Barton esters selenium

The radical substitution at selenium is efficient when the leaving radical is stabilized (e.g. benzyl, tert-butyl). This reaction has been used for the preparation of selenium containing heterocycles [117]. In Eq. 50, the conversion of the Barton ester 156 to the tetrahydroselenophene derivative 157 is described [118, 119]. Mechanistic studies and ab initio calculations indicate that the substitution does not involve hypervalent selenium derivatives. 

Selenopenams have been synthesized via free radical homolytic substitution of aryl or alkyl radicals at the selenium atom. Refluxing the Barton ester 119 R = Me in benzene gave the selenocycle 120 R = Me in low yield. Alternatively, if 119, R = H, is irradiated (250W, tungsten lamp) the selenopenam 120 R = H is produced in moderate yield. 

The reaction between benzyne derivatives and selenium analogues of Barton s thiopyridone esters provided a convenient entry into complex, fused benzo[3]selenophenes <2004JHC13, 2004ARK51>. For example, the generation of the benzyne 86 in the presence of selenoester 85 provided benzo[. ]seleno[2,3-. ]pyridine 87, presumably via a single electron transfer (SET) pathway (Equation 11). This methodology was examined utilizing a number of benzyne precursors (anthranilic acids, iodium triflates, and trimethysilyl triflates) and provided access to an impressive number of fused benzo[. ]selenophenes. 

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