbon homologation of alkyl halides, stability of the WA functionality, its ease of can be easily prepared from the reaction of sodium salt preparation, the scalability of its reactions and its of phenylsulfinic acid with α-chloro-N-methoxy-N- predictable reactivity are the four main reasons for the methylacetamide4. Reagent 4 undergoes a clean increasing confidence that synthetic organic chemists alkylation at the active methylene group with various have with regard to the use of the WA in various alkyl halides, especially from the carbohydrate domain 3 synthetic endeavors . under the mild reaction conditions of K2CO3 in DMF (Figure 3). Subsequent reductive desulphonylation with Concept of WA-based building block from our sodium amalgam leads to a two carbon homologated group during late nineties product 6. Reduction of the WA functionality in 6 to Synthesis of new building blocks based upon aldehyde renders the building block 4 equivalent to an Weinreb amide functionality a pursuit initiated by us acetaldehyde carbanion (synthon A) 4. 1750 INDIAN J. CHEM., SEC B, DECEMBER 2009

O Li O O M This approach of homologating halides through the OMe OMe OMe use of reagent 4, becomes very important because two R N R N R N H R Me Me 1 Me carbon homologation of aldehydes with α-stereo- centers do have a fear of epimerization especially 231 with Wittig based reagents under basic conditions. The two carbon homologation of threo-configured iodide 7 enabled efficient synthesis of 4,5-O-isoprop- O O ylidine protected L-rhodinose5 an important trideoxy R H R R1 sugar and that of erythro-configured iodides 8 and 9 Figure 1 have enabled synthesis of another trideoxy sugar, D-amicetose6. With the use of arabino-configured iodide 10 as the alkylating halide for the 4, it Me O furnished the synthesis of 2,3-dideoxy-4,5: 6,7-di-O- N OMe PhO2S OMe Br N isoprop-ylidene-D-arabino-heptose 11, which corres- 4 O 5 Me ponded to C3-C9 fragment of a natural product (+)- aspicillin4. The convenient availability of 4 on multi- gram scale through simple reactions and its successful H O use in two carbon homologation, afforded itself a place in Aldrich catalogue7. Although synthetic O equivalent 4 could chain extend open-chain primary A B halides conveniently, it failed to do the same during an attempt to execute C-C bond formation at the Figure 2 anomeric carbon in the pyranosyl iodide 12. This was

O O 1. Alkylation, RX PhO S OMe R OMe 2 N N 2. Desulphonation 6 Me 4 Me 69%-82%

O OTBS O

I I I O OBn O RX = 7 8 9 O O O I

O 10

OH OH O O O O O O OH OH O L-rhodinose D-amicetose [ Acyclic forms of 2,3,6-Trideoxyhexoses 11 L-rhodinose and D-amicetose]

OBn OBn O B BnO O SO Ph BnO 2 BnO + 4 BnO X BnO I BnO CON(OMe)Me 12 B: K2CO 3/NaH

Figure 3 SIVARAMAN et al.: WEINREB AMIDE BASED BUILDING BLOCKS 1751

probably due to the poor stability of iodide 12 under N-disubstituted)amino ketones 15 (ref. 9, Scheme I). the reaction conditions (Figure 3). Successful alkylation of the carbanion 16 derived from The genesis of synthetic equivalent 5, N-methoxy-N- α-aminonitriles 13, an acyl anion equivalent with 5 is methyl-3-bromopropionamide, derived from 3- the key step. Successful addition of the variety of bromopropanoic acid was a result of our interest in the Grignard reagents onto the alkylated intermediate 17 use of α-aminonitriles 13 as acyl anion equivalents. and unmasking the carbonyl group in the product 18 Synthetic equivalent 5, equivalent to synthon B has under acidic hydrolytic conditions paved the way for been applied to the synthesis of two different targets, the synthesis of 1,4-diketones 14. Nucleophilic an unsymmetrical 1,4 -diketones 14 (ref. 8) and β-(N, displacement of bromine in 5 with various secondary amines and subsequent Grignard addition on to β- O O RMgX amino WA 19 furnished the β-amino ketones 15.

