ALLYLIDENETRIPHENYLPHOSPHORANE AS A BIFUNCTIONAL REAGENT - SYNTHESIS OF CYCLOPENTENONES AND ALPHA,BETA-UNSATURATED KETONES WITH NYL)-2-ETHOXY-2-PROPENYLIDENE)TRIPHENYLPHOSPHORANE

When (3-(ethoxycarbonyl)-2-ethoxy-2-propenylidene) triphenylphosphoran e (6) was allowed to react with cu-bromo ketones 8a-d in dichlorometha ne in the presence of Cs2CO3 at room temperature, a [3 + 2] annulation occurred and led to the formation of the corresponding 2-ethoxycyclop entadienes 9a-d in excellent yields. Similarly, bromo thioester 8g und erwent the annulation to give 4-(ethylthio)-cyclopentadiene 9g. Second ary bromides 2-bromo-3-pentanone and 2-bromocyclohexanone also afforde d 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 gamma position of 6 a nd an intramolecular Wittig reaction because of the fact that intermed iate 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 v ia an alkylation-Wittig reaction sequence. nyl)-2-ethoxy-2-propenylide ne)triphenylphosphorane (7) reacted first with alkyl halides and then with aldehydes in the presence of Cs2CO3 to give enol ethers 23a-f, wh ich were converted into alpha,beta-unsaturated ketones 20, 21, and 25c -f by hydrolysis of the enol ether and then decarboxylation. In this w ay, shogaol (29), the pungent principle component of ginger, was conve niently synthesized starting from 2-methoxy-4-methylphenol.