Herein we report that borane reductions of acylpyrroles and acylindoles that contain a pendant phenol take two different paths. Acylpyrroles undergo a reductive cyclization to make unusual chromanyl pyrroles. Treatment of related acylindoles under identical conditions results in deoxygenation without cyclization. The results are interpreted in terms of relative rates of cyclization and reduction of intermediate carbenium ions, where cyclization of the indole-stabilized carbenium ion is slower.
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Li et al. (2008) studied this question.
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