Mass spectral fragmentation of 18-norsteroids and 12-oxo-20-hydroxysteroids

Comparison of the mass spectra of 18-norsteroids with those of the corresponding normal steroids revealed that the absence of the 18-methyl group has little effect on the fragmentation pattern, especially on the D-ring and side-chain cleavage. The results indicate that the stability-of the cation or radical species, due to the quarternary nature of C-13, is not a major driving force in the key 13-17 cleavage. Hydrogen transfer observed in the D-ring fragmentation of 20-hydroxy-18-nor-5 alpha-pregnan-13-ones was not found to be specific to the 18-norcompounds. Deuterium labeling at the 20-OH and 20 alpha-H has suggested the involvement of a unique triple hydrogen rearrangement in this major cleavage. Migration of the 20-hydrogen to the 12-keto oxygen was also observed in the fragmentation of dammarane derivatives and was likewise confirmed by deuterium labeling.