The mass spectrometric behavior of 3α,5-cyclo-5α-cholestan-6β-yl alkyl ethers

Abstract

The 3α,5-cyclo-6β-methoxy (i-methyl ether) moiety is a common protective grouping in steroid partial synthesis. Therefore, the mass spectrometric behavior of 6β-hydroxy-3α,5-cyclo-5α-cholestane, the corresponding methyl, ethyl, and tert-butyl ethers, the analogous ketone, and the parent hydrocarbon has been investigated in order to determine the mechanisms of the characteristic fragmentations of these compounds. Such knowledge is essential for unequivocal structure elucidation by mass spectrometry and also bears on the current interest in the stereospecificity of electron impact induced eliminations. Deuterium labeling of carbon atoms 1,2,3,4,7,8,9, and 19 of 6β-methoxy-3α,5-cyclo-5α-cholestane established the course of methanol extrusion and the identity of the highly diagnostic A ring cleavage ion [Formula: see text] Mechanisms are proposed for these key fragmentations. The mass spectra of the methyl, ethyl, and tert-butyl ethers of cholesterol are analyzed, and the features distinguishing these compounds from the isomeric 3,5-cyclosteroids are noted.