Synthesis of Functionalized Indole-1-oxide Derivatives via Cascade Reactions of Allenynes and tBuONO.

This paper presents a novel access to 5-oxo-2 H-benzo[ g]indole-1-oxides/functionalized naphthalene-1,2-diones via the cascade reaction of allenynes with alcohols/amines and tBuONO without using any catalyst. Mechanistically, the formation of 5-oxo-2 H-benzo[ g]indole-1-oxides involves a cascade process combining [2 + 2] cycloaddition, 1,6-addition, and ring expansion of the in situ formed cyclobutene intermediate. The construction of naphthalene-1,2-diones should undergo a ring-opening pathway. Moreover, the utility of benzoindole-1-oxides was demonstrated by their easy conversion into pharmaceutically significant 1 H-benzo[ g]indol-5-ol derivatives.