Amphidinolides T2, T3, and T4, New 19-Membered Macrolides from the Dinoflagellate Amphidinium sp. and the Biosynthesis of Amphidinolide T1

Three new 19-membered macrolides, amphidinolides T2 (2), T3 (3), and T4 (4), structurally related to amphidinolide T1 (1) have been isolated from two strains of marine dinoflagellates of the genus Amphidinium. The structures of 2−4 were elucidated on the basis of spectroscopic data. The absolute configurations at C-7, C-8, and C-10 of 1−4 were determined by comparison of NMR data of their C-1−C-12 segments with those of synthetic model compounds for the tetrahydrofuran portion. The biosynthetic origins of amphidinolide T1 (1) were investigated on the basis of ¹³C NMR data of a ¹³C enriched sample obtained by feeding experiments with [1-¹³C], [2-¹³C], and [1,2-¹³C₂] sodium acetates and ¹³C-labeled sodium bicarbonate in the cultures of the dinoflagellate. These incorporation patterns suggested that amphidinolide T1 (1) was generated from four successive polyketide chains, an isolated C₁ unit formed from C-2 of acetates, and three unusual C₂ units derived only from C-2 of acetates. Furthermore, it is noted that five oxygenated carbons of C-1, C-7, C-12, C-13, and C-18 were not derived from the C-1 carbonyl, but from the C-2 methyl of acetates.