Synthesis and epimerization of phenylalanyl 4-aminocyclophosphamides

Peptide and amino acid conjugates of (4R)- and (4S)-4-aminocyclophosphamides (4-NH₂-CPA, 3) were designed as prodrug forms of phosphoramide mustard. Four diastereomers of Boc-Phe-4-NH-CPA (6) were synthesized stereospecifically from homoserine (R or S) and the protection strategy was optimized for the homoserine hydroxyl group during the construction of the 1,3,2-oxazaphosphorinane ring. The Phe-4-NH-CPA isomers of the trans-configuration ((2S,4R)- and (2R,4S)-) were found to be less stable than the corresponding isomers of the cis-configuration ((2R,4R)- and (2S,4S)-) and to undergo epimerization of the C-4 chiral center in the presence of 25% TFA used during Boc deprotection. The synthetic route developed should be applicable to the synthesis of a variety of peptide and amino acid conjugates of 4-aminocyclophosphamide.