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 4β,5α‐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‐d6‐octane‐4,6‐dione‐d3 [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.