Isorhizochalin: a Minor Unprecedented Bipolar Sphingolipid of Stereodivergent Biogenesis from the Rhizochalina incrustata

Isorhizochalin (1) was isolated as its peracetate from the EtOH extract of the sponge Rhizochalina incrustata. Its structure and absolute stereochemistry were elucidated as (2S,3R,26R,27R)-2,27-diamino-3-O-β-d-galactopyranosyl-oxy-26-hydroxyoctacosan-18-one by extensive NMR, MS studies, chemical transformations, including micromolar-scale Baeyer–Villiger oxidation, and by analysis of CD spectra of isorhizochalinin perbenzoate (2b). Isorhizochalin is an unprecedented C-2 epimer of rhizochalin having an erythro configuration at the glycosylated 2-amino-3-alkanol α-terminus in contrast with a canonical threo configuration for other representatives of this structural group. Probable biogenesis of 1 is discussed in the context of known sphingolipid biosynthesis beginning with condensation of alanine with a fatty acyl CoA thioester. The aglycone, isorhizochalinin (2a), shows cytotoxicity against human leukemia HL-60 and THP-1 cells with IC50 values of 2.90 and 2.20 μM, respectively.