A One-Pot Approach to Novel Pyridazine C-Nucleosides

molecules

Communication A One-Pot Approach to Novel Pyridazine C-Nucleosides

Flavio Cermola 1,* , Serena Vella 2, Marina DellaGreca 1 , Angela Tuzi 1 and Maria Rosaria Iesce 1

1 Dipartimento di Scienze Chimiche, Università di Napoli Federico II, Complesso Universitario di M. Sant’Angelo, Via Cintia, 80126 Napoli, Italy; [email protected] (M.D.); [email protected] (A.T.); [email protected] (M.R.I.) 2 Erbagil s.r.l., Via L. Settembrini, 13, 82037 Telese Terme, Italy; [email protected] * Correspondence: [email protected]

Abstract: The synthesis of glycosides and modiﬁed nucleosides represents a wide research ﬁeld in organic chemistry. The classical methodology is based on coupling reactions between a glycosyl donor and an acceptor. An alternative strategy for new C-nucleosides is used in this approach, which consists of modifying a pre-existent furyl aglycone. This approach is applied to obtain novel pyridazine C-nucleosides starting with 2- and 3-(ribofuranosyl)furans. It is based on singlet oxygen [4+2] cycloaddition followed by reduction and hydrazine cyclization under neutral conditions. The mild three-step one-pot procedure leads stereoselectively to novel pyridazine C-nucleosides of pharmacological interest. The use of acetyls as protecting groups provides an elegant direct route to a deprotected new pyridazine C-nucleoside.

Keywords: glycosyl furans; singlet oxygen; [4+2] cycloaddition; C-nucleosides; photooxygenation; pyridazines; reduction

Citation: Cermola, F.; Vella, S.; 1. Introduction DellaGreca, M.; Tuzi, A.; Iesce, M.R. A The synthesis of nucleoside analogues has a prominent role in the field of organic One-Pot Approach to Novel chemistry and biology [1]. Among modified nucleosides, C-nucleosides represent a special Pyridazine C-Nucleosides. Molecules moiety of this compound class due to their higher stability towards enzymatic and chemical 2021, 26, 2341. https://doi.org/ hydrolysis than that of natural N-nucleosides, as well as owing to the interesting biolog- 10.3390/molecules26082341 ical and pharmacological properties of some of their derivatives [1,2]. The first natural C-nucleoside isolated was pseudouridine [3], followed by showdomycin [4], and oxazino- Academic Editor: Axel G. Griesbeck mycin [5]—all characterized by important pharmacological activities, from antitumor to antibacterial. C-nucleosides comprise a sugar moiety and a non-natural heterocyclic base Received: 1 April 2021 connected by a carbon–carbon bond. The stability of this bond and the broad structural Accepted: 13 April 2021 Published: 17 April 2021 variation in the heterocyclic base account for the high interest in this compound class [6–9]. The main synthetic approach for C-glycosides is based on coupling reactions between a

Publisher’s Note: MDPI stays neutral glycosyl donor and an acceptor in the presence of a promoter [10]. Alternatively, structural with regard to jurisdictional claims in elaborations on a pre-existing aglycone are performed to afford the final C-glycoside [10]. In published maps and institutional affil- this context, the use of the furan as starting aglycone is of particular interest due to the easy iations. preparation and high versatility of this heterocycle [11]. We elaborated a green procedure based on the use of a furan aglycone as a building block and the [4+2] cycloaddition of singlet oxygen [12] by dye-sensitized photooxygenation as a strategy for providing the final desired structures. Indeed, singlet oxygen addition to furan systems offers various elegant routes to differently functionalized derivatives through structural elaborations of Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. the initial endoperoxides, the 2,3,7-trioxabicyclo[2.2.1]hept-5-enes [13]. Although the furan This article is an open access article endoperoxides are thermally unstable, their reactivity can be controlled and opportunely distributed under the terms and addressed by working at low temperatures. With this strategy, it is possible to synthe- conditions of the Creative Commons size novel glycosyl derivatives, such as glycals [14] and spiroketals of monosaccharides, Attribution (CC BY) license (https:// which are structural motifs of many products characterized by important and assorted creativecommons.org/licenses/by/ biological properties [15]. Pyridazine [16], pyrazoline [14], bis-epoxide [17], and spiro- 4.0/). cyclic C-nucleosides [17] were also obtained. Key intermediates are often unsaturated

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epoxide [17], and spirocyclic C-nucleosides [17] were also obtained. Key intermediates are Zoften-1,4-enediones, unsaturated easily Z-1,4-enediones, obtained by easily photooxygenation obtained by photooxygenation and followed by in and situ followed reduction by ofin thesitu corresponding reduction of the endoperoxides corresponding [14– 17endo] atperoxides low temperatures. [14–17] at In low many temperatures. cases, the pho- In tochemicalmany cases, approach the photochemical represents approach an alternative repres toents other an methods.alternative As to an other example, methods. Maeba As describedan example, the synthesisMaeba ofdescribed 3-(20,30,50 -tri-theO -benzoyl-synthesisβ -ribofuranosyl)pyridazineof 3-(2′,3′,5′-tri-O-benzoyl- throughβ-ribo- a seriesfuranosyl)pyridazine of reactions involving through the a oxidation series of ofreactions the furan involving ring by brominethe oxidation and methanol of the furan [18]. Wering obtained by bromine an analogue and methanol 4-(20,3 0[18].,50-tri- WeO-acetyl- obtainedβ-ribofuranosyl)-3,6-dimetylpyridazine an analogue 4-(2′,3′,5′-tri-O-acetyl-β-ri- in 70%bofuranosyl)-3,6-dimetylpyridazine yield (based on starting furan) by in oxygenation, 70% yield (based followed on starting by diethyl furan) sulﬁde by reductionoxygena- andtion, hydrazine followed by hydrochloride diethyl sulfide cyclization reduction [ 16and]. Thehydrazine efﬁcient hydrochloride formation of thiscyclization compound [16]. promptedThe efficient us toformation explore theof this possibility compound of extending prompted this us approach to explore to novelthe possibility furans in of order ex- totending synthesize this approach 3- and 4-(ribofuranosyl)pyridazines. to novel furans in order to synthesize The pyridazine 3- and ring4-(ribofuranosyl)pyr- is recognized as aidazines. versatile The pharmacophore. pyridazine ring Inis recognized the last years, as a particularversatile pharmacophore. attention has been In the devoted last years, to developingparticular attention novel synthetic has been approaches devoted to thisdeve system,loping novel starting synthetic with new approaches precursors to or this by utilizingsystem, starting green methodologies with new precursors [19–22]. or by utilizing green methodologies [19–22].

