. Can. J. Chem. 58,2531Chem. 58, (1980. The cyclization under acidic conditions of bicyclic and monocyclic hydroxypropyl methoxytetrahydropyrans 1 and 4 is reported. At room temperature, compound 1 gives only cis tricyclic acetal2. At reflux, compound 2 equilibrates to yield a mixture of cis and trans tricyclic acetals 2 (45%) and 3 (55%). Compound 4 gives directly the equilibrium mixture of cis and trans bicyclic acetals 5 (55%) and 6 (45%) at room temperature. The specific formation of 2 from 1 constitutes strong experimental evidence that acetal formation is controlled by stereoelectronic effects. The anomeric effect can be estimated (= 1.5 kcal/mol) from the equilibrium study of 2 and 3. This value confirms the result previously obtained by Descotes (9).NORMAND BEAULIEU, ROBERT A. DICKINSON et PIERRE DESLONGCHAMPS. Can. J. Chem. 58,2531Chem. 58, (1980. On rapporte lacyclisatlon, sous conditions acides, des hydroxy-propylmethoxytCtrahydropyrannes bicyclique et monocyclique 1 et 4. A temperature ambiante, le compose 1 donne uniquement I'acetal tricyclique cis 2. A reflux, le compose 2 forme un melange des acetals tricycliques cis 2 (45%) et trans 3 (55%). Le compose 4, a temperature ambiante, donne directement le melange 1 I'equilibre des acetals bicycliques cis 5 (55%) et trans 6 (45%). La formation specifique de 2 a partir de 1 constitue une preuve convaincante demontrant que la formation d'acetal est contrBlCe par des effets stbreotlectroniques. L'effet anomere peut &re Cvalue a partir de I'analyse du mtlange a I'equlllbre de 2 et 3; la valeur trouvke de = 1.5 kcallmol s'accorde bien avec les rksultats deja obtenus par Descotes (9).

A diene synthesis using 2,6-xyloquinone as dienophile and boron trifluoride as catalyst does not follow the normal orientation rules and leads to a facile synthesis of ring A-aromatic steroids. A tetracyclic adduct prepared by a similar reaction and containing the steroidal C19-methyl group is also reported.

Lewis acid catalysts cause an orientation reversal in the Diels–Alder additions of 2,6-di-methyl-1,4-quinone (1) and toluquinone (12) to 1-substituted 1,3-butadienes and to 1,2- and 1,3-dialkyl dienes. These results are explained by the preferential formation of one reactive quinone–Lewis acid salt.

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