The recent increasei ni nterest in the chemistry of azoles has been owing to their paramount importance in several essential fields of research, such as pharmacology, medicine, biology, the organic synthesis of fine chemicals, agriculturalc hemistry, and the chemical industry.T his Focus Review summarizes and highlights recently developed methods for the synthesis of five-membered N,N-, N,O-, andN ,Sheterocycles based on the transformations of 2-alkenals. Over the last 15 years, these reactions, which often differ from reactions involving other a,b-unsaturatedc arbonyl compoundsi nt erms of methoda nd results, have gathered significant momentum. The multitude of catalytic and tech-nological novelties discussed in this Focus Review,s uch as asymmetricm etal-and organocatalysis,o ne-pot multicomponent reactions that proceed through domino, cascade, or tandem sequences, and microwave activation,p romote the formation of ad iverse range of target products, the structures of which becomec lear from the mechanistic schemes given in this Focus Review.T he mechanisms discussed reflect great advances in the chemo-, regio-, and stereoselectivity of the reactions of 2-alkenals, which allows new azoles and related compounds (imidazoles, pyrazoles, oxazoles, thiazoles, and their hydrogenatedd erivatives) to be synthesized.[a] Prof.The ORCID identification number(s) for the author(s) of this articlecan be found under http://dx.Scheme2.One-potsynthesis of imidazoles 8 and tentativemechanism of the reaction.T s= 4-toluenesulfonyl, TMS = tert-butyltrimethylsilyl, Bn = benzyl, Lg = leaving group.Scheme3.Enantioselective synthesiso f3-hydroxyalkyl and 3-aminoalkyli midazo[1,2-a]pyridines (9)through an asymmetricorganocatalytic [3+ +2] annulation strategy.Alk = alkyl.Scheme4.One-potsynthesis of imidazole-containingc ondensed heterocycles by the reaction of 2-alkoxypropenalswith N-binucleophiles.Scheme9.Aroute to 2-(1'-alkoxyvinyl)imidazolidines 27 through the condensation of alkoxy propenals.

A configurational assignment of the isomeric methylglyoxal bisdimethylhydrazones derived from the 2-ethoxypropenal precursor has been performed based on experimental measurements and high-level ab initio calculations of 1J(C,C) and 1J(C,H) couplings. The results reveal the marked stereochemical dependence upon the orientation of the lone pairs of both nitrogen atoms in different isomers. Methylglyoxal bisdimethylhydrazone is shown to exist in a mixture of the EE and ZE isomers (ca. 75:25), both of which adopt predominant s-trans conformations with minor (up to 8°) out-of-plane deviations.

The ever-increasing interest in the chemistry of five-membered heterocyclic compounds is due to their paramount importance in such essentialf ields as pharmacology, medicine, biology,o rganic synthesis of fine chemicals, agricultural chemistry,a nd the chemical industry.T his Review summarizesa nd highlights methodsf or the synthesis of five-membered O-, N-, and S-heterocycles based on transformations of a,b-enals. Over the last decade, these reactions, which often differ from the processes involving other a,bunsaturated carbonyl compounds in methodology andr esults, have been gathering momentum. Many of the catalytic and technological novelties discussed in the review,s uch as asymmetricm etal-and organocatalysis, multiple catalysts, one-pot multicomponent reactions proceeding through domino,c ascade, or tandem reactions, andm icrowave activation, promote the formation of diverset arget products, the structures of which becomes clear from the mechanistic schemesg iven in the review.T he mechanisms discussed reflect great advances in the chemo-, regio-, and stereoselectivity of the reactions with participation of a,b-alkenals, achieved in the last 15 years, which allowsn ew five-membered heteroarenes and related compounds (pyrroles, dihydropyrroles, pyrrolidines,f urans, thiophenes) to be synthesized.

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1H and 13C NMR spectra of N,N‐disubstituted α‐amino‐α,β‐unsaturated aldehydes and their azomethines and enammonium salts were studied. The spectra reflect the degree of p–π conjugation between the nitrogen lone pair and the π‐electrons of the carbon–carbon double bond. In this respect, the title 2‐aminoenals are shown to be in an intermediate position between α‐unsubstituted aminoalkenes and α,β‐unsaturated aldehydes. The degree of conjugation depends on the nature of the amine moiety and the activating group. © 1997 John Wiley & Sons, Ltd.