TAXOL TOTAL SYNTHESIS - PREPARATION OF A CHIRAL RING-A MOIETY VIA BIOMIMETIC CYCLIZATION AND EVALUATION OF A TANDEM NITRILE OXIDE STRATEGY FOR RINGS-B/C

A fully functionalized taxol ring A moiety 7 was efficiently prepared via biomimetic cation-olefin cyclization of chiral geraniol epoxide 2b using SnCl4 in toluene. Fractional crystallization provided endo-3 (50% yield from 2b) that was converted to aldehyde 5 by stereospecific epoxidation, secondary alcohol protection, and PCC oxidation. NaOMe-mediated ring opening secured enal 6. Addition of lithium dithiane to 6 at low temperature provided 7 as the sole product in good yield. A tandem nitrile oxide cycloaddition strategy for creating the remaining B/C carbocycles as well as key functionality present in both rings was validated, in part, by cyclization of 14 to tricyclic isoxazoline 15.