2. Results and Discussion For thisthis purpose,purpose, the the suitable suitable furans furans2a ,2ab were,b were initially initially prepared prepared by the byβ the-stereoselective β-stereose- reductionlective reduction [23] of the [23] corresponding of the corresponding furyl ketoses furyl1a ketoses,b (Scheme 1a,b 1(Scheme). The latter 1). The were latter obtained were byobtained coupling by reactionscoupling betweenreactions 2,3,5-tri- betweenO -benzyl-2,3,5-tri-O-D-ribono-1,4-lactonebenzyl-D-ribono-1,4-lactone as the glycosyl as the donor, gly- andcosyl 2- donor, or 3-furyllithium, and 2- or 3-furyllithium, according to aaccord previouslying to reporteda previously procedure reported [15 procedure]. [15].

OBn O BnO O Et SiH, CH CN O BnO 3 3 BF Et O, 40 °C r.t. OH OBn O 3 2 overnight BnO OBn 1a 2a

O OBn O BnO Et SiH, CH CN O BnO 3 3 BF Et O, 40 °C r.t. OH OBn 3 2 O overnight BnO OBn 1b 2b Scheme 1. Stereoselective synthesis of 2-2- andand 3-(3-(ββ-ribofuranosyl)furans 2a,,b..

The reduction times were particularly long (overnight), probably owing to the open- chain structuresstructures of of the the furyl furyl derivatives derivatives1a 1a,b,inb in equilibrium equilibrium with with very very small small amounts amounts of the of 1 correspondingthe corresponding cyclic cyclic structures, structures, as evidenced as evidenced by their by their proton proton spectra. spectra. Although Although the theH- NMR1H-NMR spectra spectra of both of both crude crude reaction reaction mixtures mixtures showed showed only the only presence the presence of products of products2a,b, a considerable2a,b, a considerable loss of loss material of material occurred occurred during during puriﬁcation purification by silica by gelsilica chromatography, gel chromatog- even if performed under N , as reported in the literature for 2a [24]. Firstly, 2a was photo- raphy, even if performed under2 N2, as reported in the literature for 2a [24]. Firstly, 2a was oxygenated at −20 ◦C in dichloromethane with methylene blue as the sensitizer (Scheme2 ). photo-oxygenated at −20 °C in dichloromethane 1with methylene blue as the sensitizer When the reaction was complete (90 min, TLC or H-NMR), Et12S (1.2 equiv.) was added (Scheme 2). When the reaction was complete (90 min, TLC or H-NMR), Et2S (1.2 equiv.) − ◦ towas the added crude to mixture, the crude which mixture, was which kept atwas 20keptC at to −20 avoid °C to thermal avoid thermal rearrangement rearrangement of the intermediate endoperoxide [12–17]. of the intermediate endoperoxide [12–17].

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O O BnO O BnO O BnO 1 BnO O O 1O2, DCM O O O2, DCM O 20 °C BnO OBn BnO OBn 2a 3a Et2S Et2S 20 °C O N O N BnO O BnO O BnO O BnO O BnO N O NH2NH2 HCl O NH2NH2 O O NH2NH2 HCl O NH2NH2 O THF, r.t. MeOH, r.t. THF, r.t. BnO OBn BnO OBn BnO OBn BnO OBn BnO OBn 4a' 4a 5a 4a' 4a 5a SchemeScheme 2.2. Dye-sensitizedDye-sensitized photooxygenation of ribofuranosyl furan 2a2a,, followed followed by by reduction reduction and and hydrazinehydrazine treatment.treatment.

After 2 h, the solvent and unreacted Et2S were removed under reduced pressure, and AfterAfter 22 h,h, thethe solventsolvent andand unreactedunreacted EtEt22S were removed underunder reducedreduced pressure,pressure, andand thethe crudecrude 4a4a waswas treatedtreated withwith hydrazinehydrazine hydrochloride,hydrochloride, asas reportedreported inin [[16].16]. UnderUnder thesethese conditions,conditions, compoundcompound 4a4a rapidlyrapidly isomerizedisomerized intointo thethe moremore stablestable EE-isomer-isomer 4a’4a’,, owingowing toto thethe configurationalconfigurational instability instability of of the the Z-enedione Z-enedione4a 4aunder under acidic acidic conditions, conditions, as observedas observed in similarin similar cases cases [12 [12,16],16] (Scheme (Scheme2). However,2). However, we we found found that that when when a solution a solution of hydrazine of hydrazine in THFin THF (2.0 (2.0 M) M) was was added added to the to crudethe crude glycosyl glycosyl enedione enedione4a, the 4a, expected the expected pyridazine pyridazine5a was 5a promptlywas promptly obtained obtained (Scheme (Sch2eme). The 2). synthesisThe synthesis of 5a ofwas 5a was then then realized realized through through a one-pot a one- β three-steppot three-step procedure procedure in good in good total total yield, yield, as shown as shown in Scheme in Scheme3. The 3. The-configuration β-configuration of pyridazineof pyridazine5a was5a was confirmed confirmed by theby the 2D-NOESY 2D-NOESY experiment, experiment, which which evidenced evidenced correlation correla- of pyridazine 5a was0 confirmed0 by the 2D-NOESY experiment, which evidenced correla- betweention between the H-1the H-1and′ and the the H-4 H-4of′ theof the sugar sugar ring ring (Figure (Figure1). 1). With With the the exception exception of of the the 5a protectingprotecting groups,groups, thethe 3-(ribofuranosyl)pyridazine3-(ribofuranosyl)pyridazine 5a was the samesame asas thatthat reportedreported byby Maeba et al. [18]. Maeba et al. [18].

1 1. 1O2, DCM, 20 °C, 90 min N 1. O2, DCM, 20 °C, 90 min N BnO O 2. Et2S, 20 °C, 2 h BnO N BnO O 2. Et2S, 20 °C, 2 h BnO N O 3. NH2NH2, THF, r.t., 1 h O O 3. NH2NH2, THF, r.t., 1 h O

BnO OBn BnO OBn 2a 5a (68 % from 2a) (68 % from 2a) 0 0 0 SchemeScheme 3.3. One-pot synthesis ofof thethe 3-(23-(2′,3,3′,5′-tri--tri-O-benzyl--benzyl-β--DD-ribofuranosyl)pyridazine-ribofuranosyl)pyridazine ( (5a5a).). Scheme 3. One-pot synthesis of the 3-(2′,3′,5′-tri-O-benzyl-β-D-ribofuranosyl)pyridazine (5a).

