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The phenylhydrazones of an unsaturated d-seco-pregnene aldehyde underwent BF3·OEt2-induced intramolecular 1,3-dipolar cycloaddition to afford new pyrazoline-fused Δ5-androstene derivatives under extremely mild conditions. The ring closures occurred stereoselectively in good to excellent yields via the corresponding azomethine imine intermediates and showed marked dependence on the electronic feature of the p-phenyl substituent.
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Frank et al. (2007) studied this question.
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