A Useful Hydrazone Derivative Usable for a Neutral Formyl Anion Equivalent

Formaldehyde dimethylhydrazone (1) is a useful neutral formyl anion equivalent and it can be used for the 1,4-addition of enones. For example, a hydrazone-moiety substituted silyl enol ether (2) is formed by the 1,4-addition of 3-methyl-2-cyclopentenone with 1. Further treatment of 2 with TBAF affords the corresponding 1,4-adduct (3).¹⁾ The hydrazone moiety of 3 is converted into a formyl or a cyano group, by the acid-mediated hydrolysis or the MMPP-oxidation, respectively. In a case of 1,4-additions using formyl anion equivalents such as a lithiated 1,3-dithiane, the regioselectivity and the yield have been decreased by their own strong basicity. On the other hand, the 1,4-addition using 1 can be performed under mild conditions. Thus, 1 is a useful neutral formyl anion equivalent.