NOE

H H 5' H H 5' O BnO 4' 1' BnO 4' 1' N 3' 2' N 3' 2' N

BnO OBn 5a

FigureFigure 1.1. NOESYNOESY correlationcorrelation inin 5a5a.. The novel conditions were applied to furan 2b and, in good total yield, led to the new The novel conditions were applied to furan 2b and, in good total yield, led to the new 4-(ribofuranosyl)pyridazine 5b (Scheme4). The β-conﬁguration was also conﬁrmed for 5b 4-(ribofuranosyl)pyridazine 5b (Scheme 4). The β-configuration was also confirmed for by the 2D-NOESY experiment. 5b by the 2D-NOESY experiment.

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N O 1. 1O , DCM, 20 °C, 90 min N 1. O2, DCM, 20 °C, 90 min N BnO 2. Et S, 20 °C, 2 h BnO BnO 2. Et2S, 20 °C, 2 h BnO O 3. NH NH , THF, r.t., 1 h O O 3. NH2NH2, THF, r.t., 1 h O

BnO OBn BnO OBn 2b 5b (72 % from 2b) (72 % from 2b) 0 0 0 SchemeScheme 4.4. One-pot synthesis ofof thethe 4-(24-(2′,3,3′,5,5′-tri--tri-OO-benzyl--benzyl-ββ--DD-ribofuranosyl)pyridazine-ribofuranosyl)pyridazine ( (5b5b).).

TheThe relevance of of the the protecting protecting groups groups in inthe the glycoside glycoside chemistry, chemistry, as well as well as the as pos- the possibilitysibility of using of using easily easily removable removable groups, groups, moti motivatedvated us us to to extend extend the the one-pot one-pot route toto aa sugarfuransugarfuran protectedprotected with with acetyls. acetyls. Hence, Hence, we we carried carried out out a coupling a coupling reaction reaction with thewith 2,3,5- the tetra-1-2,3,5-tetra-1-O-acetyl-O-acetyl-β-D-ribofuranoseβ-D-ribofuranose as the as glycosyl the glycosyl donor donor and methyland methyl furan-2-carboxylate furan-2-carbox- asylate the as acceptor—both the acceptor—both commercially commerciall available—iny available—in the presence the presence of SnCl of4 SnClas the4 as promoter, the pro- accordingmoter, according to a reported to a reported procedure procedure [17]. [17]. ThisThis syntheticsynthetic approachapproach was was described described to to lead lead mainly mainly to toβ-anomers β-anomers due due to theto the partici- par- 0 pationticipation of the of neighboringthe neighboring acetyl acetyl group group at the at C-2 the ofC-2 the′ of sugar the sugar ring during ring during the departure the depar- of ticipation of the neighboring0 acetyl group at the C-2′ of the sugar ring during the depar- theture leaving of the leaving group at group C-1 [at25 C-1,26].′ [25,26]. α Surprisingly,Surprisingly, the the reaction reaction afforded afford onlyed only the ribofuranosylthe ribofuranosyl furan furan2c, whose 2c, whose-configuratio α-config-n wasuration assigned was assigned by the 2D-NOESY by the 2D-NOESY experiment experiment (Scheme5 (Scheme). Indeed, 5). a Indeed, NOE effect a NOE between effect the be- uration0 was assigned0 by the 2D-NOESY experiment (Scheme 5). Indeed, a NOE effect be- H-1tweenand the the H-1 H-3′ andof the sugarH-3′ of ring the insugar C6D 6ringwas in evidenced C6D6 was(Figure evidenced2). In (Figure this anisotropic 2). In this tween the H-1′ 0and the H-30 ′ of the sugar ring in C6D6 was evidenced1 (Figure 2). In this solvent, the H-3 and H-4 signals do not overlap as it occurs when the H-NMR spectrum1 anisotropic solvent, the H-3′ and H-4′ signals do not overlap as it occurs when the 1H- is recorded in CDCl3 (aromatic-solvent-induced shift (ASIS) effects) [27]. The previously NMR spectrum is recorded in CDCl3 (aromatic-solvent-induced shift (ASIS) effects) [27]. incorrect β-conﬁguration for 2c was assigned on the basis of the constant value of the The previously incorrect β-configuration for 2c was assigned on the basis of the constant coupling, in comparison with those of the two anomers of showdomycin, and the synthetic value of the coupling, in comparison with those of the two anomers of showdomycin, and procedure used [17]. the synthetic procedure used [17].

AcO AcO AcO OAc AcO O CO2Me O O CO2Me O SnCl , DCM, 0 °C r.t., 4 h SnCl4, DCM, 0 °C r.t., 4 h O AcO OAc AcOAcO AcO OAc AcOAcO CO Me CO2Me 2c 2c SchemeScheme 5.5. Synthesis of thethe ribofuranosylribofuranosyl furanfuran αα--2c2c..

NOE AcO AcO 5' H 5' H O O 1' 4' 1' 3' 2' O AcOAcO AcOAcO CO Me CO2Me 2c 2c FigureFigure 2.2. NOESY correlation inin 2c2c..

