Facile synthesis of diiodoheteroindenes and understanding their Sonogashira cross-coupling selectivity for the construction of unsymmetrical enediynes

Abstract

Electrophile-promoted cyclizations of functionalized alkynes offer a useful tool for constructing halogen-substituted heterocycles primed for further derivatization. Preinstallation of an iodo-substituent at the alkyne prior to iodo-cyclization opens access to ortho di-iodinated heterocyclic precursors for the preparation of unsymmetrical heterocycle-fused enediynes. This general approach was used to prepare 2,3-diiodobenzothiophene, 2,3-diiodoindole, and 2,3-diiodobenzofuran, a useful family of substrates for systematic studies of the role of heteroatoms on the regioselectivity of cross-coupling reactions. Diiodobenzothiophene showed much higher regioselectivity for Sonogashira cross-coupling at C2 than diiodoindole and diiodobenzofuran. As a result, benzothiophene can be conveniently involved in a one-pot sequential coupling with two different alkynes, yielding unsymmetrical benzothiophene-fused enediynes. On the other hand, the Sonogashira reaction of diiodoindole and diiodobenzofuran formed considerable amounts of di-substituted enediynes in addition to the monoalkyne product by coupling at C2. Interestingly, no C3-monocoupling products were observed for all of the diiodides, suggesting that the incorporation of the 1^st alkyne at C2 activates the C3 position for the 2^nd coupling. Additional factors affecting regioselectivity were detected, discussed and connected, through computational analysis, to transmetalation being the rate-determining step for the Sonogashira reaction. Several enediynes synthesized showed cytotoxic activity, which is not associated with DNA strand breaks typical of natural enediyne antibiotics.