1,2,3 triazole aliphatic aldehydes

The mechanism of this reaction was hrst described by Breslow as early as 1958 [4], Subsequently, the natural enzyme thiamine, found in yeast, was replaced by related nucleophiles like thiazole [5,6], triazole [7] and imidazole [8], Reactions that follow this mechanism include the very important Stetter reaction (the benzoin condensation of aliphatic aldehydes), the Michael-Stetter reaction (a variant of the Stetter reaction where the aldehyde reacts with an a,P-unsaturated ketone) [1], transesteriflcations [9] or the acylation of alcohols [9,10], All four reactions are carbene catalysed nucleophilic acylation processes. 

Heterofunctionalisation of carbonyl compounds in the a-position has become an important facet of oiganocatalytic enamine-mediated reactions. In 2005, the Jorgensen group described asymmetric a-sulfenylation of aliphatic aldehydes using TMS-protected prolinol catalysts. The best sulfenylating agent was M-benzylsulfanyl-1,2,4-triazole. Other catalysts, such as proline, prolinol, prolinamide or other secondary amide were less effective. The catalyst with bulkier aromatic groups (C2a) afforded the most enantioselective reaction (Scheme 8.39). 

The synthesis of a new series of tetrahydro-oxazolo[4,5-d]-1,2,3-triazoles (102) has been reported. The triazolium imide 1,3-dipole (104) reacted with ( )-cinnamaldehyde in refluxing ethyl methyl ketone. It is significant that it was to the C=0 bond of the a,/ -unsaturated aldehyde that the cycloaddition took place, yet analogous addition to benzaldehyde or aliphatic ketones was not possible. The primary products of the cycloaddition reaction (105) underwent a sigmatropic rearrangement to furnish the new ring system (102) as shown in Scheme 23 <90JCS(P1)2527>. 

Phenylhydrazono-2-thiazolidinones are cyclized upon treatment with aliphatic and aromatic aldehydes to give thiazolo[4,3-c][l,2,4]triazol-5-ones (equation 22) (76ZN(B)380, 76ZN(B)853). Similar reactions of 2-(and 4-)phenylhydrazono-l-phenylhydantoins with aliphatic aqd aromatic aldehydes gave imidazo[2,3-c][l,2,4]triazoles and imidazo[4,3-c]-[l,2,4]triazoles (82H(19)1375>. 

A series of l,2,4-triazolo[4,3-u]pyrimidines 52 was synthesized in excellent yield by Dandia and co-workers [98] via a microwave-assisted reaction of the l//-l,2,4-triazol-5-amine, a suitable carbonyl compound and malononitrile in aqueous medium. A variety of aromatic aldehydes and cyclic and aliphatic ketones was successfully used to give the desired compounds 52 in good to excellent yields. The reactions were carried out in a domestic microwave oven (Scheme 38). 

The reaction of N-aminoazonium salts with aliphatic and aromatic aldehydes in the absence of base gives the Schiff base type compounds 28.151,153-156 Some aliphatic ketones also react with N-aminoazonium salts.40,155 Hexane-2,5-dione reacts with N-aminopyridinium perchlorate to give l-(l -pyrrolyl)pyridinium cation 29 together with some of the bisper-chlorate 30.157 Certain esters such as diethyl malonate and ethyl cyano-acetate react with N-aminopyridinium salts in the presence of base to give the corresponding N-acylimines 31.48 However, the reaction of ethyl aceto-acetate and acetylacetone with N-aminoazonium salts in the presence of base gives 1,3-dipolar cycloaddition products (Section IV,C,1).36,154,158 The reaction of ethyl acetoacetate with 1-alkyl-l,2,4-triazole 4-imine affords zwitterionic triazolo[4,3-/>]pyridazines 32.139,159... 

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