Crystal structures of 10α-gon-5-enes from the synthetic pathway to desogestrel

X-ray crystallographic studies performed on the product of the ketalization reaction of 13β-ethyl-11α-hydroxy-gon-5-ene-3,17-dione have lead to the unequivocal assignment of the 10α stereochemistry to C10, showing that an inversion of configuration occurred during formation of the 3,17-diketal. From the Swern oxidation of this compound, 11α-(methylthio)methoxy-10α-gonene was obtained as the major product instead of the desired 11-ketone. Modeling studies showed that the configurational instability at C10 is determined by the presence of the 11α-hydroxyl group.