Reaction of lithium aluminium hydride with 4,4-dimethyl-4a,5-epoxy-A-homocholestane derivatives and mass spectral fragmentation of some degradation products

Reductive opening of the epoxide ring of stereoisomeric 4,4-dimethyl-4a,5-epoxy-A-homocholestane derivatives with oxygen containing substituent (OH, OCOCH3, OCH3) in the position 3 was investigated. In the absence of other directing effects, than the stereoelectronic ones, the epoxide ring of 4aα,5α-epoxides is opened at the side of the less substituted carbon C(4a), under formation of 5α-hydroxy derivatives, while in the case of 4aβ,5β-epoxides both the cleavage of the C(4a)-O bond, leading to the formation of 5β-hydroxy derivatives, and the cleavage of the C(5)-O bond, leading to the formation of 4aβ-hydroxy-5,6-unsaturated derivatives take place. The participation of the substituent in the position 3 leads to an abnormal cleavage both in 4aα,5α-and 4aβ,5β-epoxides, i.e. to the cleavage of the C(5)-O bond under formation of 3,5-epoxides. The effect of the character of the substituent in the position 3 on the direction of the reductive cleavage of the epoxide ring is also discussed from the point of view of conformational and electronic effects. In mass spectrometry the main product of the fragmentation of the molecular ions of the 4a-hydroxy-3,5-epoxides XIII and XXV and the 4a-ketones XVII and XXVIII is the ion [C23H40O]+. which is formed after the breaking of the C(3)-O bond by the splitting off of the ring A. The fragmentation of the molecular ion of the semi-ketal XVIII is determined by the cleavage of the epoxide bond O-C(5).