Arylimines are converted into 9-azaphenanthrene derivatives (38-46%) when photolyzed in the presence of boron trifluoride etherate. Recently, structurally similar pentacyclic aromatic compounds showing interesting biological activity have been isolated from different sp0nges.l Amphemidine 1, petrosamine 2 and 2-bromoleptoclinidinone 3 represent this novel class of alkaloids. The angular diazaphenanthrene nucleus common to these compounds presents an interesting synthetic challenge. Our efforts in preparing the tricyclic substructure are described herein.
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The stilbene to phenanthrene (Eq.1; X=CH) transformation (4a-6a) is one of the most general and successful photochemical reactions available to the organic chemist.2 However, the simple extension to the imine analog of stilbene 4b fails to produce 9-azaphenanthrene under normal conditions.3a4 TO overcome this problem, researchers conduct the arylimine photocyclization in 98-99% sulfuric acid.5s6 Unfortunately, these harsh conditions limit the functional groups present, complicates workup and is cumbersome to employ. Despite these drawbacks, the arylimine to 9-azaphenanthrene photoconversion 5213