Cyclization of alkynyl benzoates and generation of dioxolenylium ions

Article abstract of DOI:10.1021/ja00180a033

Reactions of l-ethynyl benzoates RC≡CO₂CC₆H₄X in H₂O and CH3OH have been studied. In neutral H₂O, reaction rates for CH₃C≡CO₂CC₆H₄X depend on σ_p of X with p of 1.3. Reaction of CH₃C≡CO₂CPh (1) in ¹⁸O-labeled H₂O gives CH₃CH₂CO₂H and PhCO₂H in 54% relative yield with 90 and 83%, respectively, incorporation of a single ¹⁸O in the acids and CH₃COCH₂O₂CPh (2) in 46% relative yield with 100% incorporation of ₁₈O in both carbonyl oxygens. These results arc explained by the hypothesis that at least 46% of 1 reacts by cyclization to an intermediate 2-hydroxy-1,3-dioxolene 18. Mcthanolysis of 1 and other alkynyl benzoates gives 2-methoxy-1,3-dioxolenes, confirming the cyclization pathway. Reaction in 44% H₂SO₄ of 2-methoxy-2-phenyl-4-tert-butyl-1,3-dioxol-4-ene (8), prepared in this way, gives the 2-phenyl-4-tert-butyl-1,3-dioxol-4-enylium cation (16), directly observable by UV, which hydrolyzes to t-BuCOCH₂O₂CPh (12).