Stereoselective synthesis of the 5′-hydroxy-5′-phosphonate derivatives of cytidine and cytosine arabinoside

Both the (R)- and (S)-5′-hydroxy 5′-phosphonate derivatives of cytidine and cytosine arabinoside (ara-C) have been prepared via phosphite addition or a Lewis acid mediated hydrophosphonylation of appropriately protected 5′-nucleoside aldehydes. Phosphite addition to a cytosine aldehyde protected as the 2′,3′-acetonide gave predominately the 5′R isomer, while phosphite addition to the corresponding 2′,3′-bis TBS derivative favored the 5′S stereochemistry. In contrast, phosphite addition to the 2′,3′-bis TBS protected aldehyde derived from ara-C gave only the 5′R adduct. However, TiC₄-mediated hydrophosphonylation of the same ara-C aldehyde favored the 5′S stereoisomer by a 2:1 ratio. Once all four of the diastereomers were in hand, the stereochemistry of these compounds could be assigned based on their spectral data or that obtained from their O-methyl mandelate derivatives. After hydrolysis of the phosphonate esters and various protecting groups, the four α-hydroxy phosphonic acids were tested for their ability to serve as substrates for the enzyme nucleoside monophosphate kinase and for their toxicity to K562 cells.