Structure Determination of 1,3-Dioxolane-Containing Lipids from the Marine Sponge Leucetta sp. Using Chiral 1H NMR Analysis of Model Systems

Abstract

Chemical investigation of n-hexane extract from the marine sponge Leucetta sp. led to the isolation of five new lipids, 1−5, each characterized by a substituted dioxolane core. The structures of 1−5 were established based on the interpretation of NMR and HRESIMS data. To assign the absolute configuration at C-1′, model systems consisting of diastereomers at C-2, C-4, and C-1′ of the dioxolane core were prepared from a chiral glycerol dimethylacetal. 1H NMR inspection of model compounds revealed that a pair of C-1′ epimers, 11a/c and 11b/d, was indistinguishable, restricting structural assignment by direct comparison of NMR data. In addition, the lack of chromophores in the dioxolane core resulted in unreliable ECD results, with Cotton effects appearing below 190 nm. As an alternative, a chiral NMR method using Eu(hfc)3 revealed notable lanthanide-induced shifts, allowing the spectroscopic discrimination of 11a/c and ent-11a/c. Therefore, the absolute configuration of all five new lipids was determined to be 2S, 4S, 1′S by direct comparison with the Eu(hfc)3-induced 1H NMR data.