Allylidenetriphenylphosphorane as a Bifunctional Reagent: Synthesis of Cyclopentenones and α,β-Unsaturated Ketones with (3-(Alkoxycarbonyl)-2-ethoxy-2-propylidene)triphenylphosphorane

Article abstract of DOI:10.1021/jo00080a019

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 Cs2CO3 at room temperature, a <3 + 2> annulation occured 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 belived 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, allylidenetriphenylphosphorane 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 Cs2CO3 to give enol ethers 23a-f, which were converted into α,β-unsaturated 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.