Stereoselective bidirectional synthesis of spiroacetals via Rh(II)-catalysed double C-H insertion

The stereoselective synthesis of 5,5-spiroacetals by Rh(II)-catalysed double C–H insertion of methylene acetals bearing two diazo ester substituents is described. The diastereoselectivity of this bidirectional approach can be tuned through appropriate choice of catalyst, and chiral catalysts lead to products with up to 89% ee. The diastereoselectivity of the first cyclisation suggests that the second cyclisation proceeds via a non-concerted pathway.