Aplisulfamines, Unique Natural Sulfoxides from Aplidium sp.: Assignment of Sulfur and Carbons Absolute Configuration through an Unprecedented Combination of Spectroscopic and Computational Methods

The occurrence of sulfoxides in nature is limited, being confined mostly to peptide derivatives containing methionine- or b,b-dimethylmethionine S-oxides, podolactones, metabolites from onions and from other Allium species as well as from a few marine organisms. In the frame of a research project devoted to the study of secondary metabolites of Mediterranean tunicates, we have isolated two novel natural sulfoxides, aplisulfamines A e B (1 and 2), from an Aplidium sp. collected in the Bay of Naples. In the present communication we report about the isolation and structure determination of these two unique compounds. The assignment of absolute configuration of chiral sulfur and carbon atoms present in the molecules 1 and 2 was achieved by an unprecedented approach based on a combination of NMR experiments and CD studies assisted by computational methods.