Selective conformational changes of cyclic systems—XII : Enhanced reactivity of ten membered ring ketones due to transanular steric compression: hydrates and hemiketals from oxo-[2.2]metacyclophanes

Abstract

The rigid, 10-membered ring Ketones incorporated in [2.2]metacyclophanes exhibit a pronounced tendency towards adduct formation with nucleophiles. [2.2]Metacyclophane-1,10-dione ( 1) readily adds water and alcohols, e.g. in dioxane solution containing 20 n n water an equilibrium is attained between 1 (9%) and the mono- and dihydrates 1' (55%) and 1″ (36%) respectively. In pure water the equilibrium is further shifted towards the adducts, as revealed by an extrapolation. Equilibration of the dihydrate 1″ which can be isolated in crystalline form gives corresponding results. In methanol the equilibrium mixture consists mostly of the diastereomeric bishemiketals 1aa, 1ee and 1ae. A similar behaviour is found for the monoketone 2, 1-propylenethioketal of [2.2]1metacyclophane-l,10-dione. The reactions studied are fully reversible on evaporation of the solvents and subsequent warming under reduced pressure. The results are gained from a combinatory evaluation of 1H NMR and absorption spectra of 1 and 2 in solution. The enhanced reactivity of the CO groups in 1 and 2 can be accounted for by a relief of steric strain on rehybridization sp 2→sp 3 due to a simultaneous conformational change. In dodecahydro-[2.2]metacyclophane-1,10-dione ( 3), where a similar relief of strain is not operative an anomalous carbonyl reactivity is not observed.