AttemptsAttempts to synthesizesynthesize thethe ββ-anomer of 2c byby carryingcarrying outout thethe couplingcoupling reactionreaction inin acetonitrileacetonitrile and/orand/or byby usingusing differentdifferent promoters promoters (BF (BF3 3or or TMSOTf) TMSOTf) failed, failed, and and only only the theα- 1cα- was1c was obtained, obtained, as evidencedas evidenced spectroscopically spectroscopically and and chromatographically. chromatographically. AA possible rationalizationrationalization ofof thisthis unexpectedunexpected resultresult isis thatthat thethe couplingcoupling proceedsproceeds herehere 0 throughthrough anchimericanchimeric assistanceassistance byby thethe acetylacetyl groupgroup atat thethe C-5C-5.′. AA similarsimilar stereochemicalstereochemical trendtrend isis describeddescribed inin thethe iodinationiodination ofof somesome acetylatedacetylated oxathiolanesoxathiolanes [[28].28]. Otherwise,Otherwise, thethe β useuse of the the only only β-anomer-anomer of of 1,2,3,5-tetra- 1,2,3,5-tetra-OO-acetyl--acetyl-D-ribofuranoseD-ribofuranose as asa glycosyl a glycosyl donor donor to- togethergether with with the the observed observed full full stereoselectivity stereoselectivity suggest suggest that that the the reaction reaction to tomethyl methyl furan-2- furan- α 2-carboxylatecarboxylate could could occur occur through through a conc a concertederted pathway pathway leading leading to tothe the only only α-anomer-anomer of 2c of. α 2cThis. This hypothesis hypothesis could could be confirmed be conﬁrmed by bythe the use use of ofthe the only only α-anomer-anomer of of1,2,3,5-tetra- 1,2,3,5-tetra-O- O-acetyl-D-ribofuranose as a glycosyl donor, which should lead to the β-anomer of 1c. acetyl-D-ribofuranose as a glycosyl donor, which should lead to the β-anomer of 1c. Con- Control experiments showed that the peracetylation reaction in pyridine or catalyzed by the trol experiments showed that the peracetylation reaction in pyridine or catalyzed by the

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TMSOTfTMSOTfTMSOTf afforded affordedafforded the thethe peracetylated peracetylatedperacetylated pyranosic pyranosicpyranosic form foformrm of theofof thethe sugar sugarsugar as the asas main thethe mainmain product, product,product, as well asas wellaswell the asasβ the-anomerthe ββ-anomer-anomer-anomer of 1,2,3,5-tetra- ofofof 1,2,3,5-tetra-1,2,3,5-tetra-1,2,3,5-tetra-O-acetyl-OO-acetyl--acetyl--acetyl-D-ribofuranose,DD-ribofuranose,-ribofuranose,-ribofuranose, which whichwhichwhich is the isisis only thethethe onlyonly commerciallyonly commer-commer-commer- ciallyavailablecially available available form. form. form. 0 0 0 TheThe mildmildmild procedure procedureprocedure for forfor pyridazine pyridazinepyridazine C-nucleoside C-nuC-nucleosidecleoside was waswas later laterlater applied appliedapplied to thetoto thethe 2-(2 2-(22-(2,3 ,5′′,3′,3,3-tri-′′,5′,5,5′′-′-- α D Otri-tri-tri--acetyl-OO-acetyl--acetyl--acetyl-- α-ribofuranosyl)furanα---DD-ribofuranosyl)furan-ribofuranosyl)furan-ribofuranosyl)furan (2c). (((2c Unexpectedly,2c).).). Unexpectedly,Unexpectedly,Unexpectedly, the sequence thethethe sequencesequencesequence of reactions ofofof reactionsreactionsreactions on 2c led ononon 2cin2c ledhighledled in inin yield high highhigh to yield yieldyield the deprotectedtototo the thethe deprotected deprotecteddeprotected pyridazine pyridazine pyridazinepyridazine C-nucleoside C-nucleoside C-nucleosideC-nucleoside5c (Scheme 5c 5c (Scheme (Scheme(Scheme6). 6). 6).6).

11 AcO 1. 1O 2,, DCM,DCM, 2020 °C,°C, 9090 minmin AcOAcO 1. O22, DCM, 20 °C, 90 min HOHO O 2. Et 2S, 20 °C, 2 h O O 2. Et22S, 20 °C, 2 h O 3. NH NH , THF, r.t., 1 h 3.3. NH NH22NHNH22, ,THF, THF, r.t., r.t., 1 1 h h N OO 2 2 N AcOAcO HO HO NN AcOAcO CO 2Me HO HO CO22Me 2c 5c CONHNH 2 2c 5c5c CONHNH22 (80(80 %% fromfrom 2c)) (80 % from 2c) D SchemeScheme 6.6. One-pot One-pot synthesis synthesis of of the the 6-(6-(αα---DD-ribofuranosyl)pyridazine-3-carbohydrazide-ribofuranosyl)pyridazine-3-carbohydrazide-ribofuranosyl)pyridazine-3-carbohydrazide (((5c5c).).).

WithWith thethethe aimaimaim ofofof preservingpreservingpreserving thethethe methylmethylmethyl esteresterester function,function,function, as asas well wellwell as asas the thethe protecting protectingprotecting acetyl acetylacetyl groupsgroups onon thethethe sugarsugarsugar ring,ring,ring, thethethe reactionreactionreaction waswaswas carriedcacarriedrried out outout under underunder the thethe same samesame conditions conditionsconditions except exceptexcept forforfor thethethe useuseuse ofofofof 1.21.21.21.2 equiv. equiv.equiv.equiv. ofofofof hydrazine. hydrazine.hydrazine.hydrazine. In InIn this thisthis case,case,case, thethethe one-potone-potone-potone-pot procedureprocedureprocedureprocedure affordedaffordedaffordedafforded thethethethe protectedprotectedprotected pyridazinepyridazinepyridazine C-nucleosideC-nucleosideC-nucleoside 5d5d goodgoodgood yield,yield,yield, showingshowingshowing thatthatthat the thethethe cyclization cyclizationcyclizationcyclization proceeds proceedsproceedsproceeds more rapidly than the attack to the ester functions (Scheme7). moremore rapidly rapidly than than the the attack attack to to the the ester ester functions functions (Scheme (Scheme 7). 7).

11 1. 1O 2,, DCM,DCM, 2020 °C,°C, 9090 minmin 1. O22, DCM, 20 °C, 90 min AcO 2. Et 2S, 20 °C, 2 h AcO AcOAcO 2. Et22S, 20 °C, 2 h AcOAcO O 3. NH 2NH 2 (1.2(1.2 equiv.),equiv.), O O 3. NH22NH22 (1.2 equiv.), OO THF, r.t., 1 h THF, r.t., 1 h N OO N AcOAcO AcO AcO NN AcOAcO CO 2Me AcO AcO CO22Me 2c 5d CO 2Me 2c 5d5d CO22Me (75(75 %% fromfrom 2c)) (75 % from 2c) 0 0 0 αD SchemeScheme 7.7. One-pot One-pot synthesis synthesis of of the the 3-(2 3-(2′′,3,3′,3,3′′,5,5′,5,5′′-tri--tri-′-tri-OO-acetyl--acetyl--acetyl-αα---DD-ribofuranosyl)-6-(methoxycar--ribofuranosyl)-6-(methoxycar--ribofuranosyl)-6-(methoxycar--ribofuranosyl)-6-(methoxycarbonyl) pyridazinebonyl)pyridazinebonyl)pyridazine (5d). ( (5d5d).).).

