Syntheses of 1-alkyl-1,25-dihydroxyvitamin D3

Article abstract of DOI:10.1021/jo00111a047

1-Alkylated analogs of 1α,25-(OH)₂D₃ were synthesized to investigate the effect of the alkyl group on the A-ring conformation and the biological potency. The analogs were synthesized via two routes. In the first approach, alkylation of 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) adduct of 1-oxoprovitamin D (4) was used as the key step to synthesize 1β-methyl-1α,25-dihydroxyprovitamin D₃ (OH)₂D₃ (16a) efficiently and stereoselectively. The photolysis of the provitamin D (16a), however, gave the desired previtamin D (17a) only as a minor product (<5%) and an unusual 1,10-bond cleavage product (18a) occurred in high yield (79%). As an alternative C(1)-epimeric pairs of 1-alkyl-1,25-(OH)₂D₃ were synthesized conveniently from 25-hydroxy-1-oxoprevitamin D₃ (19) by reaction with an alkyllithium ollowed by thermal isomerization. In the alkylation, the alkyllithium attacked the ketone preferentially from the side of the 3β-hydroxyl group to afford the 1β-alkyl-1α-hydroxy epimer in a 1.6-2.7 to 1 ratio over the 1α-alkyl-1β-hydroxy isomer. Introduction of a 1β-methyl group to 1α,25-(OH)₂D₃, shifted the equilibrium between the two chair conformations of the A-ring preferentially to the side of the α-form (4:1) and reduced considerably the activity to bind to the VDR.