APPROACH TO 6A-EPIPRETAZETTINE AND 6A-EPIPRECRIWELLINE VIA AN INTRAMOLECULAR 2-AZAALLYL ANION CYCLOADDITION REACTION

An intramolecular 2-azaallyl anion cycloaddition with a diene produced the 2,3,4,6,7,7a-hexahydroindol 33 with complete control of relative stereochemistry, thus providing the first example of the e use of such a cycloaddition in an approach to a relatively complex target molecul e. The 2-azaallyl anion was generated by tin-lithium exchange of the ( 2-azaallyl)stannane 32. The stannane was prepared by a convergent rout e using a Stille coupling of the vinyl bromide 14 with the vinylstanna ne 28, providing the diene 29. An unexpected isomerization occurred in the Stille coupling. Transformation of the cycloadduct 33 to the ally lic methyl ethers 36 and 37 produced potential precursors of 6a-epipre criwelline (4) and 6a-epipretazettine (3), respectively. The inability to carry out an oxidative desilylation thwarted the completion of the syntheses of these alkaloids.