| This thesis describes efficient catalytic enantioselective routes to three compounds containing quaternary carbon stereocenters: the naturally-derived antiulcer substance (+)- cassiol (II-4), the naturally-occurring potent angiogenesis inhibitor ({dollar}-{dollar})-ovalicin (III-1), and a key intermediate (VII-3) in the synthesis of the coffee bean-derived toxin ({dollar}-{dollar})-atractyligenin. These efforts required the mechanistically-guided development of new applications for three catalytic enantioselective reactions.; The synthesis of II-4 is based on the enantioselective Diels-Alder reaction between the trisubstituted electron-rich open-chain diene II-14 and 2-methylacrolein catalyzed by oxazaborolidine II-1d. The high enantioselectivity of the key cycloacidition is the collective result of critical modifications to the substrate, catalyst and reaction solvent.*; The key step in the synthesis of III-1 is a novel application of the Sharpless asymmetric dihydroxylation (AD) in which a 4-methoxybenzoyl group is attached to the allylic alcohol substrate, such as in III-3b, to produce a dramatic enhancement in enantioselectivity. The reaction has been extended to the highly selective dihydroxylation of a variety of allylic 4-methoxybenzoates, homoallylic 4-methoxyphenyl ethers, and other related compounds. The AD of several enantiomerically pure 4-substituted allylic 4{dollar}spprime{dollar}-methoxybenzoates proceeded with excellent reagent-controlled diastereoselectivities, and was applied to the stereocontrolled total synthesis of various vic-polyols. The kinetic resolution of 1-substituted allylic 4{dollar}spprime{dollar}-methoxybenzoates was accomplished with relative rate constants as high as 79 using a specifically designed dihydroxylation catalyst. The mechanistic rational which guided each of these advances is discussed.*; The challenging quaternary carbon center in VII-3 was established by the enantioselective oxazaborolidine-catalyzed (CBS) reduction of {dollar}beta{dollar}-tri-n-butylstannyl enone VII-6c followed by chirality transfer from the resulting secondary alcohol to the desired quaternary site via an Ireland-Claisen rearrangement.*; The findings of these investigations have provided critical information that significantly expands the scope of the catalytic processes studied. ftn*Please refer to the dissertation for diagrams. |
