Catalytic Intramolecular Formal [3 + 2] Cycloaddition for the Synthesis of Benzobicyclo[4.3.0] Compounds

In the presence of 20 mol % of tributylphosphine, tert-butyl carbonate substrate 3a undergoes smoothly an intramolecular formal [3 + 2] cycloaddition reaction at room temperature to give benzobicyclo[4.3.0] compounds in 99% yield with a 19/81 ratio of 2a and 2a'. The mechanism of the isomerization of the product 2a into 2a' has been investigated in detail. On the basis of this mechanism, two strategies, using 20 mol % of triphenylphosphine or 10 mol % of tributylphosphine in the presence of 20 mol % of Ti(O(i)Pr)(4), have been established for the selective construction of benzobicyclo[4.3.0] compounds. Under neutral conditions, the reactions of compounds 3a-g afford benzobicyclo[4.3.0] compounds 2a-g with high selectivities in good to excellent yields. In addition, alpha-methyl alpha,beta-unsaturated ester 3h also works well to give the corresponding product 2h with one quaternary carbon center in 99% yield under neutral and room temperature conditions.