Intramolecular free radical cyclisations onto enol ethers. A general synthesis of α-alkyl-β-oxy- and α-methylene-β-oxy-γ-butyrolactones

Radical cyclisation of the enol ether bromoacetals (26), (28a–c), (29), and (37) in the presence of tributylstannane, produces precursors [viz. (30), (31a–c), (32), and (38)] to the β-oxy-γ-butyrolactones (33), (34a–c), (35), and the α-methylene-β-oxy-γ-butyrolactone (39) in high overall yields. By contrast, treatment of (26) and (28a–b) with the cobalt(I) reagent derived from bis(dimethylglyoximato)(pyridine)cobalt(III) chloride (40), followed by oxidation of the intermediates (43) leads to the corresponding unsaturated β-oxy-γ-butyrolactones (42).