On the construction of the C/D ring systems of chasmanine and napelline by diene addition

Abstract

The construction of the C/D ring system of the alkaloids chasmanine and napelline by modification and rearrangement of a common intermediate is explored on model compounds.The phenolic acid 9 was oxidized to the spirolactone 10. Addition of benzyl vinyl ether to this material gave the adduct 28 in high yield. This central 'nordenudatine' type intermediate was then modified (in a very few stereospecific steps) to the 'napelline' model compound 47 and to the keto acetal 38. Compound 38 is an intermediate of our photochemical route and we have already described its conversion to a chasmanine model derivative.The new syntheses of chasmanine and napelline foreshadowed by the present model studies are expected to be completely stereo- and regiospecific, almost 10 steps shorter and an order of magnitude more efficient than the total syntheses of these two alkaloids which we have reported in the recent past.Incidentally, with these model studies, the methoxy ketone 20 was prepared. The methoxyl in this compound is epimeric to the corresponding (C16) methoxyl of the aconite alkaloids. Some relevant observations on the nmr spectra and allylic rearrangement (pyro–isopyro rearrangement) of this material are also reported.