The Westphalen and Backbone Rearrangements

Leboeuf, A. Cave, and R. Goutarel, Bull. Soc. chim. France, 1969, 1628. 

The 10 -ethyl derivative (552) undergoes Westphalen rearrangement 5.3 times more rapidly than the 10/5-methyl compound, probably indicating 10 8-alkyl participation in ionisation at C-5. The need for such alkyl participation explains the failure of 5j8-hydroxy-steroids to undergo Westphalen rearrangement. 

The factors controlling the formation of Westphalen (A ) products, and the products of partial [A or or total backbone rearrangement are 

The differing effects of various polar substituents in rings a and b are only partially understood. Under normal Westphalen conditions the 4jS,5(X-diol 

4-acetate (555) affords the A -compound (556 60%), the A -compound (557 14%) and the A -compound (558 3 %), whereas a 6j5-substituted compound gives no products of rearrangement beyond C-9. The absence of an electronegative 6 -substituent presumably favours migration of the centre of positive 

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Contraction and Expansion of Steroid Rings The Westphalen and Backbone Rearrangements Epoxide Rearrangements Aromatization... 

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