Restricted Rotation Involving the Tetrahedral Carbon. I. Some Diels-Alder Adducts Derived from 9-Substituted Anthracenes

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Abstract
9-t-Butylanthracene was treated with dimethyl acetylenedicarboxylate and benzoquinone to give Diels-Alder adducts. The NMR spectra of these adducts suggest that the internal rotation about the C1–CBu bond is not taking place on the NMR time scale. On the other hand, a Diels-Alder adduct prepared from 9-isopropylanthracene and dimethyl acetylenedicarboxylate shows the coalescence phenomenon of the NMR signals when the temperature is raised. The possibility of optical resolution due to hindered rotation about the sp3–sp3 carbon bond is pointed out.