| Abstract
| - New synthetic pathways to 11H-benzo[b]fluoren-11-ols, 1H-cyclobut[a]indenes, and related compounds via biradicals generated from benzoenyne−allenes were developed. Treatment of thediacetylenic propargylic alcohols 13, derived from condensation between benzophenones and thelithium acetylide of 1-(2-ethynylphenyl)-2-phenylethyne, with thionyl chloride produced the 11-chloro-11H-benzo[b]fluorene 14 and, after hydrolysis, the corresponding 11H-benzo[b]fluoren-11-ols 15. The transformation involved a sequence of reactions, including a biradical-forming C2−C6cyclization (Schmittel cyclization) reaction of the chlorinated benzoenyne−allene intermediatesfollowed by an intramolecular radical−radical coupling to form the formal Diels−Alder adducts.Interestingly, in the case of the diacetylenic propargylic alcohol 26, obtained from dibenzosuberenone(25), an intramolecular [2 + 2] cycloaddition reaction of the chlorinated benzoenyne−alleneintermediate occurred, furnishing the 1H-cyclobut[a]indene 27 exclusively. The dramatic changeof the reaction pathway could be attributed to the emergence of a steric strain due to the nonbondedinteractions with the chloro substituent along the pathway toward the formal Diels−Alder adduct31. On the other hand, the non-chlorinated benzoenyne−allene, derived from prototropic isomerization of the diacetylenic hydrocarbon 60, underwent a formal Diels−Alder reaction to furnishthe 11H-benzo[b]fluorene-type hydrocarbon 61 exclusively.
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