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

摘要

In the presence of 20 mol % of tributylphosphine, tert-butyl carbonate substrate 3a undergoes smoothlyan 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 theproduct 2a into 2a' has been investigated in detail. On the basis of this mechanism, two strategies, using20 mol % of triphenylphosphine or 10 mol % of tributylphosphine in the presence of 20 mol % ofTi(O~iPr)_4, have been established for the selective construction of benzobicyclo[4.3.0] compounds. Underneutral conditions, the reactions of compounds 3a—g afford benzobicyclo[4.3.0] compounds 2a—g withhigh selectivities in good to excellent yields. In addition, a-methyl α,β-unsaturated ester 3h also workswell to give the corresponding product 2h with one quaternary carbon center in 99% yield under neutraland room temperature conditions.