epoc ABSTRACT: Three stable ,-dimesityl enols with heteroaromatic rings in the -position were synthesized to study the effect of OH Á Á ÁN hydrogen bonding on the oxidation potentials of enols. In contrast to its solid-state structure, enol E1 exists predominantly as intramolecularly hydrogen-bonded species in solution. For enol E2 an intermolecular hydrogen bond and for E3 a partial proton transfer were established based on NMR, dilution experiments, solvent dependence and UV-visible spectroscopic studies. Cyclic voltammetric investigations revealed that OH Á Á ÁN hydrogen bonding may shift the oxidation potentials of enols by up to 510 mV cathodically. RESULTS AND DISCUSSIONEnols E1-E3 were prepared by the reaction of ,dimesitylketene 8 with the corresponding heteroaryl lithium reagent, in yields ranging from 24 to 40%. They were fully characterized by 1 H and 13 C NMR, IR and elemental analysis. Moreover, the solid-state structure of E1 (see below) confirms the enol structure of E1-E3.

SummaryA one-pot transformation, which involves the reaction of ketones with aldehydes in the presence of metal halides to furnish tetrahydro-2H-pyran-2,4-diols in a highly diastereoselective manner, is investigated thoroughly by experiments and computations. The reaction was also successfully implemented on a flow micro reactor system.

The synthesis of 1,2-naphtho-annelated barrelene derivatives, namely dimethyl-7,12-dihydro-7,12-ethenobenzo[a]anthracene-13,14-dicarboxylate (4a) and dimethyl-7,14-dihydro-7,14-ethenodibenzo[a,j]-anthracene-15,16-dicarboxylate (4b), and the investigation of their photoreactivity in solution and in the solid state is reported. The irradiation of 4a and 4b resulted in regioselective di-p-methane rearrangements to give semibullvalene products; however, the product distribution is inverted upon changing the reaction medium from solution to the solid state. In solution, an a-naphtho-vinyl bridging predominates as the initial photochemical step because of the selective stabilization of the corresponding transition state, as discussed on the basis of the structures of the related biradical intermediates, whereas the solid-state photoreaction is significantly influenced by the steric constraints within the crystal lattice that only allow a b-naphtho-vinyl bridging.Résumé : On a réalisé la synthèse de dérivés condensés 1,2-naphtobarrèlènes, à savoir le 7,12-dihydro-7,12-éthénoben-zo[a]anthracène-13,14-dicarboxylate de diméthyle (4a) et le 7,14-dihydro-7,14-éthénobenzo[a,j]anthracène-15,16-dicarboxylate de diméthyle (4b), et on a étudié leur photoréactivité en solution et à l'état solide. L'irradiation des composés 4a et 4b conduit à des réarrangements di-p-méthane régiosélectifs et à la formation de produits semibullvalènes; toutefois, la distribution des produits est inversée lorsque le milieu réactionnel passe de la solution à l'état solide. En solution, un pont a-naphtovinyle prédomine comme étape photochimique initiale en raison de la stabilisation sélective de l'état de transition correspondant, tel que discuté sur la base des structures des intermédiaires biradicalaires apparentés, alors que la photoréaction à l'état solide est fortement influencée par des contraintes stériques dans le réseau cristallin qui ne permettent que la formation d'un pont b-naphtovinyle.Mots-clés : réarrangement di-p-méthane, dérivés du barrèlène, photochimie à l'état solide.[Traduit par la Rédaction]