Conversion of isomeric 2:3 adducts (aminoacid–formaldehyde) to N-acyl-pseudoprolines derivatives

Reaction of acyl chlorides or acid anhydrides with isomeric 2:3 adducts derived from condensation of l-serine (1a), l-threonine (1b) and l-cysteine (1c) methyl esters with formaldehyde afforded N-acyl-pseudoprolines 7–19 in various yields. These 2:3 adducts can be considered as synthetic equivalents of oxaproline and thiaproline moieties. The present work revealed the versatile behaviour of the two 2:3 adducts as a consequence of the ring-chain tautomerism occurring in the five- and/or seven-membered rings.