Caloundrin B and Funiculatin A: New Polypropionates From Siphonariid Limpets

Two new polypropionates, caloundrin B (11), systematic name (1″R,2R,3″R,4R,5S,5″R,6S,-7″R,8″S,9″S,10″R)-2-(6′-ethyl-3′,5′-dimethyl-4′-oxopyran-2′-yl)-6-(5″-ethyl-7″-hydroxy-8″,9″,10″-trimethyl-2″,4″,6″-trioxatricyclo[3.3.1.13,7]dec-3″-yl)-5-hydroxy-4-methylheptan-3-one, and funiculatin A (12), systematic name (2ξ,1′S,4R,5′S,6′S)-2-(1′-ethyl-4′,6′,8′-trimethyl-2𔈀,9′-dioxabicyclo[3.3.1]nona-3′,7′-dien-3′-yl)-4,6-dimethylnon-6-en-3-one, have been isolated from Siphonaria zelandica and S. funiculata respectively. The structures of the new compounds were deduced by two-dimensional n.m.r. spectroscopy, particularly long-range 13C- 1H correlation spectroscopy (HMBC), by biosynthetic reasoning and by comparison with the known polypropionates denticulatin A (5), siphonarin B (8), muamvatin (9) and baconipyrone C (14). Information about the relative stereochemistry of the tricyclic ring system of (11) and the bicyclic ring system of (12) was deduced from coupling constant values, and by n.O.e. difference and NOESY experiments, and was confirmed by molecular modelling studies. The relative stereochemistry of the side chains and the absolute stereochemistry were inferred from biosynthetic comparison with the above known polypropionates, and by correlation of funiculatin A with denticulatin A (5) of known absolute stereochemistry. The stereochemistry at C10 of funiculatin A (position 2 of the non-6-en-3-one chain) could not be unambiguously determined. A third new polypropionate funiculatin B, epimeric with funiculatin A at C10, was isolated and partially characterized.