Selective Cleavage of the Allyl and Allyloxycarbonyl Groups through Palladium-Catalyzed Hydrostannolysis with Tributyltin Hydride. Application to the Selective Protection-Deprotection of Amino Acid Derivatives and in Peptide Synthesis

N-Allyloxycarbonyl (Alloc) derivatives of amines and amino acids are quantitatively and very rapidly converted to free amino compounds by palladium-catalyzed hydrostannolytic cleavage with tributyltin hydride in the presence of a proton donor (acetic acid, p-nitrophenol, pyridinium acetate, water). A similar procedure can be used for the deprotection of allyl (All) carboxylates and allyl aryl ethers. Deprotection experiments were performed on various mixed N-Alloc and O-Bzl, N-Z and 0-All, N-Alloc and O-t-Bu, and N-alloc- and N-Boc-protected amino acid derivatives. The palladium-catalyzed hydrostannolytic cleavage is fully compatible with the Bzl and Z protecting groups; furthermore the BOC and t-Bu groups and the Alloc and All groups appear to be orthogonal. The reliability of the Alloc methodology for temporary protection of the a-amino functions is illustrated by the solid-phase synthesis of the biologically active undecapeptide substance P.