Sterpuric acid

Synthetic application includes Paquette s recent s lication in work directed toward the total synthesis of sterpuric acid. Exposure of enol ether (55) to peracid provided a single diastereomer of the silyloxy compound (56) in good yield. It was from this substrate (55) that the first stable trimethylsilyloxy epoxide was obtained (57) and examined by X-ray crystallography. Similarly stereoselective oxygenation of p-keto ester (49) via the corresponding silyl enol ether provided (50), also in 76% yield. Lastly efficient and highly stereoselective a-hydroxylation by this method was employed during studies towards the synthesis of helenanolides (58 to 59). ... 

The silver-leaf disease of fruit trees is caused by the fungus Stereum pur-pureum. The phytotoxic metabolites such as sterpuric acid (5.82) possess the sterpurane skeleton which may also be derived from a protoilludane by a series of rearrangements. 

Support for this mechanism came from an unexpected quarter. In a synthesis of sterpuric acid 215, Paquette planned to put in the alkene at the end by a Julia olefination.35 Addition of MeLi to the ketone 216 and non-stereospecific elimination (chapter 15) looked a good strategy. The 1,2-relationship between OH and ketone in 216 is awkward and the decision was made to try a hydroxylation on the silyl enol ether of the parent ketone 217. 

When the epoxide 219 was treated with even weak acid, or when the silyl enol ether 218 was epoxidised with unbuffered mCPBA, no epoxide could be found. Instead a 76% yield of the silyl ether 220 of the hydroxylated ketone 216 was formed. The stereochemistry of 220 is of course determined by that of 219 and the requirement for ds-fused four- and five-membered rings. Sterpuric acid 215 has the same stereochemistry. 

Santamarine 11)5,13-Dihydro, in S-60007 Shonachalin D, S-70040 Sterpuric acid, S-60053 Temisin, T-80006 Warburganal, W-60001... 

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