The mechanism for the spiroacetalization of enol-ethers 1 and 2 promoted by chiral phosphoric acid (CPA) catalyst (I) and by chiral imidodiphosphoric acid catalyst (II) has been investigated by QM/MM methods. The computed levels of enantioselectivity following exhaustive conformational analysis is in close agreement with the sense and magnitude of experimental results. Small substrates fit inside catalyst I to yield both enantiomers, in agreement with the absence of asymmetric induction for this reaction, while for catalyst II chiral discrimination between TS structures is possible. Unlike reactions catalysed by CPA or CPA derivatives in which steric effects and substrate distortion controls enantioselectivity, we show that chiral discrimination results from the restricted area and direction of possible hydrogen-bonding interactions with a more confined catalyst structure.