Hexamethylphosphoramide-Mediated Conjugate Addition of (Alkylthio)-, (Phenylthio)-, and (Phenylseleno)allyllithium Reagents to 2-Cyclopentenone

(Methylthio)- and (tert-butylthio)allyllithium react irreversibly with 2-cyclopentenone by β- and γ-1,2-addition in THF at -78 °C. (Phenylthio)- and (phenylseleno)allyllithium also react in this way but in addition display a small tendency to undergo α- and γ-l,4-addition. In the presence of 1 equiv of HMPA, the major reaction pathway for all anions is α-1,4-addition, with barely detectable γ-1,4-addition (~5%) also taking place. The enolate resulting from the HMPA-mediated conjugate addition of (methylthio)allyllithium to 2-cyclopentenone reacts readily with 3-iodo-l-(trimethylsilyl)-l-propyne to produce the corresponding 2,3-disubstituted and 2,3,5-trisubstituted cyclopentanones in 75% and 4% yields, respectively, from 2-cyclopentenone.