New functionalized cycloheptatrienes via tropone σ-adducts

Abstract

We report that, on treatment with phenyl-lithium, followed by triethyloxonium tetrafluoroborate in diethyl ether at –78°C, (i) 2-chloro- and 2-bromo-tropone give the corresponding 1-ethoxy-2-halogeno-7-phenylcycloheptatriene; (ii) 2-fluorotropone gives 2-ethoxy-3,7-diphenylcycloheptatriene; and (iii) tropone affords 1-ethoxy-2-phenylcycloheptatriene. Conceivably, in case (i) the tropone undergoes phenyl-lithium attack at C-7, followed by O-alkylation of the enolate, while in case (iii) the intermediate enolate suffers base-catalysed rearrangement and, finally, in case (iii) nucleophilic replacement of fluorine by phenyl-lithium from the tropone precedes phenyl-lithium attack at C-7 of the resulting 2-phenyl-tropone. This gives a better synthesis, than those available so far, of 2-chloro-7-aryltropones; in addition, a 1-alkoxy-2-chloro-7-phenylcycloheptatriene undergoes selenium dioxide oxidation to 2-chloro-7-phenyltropone.