Highly Diastereo- and Enantioselective Synthesis of Cyclohepta[b]indoles by Chiral-Phosphoric-Acid-Catalyzed (4+3) Cycloaddition.

A highly enantio- and diastereoselective formal (4+3) cycloaddition of 1,3-diene-1-carbamates with 3-indolylmethanols in the presence of a chiral phosphoric acid catalyst is reported. The approach described herein provides efficient access to 6-aminotetrahydrocyclohepta[b]indoles in good yields with mostly complete diastereoselectivity and excellent levels of enantioselectivity (>98:2 dr and up to 98 % ee). Mild reaction conditions, facile scale-up, and versatile derivatization highlight the practicality of this methodology. A mechanistic study suggests that cycloaddition occurs in a stepwise fashion, after the formation of an ion pair between the chiral catalytic phosphate and the intermediate carbocation.