Concise synthetic route to both enantiomeric forms of 2,3,4,4a-tetrahydro[1,3]dioxolo[4,5-j]phenanthridin-6(5H)-one, the tetracyclic skeleton associated with the narcissus alkaloids lycoricidine and narciclasine

Abstract

Both enantiomeric forms, (S)-4 and (R)-4, of the tetracyclic skeleton associated with the title alkaloids 1 and 2 have been prepared; the key step involved silver isocyanate-promoted ring-opening of gem-dibromocyclopropane 5 and trapping of the resulting allylic isocyanate (±)-6 with (–)-menthol.