N O N O OMe Our failure to accomplish the C-C bond formation R N R 1 R = alkyl, R1 at the anomeric center using 4, inspired us to explore CN Me CN aryl or a strategy similar to the one developed by Kametani et 70%-78% 17 18 hetero aryl CuSO4, aq. MeOH al.10 wherein the phenylthioglycoside 20 had been 16 reflux, 3 h O reacted cleanly with diazo derivative of dimethyl 5 R malonate 21 in the presence of rhodium(II) acetate. R1 R =R =Bn, This lead to the proposal of synthetic equivalent 22 R2 2 3 14 O R2=R3=alkyl, towards the possible accomplishment of the C-C bond HN 68%-79% R2=R3=-(CH2)3- formation under the mild and neutral condition using R3 R =R =-(CH) -O-(CH ) - 2 3 2 2 2 2 rhodium(II) acetate as a catalyst. The synthetic R2 Me RMgX R2 equivalent 22, a crystalline orange colour solid (m.p. -40 oC-0 oC N N N R 82-84ºC) could be readily prepared in 72% yield by R OMe 1.5 h 3 R3 reacting the sulfone WA 4 with tosyl azide in the 19 O R=alkyl, 15 O 11 75%-91% aryl or 65%-83% presence of DBU as a base . When synthetic hetero aryl equivalent 22 was reacted with thioglycoside 20, CN CN using rhodium (II) acetate as a catalyst at RT in O N NaH/ DMF O N dichloromethane, it was found that thioglycoside 20 R R1 1 R =arylor remained unreacted and was completely recovered, R1 =arylor 1 heteroaryl heteroaryl while the diazo compound 22 was completely 13 16 consumed (Scheme II). The intermediate of rhodium Scheme I carbenoid 23 formed under the reaction conditions,

N2 OMe TsN3,DBU,DCM 0 oC, 4 h, 72% N 4 PhO2S Me 22 O OAc OAc O 22;Rh(OAc)4.6H2O O SPh AcO SPh X AcO SO 2Ph AcO AcO OAc DCM, 1 h OAc CON(OMe)Me r.t. 20 2 O O

MeO OMe O

Rh(OAc) N Me N2 OMe PhO2S 21 N PhO S Me O OAc 2 24 O O SPh AcO COOMe 23 AcO OAc COOMe

Scheme II 1752 INDIAN J. CHEM., SEC B, DECEMBER 2009

underwent a facile intramolecular C-H insertion at the buiding block 4, or with the reaction of diazo C-H bond of –OMe group to furnish 24. The product compound 22 derived thereof with phenylthioglycol- obtained was evident from 1H NMR data. It showed side 20, prompted further search towards alternatives the presence of three doublet of doublets at δ 4.30, in achieving this objective. Since appending of WA- 4.60 and 4.90 for three protons. Also 13C NMR, along containing fragment at the anomeric position would with DEPT studies showed the presence of methylene enable further derivatization, this objective was carbon at δ 66.4 and methine carbon at δ 67.5 (ref. important, novel and worthy of persuasion. Besides 11a). To confirm this observation and result, the diazo the afore mentioned unaccomplished task, our global compound 22 was subjected to the same reaction objective towards developing novel WA based conditions, but devoid of thioglycoside 20 as a co- building blocks for varied synthetic pursuits remained reactant. The reaction once again furnished same C-H uninterrupted (Figure 4). inserted cyclized product 24 in 67% yield. The potential and facile possibility of an intramolecular hetero-Michael reaction in the scaffold New WA-based building block from our group 25 to furnish not only the C-gluco configured during the recent years (2006-present) pyranoside derivative, but also provide a handle in the Our failure to accomplish C-C bond formation at form of WA-functionality, lead to the proposal of the anomeric carbon in pyranoside iodide 12 with building block 26. Simultaneously, another clinically important molecule, FTY-720 as an immuno- OP O O O suppressant12 attracted our attention. The synthesis of S OMe N N PO OH this important target through the envisaged discon- PO CON(OMe)Me S Me PO nection prompted three possible WA-based building 25 26 blocks, 27, 28 and 29 for the central core of FTY-720. P = Suitable Protection O The synthetic equivalent 26, combines the OMe N usefulness of WA-chemistry with Julia olefination 13 HO Me protocol . Clean reaction of benzothiazole with 6 HO X α-chloro-N-methoxy-N-methylacetamide followed by NH Cl oxidation of sulfide to sulfone paved the way for the 3 -P(Ph) 27:X= 3 Br reagent 26. Successful olefination of aldehydes with FTY-720 28:X=-P(O)(OEt)2 Immunosuppresant the reagent 26, especially from the domain of [Phase III Clinical trials] O O S carbohydrates under NaH in DMF conditions 29:X= S N (Scheme III) facilitates the synthesis of α, β-unsatu- rated WA structural unit 30. This reagent is a viable 14 15 Figure 4 alternative to the Wittig or HWE methodology for

