[2 + 2 + 1] Cycloaddition between Alkynes, Acrylates, and Titanocene(II): Regioselective Formation of 5-Alkoxycarbonyltitanacyclopent-2-enes and Their Addition to Carbonyls

5-Alkoxycarbonyltitanacyclopent-2-enes were produced regioselectively by the [2 + 2 + 1] cycloaddition between alkynes, acrylates, and titanocene(II) reagent Cp2Ti[P(OEt)3]2. Although the titanacycles were inactive toward carbonyl compounds, their addition proceeded in the presence of titanocene(II).