Chiral allyl cations are captured by furan with 100 % stereoselectivity: Synthesis of enantiopure 2-alkoxy-8-oxabicyclo[3.2.1]oct-6-en-3-ones by low-temperature [4+3] cycloaddition

A low-temperature (-95 degrees C) protocol for intermolecular cycloaddition s of furan to chiral silyloxyallyl cations in dichloromethane is described. Key precursors are open-chain, mixed alpha-ketoacetals, which are chiral. The resulting [4+3] cycloadducts are densely functionalized and are isolate d as single enantiomers in high chemical yield. The yield of the cycloadduc ts increases with increasing dilution. Three and four stereogenic centres a re created in one single step.