4-Phenyl-1,2,4-triazoline-3,5-dione (PTAD) reacts with 1 -methoxy-4-methylnaphthalene to give the 1,4-addition product in the presence of methanol; the reaction does not proceed in the absence of the alcohol. Methoxy exchange (with CD3OD) was also observed during the reaction. Reactions of PTAD with 1-(3-hydroxypropoxy)-4-methylnaphthalene and related naphthalenes afford stereoselectively 1,4-adducts (70-98% of the major diastereomer). The stereoface-selective addition of PTAD at C-4 with concurrent formation of an acetal at C-1 takes place in a syn manner, which is induced by the hydrogen-bonding interaction between PTAD and the hydroxy group. The alpha -methyl substitution on the propoxy side chain strongly enhances the stereodifferentiation (90% de) and accelerates the cyclization by the Thorpe-Ingold effect. The alkoxy moiety of the adduct is easily removed to give 4-methyl-4-amino-4H-naphthalen-1-one with high enantiomeric excess. The gamma -methyl group of the side chain also affects the stereodifferentiation. Thus, the two stereogenic centers of the (1S,3R)-3-hydroxy-1-methylbutoxy side chain work together to achieve the high stereodifferentiating 1,4-addition from the Si-Re face (up to 96% ee). Epimerization of the cyclic acetal of a minor adduct was observed during the reaction of 1-(3-hydroxybutoxy)-4-methylnaphthalene, indicating that the minor component of the final products is derived from the initial minor syn adduct formed from the opposite face. The syn selectivity of the addition is achieved completely in the initial stage of formation of both the major and the minor adducts.