Intramolecular Diels‐Alder Reaction with Furan‐Diene. 1. A Novel Entry to the BCD‐Ring System of 11‐Ketosteroids

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Abstract
A novel approach for the synthesis of the BCD‐ring system in steroids is described. The sequence centers about the intramolecular Diels‐Alder reaction of furan 15, which, depending on the reaction conditions, leads predominantly to adduct 26 (kinetic control) or 27 (thermodynamic control). The precursor 15 is obtained from enol ether 2, prepared via dissolved metal reduction of 5. Only in reduced adduct 34 is the oxygen bridge opened without concomitant epimerization at C‐14. The synthesis of other dienophiles (17, 19, 22 and 25) and their Diels‐Alder reactivity is discussed.