A General Synthesis of Methylenecyclopentanes by a Stereoselective [3 + 2] Approach

The [3 + 2] cyclopentanation involving 3-(phenylsulfonyl)-2-(bromomethyl)-1-propene ( 1 ) and representative ( E ) α,β-unsaturated acyclic esters and ketones has been studied. High yields and complete stereoselectivity were observed in all reactions leading to tri- or tetrasubstituted methylenecyclopentanes. The anti -diastereoselectivity in the first, Michael addition step is rationalized by a chelation-controlled transition state in which MO interactions of the two π systems are involved. The Michael reactions of methallyl sulfone 8 with ( E ) enoates in the absence and presence of HMPA confirms the influence of chelation on the diastereomeric ratio of adducts. Cyclopentanations involving 1 with cyclohexenone, 2(5 H )-furanone, and 5,6-dihydro-2-pyranone, respectively, were also studied with emphasis on the factors influencing the stereochemical outcome of the annulation process.