Rings, bicyclic, stereoselectivity fragmentation

With compound 64 available, vanadyl acetylacetonate catalyzed epoxidation [44] accompanied by simultaneous cyclization, afforded the corresponding tetrahydrofuran and its diastereomer in a 4 1 ratio (Scheme 12). Ring expansion of the corresponding mesylate 65 with silver (I) carbonate afforded compound 66 in a 42% yield for the two steps [45]. Extension of the side chain in six steps, followed by an asymmetric epoxidation, gave product 67 stereoselectively. The cyclization of 67 with titanium tetraisopropoxide in a manner consistent with model studies [27d], afforded bicyclic ether 68 in 65% yield. Transformation to the epoxide under standard conditions afforded fragment 69 ready to be coupled with the D-ring side chain. 

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