Total synthesis of (-)-epibatidine using an asymmetric Diels-Alder reaction with a chiral N-acylnitroso dienophile

An asymmetric total synthesis of(-)-epibatidine (1), isolated from the skin of the Ecuadorian poison frog, Epipedobates tricolor, of the family Dendrobatidae, has been achieved by virtue of the development of asymmetric hetero Diels-Alder (D-A) cycloaddition with an N-acylnitroso dienophile bearing the optically active 8-arylmenthol as a chiral source. Thus, in situ oxidation of the hydroxamic acid ent-laf incorporating the (1S,2R,5S)-8-(2-naphthyl)menthyl auxiliary was performed using the Swern conditions to produce the acylnitroso dienophile, which reacted at once with 2-chloro-5-(1,5-cyclohexadienyl)pyridine (7) to provide the (1S,4R)-meta-aza cycloadduct 24 as a major diastereoisomer. The observed facial diastereoselectivity is consistent with a transition-state model with the naphthyl group in "stacked" position and with the acylnitroso group in the s-cis conformation, wherein pi attractive interaction between the naphthyl and nitrosocarbonyl groups may contribute to facial control. Compound 24 underwent hydrogenation followed by removal of the chiral auxiliary with LiH2NBH3 and reductive cleavage of the N-O bond with Mo(CO)(6) to give the amino alcohol derivative 29, which was converted to (-)-epibatidine via bromination followed by cyclization.