Synthesis of 3‐Methoxy‐16α‐nitro‐14,17‐ethenoestra‐1,3,5(10)‐trien‐17β‐yl Acetate and Fragmentation‐Mediated Pathways to 14β,15β‐Fused N ‐Heterocycles and 14β‐Functionalised Alkyl Derivatives

The Diels–Alder reaction of 3‐methoxyestra‐1,3,5(10),14,16‐pentaen‐17‐yl acetate ( 1 ) with nitroethylene gives 3‐methoxy‐16‐nitro‐14,17‐ethenoestra‐1,3,5(10)‐trien‐17β‐yl acetate ( 2 ). Cycloadduct 2 can be reduced with TiCl 3 , giving oxazine 3 as a result of sigmatropic rearrangement, whereas the treatment of 2 with Lewis acids (TiCl 4 , SnCl 4 ) leads to a derivative of the cyclic hydroximic acid 10 ; the latter is viewed as a product of intramolecular rearrangement. Cycloadduct 2 suffers a weak base‐induced cleavage of the C(16)–C(17) bond, releasing the nitrile oxide intermediate, which can be trapped by a dipolarophile or reduced with triphenylphosphane. Thus, the trapping of the nitrile oxide with propargyl alcohol gives the isoxazole 12 in 50 % yield, and the reduction of the nitrile oxide with triphenylphosphane gives nitrile 9 in 94 % yield. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)