Synthese der Phoracantholide K, O und M

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Abstract
Synthesis of Phoracantholide K, O and MTwo 14membered ring lactones 3 and 4 and a 12membered ring lactone 5 isolated from the metasternal secretion of the eucalypt longicorn Phoracantha synonyma have been synthesized as racemic mixtures by the following method. Reaction of the dilithium derivative of 5‐hexynoic acid (6) with threo‐8‐bromo‐2,4‐isopropylidenedioxyoctane (7), followed by removal of the protecting group and esterification with diazomethane gave methyl‐threo‐11, 13‐dihydroxy‐5‐tetradecy‐noate (8) (s. Scheme 2). Partial hydrogenation of the triple bond in 8 with Lindlar Pd‐catalyst, followed by saponification lead to (threo, Z)‐11, 13‐dihydroxy‐5‐tetra‐decenoic acid (10). The dihydroxy acid 10 was converted into the S‐(2‐pyridyl) thioate and cyclized in diluted benzene solution under the influence of silver ions to yield the corresponding 12‐ and 14‐membered lactones in approximately equal amounts. Isomerization of the mixture with p‐toluenesulfonic acid in methylene chloride yielded the 14‐membered lactone (cis‐11, 13–5Z)‐11‐hydroxy‐5‐tetradecen‐13‐olide almost exclusively. It proved to be identical in its properties with natural phoracantholide K (3). With 5‐hexynoic acid and 7‐tetrahydropyranyloxy‐octyl bromide or 5‐tetrahydropyranyloxy‐hexyl bromide as starting materials (±)‐phoracantholide O (4) and M (5) have been synthesized in an analogous manner.