Acyl transfer agents selenol esters

As well as thiol esters, selenol esters (1) frequently exhibit a high and selective reactivity toward nucleophiles, which is enhanced even further by activation with heavy ions or oxidizing agents. These properties make selenol esters valuable acyl transfer agents. This review deals with general methods for the synthesis of selenol esters and their reactivity as acyl transfer agents. Furthermore, selenoesters (2), isomeric compounds of selenol esters, and their derivatives selenoamides (3) are also described. These compounds show the characteristic reactivity based on the carbon-selenium double bond. 

Selenol esters are expected to be a more reactive species than the corresponding thiol esters or 0x0 esters due to the comparatively weak bonding between carbon and selenium. The ability of the selenol esters to serve as active acyl transfer agents has been readily demonstrated by the easy conversion of the selenol ester (34 equation 15) to its corresponding acid (35), ester (36) or amide (37). This acyl-selenium bond cleavage has also been promoted by Cu and Cu" salts. - The isopropylidene derivative (38 equation 16) of ribofuranosylacetate has been converted to a lactone (40) in good yield via the selenol ester (39). 

Scheme 22 Selenol ester as acyl-transfer agent...

A further example of the value of thioesters as acyl transfer agents is the production of esters from these reagents by reaction with an alcohol and N-bromosuccinimide. ° Both thio- and seleno-esters are readily available from free acids via the intermediacy of the acid imidazolide or 1,2,4-triazolide, which react smoothly with thiols or selenols to give the desired esters in high overall yields. Thioesters can also be prepared directly from free acids and thiols by condensation in the presence of either diphenyl phosphorazidate or diethyl phos-phorocyanidate. S-Ethylthioesters can be obtained, generally in high yields, by... 

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