Über die absolute Konfiguration der Visnagane

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Abstract
(+)‐cis‐Khellactone methyl ether (4) and (−)‐trans‐khellactone methyl ether (6) had earlier been assigned the absolute configurations 3′‐S; 4′‐S and 3′‐S; 4′‐R, respectively, on the basis of the FREUDENBERG, rule. Both compounds together with their defunctionalised derivatives (−)‐7 and (+)‐8 (=(+)‐lomatin), obtained from a mixture of (+)‐visnadin (1) and (+)‐samidin (2), were investigated by the HOREAU method. A conformational analytical study showed that the optical yield should rise in the order 4 < 6 < 7, 8. This order was found and the α‐phenylbutyric acid liberated was always dextrorotatory. The centre 3′ of the khellactones and their derivatives must be R‐chiral and not S. Treatment of (−)‐6 with pyridinium perbromide gave (−)‐trans‐3‐bromokhellactone methyl ether (11) as orthorhombic crystals. The X‐ray crystal structure determination was made using the anomalous scattering of the Mo‐K α radiation by Br. The result, — centre 3′ R‐chiral (fig. g) — showed that the HOREAU method was correct.