Ruthenium-Catalyzed Cycloisomerization−Oxidation of Homopropargyl Alcohols. A New Access to γ-Butyrolactones

Vinylidenemetal species, which readily form from terminal alkynes under mild conditions, have rarely been utilized as reactive intermediates in a catalytic cycle. The conversion of homopropargyl alcohols via such intermediates to metal-complexed oxacarbenes led to the development of an “oxidant” compatible with a ruthenium complex capable of performing the cycloisomerization, that would convert them to lactones. None of the oxidants known to stoichiometrically convert isolated metallooxacarbenes to esters are effective. The unconventional “oxidants”, N -hydroxyimides, proved to be capable of effecting the desired transformation, with N -hydroxysuccinimide being the “oxidant” of choice. The procedure of choice employs cyclopentadienyl (1,4-cyclooctadiene) ruthenium chloride and trifuryl phosphine as the precatalyst in the presence of tetra- n -butylammonium bromide or hexafluorophosphate with N -hydroxysuccinimide as the oxidant in DMF−water at 95°. In this way, a wide diversity of homopropargyl alcohols were converted to γ-butyrolactones with excellent chemoselectivity. Lactones synthesized include an intermediate toward a platelet aggregation inhibitor, a fruit flavor principle, an inhibitor of binding of phorbol esters to PKC-α, a tobacco constituent, a wood constituent (quercus lactone), an aldosterone antagonist (spironolactone) precursor, and an acetogenin known for pesticidal and antitumor activities (muricatacin).