Sesquiterpenoids. Part XIV. X-Ray study of 13β-p-bromophenylthio-11α,13-dihydropulchellin-C diacetate (2α,3β-diacetoxy-13β-p-bromophenylthio-11α,13-dihydroisolantolactone): stereochemistry of addition of thiols to sesquiterpenoid α-methylene γ-lactones

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Abstract
The molecular structure of the p-bromobenzenethiol adduct of the sesquiterpenoid pulchellin-C diacetate (2α,3β-diacetoxyisolantolactone) has been determined by X-ray diffraction. In the formation of the adduct addition of the hydrogen atom to C(11) of the α-methylene γ-lactone took place cis to the hydrogen atom at C(8), causing the bulky RSCH2-group at C(11) to adopt a quasi-equatorial orientation on the envelope-like lactone ring. The cyclohexane ring A approximates closely to an ideal chair form with torsion angles in the range 56–62, mean 60°, but ring B, because of cis-fusion to the γ-lactone, is considerably distorted from ideal chair form and has torsion angles in the range 41–64, mean 51°. The adduct crystallizes in the orthorhombic space group P212121, with Z= 4, and a= 11·076(3), b= 7·194(2), c= 31·189(6)Å. The structure was solved, from X-ray data measured on an automatic diffractometer, by Patterson and Fourier methods, and refined by least-squares techniques to R 6·0% for 1980 reflections. The absolute configuration of the sesquiterpenoid was established by the anomalous-dispersion effect.

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