TheThe structure structure of of compound compound compound 5d 5d5d,, , assigned assignedassigned, assigned by byby by NMR NMRNMR NMR data, data,data, data, was waswas was confirmed confirmedconfirmed conﬁrmed by byby X-ray X-rayX-ray by X-ray crys- crys-crys- tallographiccrystallographictallographictallographic analysis analysisanalysis analysis (Figure (Figure(Figure (Figure 3). 3).3). 3).

Figure 3. Molecular structure of 5d5d showingshowing thethe α-configuration-configuration (Ortep(Ortep viewview withwith ellipsoidsellipsoids FigureFigure 3.3.Molecular Molecular structurestructure of of5d 5dshowing showing the theα α-conﬁguration-configuration (Ortep (Ortep view view with with ellipsoids ellipsoids drawn drawndrawn at at 30% 30% probability probability level). level). at 30% probability level). Pyridazine 5d was quite stable; however, it slowly aromatized after several days at PyridazinePyridazine 5d5d waswas quitequite stable;stable; however,however, itit slowly slowlyslowly aromatized aromatizedaromatized afterafter several severalseveral days daysdays at atat room temperature, leading to the corresponding furan 6d (Scheme 8). Similar elimination roomroom temperature,temperature, leading leading to to the the corresponding corresponding furanfuran furan 6d6d 6d (Scheme(Scheme (Scheme8 8). ).8). Similar Similar Similar elimination elimination elimination was previously observed in a benzoylated pyridazine C-nucleoside [18]. waswas previouslypreviously observed observed inin in aa a benzoylatedbenzoyla benzoylatedted pyridazinepyridazine pyridazine C-nucleosideC-nucleoside C-nucleoside [[18]. 18[18].].

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AcO AcOH O N N N O AcO CO Me AcO AcO N 2 6d 5d CO2Me Scheme 8. Aromatization of 5d intointo 6d6d..

