Allylidenetriphenylphosphorane as a bifunctional reagent: synthesis of cyclopentenones and .alpha.,.beta.-unsaturated ketones with [3-(alkoxycarbonyl)-2-ethoxy-2-propenylidene]triphenylphosphorane

Abstract

When (3-(ethoxycarbonyl)-2-ethoxy-2-propenylidene)triphenylphosphorane (6) was allowed to react with α-bromo ketones 8a-d in dichloromethane in the presence of Cs 2 CO 3 at room temperature, a [3+2] annulation occurred and led to the formation of the corresponding 2-ethoxycyclopentadienes 9a-d in excellent yields. Similarly, bromo thioester 8g underwent the annulation to give 4-(ethylthio)-cyclopentadiene 9g. Secondary bromides 2-bromo-3-pentanone and 2-bromocyclohexanone also afforded tetrasubstituted cyclopentadienes 9e and 9f in moderate yields when 2 equiv of 6 was used. The annulation is believed to proceed through a sequence involving a stepwise alkylation at the γ position of 6 and an intramolecular Wittig reaction because of the fact that intermediate 11 was isolated. The resulting 2-ethoxycyclopentadienes 9a-g were converted quantitatively into the corresponding cyclopentenones 10a-g upon mild acid treatment.Furthermore, alkylidenetriphenylphosphorane underwent a carbon elongation at both ends of the three-carbon unit via an alkylation-Wittig reaction sequence. (3-(tert-Butoxycarbonyl)-2-ethoxy-2-propenylidene)triphenylphosphorane (7) reacted first with alkyl halides and then with aldehydes in the presence of Cs 2 CO 3 to give enol ethers 23a-f, which were converted into α,β-unsatured ketones 20, 21, and 25c-f by hydrolysis of the enol ether and then decarboxylation. In this way, shogaol (29), the pungent principle component of ginger, was conveniently synthesized starting from 2-methoxy-4-methylphenol