Me R N N Me OMe N S S OMe RCHO O O O O R = Alkyl, Aryl, Sugar 30 26 44-72%

O O NaH, THF O r.t., 50% O + 26 BnO BnO BnO BnO CHO CON(OMe)Me BnO BnO 31 32 OH Zn(NO3)2.6H2O O o BnO CH3CN, 70 C BnO CON(OMe)Me 75% BnO H 33 Scheme III SIVARAMAN et al.: WEINREB AMIDE BASED BUILDING BLOCKS 1753

HO NHBoc 6 O O HO O NH2..HCl FTY-720 34 polar head group O OMe O N 34 Me 28 or 29 O K2CO3,DMF 70% from 28 NHBoc 35 65% from 29 34 27 35 + Z-isomer O K2CO3,DMF 70% OMe O N

H2,Pd/C Me 35 + Z-isomer O 90% NHBoc 36

n-C7H15MgBr n-C7H15MgBr 35 36 o 0 oC, 3 h 0 C, 3 h O O

O O 6 6 O O NHBoc 37 NHBoc 38

1. NaBH4 1. NaBH4 2. H2,Pd/C 2. H2,Pd/C ACOH, EtOAc O ACOH, EtOAc 85% 6 90% O NHBoc 39 Scheme IV the conversion of aldehydes to α, β-unsaturated N- removal and subsequent in situ intramolecular methoxy-N-methyl amides. This would allow all Michael reaction. possible synthetic manipulations associated with Synthesis of three bi-functional synthetic α, β unsaturated system and those innate with WA equivalents 27, 28 and 29 using commercially functionality. The synthetic equivalent 26, offers a available p-toluic acid was triggered by the significant advantage during purification of the importance of FTY-720 as an immunosuppressant products, in contrast to the tedious removal of (Scheme IV, ref. 20). The polar head group of FTY- triphenylphosphine oxide while using the Wittig 720 was incorporated on the central aromatic core reaction towards the same objective. In the light of the presented in these building blocks through Julia, fact that α, β unsaturated aldehydes and ketones being Wittig and HWE reactions by reaction with the poor substrates for the asymmetric dihydroxylation requisite aldehyde, tert-butyl 5-formyl-2,2-dimethyl- (AD) process16 and cyclopropanation reaction17, the 1,3-dioxan-5-ylcarbamate 34 (ref. 21). The WA α, β unsaturated N-methoxy-N-methyl amides would functionality in the olefinated product 35 or further allow a great advantage for indirect access to these hydrogenated product 36 provided the necessary functionalities. The successful olefination of the handle for a complete control at the length of the gluco-configured aldehyde 31 (ref. 18) resulting in the lipophilic side chain. formation of 32, has paved the way for the synthesis All the reactions and conditions en-route to the of much desired C-glucoside building 33 (ref. 19) target molecule are simple and good yielding and through acid catalyzed terminal isopropylidene therefore hold significant promise in industries. The 1754 INDIAN J. CHEM., SEC B, DECEMBER 2009

