Stereochimie de l'hydrogenation des (±) Δ3(3a)-hydrindenones-4. Comparaison avec le cas de la (+) methyl-3 Δ3(3a)-hydrindenone-4 et de ses dioxolannes

We show that the four studied 4-hydrindenones (with or without a methyl substituant at C3 or C7a) give by hydrogenation on Raney nickel or on Pd/C, in various solvants, the cis 4-hydrindanones, by a kinetically controlled process. During the hydrogenation, the double bond can migrate only in the positions conjugated to the carbonyl group.In contrast, when the carbonyle is protected as a dioxolanne, the double bond is free to migrate around the five membered ring. In spite of this mobility the optical purity of the saturated dioxolannes formed is high. In the case of the (+)3-methyl Δ^3(3a) 4-hydrindenone, the enantiomeric hydrindanones $\text{9a}$ obtained by direct hydrogenation or with isomerization of the double bond are not the same. Therefore, the optical purity of the product is low, but its value allows an estimation of the relative importance of the two reaction pathways.