Receiwd 20 A'ooember 1978) R&m& -La reduction $kctrochimique des chloro-3 acyl-2 dihydro-5,6 4H pyrannes, sur cathode de mercure et en milieu hydroorganique. permet~d'&liminer s&ctivemeot l'halog&ne et d'obtenir quantitativement ks acyl-2 dihydro-56 4U pyrannw. La reduction de ccs dCrivb non chlor$s est igalement Selective en milieu acide. car a &tC de traces de prod&s de dim&r&&m on obtient les acyl-2 titrahydropyrannes avec de bons rendements. En milieu basique, la s&ctivit6 n'est bonne que si le groupentent R de Pacyle a un coefficient positif dans l'ichelle de Taft(R -C,H,,CH,-C,H,);onobtientalorslesdihydto-5,64H pyrannyl-2carbinols. Silegroupement R cat 6lectrodonneur (R = CHs, C*H,), on obticnt un mtlange de &tone satur&e et d'alwol dthyl&oique oti celui-ci pr&mine. La polarographie et I'&& des courbes E,,, -f(pH) permettent d'apporter des pr6cisions sur les iaterm&diiires de rCaction.
Ah&act -The macro-scale electrochemical reduction of the 3-chloro-2-acyl-5.6-dihydro-4H-pyrans, on mercury cathode in aqueous medium, is a good method to obtain in quantity the 2-acyl-5.6-dihydro-4Hpyrans, after elimination of the halogen group.
The reduction of the 2-acy~-5.+lihydro-4H-pyrans is also selective in acidic medium and besides very small amounts of dimeric products, we can get 2-acyltetrahydropyrans in good yields. In basic medium selectivity is good, and 5&dtbydrc+4H-pyran-2-yl carbinols are obtained only if R, in the acyl group, has a positive Taft's coe&cient (R = C*H,, CH,-C.Hs). When R has an electrodonating effect (R = CH,, CIHs), we have got a mixture ofsaturated ketone and ethylenic alcohol, where the last is the most important.
Polarography and curves I?,,1 =/(pH) give some information about intermediary species.