C, with opposing polarity. These synthetic equi- n valents were enisaged for the synthesis of mono- O Me O SS N protected α-diketones 42 (Scheme V, ref. 23). Acid H COOH H OMe catalyzed thioketalization of the aldehyde function- 43 O O ality in glyoxalic acid 43 with 1,2-ethane or 1,3- 40:n=0 C propane dithiol and later converting the carboxyl 41:n=1 24 RMgX R X group to WA using the mixed anhydride approach o 1 0 C, 3 h 0 oC, 2 h yielded the synthetic equivalent 40 and 41 SS SS 40 65%-90% R 54%-72% R respectively. Nucleophilic addition on to the amide R R = alkyl, H 1 alkyl, Benzyl, functionality in 40 followed by alkylation at the C2 aryl or O allyl, propargyl O 44 42 position of the dithiolane ring in 44 furnished the hetero aryl ethyl bromo acetate targeted mono-protected α-diketones 42. Scheme V An interesting application of this new protocol is the successful synthesis of 6-(2-methyl-1,3-dithiolan- 2-yl)-2,3,4,5-tetrahydropyridine 45, a dithioacetal 1. THPO(CH2)4 MgBr, THF protected derivative of an important target molecule 0 oC, 3 h, 85% SS 40 OTHP 46, a tautomer of a compound responsible for the Me 25 2. MeI, NaH/ DMF 0 oC, 3 bread flavour . The requisite carbon skeleton to O 1 h, 75% 47 arrive at compound 45 was easily assembled in good yields by nucleophilic addition of THPO(CH2)4MgBr on 40 followed by methylation at C2 position to furnish 47. Further functional group interconversion O SS SS allowed convenient access to the azido ketone 48 as a P(Ph)3 Me Me Me key intermediate which underwent phosphine 46 N 45 N 48 O mediated cyclization affording the target 45 THF, reflux (Scheme VI, ref. 23). 6 h, 80% N3 Towards the synthesis of pharmacologically Scheme VI important 4-aryl-1,2,3,4-tetrahydro isoquinoline deri- vatives 49 two WA-based synthetic equivalents 50 olefination of TRIS aldehyde 34 with synthetic and 51 were envisaged as useful template for the equivalents 27 and 28 using 3 equiv of K2CO3 in general synthesis of 4-aryl-1,2,3,4-tetrahydroiso- DMF/THF (1:3) mixture at 70°C yielded the product quinoline skeleton. Both these synthetic equivalents 35 as E-isomer in 70 and 60% yields respectively. for the synthon D were easily accessible from glycine The same reaction using Wittig salt 27 afforded the in two steps. Although the N-phenylsulfonyl desired product 35 accompanied by its Z-isomer protected, WA-based building block 50 (ref. 26), (E/Z= 1:3) in 70% yield. However the simple underwent clean N-benzylation to afford 52 the key hydrogenation of E/Z product mixture to compound intermediate towards the target, our recent studies 36 makes the formation of geometrical isomers showed that N-Boc protected analogue 51 (ref. 27) inconsequential. The addition of n-C7H15MgBr on 35 failed to undergo similar N-benzylation in an attempt and 36 furnished the ketones 37 and 38 in 70% yields, to arrive at 53 (Figure 5). which on reduction using NaBH4 and subsequent Nucleophilic addition of a variety of ArMgX onto hydrogenolysis afforded the target compound 39 in WA group in 52 furnished the arylketones 54 which good yields20. The synthetic equivalent 27 has been on reduction and acid promoted cyclization afforded applied towards preparing di-aryl ketones having the N-sulfonyl protected 4-aryl-1,2,3,4-tetrahydro- highly functionalized appendages and for preparing isoquinoline 55 derivatives26. Realizing the con- new analogues of phenstatins, a promising anti venience of N-Boc removal against N-phenylsulfonyl, mitotic agent22. in the final step towards the target 49, synthesis of 53 The synthetic equivalents N-methoxy-N-methyl-1, became important. The compound 53 was prepared in 3-dithiolane-2-carboxamide 40 and N-methoxy-N- three steps using benzylamine and ethyl bromo- methyl-1, 3-dithiane-2-carbo-xamide 41, derived from acetate. The steps involved were mono-alkylation of glyoxalic acid represents α-dicarbonyl unit, synthon benzyl amine 56 with ethyl bromoacetate 57, SIVARAMAN et al.: WEINREB AMIDE BASED BUILDING BLOCKS 1755

