Synthetic studies toward astins A, B and C. Efficient syntheses of cis-3,4-dihydroxyprolines and (−)-(3S,4R)-dichloroproline esters

Efficient syntheses of the esters of cis-3,4-dihydroxyprolines and the first synthesis of the ester of the rare amino acid (−)-(3S,4R)-dichloroproline from trans-4-hydroxy-L-proline are presented. The synthetic route provides easy access to several substituted prolines and cis-hydroxylated proline esters. Various types of biological activities have been associated with these substituted prolines. The role of (−)-(3S,4R)-dichloroproline in astins A, B and C is under investigation.