Epimerization of esters of stereoisomeric 8-ergolinecarboxylic acids on carbon C(8)

Abstract

Esters of 8β-ergolinecarboxylic acids, I-XI, exposed to strong bases, such as lithium diisopropylamide, in polar aprotic solvents gave enolates, which were decomposed by suitable proton donors to a mixture of epimers. This contained, apart from the starting 8β-esters, the corresponding 8α-esters, Ia-IVa and VIa-XIa (65-80%) and Va (about 16%). Exposure of 8α-ester XIIa to these conditions produced epimerization on C(8) (about 54%) and, to a small extent, isomerization on C(10), affording ester I (c. 1%) and Ia (c. 5%).