Stereoselective synthesis of threo and erythroβ-hydroxy and β-disubstituted-β-hydroxy α-amino acids

Optically pure N-protected serine aldehyde equivalents can be prepared by the protection of the carboxylic group of serine by a cyclic ortho ester. Alkylation of N-Cbz-, N-Fmoc- or N-Boc-protected serine with oxetane tosylate 1 or bromide 2 gives the corresponding oxetane esters 4a-c which can easily be converted to the cyclic ortho esters 5a-c. A variety of unusual threo beta-hydroxy amino acids have been synthesized by Grignard addition to these optically pure serine aldehyde equivalents. The erythro diastereomers can be obtained by oxidation of the initial three adduct followed by reduction with LiBH4. Also described is a general approach for the diastereoselective synthesis of optically pure beta,beta-dialkyl-beta-hydroxy alpha-amino acids. These highly substituted amino acids are prepared by a sequence of Grignard addition to the optically active serine aldehyde equivalent, followed by oxidation of the initial adduct, and a second Grignard addition to the resulting ketone. The hydroxy adduct is obtained with very high diastereoselectivity (84-96% de). All four diastereomers can be selectively synthesized by varying the order of the Grignard additions and the chirality of the initial synthon. Removal of the protecting groups can be effected in very mild conditions, giving excellent yields of highly substituted amino acids in high diastereomeric purity.