Structure and stereochemistry of aplyolides A-E, lactonized dihydroxy fatty acids from the skin of the marine mollusk Aplysia depilans

The opisthobranch Aplysia depilans contains in its dorsum five unprecedented C-16 and C-18 fatty acid lactones (2-6). Their structures were assigned on the bases of chemical and spectral methods. Lactone 2 is derived by cyclization of 15(S)-hydroxyhexadeca-4(2),7(2),10(2),13(Z)-tetraenoic acid. The absolute stereochemistry at C-15 was determined by Mosher's method after opening of the lactone ring. Two other lactones (3 and 5) result from the cyclization either at C-15 or C-16 of 15,16-dihydroxyoctadeca-9(Z),12(Z)-dienoic acid. They differ from the remaining pair (4 and 6) by the absence of an additional double bond at C-6. S absolute stereochemistry of the free carbinols in 3-6 was suggested by applying Mosher's method. The same absolute stereochemistry was assigned at all lactonized carbinol centers by isomerization of the lactones by ring opening and subsequent enzymatic cyclization.