Efficient synthesis of a novel 4-hydroxy-2,3-dioxocyclobut-1-enyl group containing amino acids

[GRAPHICS] Syntheses of novel amino acids possessing a squaryl group, 1-3, in an optic ally active form are described. The syntheses of 1-3 were conducted by a co ncise route involving (1) an aldol addition of an enolate derived from Gly, L-Asp, or L-Glu to diisopropyl squarate (4) and (2) an efficient decarboxy lation of the aldol adducts based on the electron-withdrawing property of t he squaryl group. Introducing N-protected group to 2 and 3 and peptide form ation reactions are also described.