Synthesis of α-Amino Acids with β,γ-Unsaturated Side Chains.

Abstract α-Amino acids with allenyl, vinyl, and acetylenic side chains can be synthesized using non-enolate based strategies. The ester enolate-Claisen rearrangement applied to propargylic esters of N -protected α-amino acids ie of limited utility since only poor yields of allenic product are obtained with the N-Boc glycine esters, the system which give the most reproducible results. However, α-allenyl-α-amino acids that are fully functionalized on the α-carbon are available through the agency of 4-allenyl-2-phenyloxazolones 4 (obtained from propargyl esters of N-benzoyl protected amino acids via cyclization and Claisen rearrangement) provided that Meerwein's reagent is used to facilitate hydrolysis of the benzamide function in 5 . A variety, of α-substituted glycinates, including those with α-vinyl and α-acetylenic functions, can be prepared using a two-step sequence involving condensation of the cationic glycine synthon 22 with various organomagnesiu reagents, followed by hydrolysis.