Eremophilanes, Vetispiranes, Valeranes, and Cyperanes

Angular methyl migrations of this type have been observed in the heterolyses of 5a-substituted steroids, a process known as the Westphalen rearrangement 234), As an example, the nitrous acid deamination of diacetoxy amine 148 is shown below 235), The presence of an electronegative substituent in the 6P-position is apparently usually a prerequisite for the Westphalen rearrangement. 

Since lactonization of 149 (R = H) with a deuterium located at position 2 occurred without loss of the label, a mechanism involving simple epimerization of the carboxyl group is excluded. Further the facile lactonization of unsaturated spiro acid 152 to the isomeric decalin y-lactones tends to support a mechanism for the lactonization of 149 (R = H) via a spiro intermediate. 

Dunham and Lawton have suggested an alternative biogenesis for the eremophilane sesquiterpenes via a series of four decalyl spiro rearrangements, a process which effectively shifts the position of the methyl group from one angular position to the other (238). This proposal has the disadvantage of requiring a cw-fused eudesmane precursor, a known (occidentalol), but rare stereochemical variant, and, at least in the case of capsidiol is excluded by recent biosynthetic results (229). 

Both spiro and methyl migration rearrangements are proposed to occur when the pentamethyloctalin 153 is subjected to reaction with sulfuric acid in acetic acid (239). Isomers 154 and 155 are formed by decalyl spiro rearrangement followed by angular methyl shift and elimination. The minor product 156 however, arises by direct methyl migration. 

The biogenesis of the relatively rare valerane and cyperane (e.g., faurinone and cyperolone) sesquiterpenes involves rearrangement of a eudesmane precursor by shift of the C4 methyl group or ring contraction to a hydrindane nucleus, respectively. Since all of the known natural products bear oxygen functions at the site of rearrangement, an epoxide precursor is usually proposed. 

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