Synthesis, structure and comparative stability of beta-hydrazono, oximino methyl ether and imino boronates

beta-Hydrazono and oximino ether boronates have been prepared by the sequen tial lithiation of the corresponding methyl hydrazone or oxime methyl ether , followed by reaction with an iodomethylboronate ester, typically in the f orm of the pinacol ester. The resulting products have contrasting hydrolyti c stabilities. beta-Hydrazono boronates are highly sensitive to intramolecu larly catalysed hydrolysis, providing the corresponding beta-keto boronates in generally high yields; beta-oximino ether boronates are stable to. sili ca gel chromatography and show evidence of E-Z-isomerisation. Stable homoch iral boronate ester derivatives of beta-oximino ethers can be readily prepa red by a double transesterification process, via a diethanolamine-mediated pinacol ester exchange, followed by diethanolamine ester hydrolysis-reester ification process with a homochiral diol. beta-Imino boronates can be gener ated in situ by condensation of the corresponding beta-keto boronate with a primary amine, however the resulting imine function is highly hydrolytical ly unstable and cannot be isolated in a pure form.