Hydroxyl-Directing Effects on -Sigmatropic Hydrogen Migrations

Article abstract of DOI:10.1021/jo00300a015

Previous stereomechanistic investigations of thermally induced <1,7>-sigmatropic shifts of cis-isotachysterol analogues 8 and 14 revealed that an allylic hydroxyl exerts a syn-directing effect on the helicity of this antarafacial process.Studies of cis-isotachysterols 17 and 18, wherein the allylic hydroxyl control element at C₁ is relocated to a new position on the steroid, namely C₄, were undertaken to develop a better understanding of this eight-electron pericyclic reaction.The rearrangement in isooctane at 98.4 deg C of 17 to 24 and of 18 to 26 and 27 and their equilibrations were studied quantitatively.The results reveal that the hydroxyl syn-facial directing effect on the antarafacial helicity of this rearrangement is retained for 17 and 18 and that the magnitude of this ?-facial selectivity is similar to that observed for 8 and 14.