3. Materials and Methods 3.1. General Information 3.1. General Information Melting points are uncorrected. The 1H- and 13C-NMR spectra were recorded at Melting points are uncorrected. The 1H- and 13C-NMR spectra were recorded at 500 500 and 125 MHz, respectively, on a Fourier Transform NMR Varian 500 Unity Inova and 125 MHz, respectively, on a Fourier Transform NMR Varian 500 Unity Inova spec- spectrometer. The carbon multiplicity was evidenced by DEPT experiments. The proton couplingstrometer. The were carbon evidenced multiplicity by 1H-1 Hwas COSY eviden experiments.ced by DEPT The experiments. heteronuclear The chemical proton shiftcou- 1 1 correlationsplings were evidenced were determined by H- byH COSY HMQC experiments. and HMBC The pulse heteronuclear sequences. 1 chemicalH-1H proximities shift cor- 1 1 throughrelations space were within determined a molecule by HMQC were determined and HMBC by pulse NOESY sequences. experiments. H- X-rayH proximities analysis wasthrough performed space within on a Bruker-Noniusa molecule were Kappadetermined CCD by (Nonius NOESY BV, experiments. Delft, The X-ray Netherlands) analysis was performed on a Bruker-Nonius Kappa CCD (Nonius BV, Delft, The Netherlands) dif- diffractometer (graphite monochromated Mo Kα radiation, λ = 0.71073 Å, CCD rotation images,fractometer thick (graphite slices, ϕ andmonochromatedω scans to fill Mo asymmetric Kα radiation, unit). λ Analytical= 0.71073 Å, TLC CCD was rotation performed im- onages, precoated thick slices, silica φ gel and plates ω scans (Macherey-Nagel, to fill asymmetric Düren, unit). Germany) Analytical with TLC 0.2 was mm performed film thick- on precoated silica gel plates (Macherey-Nagel, Düren, Germany) with 0.2 mm film thick- ness. Spots were visualized by UV light and by spraying with EtOH/H2SO4 (95:5 v/v), followedness. Spots by were heating visualized for 5 min by at UV 110 light◦C. Columnand by spraying chromatography with EtOH/H was performed2SO4 (95:5 onv/v silica), fol- gellowed (0.063–0.2 by heating mm) for (Macherey-Nagel). 5 min at 110 °C. Reagent-gradeColumn chromatography commercially was available performed reagents on silica and solventsgel (0.063–0.2 were used.mm) (Macherey-Nagel). Reagent-grade commercially available reagents and solvents were used. 3.2. Synthesis of 2a,b 3.2. SynthesisA solution of 2a,b of 243 mg (0.5 mmol) of 1a [15] in 5.2 mL of acetonitrile, cooled to −40 ◦C, was addedA solution to 240 of 243µL (3mg equiv.) (0.5 mmol) of triethylsilane of 1a [15] in and5.2 mL 70 µofL acetonitrile, (1 equiv.) of cooled BF3·Et to2 O.−40 The °C, ◦ solutionwas added was to stirred 240 µL at (3− equiv.)40 C for of 4trie h,thylsilane which was and allowed 70 µL to (1 rise equiv.) to r.t. of while BF3·Et the2O. mixture The so- waslution further was stirred stirred at overnight. −40 °C for 4 A h, saturated which was aqueous allowed solution to rise to of r.t. K2 whileCO3 was the mixture later added was (10further mL), stirred and theovernight. mixture A was saturated kept underaqueous stirring solution for of 10 K2 min.CO3 was The later organic added layer (10 mL), was extractedand the mixture with diethyl was kept ether under (3 × 30 stirring mL), washed for 10 min. with The brine, organic dried layer over anhydrouswas extracted Na2 withSO4, anddiethyl filtered. ether (3 The × 30 solvent mL), washed was removed with brine, under dried reduced over anhydrous pressure, andNa2SO the4, residueand filtered. was chromatographedThe solvent was removed on flash under silica reduced gel (n-hexane/ether pressure, and 1:1 thev/ residuev), affording was chromatographed the C-nucleoside βon-2a flash[24] silica with gel 35% (n yield.-hexane/ether 1:1 v/v), affording the C-nucleoside β-2a [24] with 35% 1 0 0 0 yield.β -2a: oil; H-NMR; δ = 3.61 (m, 2 H, H-5 A and H-5 B), 4.05 (dd, J = 4.9 Hz, 1 H, H-3 ), 0 0 4.18 (dd,β-2aJ: oil;= 6.5, 1H-NMR; 4.9 Hz, 1δ H,= 3.61 H-2 (m,), 4.30 2 H, (m, H-5 1′ H,A and H-4 H-5), 4.50–4.66′B), 4.05 (dd, (m, 6J = H, 4.9 CH Hz,2 of 1 Bn),H, H-3 5.04′), 0 × (d,4.18J =(dd, 6.5 J Hz, = 6.5, 1 H, 4.9 H-1 Hz,), 1 6.34 H, H-2 (bs,′ 2), H,4.30 H-3 (m, and 1 H, H-4), H-4 7.23–7.33′), 4.50–4.66 (m, (m, 15 H,6 H, 3 CHPh),2 of 7.34 Bn), (bs, 5.04 1 13 H,(d, H-5);J = 6.5 Hz,C-NMR; 1 H, H-1δ =′ 70.3), 6.34 (t), (bs, 72.1 2 H, (2× H-3t), 73.4and (t),H-4), 76.5 7.23–7.33 (d), 77.7 (m, (d), 15 79.9 H, (d),3× Ph), 81.5 7.34 (d), 108.9(bs, 1 (d),H, H-5); 110.3 13 (d),C-NMR; 127.5 (d),δ = 70.3 127.6 (t), (d), 72.1 127.7 (2× (d),t), 73.4 127.8 (t), (d), 76.5 128.0 (d), (d),77.7 128.3 (d), 79.9 (d), 137.7(d), 81.5 (s), (d), 138.0 108.9 (s), 138.2(d), 110.3 (s), 142.5(d), 127.5 (d), 152.2(d), 127.6 (s). (d), 127.7 (d), 127.8 (d), 128.0 (d), 128.3 (d), 137.7 (s), 138.0 (s), 138.2The (s), synthesis142.5 (d), 152.2 of 2b (s).was performed as reported above for 2a, starting from 1b [15]. β The flashThe synthesis silica gel (ofn -hexane/ether2b was performed 1:1 v /asv )reported afforded above the novel for 2aC,-nucleoside starting from- 2b1b with[15]. 30% yield. The flash silica gel (n-hexane/ether◦ 1 1:1 v/v) afforded the novel C-nucleoside β0-2b with 30% β-2b: mp 49–51 C; H-NMR; δ = 3.56 (dd, J = 10.4, 4.4 Hz, 1 H, H-5 A), 3.59 (dd, yield. 0 0 J = 10.4, 4.4 Hz, 1 H, H-5 B1), 3.84 (dd, J = 6.6, 4.9 Hz, 1 H, H-2 ), 3.99 (dd, J = 4.9, 3.8 Hz, 1 H, 0 β-2b: mp 49–51 °C;0 H-NMR; δ = 3.56 (dd, J = 10.4, 4.4 Hz, 1 H, H-5′A), 3.59 (dd,0 J = H-3 ), 4.28 (m, 1 H, H-4 ), 4.48–4.62 (m, 6 H, CH2 of Bn), 4.97 (d, J = 6.6 Hz, 1 H, H-1 ), 6.31 10.4, 4.4 Hz, 1 H, H-5′B), 3.84 (dd, J = 6.6, 4.9 Hz, 1 H, H-2′), 3.99 (dd, J = 4.9, 3.8 Hz, 1 H, H- (bs, 1 H, H-4), 7.22–7.33 (m, 15 H, 3× Ph), 7.35 (bs, 1 H, H-5), 7.40 (s, 1 H, H-2); 13C-NMR; 3′), 4.28 (m, 1 H, H-4′), 4.48–4.62 (m, 6 H, CH2 of Bn), 4.97 (d, J = 6.6 Hz, 1 H, H-1′), 6.31 (bs, δ = 70.4 (t), 71.9 (t), 72.2 (t), 73.4 (t), 75.7 (d), 77.5 (d), 81.6 (d), 82.2 (d), 108.5 (d), 124.4 (s), 1 H, H-4), 7.22–7.33 (m, 15 H, 3× Ph), 7.35 (bs, 1 H, H-5), 7.40 (s, 1 H, H-2); 13C-NMR; δ = 127.6 (d), 127.7 (d), 127.8 (d), 128.0 (d), 128.3 (d), 137.7 (s), 137.9 (s), 138.1 (s), 140.2 (d), 143.2 70.4 (t), 71.9 (t), 72.2 (t), 73.4 (t), 75.7 (d), 77.5 (d), 81.6 (d), 82.2 (d), 108.5 (d), 124.4 (s), 127.6 (d). HRMS (ESI-TOF) calcd. for C H O [M + H]+ 471.2171, found 471.2168. Anal. Calcd. (d), 127.7 (d), 127.8 (d), 128.0 (d), 30128.331 (d),5 137.7 (s), 137.9 (s), 138.1 (s), 140.2 (d), 143.2 (d). for C30H30O5: C, 76.57; H, 6.43. Found: C, 76.45; H, 6.52. HRMS (ESI-TOF) calcd. for C30H31O5 [M + H]+ 471.2171, found 471.2168. Anal. Calcd. for C30H30O5: C, 76.57; H, 6.43. Found: C, 76.45; H, 6.52.

