33 ピロネチンの合成研究(口頭発表の部)

抄録

Pironetin 1 (PA-48153C) is a novel unsaturated δ-lactone derivative isolated from the fermentation broths of Streptomyces species which shows immunosuppressive and plant growth regulating activities. Several total syntheses of 1 have been reported to date, but we show here a novel approach to 1 via a convergent route using our chiral building block, (1S,5S,6R)-5-hydroxybicyclo[4.1.0]heptan-2-one 5. We dissected pironetin at C6-C7 and employed the coupling of the epoxide 16 with the dithiane 25, which would be derived from 5, as a key step. The THP ether of 5 was methylated and the following reduction gave an inseparable mixture of four diastereomers 8. After etherification, removal of THP group afforded separable isomers 9a and 9b in a ratio of 13 to 1. The desired major isomer 9a was oxidized to ketone 6. Oxidation of 6 via silyl enol ether with osmium tetroxide gave ketol 21 in quantitative yield. Reduction of cyclopropylketone moiety of 21 under Birch condition afforded a mixture of 22a and 22b. The ketol moiety of 22a and 22b was oxidatively cleaved with lead tetraacetate in benzene-methanol to give the single aldo-ester 23. Acetalization of 23 followed by reduction of ester group gave aldehyde 24. Introduction of C2-unit via Takai's protocol as E-olefin formation followed by thioacetalization afforded the dithiane 25, as C7-C14 unit of pironetin. Meanwhile, the epoxide 16 as C1-C6 unit was prepared from known epoxy alcohol 3 in 6 steps. Coupling reaction of two units, the dithiane 25 and the the epoxide 16, successfully gave the product with the whole skeleton 26 in high yield. Mercuric ion assisted hydrolysis of the thioacetal 26 gave ketol 27. Selective reduction of β-hydroxyketone 27 by Mori's method afforded and-diol 28a predominantly (91: 9). Finally, desilylation and oxidation with manganese dioxide afforded pironetin 1.