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The mechanism of the formation of 3 and 4 can be interpreted by the front side neighbouring group participation. We assume that the first step of the transformation of 2 is the formation of the monoacetate 13. This compound is cyclised in acidic medium to the orthoester 14, which is converted by HBr to the ambident 1,3-dioxane-2-ylium ion (15) and stabilized in the form of 16b-bromomethyl-3-methoxyestra-1,3,5(10)-trien-17b-acetate (4). The same mechanism may be supposed for the reaction starting from 1 and resulting 3. Scheme 2
A reversed phase, isocratic HPLC method has been developed for separation of the reaction products. The structure of the isolated and purified compounds were established by NMR spectroscopy.