Organocatalytic asymmetric synthesis of bioactive hexahydropyrrolo[2,3-b]indole-containing tetrasubstituted allenes bearing multiple chiral elements

An organocatalytic asymmetric construction of hexahydropyrrolo[2,3-b]indole-containing tetrasubstituted allene scaffolds bearing both axial chirality and central chirality has been established via a cascade 1,8-addition/dearomatization-cyclization reaction of para-aminophenyl propargylic alcohols with tryptamines in the presence of chiral phosphoric acid (CPA), thus affording a wide range of such tetrasubstituted allenes bearing multiple chiral elements in generally good yields (up to 94%) with high diastereo- and enantioselectivities (up to 95:5 dr, 95% ee). In addition, the evaluation on the cytotoxicity of some selected products indicated that this class of chiral tetrasubstituted allenes could inhibit the growth of the pancreatic cancer cells to some extent. This work not only solved the challenging issues in enantioselective construction of tetrasubstituted allenes via alkynyl (aza)-para-quinone methides, but also represents the first example of using alkyne derivatives as electrophiles in organocatalytic asymmetric dearomatizations of tryptamines.