Cyclization of alkynyl benzoates and generation of dioxolenylium ions

Reactions of 1-ethynyl benzoates RC≡CO 2 CC 6 H 4 X in H 2 O and CH 3 OH have been studied. In neutral H 2 O, reaction rates for CH 3 C≡CO 2 CC 6 H 4 X depend on σ p of X with ρ of 1.3. Reaction of CH 3 C≡CO 2 CPh (1) in 18 O-labeled H 2 O gives CH 3 CH 2 CO 2 H and PhCO 2 H in 54% relative yield with 90 and 83%, respectively, incorporation of a single 18 O in the acids and CH 3 COCH 2 O 2 CPh in 46% relative yield with 100% incorporation of 18 O both carbonyl oxygens. These results are explained by the hypothesis that at least 46% of 1 reacts by cyclization to an intermediate 2-hydroxy-1,3-dioxolene. Methanolysis of 1 and other alkynyl benzoates gives 2-methoxy-1,3-dioxolenes, confirming the cyclization pathway. Reaction in 44% H 2 SO 4 of 2-methoxy-2-phenyl-4-tert-butyl-1,3-dioxol-4-ene, prepared in this way, gives the 2-phenyl-4-tert-butyl-1,3-dioxol-4-enylium cation, directly observable by UV, which hydrolyzes to t-BuCOCH 2 O 2 CPh