Lewis acid catalyzed and self-assembled diels-alder reactions (LACASA-DA) : a new strategy to control diels-alder reaction

Abstract

The first comprehensive account of Diels-Alder reactions occurring by a simultaneous self-assembly of the components and catalysis of the reaction via a Lewis Acid (LA) is presented in this study. The synthetic usefulness of the intramolecularity achieved by temporary connection of DA reaction components has been combined with the benefits of LA catalysis to overcome the regio- and stereochemical diversities associated with DA reactions of unsymmetrically substituted dienes and dienophiles. LA's employed in this study act both as temporary connectors (by binding to LB sites of the diene and dienophile) and as catalysts (by complexation to oxygen of carbonyl moieties of the dienophiles). Formation of the tether, cycloaddition; and removal of the transient linker all occurs in a single operation.* To illustrate the high selectivity accomplished in this approach, an experimental model system consisting of unsymmetrically substituted dienol 6 and methoxy diene 397 and an unsymmetrically substituted dienophile (methyl acrylate, MAC) was designed. Thermal combination of either of these two dienes with MAC produces nearly equimolar mixtures of all four possible adducts illustrating the unselective nature of the reactions. Use of LA's to mediate the reaction between dienol 6 and MAC results in exclusive regio- and stereoselective formation of a single adduct. In contrast, application of the same LA's for the reaction between the methoxy diene and MAC either results in the formation of two 'endo' regioisomers in low yield or no reaction at all.* Exclusive formation of a single adduct for the reactions of 6 is attributed to a process in which the LA both pre-organizes the reactants and catalyzes the cycloaddition. This phenomenon is not expected to be observed in the reactions of the methoxy diene. The difference in the behavior of the two dienes is ascribed to the difference in their interactions with the LA's. Dienol 6 is able to bind covalently and irreversibly to the LA's while methoxy diene 397 is only able to co-ordinate to the LA's in a reversible manner. Mechanistic studies strongly support the self-assembled pathway and exclude other potential routes. Reactions of dienol 6 with other structurally suitable dienophiles under similar LA mediated conditions exhibit higher selectivities and reactivities in comparison with their uncatalyzed counterparts. Use of a structurally similar chiral dienol induces a highly diastereoselective cycloadditions with different dienophiles, whereas the thermal versions of the reactions again produce all possible adducts. The strategy has also been used for DA reactions of ' 2H'-thiopyran dienols with dienophiles 'N'-acroyl-2-oxazolidinone (NAO), ('E')-3-[3-(methoxycarbonyl)propenoyl]-1,3-oxazolidin-2 -one (MCPO), and 'N'-phenylmaleimide (NPM). Use of MgII is demonstrated to be effective to stereoselectively catalyze reactions which are unselective thermally or do not proceed at all. *Please refer to dissertation for diagrams.