3.3. Synthesis of Pyridazines: General Procedure

Molecules 2021, 26, 2341 7 of 10

3.3. Synthesis of Pyridazines: General Procedure ◦ A 0.02 M solution of 2 (0.25 mmol) in dry CH2Cl2 was irradiated at −20 C with a halogen lamp (650 W) in the presence of methylene blue (MB, 1 × 10−3 mmol), while dry oxygen was bubbled through the solution. The progress of each reaction was checked by periodically monitoring (TLC, or 1H-NMR) the disappearance of 2. When the reaction was complete (ca. 90 min), 1.2 equiv. of Et2S were added to the crude solution, which was kept ◦ at −20 C for 2 h. Thus, the solvent and the unreacted Et2S were removed under reduced pressure, and 1 mL of a hydrazine solution in THF (2 M) was added (1.2 equiv. for 5d). The resulting mixture was kept under stirring at r.t. for 1 h. Subsequently, the solvent and the excess hydrazine were removed under reduced pressure to afford the crude product 5. Silica gel chromatography (n-hexane/ether) afforded the pure pyridazine derivatives 5. 0 0 0 3-(2 ,3 ,5 -tri-O-Benzyl-β-D-ribofuranosyl)pyridazine (5a), (68% yield, from 2a); oil; 1H- 0 NMR (CDCl3); δ = 3.66 (dd, J = 10.4, 3.3 Hz, 1 H, H-5 A), 3.88 (dd, J = 10.4, 2.5 Hz, 1 H, 0 0 0 H-5 B), 3.97 (dd, J = 7.7, 4.9 Hz, 1 H, H-3 ), 4.31 (dd, J =4.9, 2.7 Hz, 1 H, H-2 ), 4.36 (d, J = 12.0 Hz, 1 H, CH of Bn), 4.44 (m, 1 H, H-40), 4.50 (d, J = 11.5 Hz, 1 H, CH of Bn), 4.56 (d, J = 12.0 Hz, 1 H, CH of Bn), 4.57 (d, J = 11.5 Hz, 1 H, CH of Bn), 4.74 (d, J = 12.0 Hz, 1 H, CH of Bn), 4.86 (d, J = 12.0 Hz, 1 H, CH of Bn), 5.48 (d, J = 2.7 Hz, 1 H, H-10) 7.20 (dd, J = 8.8, 4.9 Hz, 1 H, H-5), 7.22–7.40 (m, 15 H, 3× Ph), 7.80 (dd, J = 8.8, 1.6 Hz, 1 H, H-4), 9.03 (dd, J = 4.9, 1.6 Hz, 1 H, H-6); 13C-NMR; δ = 69.1 (t), 71.7 (t), 72.1 (t), 73.4 (t), 76.3 (d), 80.9 (d), 81.5 (d), 83.3 (d), 124.9 (d), 126.6 (d), 127.7 (d), 127.8 (d), 128.2 (d), 128.3 (d), 137.7 (s), + 138.0 (s), 150.4 (d), 162.8 (s). HRMS (ESI-TOF) calcd. for C30H31N2O4 [M + H] 483.2284, found 483.2280. Anal. Calcd. for C30H30N2O4: C, 74.67; H, 6.27; N 5.81. Found: C, 74.56; H, 6.35; N 5.72. 0 0 0 4-(2 ,3 ,5 -tri-O-Benzyl-β-D-ribofuranosyl)pyridazine (5b), (72% yield, from 2b); oil; 1H- 0 NMR (CDCl3); δ = 3.57 (dd, J = 10.4, 3.3 Hz, 1 H, H-5 A), 3.65 (dd, J = 10.4, 3.8 Hz, 1 H, 0 0 0 H-5 B), 3.77 (dd, J = 7.7, 4.9 Hz, 1 H, H-2 ), 4.02 (dd, J = 4.9, 2.7 Hz, 1 H, H-3 ), 4.38 (m, 2H, H-40 and CH of Bn), 4.51 (d, J = 12.0 Hz, 1 H, CH of Bn), 4.56 (d, J = 12.0 Hz, 1 H, CH of Bn), 0 4.57(d, J = 12.0 Hz, 1 H, CH of Bn), 4.60 (s, 2 H, CH2 of Bn), 4.98 (d, J = 7.7 Hz, 1 H, H-1 ), 7.17–7.35 (m, 15 H, 3 x Ph), 7.47 (bd, J = 5.5 Hz 1 H, H-5), 9.00 (d, J = 5.5 Hz, 1 H, H-6), 9.16 (bs, 1 H, H-3); 13C-NMR; δ = 70.1 (t), 72.0 (t), 72.7 (t), 73.6 (t), 77.1 (d), 78.6 (d), 82.7 (d), 83.4 (d), 123.3 (d), 127.7 (d), 127.9 (d), 128.0 (d), 128.1 (d), 128.5 (d), 136.9 (s), 137.4 (s), 137.6 (s), + 140.3 (s), 149.7 (d), 151.0 (d). HRMS (ESI-TOF) calcd. for C30H31N2O4 [M + H] 483.2284, found 483.2281. Anal. Calcd. for C30H30N2O4: C, 74.67; H, 6.27; N 5.81. Found: C, 74.53; H, 6.37; N 5.69. 6-(α-D-Ribofuranosyl)pyridazine-3-carbohydrazide (5c), (80% yield, from 2c); m.p. 215– ◦ 1 0 0 0 217 C; H-NMR (DMSO); δ = 3.49 (m, 1 H, H-5 A), 3.66 (m, 1 H, H-5 B), 3.98 (m, 1 H, H-4 ), 4.19 (m, 2 H, H-30 and H-20), 4.77 (t, J = 4.8 Hz, 1 H, OH), 4.93 (d, J = 4.4 Hz, 1 H, OH), 5.00 (d, J = 6.6 Hz, 1 H, OH), 5.33 (d, J = 3.4 Hz, 1 H, H-10), 7.80 (d, J = 8.6 Hz, 1 H, H-5), 8.11 (d, J = 8.6 Hz, 1 H, H-4); 13C-NMR (DMSO); δ = 61.9 (t), 72.9 (d), 73.9 (d), 82.3 (d), 83.5 (d), 125.3 (d), 128.1 (d), 152.6 (s), 162.0 (s), 164.2 (s). HRMS (ESI-TOF) calcd. for C10H15N4O5 [M + + H] 271.1042, found 271.1040. Anal. Calcd. for C10H14N4O5: C, 44.44; H, 5.22; N 20.73. Found: C, 44.33; H, 5.30; N 20.69. 0 0 0 3-(2 ,3 ,5 -tri-O-Acetyl-α-D-ribofuranosyl)-6-(methoxycarbonyl)pyridazine (5d), (75% yield, ◦ 1 from 1c); m.p. 124–126 C; H-NMR (CDCl3); δ = 1.81 (s, 3 H, CH3CO), 2.02 (s, 3 H, 0 CH3CO), 2.14 (s, 3 H, CH3CO), 4.09 (s, 3 H, OCH3), 4.23 (dd, J = 11.7, 3.9 Hz, 1 H, H-5 A), 0 0 0 4.49 (m, 2 H, H-5 B and H-4 ), 5.56 (dd, J = 7.4, 4.0 Hz, 1 H, H-3 ), 5.78 (d, J = 4.0 Hz, 1 H, H-10), 5.94 (dd, J = 4.0 Hz, 1 H, H-20), 7.89 (d, J = 7.4 Hz, 1 H, H-5), 8.22 (d, J = 7.4 Hz, 1 H, 13 H-4); C-NMR (CDCl3); δ = 17.0 (q), 20.4 (q), 20.8 (q), 53.4 (q), 63.3 (t), 72.2 (d), 73.4 (d), 78.4 (d), 80.4 (d), 125.8 (d), 127.5 (d), 150.9 (s), 162.0 (s), 164.6 (s), 168.8 (s), 169.5 (s), 170.6 (s). + HRMS (ESI-TOF) calcd. for C17H21N2O9 [M + H] 397.1247, found 397.1244. Anal. Calcd. for C17H20N2O9: C, 51.52; H, 5.09; N, 7.07. Found: C, 52.33; H, 5.20; N 7.18. Molecules 2021, 26, 2341 8 of 10

