CAL-B-Catalyzed Alkoxycarbonylation of A-Ring Stereoisomeric Synthons of 1α,25-Dihydroxyvitamin D<sub>3</sub> and 1α,25-Dihydroxy-19-<i>nor</i>-previtamin D<sub>3</sub>:  A Comparative Study. First Regioselective Chemoenzymatic Synthesis of 19-<i>nor</i>-A-Ring Carbonates Mónica Díaz Vicente Gotor-Fernández Miguel Ferrero Susana Fernández Vicente Gotor 10.1021/jo010017f.s001 https://acs.figshare.com/articles/journal_contribution/CAL-B-Catalyzed_Alkoxycarbonylation_of_A-Ring_Stereoisomeric_Synthons_of_1_25-Dihydroxyvitamin_D_sub_3_sub_and_1_25-Dihydroxy-19-_i_nor_i_-previtamin_D_sub_3_sub_A_Comparative_Study_First_Regioselective_Chemoenzymatic_Synthesis_of_19-_i_nor_i_-A-Ring_Carb/3688779 A comparative study of alkoxycarbonylation processes of both 19-<i>nor</i>-A-ring and A-ring stereoisomers of 1α,25-dihydroxyvitamin D<sub>3</sub> analogues catalyzed by <i>Candida antarctica</i> lipase B (CAL-B) has been described. The presence of the methyl group in the A-ring at C-2, as in <b>3</b>−<b>6</b>, has a determining role in the regioselectivity of the biocatalysis, mainly allowing the hydroxyl group at C-5 position to react. For the 19-<i>nor</i>-A-ring stereoisomers <b>7</b>−<b>10</b>, which lack the C-2 methyl group, the configurations at C-3 and C-5 have a high influence in the selectivity exhibited by CAL-B. Thus, each couple of enantiomers showed opposing regioselectivities depending on the C-3 configuration. When C-3 possesses an (<i>S</i>)-configuration, enzymatic alkoxycarbonylations took place at the C-5-(<i>R</i>) or C-5-(<i>S</i>) hydroxyl groups. However, if the chiral centers at C-3 are (<i>R</i>), CAL-B alkoxycarbonylated the C-3-(<i>R</i>) hydroxyl group independently of the configuration at C-5. The corresponding carbonates are useful A-ring precursors of 1α,25-dihydroxyvitamin D<sub>3</sub> analogues, selectively modified at the C-1 or C-3 positions. In addition, an improved synthesis of <i>cis</i> A-ring synthons <b>5</b> and <b>6</b> is described using a Mitsunobu methodology. 2001-05-17 00:00:00 analogue Candida antarctica lipase B alkoxycarbonylation dihydroxyvitamin configuration hydroxyl group stereoisomer methyl First Regioselective Chemoenzymatic Synthesis