Synthesis of indole alkaloid analogues containing the novel hexahydropyrrolo[1′,2′,3′:1,9a,9]imidazo[1,2-a]indole skeleton by ring-closing reactions of tryptophan-derived α-amino nitriles

New indole alkaloid analogues, containing a 10b-methyl- or a 10b-hydroxy-1,2,4,5,10b,10c-hexahydropyrrolo[1′,2′,3′:1,9a,9]imidazo[1,2-a]indole skeleton, have been obtained by highly stereoselective electrophile addition–cyclization reactions of a tryptophan-derived α-amino nitriles.