3.4. Aromatization of 5d Crystals of 5d (25 mg) were kept without solvent at r.t. for one week. A 1H-NMR spectrum was then recorded, which showed the presence of the furan 6d and traces of acetic acid. TLC chromatography (n-hexane/ether 6:4 v/v) afforded the furan 6d as solid in 85% yield. ◦ 1 m.p. 169–170 C; H-NMR (CDCl3); δ = 2.12 (s, 3 H, CH3CO), 4.08 (s, 3 H, OCH3), 0 0 5.16 (s, 2 H, CH2), 6.64 (d, J = 3.5 Hz, 1 H, H-4 ), 7.49 (d, J = 3.5 Hz, 1 H, H-3 ), 7.95 (d, 13 J = 8.7 Hz, 1 H, H-5), 8.19 (d, J = 8.7 Hz, 1 H, H-4); C-NMR (CDCl3); δ = 20.8 (q), 53.2 (q), 57.9 (t), 113.6 (d), 113.7 (d), 121.6 (d), 127.9 (d), 149.4 (s), 150.7 (s), 152.7 (s), 153.1 (s), + 164.5 (s), 170.4 (s). HRMS (ESI-TOF) calcd. for C13H13N2O5 [M + H] 277.0824, found 277.0822. Anal. Calcd. for C13H12N2O5: C, 56.52; H, 4.38; N, 10.14. Found: C, 56.38; H, 4.45; N 10.09.

3.5. X-ray Crystallography of 5d X-ray analysis was performed on single crystals of 5d obtained as colorless blocks by slow evaporation of a DCM/hexane solution at room temperature. One selected crystal was mounted at ambient temperature on a Bruker-Nonius KappaCCD diffractometer (graphite monochromated Mo Kα radiation, λ = 0.71073 Å, CCD rotation images, thick slices, ϕ and ω scans to fill asymmetric unit). Semiempirical absorption corrections (SADABS [29]) were applied. The structure was solved by direct methods (SIR97 program [30]) and anisotrop- ically refined by the full matrix least-squares method on F2 against all independently measured reflections using SHELXL-2018/3 To SHELXL (version 2018/3) [31] and WinGX software (version 2014.1) [32]. All of the hydrogen atoms were introduced in calculated positions and refined according to a riding model with C–H distances in the range of 0.93–0.98 Å and with Uiso (H) equal to 1.2 Ueq or 1.5 Ueq (Cmethyl) of the carrier atom. Compound 5d crystallizes in P 21 21 21 space group with two independent molecules in the asymmetric unit (see Figure S31 of Supporting Information). The two molecules are very similar to each to the other; in molecule A, one acetyl group is split into two positions with refined occupancy factors of 0.61 and 0.39. Some restraints were introduced in the last stage of refinement to regularize the geometry. In the absence of strong anomalous scatterer atoms, the Flack parameter is meaningless, so it was not possible to assign the absolute configuration by anomalous dispersion effects with diffraction measurements on the crystal. The absolute configuration has been assigned by reference to unchanging chiral centers in the synthetic procedure. A rather high residual electronic density (0.472 eA−3) is explained by residual thermal disorder of some acetyl groups at ambient temperature. Unfortunately, it was not possible to re-collect data at low temperatures. Crystal data and structure refinement details are reported in Table S1 of SI. Figures were generated using program Ortep-3 [32]. CCDC-1493362.

4. Conclusions In summary, we highlight that novel pyridazine C-nucleosides can be easily obtained starting with protected ribofuranosyl furans. Appropriate mild conditions to preserve the Z- configuration of the intermediate 1,4-dicarbonyl compound, necessary for the final step that involves the reaction with hydrazine. It is noteworthy that the use of 2a and 2b provides the corresponding 3- and 4-(ribofuranosyl)pyridazines 5a and 5b, respectively, which would be hard to synthesize through direct coupling reactions with complete regioselectivity. Moreover, the use of an acetylated sugar provides a direct route to deprotected pyridazine- C-nucleosides. The three-step one-pot procedure is completely stereoselective and gives the product 5 the same anomeric configuration as the starting sugar furans. The ease of methodology, together with the good yields of pyridazine-C-nucleosides 5, foresee novel applications in the field of C-nucleosides synthesis since pyridazinesare useful intermediates for constructing heterocycle derivatives [33]. These systems are considered by GlaxoSmithKline as one of the “most developable” heteroaromatic rings [34] and are proposed as privileged structures for drug design [35,36]. Moreover, a broad array of Molecules 2021, 26, 2341 9 of 10

signiﬁcant biological activities has been evidenced in several compounds with pyridazine rings [37–39].

Supplementary Materials: The following are available online. 1H- and 13C-NMR, COSY and NOESY spectra for all new compounds. X-ray crystallographic data for 5d. Supplementary data associated with this article can be found in the online version, at CCDC-1493362, which contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Author Contributions: Conceptualization F.C.; methodology F.C. and S.V.; NMR mono- and bidi- mensional spectra F.C. and M.D.; X-ray crystallographic data A.T.; data curation F.C., M.R.I. and M.D.; resources M.R.I.; draft preparation F.C. and M.R.I. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data are contained within the article or the supplementary data file. Acknowledgments: NMR experiments and X-ray crystallography were run at the Dipartimento di Scienze Chimiche, Università di Napoli Federico II. Conflicts of Interest: The authors declare no conflict of interest. Sample Availability: Samples of the compounds are not available from the authors.

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