Diazoaldehyde Chemistry. Part 1. Transdiazotization of Acylacetaldehydes in Neutral‐to‐Acidic Medium. A Direct Approach to the Synthesis of α‐Diazo‐β‐oxoaldehydes

First ever non‐deformylating transdiazotization of acylacetaldehydes was achieved: the reactions of 2‐azido‐l‐ethylpyridinium tetrafluoroborate (4) with acylacetaldehydes3proceeded partially without deformylation to yield 16 new α‐diazo‐β‐oxoaldehydes1along with diazomethyl ketones2, especially in the presence of NaOAc (Scheme 1, Tables 1and2). The product distribution was substituent‐dependent and could be correlated quantitatively. This new diazotization reaction appears as an alternative, direct, and more general method for the synthesis of these diazooxoaldehydes. α‐Oxocycloalkanecarbaldehydes5gave only traces (if any) of α‐diazocycloalkanones7, and rearrangement products6were isolated (Scheme 2). Mechanisms of the reactions are discussed (Schemes 4and5).