STEREOCHEMICAL CONTROL IN THE ESTER ENOLATE CLAISEN REARRANGEMENT .2. CHAIRLIKE VS BOATLIKE TRANSITION-STATE SELECTION

The preference for chair- and boatlike transition-state geometries in the ester enolate Claisen rearrangement of straight chain, carbocyclic, and heterocyclic propanoates was investigated. A novel stereoelectronic effect in pyranoid and furanoid glycal systems leads to a significant relative stabilization of the boatlike vs the chairlike TS double-ended-dagger. The preferred transition state in six- and five-membered carbocyclic systems is highly dependent on steric factors, as the energy difference between chair- and boatlike TS double-ended-dagger tends to be small. With straight-chain substrates, a significant contribution of the boatlike TS double-ended-dagger to the rearrangement product mixture is only expected in bis-donor substituted allylic esters.