CO2 AS A BUILDING-BLOCK FOR SYNTHESES - S IDE-CHAIN SYNTHESIS OF STEROIDAL 17-BETA-PENTENOIC ACIDS VIA OXIDATIVE COUPLING OF CO2 AND STEROIDAL DIENES ON NICKEL(0) COMPOUNDS

The 2-substituted steroidal butadienes 2, obtained from the aldehydes 1 by Wittig reaction, react smoothly with (bipy)Ni(COD) and CO2 to giv e the nickelalactones 3 in high yields. Protonation and esterification of 3a gives the 4-substituted methyl 3-pentenoate 4a. Ultrasound-acti vated cross coupling of 3a with cyclohexyliodide gives the 4-substitut ed 5-cyclohexyl-3-pentenoic acid Sa. These results demonstrate the syn thetic potential of the metalla-ring closure reaction between CO2 and dienes at nickel(0): the regioselectivity is high and strongly depends on the steric hindrance of the 2-substituent, and reactive functional groups (OH, COOR) are tolerated.