Acyclal Hydrolysis. The pH-Independent Breakdown of 7-Oxo-6,8-dioxabicyclooctane

Article abstract of DOI:10.1021/jo00030a045

The hydrolysis of the bicyclic acylal 7-oxo-6,8-dioxabicyclo<3.2.1>octane in water is rapid and pH independent from pH 1-12 (k₀ = 6.0*10^-3 s^-1 at 20 deg C).This reaction proceeds at nearly the same rate in D2O as in H2O (k_H2O/k_D2O = 1.1) and is uncatalyzed by buffer.Therefore, the reaction is unimolecular breakdown to a resonance-stabilized oxocarbonium ion; i.e., the acylal is hydrolyzing like an acetal with a good leaving group and not like an ester.The 1H and 13C NMR spectra indicate a diaxial conformation for the substituents at C-1 and C-5 with moderate distortion of the tetrahydropyran ring.There is a large upfield shift for carbon at C-3 as compared with the corresponding carbon (C-4) of tetrahydropyran (8.8 ppm) or 3-ethoxytetrahydropyran (3.8 ppm).The rapid pH-independent unimolecular breakdown reaction is due to a relatively favorable ΔS^* (-2.6 eu) and the lack of effective reversibility of that reaction.