DFT investigations on the mechanism of Diels-Alder reactions of a hydroxy-ortho-quinodimethane with fumaric acid derivatives were performed to understand the origin of the syn or anti configuration of the adducts. The diene hydroxyl group and the dieneophile carboxyl group show hydrogen bonding in the transition state, significantly favouring the syn product. This reaction is poorly diastereoselective for R = CO2Me (ratio syn/anti = 57:43) and significantly improved for R = CO2H (ratio syn/anti = 71:29). The stereoselectivities are properly predicted from transition structures calculated at the B3LYP/6-31G(d) level. (c) 2005 Elsevier B.V. All rights reserved.