Stereochemistry and Regiochemistry of Electron Impact, Photolytically, and Thermally Induced Eliminations from 5α-Cholestanyl Acetates

Article abstract of DOI:10.1021/jo00175a009

Deuterium-labeled compounds were used to define the sterochemistry and regiochemistry of the electron impact induced eliminations of acetic acid from 5α-cholestanyl 3α-acetate, 4α-acetate, and 6α-acetate.Comparison of the electron impact induced eliminations to the pyrolysis and to the photolysis of the corresponding phenylacetates confirmed that the mass spectral elimination was a stepwise process proceeding through the stable chair conformation of the steroid's cyclohexyl ring.The equatorially oriented 4α- and 6α-acetates fragmented with predominant loss of asecondary trans-equatorial hydrogen, rather than the tertiary cis-axial hydrogen, despite the a priori greater migratory aptitude of tertiary hydrogens.The electron impact induced fragmentation of the 3α-acetate occured with predominant loss of a C-4 hydrogen; in contrast, the photolysis of the corresponding 3α-phenylacetate results in loss of a C-2 hydrogen.This results can be attributed to the reversibility of the photolytically induced hydrogen-abstraction step.