Über die absolute Konfiguration der Visnagane

(+)‐ 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 F REUDENBERG , 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 H OREAU 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 H OREAU method was correct.