USE OF 1,3-DIOXIN-4-ONES AND RELATED-COMPOUNDS IN SYNTHESIS .46. -MENTHYLOXYCARBONYL)-2,6-DIMETHYL-1,3-DIOXIN-4-ONE AND RELATED-COMPOUNDS -RELATIONSHIP BETWEEN FACIAL SELECTIVITY AND PYRAMIDALIZATION

Preparation and ground-state reactions (conjugate addition and catalyt ic 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), a re reported. The X-ray structure of the 2(methoxycarbonyl)-2-phenyl de rivative (4b) was also determined. The result, when combined with prev iously 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 py ramidalization of the enone function in the ring, Conjugate addition r eaction of 5a leads to exclusive addition on the bottom face, while ad dition to 7a gives the product corresponding to the top face addition, Catalytic hydrogenation of 6a and ga is also reported. Though the sel ectivity is much lowered as compared with the conjugate addition, the same facial selectivities are again observed. A comparison with the ex clusive 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 additio n and catalytic hydrogenation) based on pyramidalization is not always correct. A novel hypothesis which accounts for all of the above resul ts (sofa conformation of the hetero ring, pyramidalization at the enon e portion, and facial selectivity) is presented.