1α,25-Dihydroxyvitamin D3 A-Ring Precursors: Studies on Regioselective Enzymatic Alkoxycarbonylation Reactions of Their Stereoisomers. Chemoenzymatic Synthesis of A-Ring Synthon Carbamate Derivatives, Including Carbazates and Polyamino Carbamates

Article abstract of DOI:10.1021/jo990813+

The stereoisomers of 1α,25-dihydroxyvitamin D₃ A-ring synthon 3a, named 3b-d, were subjected to a very comprehensive regioselective enzymatic study with Candida antarctica lipase (CAL). From this, it emerged that 3b, the enantiomer of the natural A-ring synthon, was a very good substrate for CAL in toluene, dioxane, or THF, showing in all cases conversions close to 100% and regioselectivities between 95% and 99% toward the C-5-(S) hydroxyl group. The best results for the regioselective enzymatic transformation of stereoisomer 3c were achieved with toluene at 30 °C or with THF at 60°C. The regioselectivity displayed a preference toward the C-5-(S) hydroxyl group. The 1:10 ratio (3c:4) was mandatory so as to obtain an acceptable degree of conversion (in dioxane or THF). The A-ring synthon 3d has a surprising conduct, suffering C-5-(R) enzymatic alkoxycarbonylation, whereas in the acylation process with Chromobacterium viscosum lipase, it showed behavior opposite to that observed for 3a-c. In addition to the above, an efficient chemoenzymatic synthesis of A-ring synthon carbamate derivatives 15a,c-17a,c, including carbazates 15b-17b, and polyamino carbamates 15d-17d was accomplished by a two-step strategy, involving the regioselective enzymatic synthesis of carbonates 5, 7, and 10, followed by reaction with (poly)amino derivatives 14.