Synthesis of Diversely Functionalized Hexahydropyrrolo[2,3-b]indoles Using Domino Reactions, Olefination, Isomerization and Claisen Rearrangement Followed by Reductive Cyclization

Hexahydropyrrolo[2,3-b]indoles 6 were synthesized in five steps from indolin-3-one 8 by a general and efficient method, in which elements of molecular diversity were readily added onto the 3a-position of the pyrrolo[2,3-b]indole ring system. Horner−Wadsworth−Emmons reaction of 2-allyloxyindolin-3-ones 7, derived from indolin-3-one 8 and a variety of allylic alcohols, smoothly proceeded with successive Claisen rearrangement to give the corresponding 3-allyl-3-cyanomethylindolin-2-ones 15. Indolin-2-ones 15 were converted into pyrrolo[2,3-b]indoles 6 using partial hydrolysis followed by reductive cyclization with LiAlH4. Synthesis of N-methylated pyrrolo[2,3-b]indole derivatives 23 and 26 is also described.