CONCERTED APPLICATION OF A SHIFT-REAGENT AND 2D-NOESY TO THE STRUCTURE DETERMINATION OF NEW NATURAL-PRODUCTS FROM THE TROPICAL BROWN ALGA DICTYOPTERIS-DELICATULA

From the marine brown alga Dictyopteris delicatula Lamaouroux, two new (1, 3) and two previously reported sesquiterpenes (2, 4) were isolated and characterized. Compound 1 was identified as 4 beta,5 alpha-dihydroxycubenol and 2 was found to be a non-racemic mixture of (+/-)-torreyol. Compound 3 was characterized as cubenol-3-one and 4 was identified as cubenol. For the assignment of the complete relative stereochemistry of compounds 1 and 2, it was necessary to employ extensive 2D NMR methodologies in combination with the lanthanide shift reagent europium(III) tris(1,1,1,2,2,3,3)-heptafluoro-7,7-dimethyl-d(6)-octane-4,6-dione-d(3) [Eu(fod)(3)]. Anomalous proton shielding effects which occurred on addition of Eu(fod)(3) to compound 1 suggested that classical lanthanide-induced shift (LIS) analysis for predicting proton chemical shifts was not applicable. Application of 2D NOESY measurements to NMR samples of 1 and 2 containing Eu(fod)(3) clearly demonstrated an alternative to classical methods for interpreting lanthanide-induced shifts. The enhanced value of shift reagnents of this type as an aid to structure determination and as for assignment purposes is demonstrated.