 This is prevented, with retention of activity, by methylation to give the C-15 tertiary carbinol series.This molecular feature is readily introduced at the stage of the Corey lactone (1) by reaction with methyl Grignard reagent or trimethylaluminium.The resulting mixture of tertiary carbinols (2) is transformed to oxytocic carboprost (3) by standard transformations, including sepoaration of diastereomers, so that the final product is the C-15 analogue.