Reaction of N-acyl-α-triphenylphosphonio-α-amino acid esters with organic bases:: Mechanism of the base-catalyzed nucleophilic substitution of the triphenylphosphonium group

Reactions of N-acyl-alpha-triphenylphosphonio-alpha-amino acid methyl esters with organic bases (triethylamine or DBU) were investigated as the crucial step of the base-catalyzed displacement of the triphenylphosphonium group by nucleophiles. It was proved that N-acyl-alpha-triphenylphosphonioglycinates are transformed to an equilibrium mixture of the corresponding N-acyliminoacetates and N-acyl-alpha-triphenylphosphoranylidene glycinates by bases. In the case of N-acyl-alpha-triphenylphosphonio-alpha-amino acid esters with quaternary alpha-carbon, the alpha-substituted homologues of the N-acyliminoacetates were detected to be the only primary reaction product which, however, can undergo further tautomerization to the Corresponding alpha,beta-dehydro-alpha-amino acid derivatives. In both these cases the reaction of N-acyl-alpha-triphenylphosphonio-alpha-amino acid esters with nucleophiles proceeds via the addition of a nucleophile to the activated C=N double bond of the N-acylimino intermediate.