according to the reported procedure28, followed N- derivative containing Weinreb amide functionality, Boc protection with di-tert-butyl carboxy anhydride, 61a, as a representative example was synthesized by ester hydrolysis and in situ conversion of acid to WA the Julia reaction between the sulfone building block through mixed anhydride approach (Scheme VII). 29 and commercially available terephthaladehyde 62 Compound 53 underwent similar set of reactions (ref. 32). This DSB derivative 61a was found to be (53 → 58 → 60) as described for 52, with an efficient laser dye than a standard dye (POPOP)33 at additional advantage that during acid promoted P cyclization, the N-Boc protection was easily removed. ArMgBr N The final 4-aryl-1,2,3,4-tetrahydro isoquinolines 49 O 0 oC were converted to hydrochloride salts for the 52 or 53 Ar convenience of isolation (Scheme VIII, ref. 29). 60-85% f or 54 The importance of end-substituted distyrylbenzene 75-80% f or 58 54: P=SO2Ph (DSB) and their derivatives in electro-optic and solar 58:P=Boc applications30 and as potential energy transporters in one dimensional channel31, we became interested in NaBH4 1h the synthesis of distyrylbenzene derivative 61a-c with MeOH Weinreb amide functionality as one of the end- P TFA P N substituent. Since the modifications in the (a) nature Con.H2SO4 N of terminal substituents on the distyrylbenzene 65-97% for 55 HO scaffold have been found to be very useful and also Ar 92-95% for 49 (b) that the use of Weinreb amide functionality as one Ar 55:P=SO2Ph of the terminal substituents has never been made, it 49:P=H 59: P=SO2Ph 60: P=Boc provided all the basis for the synthesis of compounds

61 as an interesting functional material worthy of investigations in optics (Scheme IX). The DSB Scheme VIII

P NH NP Me HN N MeO R1 Ar O O R1 R R 50: P=SO Ph 2 2 49 D 2 51: P=Boc R R2 3 61

a:R1 =CON(OMe)Me,R2 =R3 =H 50 NP 51 b:R2 =CON(OMe)Me,R1 =R3 =H X c:R =CON(OMe)Me,R =R =H NaH, DMF O 3 1 2 K CO ,5h 2 3 O 60 oC, 80% N MeO Me 29 + O 61a 52: P=SO2Ph 53 P=Boc 62 :

Figure 5 Scheme IX

1.Saponification NH2 1.THF, TEA Boc r.t., 90% Boc 2. PivCl, TEA N 56 N 3. DMHA.HCl O + O OEt TEA,THF Br N 2.(Boc)2O, TEA OEt 76% MeO Me 57 O DCM, r.t., 91% 53

