Cyclopropane ketal synthesis

The rational synthesis of 11 started from 4,5-benzocycloheptenone ethylene ketal 16 which was reduced to the dihydrocompound 17 with lithium in liquid ammonia. Cyclopropanation of the latter with dichlorocarbene then gave the adduct 18, the ketal oxygens of 7 7 presumably coordinating with the carbene and directing it... 

Recently reported uses of optically pure stilbene diol in asymmetric synthesis include. (1) the dimethyl ether as a ligand for effecting enantioselective conjugate addition (2) the preparation of a,p-unsaturated ketals for achieving diastereoselective Simmons-Smith cyclopropanation <3 (3) the preparation of enantlomercially pure p-halohydrins i and (4) the preparation of chirai crown... 

A synthesis of the related sesquiterpenoid pentalene, 26, also began with the Weiss-Cook reaction product 12 (R = R" = H). Wittig olefination was carried out on the mono-ketal of 12 followed by hydrolysis of the ketal. The resultant ketone was reduced to the corresponding alcohol and the exocyclic alkene underwent cyclopropanation to produce 23. Oxidation of the alcohol generated a ketone that could be converted to the enone using the... 

Cheap and readily available cyclopentadecanone is alpha-brominated to (30) and ketalised with (29), and the resulting a-bromoketal (31) subjected to base induced elimination of HBr, giving (32). The key asymmetric step is now at hand. The Simmons-Smith cyclopropanation proceeds with very high facial selectivity, giving almost entirely the diastereomer of (33) shown. The result is rationalised on the basis of zinc chelation by the oxygens of the ketal. The synthesis is completed by acid hydrolysis of the ketal (the chiral diol may be recovered for reuse), reductive opening of the cyclopropane ring under Birch conditions, and re-oxidation of the alcohol back to the ketone. 

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