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peri-Bridging of naphthalene by a boron atom causes a large distortion in the sp2 bond angles at C1 and C8 in the naphtho[l,8-bc]boret 1. The resulting ring strain is evident in the chemistry of 1: on treatment with electron-poor boron compounds BX3 an unexpected ring expansion to 2 (X = Cl, Br, OEt) is observed.
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Hergel et al. (1994) studied this question.
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