22 Synthesis of Hommothallin II
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The fungus Trichoderma koningii Oud. agg. produces volatile metabolites which stimulate sexual reproduction in one of the two mating types of phytopathogenic fungus Phytophthora cinnamomi Rands. The compounds responsible for this phenomenon have been identified as the novel isocyanides, homothallins I (1) and II (2). A model compound (3) was synthesized from 5 by formylation followed by dehydration with phosgene/triethylamine to examine the chemical properties of the vinylogous acyl-isocyano group. The product is very unstable and easily polymerizes, clarifying the extreme instability of 2. Homothallin II (2) was synthesized from 1,2,4-cyclopentantrione in six steps. Selective hydrolysis of 8 was not successful, but the amino compound (11) gave an enaminoketone (13) in a good yield. Vinylation of 13 gave 15 in a very low yield. An alternative synthesis of 15 was achieved as follows. Alkylation of 4 with acetaldehyde followed by dehydration yielded 19 as a main product. Compound 19 was not oxidized with H_2O_2 but with m-chloroperbenzoic acid to the epoxide (21). Opening of the epoxide with a selenolate anion gave a selenide (22) as well as a considerable amount of a diastereoisomeric mixture of 16. Amination followed by formylation gave 24. Oxidative elimination reaction of 24 followed by dehydration by the phosgene method yielded, though in a very low yield because of polimerization, (±)-homothallin II (2).
