Toward the Development of a General Chiral Auxiliary. 5. High Diastereofacial Selectivity in Cycloadditions with Trienol Silyl Ethers: An Application to an Enantioselective Synthesis of (-)-Cassioside

摘要

As part of a program directed at the development of covalently bound and catalytic controllers for the dias-tereofacially selective construction of carbon—carbon bonds, we have developed a family of terpene-derived bicyclic lactams 1-3 useful as chiral controller molecules in a variety of applications. These controller molecules have proven particularly valuable for the diastereofa-cially selective construction of quaternary carbon centers via [4 + 2] cycloaddition reactions, a significant shortcoming of a number of the most widely employed chiral auxiliaries. Nevertheless, limitations in the scope of cycloaddition methodology using 1-3 were still apparent. Use of highly reactive oxygen-substituted dienes has not been generally possible, although a few isolated cases have been reported, owing principally to the sensitivity of the dienes to the Lewis acids required to catalyze the cycloaddition and to the lower diastereofacial selectivity generally observed. We have addressed these problems by appropriate choice of protecting groups for the dienes and modifications to the Lewis acids that minimize the tendency toward destruction of the diene.