New Phosphonate Reagents for Aldehyde Homologation

New phosphonate reagents were developed for the two‐carbon homologation of aldehydes to unbranched or methyl‐branched unsaturated aldehydes. The phosphonate reagents, diethyl methylformylphosphonate dimethylhydrazone and diethyl ethylformyl‐2‐phosphonate dimethylhydrazone, contained a protected aldehyde group instead of the usual ester group. A homologation cycle entailed condensation of the reagent with the starting aldehyde, followed by removal of the dimethylhydrazone protective group with a biphasic mixture of 1 M HCl and petroleum ether. This robust two‐step process worked with aliphatic, α,β‐unsaturated and aromatic aldehydes. Isolated yields for the condensation step ranged from 77% to 89%, and yields for the deprotection step ranged from 81% to 96%.