Asymmetric synthesis of a tricyclic core structure of the securinega alkaloids virosecurinine and allosecurinine

A concise asymmetric synthesis of tricyclic core structures of virosecurinine [(+)-] and allosecurinine [(−)-] is presented. An asymmetric electrophilic α-amidoalkylation reaction employing a chiral enamide gave access to enantiopure (S)-2-anisylpiperidine with the diastereoselectivity (d.s.) of 93/7. The latter was transformed into the target compounds, with the main steps involving a Birch-reduction followed by an ozonolysis of the resulting 1,4-cylohexadiene and a final spirocyclization reaction.