Scheme VII 1756 INDIAN J. CHEM., SEC B, DECEMBER 2009

the pump wavelength of 355 nm and its third-order 16 Bennani Y L & Sharpless K B, Tetrahedron Lett, 34, 1993, optical non linearity is as large as the standard 2079 and references cited therein. reference molecule carbon disulphide. The synthesis 17 Rodriques K E, Tetrahedron Lett, 32, 1991, 1275. 18 Paulsen H & Von Deyn W, Liebigs Ann Chemie, 1987, 125. and study of other isomers 61b and 61c is currently 19 Manjunath B N, Ph.D. Thesis, IIT Madras, Chennai, India, underway. 2009. (under preparation) 20 Sivaraman B, Senthilmurugan A & Aidhen I S, Synlett, 2007, References 2841. 1 Nahm S & Weinreb S M, Tetrahedron Lett, 22, 1981, 3815. 21 Ooi H, Ishibashi N, Iwabuchi Y, Ishihara J & Hatakeyama S, 2 (a) Sibi M P, Org Prep Proced Intl, 25, 1993, 15; (b) Mentzel J Org Chem, 69, 2004, 7765. M & Hoffmann H M R, J Prakt Chem, 339, 1997, 517. 22 Sivaraman B & Aidhen I S, Manuscript under preparation. 3 (a) Singh J, Satyamurthi N & Aidhen I S, J Prakt Chem, 342, 23 Sivaraman B & Aidhen I S, Synlett, 2007, 959. 2000, 340; (b) For achievements from other groups and for a 24 Raghuram T, Vijaysaradhi S, Singh J & Aidhen I S, Synth recent comprehensive review on the chemistry of Weinreb Commun, 29, 1999, 3215. amide see: Sivaraman Balsubramanian & Aidhen I S, 25 Adams A & De Kimpe N, Chem Rev, 106, 2006, 2299. Synthesis, 2008, 3707. 26 Selvamurugan, V, Ph.D. Thesis, IIT Madras, Chennai, India, 4 Satyamurthi N, Singh J & Aidhen I S, Synthesis, 2000, 375. 2001 5 Satyamurthi N & Aidhen I S, Carbohydrate Lett, 3, 1999, 27 Coffin A R, Rousell M A, Tserlin E & Pelkey E T, J Org 355. Chem, 71, 2006, 6678. 6 Aidhen I S & Satyamurthi N, Indian J Chem B, 2008, 1851. 28 Philip C, Bulman Leach, David C, Hayman C M., Hamzah A, 7 Satyamurthi N & Aidhen I S, Aldrichimica Acta, 35, 2002, 1. Sazali Allin, Steven M & McKee V, Synlett, 2003, 1025. 8 Selvamurugan V & Aidhen I S, Tetrahedron, 57, 2001, 6065. 9 Selvamurugan V & Aidhen I S, Synthesis, 2001, 2239. 29 Harikrishna K, Sivaraman B & Aidhen I S, Manuscript under 10 Kametani T, Kawamura K & Honda T, J Am Chem Soc, 109, preparation. 1987, 3110. 30 Sancho-Garcia J C, Poulsen L, Gierschner J, Martinez- 11 (a) Satyamurthi N, Ph.D. Thesis, IIT Madras, Chennai, India, Alvarez R, Hennebicq E,.Hanack M, Egelhaff H.-J, Oelkrug 2001; (b) Maheswaran K S, M.Sc. Thesis, IIT Madras, D, Beljonne D, Bredas J.-L & Cornil J, J Adv Mater, 16, Chennai, India, 1999. 2004, 1193. 12 Kim S, Lee H, Lee M & Lee T, Synthesis, 2006, 753 and 31 Sancho-Garcı´a J C, Bre´ das J.-L, Beljonne D, Cornil J, references cited therein. Martı´nez-A´ lvarez R, Hanack M, Poulsen L, Gierschner J, 13 Manjunath B N, Sane N P & Aidhen I S, Eur J Org Chem, Mack H.-G, Egelhaaf H.-J & Oelkrug D, J. Phys Chem B, 2006, 2851. 109, 2005, 4872. 14 Evans DA, Kaldor S W, Jones T K, Clardy J & Stout TJ, J Am 32 Basanth S K, Prem B B, Senthilmurugan A & Aidhen I S, J Chem Soc, 112, 1990, 700. Lumin, 129, 2009, 1094. 15 (a) Nuzillard J.-M, Boumendjel A & Massiot G, Tetrahedron 33 POPOP=1,4-di[2-(5-phenyloxazolyl)]benzene,see: Lett, 30, 1989, 3779; (b) Netz D F & Seidel J L, Tetrahedron Brackmann U, Lambdachrome Laser Dyes Data Sheets, Lett, 33, 1992, 1957. Lambda Physik, Germany, 1986.