Stannyl radical-mediated cleavage of pi-deficient heterocyclic sulfones. Synthesis of alpha-fluoro esters

Treatment of ethyl 2-(pyridin-2-ylsulfonyl)hexanoate with tributylstannane and azobis(2-methyl-2-propanitrile) (AIBN) in benzene at reflux for 36 h re sulted in hydrogenolysis to give ethyl hexanoate (60%), whereas no reaction was observed after 48 h at reflux with ethyl 2-(phenylsulfonyl)hexanoate. Ethyl 2-(pyrimidin-2-ylsulfonyl)hexanoate underwent quantitative hydrogenol ysis within 1 h under these conditions. This represents a mild new methodol ogy for removal of the synthetically useful sulfone moiety. Substitution of Bu3SnD for Bu3SnH gave access to alpha-deuterium-labeled esters. Treatment of the alpha-(pyrimidin-2-ylsuffonyl) enolates derived from several esters with Selectfluor gave high yields of the 2-fluoro-2: (pyrimidin-2-ylsulfon yl)alkanoates, which were smoothly desulfonylated [Bu3SnH (2 equiv)/AIBN/be nzene/Delta] to give 2-fluoroalkanoates. "Catalytic" tin hydride, generated from tribuytltin chloride (0.15 equiv) and excess polymethylhydrosiloxane in the presence of potassium fluoride, also effected removal of the pi-defi cient alpha-(pyrimidin-2-ylsulfonyl) moiety from acid derivatives in high y ields. Desulfonylation is suggested to proceed via alkoxy ketyl-type radica ls and tin enolates.