Abstract
In our continuing studies of synthesis of Amaryllidaceae alkaloids, an alternative synthesis of (±)-lycorine was accomplished. Intramolecular Diels-Alder reaction of triene-ester(6) gave cis-δ-lactone(7a) in 86% yield, which was converted to 10 in six steps. Jones oxidation of 10, followed by Curtius rearrangement afforded a carbamate(12), which was treated with TFA and succesively with NaOMe to give 13 in good yield. Epoxidation of t-butyldimethylsilyl ether of 13 proceeded stereoselectively to give 16 as a sole product. In desilylation of 15 with n-Bu_4NF, Payne rearrangement occurred to give two isomeric epoxy-alcohols(16a,b), acetylation of which afforded 17a and 17b, respectively. The same rearrangement was observed in the reaction of acetate(17a) with aq. K_2CO_3 in MeOH. Furthermore, 17b was converted to 18a by the treatment with phenyl selenide. Deselenylation of 18a in a usual manner afforded 19. Although attempts to construct a B ring by cyclization of 19 was fruitless, cyclization of 18a by N-hydroxy-methylation, followed by TFA treatment gave 22. Finally, 22 was treated with NalO_4 to give 23, whose ^1HNMR spectrum was identical with that of an authentic sample prepared for conversion to (±)-lycorine by Sano et al.
