Preparation and ground-state reactions (conjugate addition and catalytic hydrogenation) of a 1,3-dioxin-4-one having an l-menthyloxycarbonyl group at the 2-position as the only substituent (7a) and its 6-methyl derivative (8a), as well as their 2-methyl derivatives (5a and 6a), are reported. The X-ray structure of the 2(methoxycarbonyl)-2-phenyl derivative (4b) was also determined. The result, when combined with previously reported X-ray crystallographic analyses on related dioxinones, suggests that all of the dioxinones take, a sofa conformation of the six-membered ring, whose top face is always on the same side as the pyramidalization of the enone function in the ring, Conjugate addition reaction of 5a leads to exclusive addition on the bottom face, while addition to 7a gives the product corresponding to the top face addition, Catalytic hydrogenation of 6a and ga is also reported. Though the selectivity is much lowered as compared with the conjugate addition, the same facial selectivities are again observed. A comparison with the exclusive top face attack on 2-tert-butyl-1,3-dioxin-4-one (1) and its 2-methyl derivative 2 and bottom face attack on 2-(l-menthyloxycarbonyl)-2-phenyl-1,3-dioxin-4-one (3a) makes it clear that the prediction of facial selectivity in these ground-state reactions (conjugate addition and catalytic hydrogenation) based on pyramidalization is not always correct. A novel hypothesis which accounts for all of the above results (sofa conformation of the hetero ring, pyramidalization at the enone portion, and facial selectivity) is presented.