Towards the asymmetric synthesis of (+)-maritidine

Abstract

Two new routes towards the asymmetric synthesis of (+)-maritidine have been proposed and investigated. In both of our routes, the aim was to synthesise the 5,10b-ethanophenanthridone core structure of the target via the tricyclic dihydrodibenzofuran core structure present in galanthamine type compounds. Synthesis of the product in this way would allow formation of the quaternary stereocentre 10b using the efficient and facially selective intramolecular Heck reaction used by multiple groups in the synthesis of galanthamine. Route one attempted the ring opening oxidation of the benzofuran moiety to its respective para-quinone using hypervalent iodine reagents such as (diacetoxyiodo)benzene. From here, reduction of the quinone would be followed by the formation of the final ring structure. Unfortunately, the oxidation only served to form an unwanted tricyclic orthoquinone. The second route employed was inspired by a bidirectional system from Treu et al. for conversion between galanthamine and crinine type alkaloids. Their retro-Michael-Michael reaction however, formed an acyclic intermediate, losing the relevant stereochemistry for the target molecule. We aimed to oxidise the allylc position of the tricyclic scaffold and form a tetracyclic pyrollidine. This intermediate would then be able to undergo a retro-Michael reaction, leaving the stereochemistry of the quaternary centre intact. Progress was made to this end and an advanced intermediate of this route synthesised