The [4+2] heterocycloaddition reaction between (E)-γ-substituted β-unsaturated α-oxo esters 1a−d and cyclic and acyclic ketone enol ethers 2a−l afforded heteroadducts 3−5 in high yields when either [Eu(fod) 3 ] or SnCl 4 were used as the catalyst. With methyl (E)-benzylidenepyruvate (1a), the two catalytic modes displayed divergent stereoselectivities. In the case of the heterocycloaddition of silyl enol ether 2h, X-ray investigations established the relative configurations of the bicyclic adducts 3h and 5h, each obtained as the major isomer with [Eu(fod) 3 ] and with SnCl 4 , respectively. These as- [a] signments provided valuable information concerning the concerted vs. nonconcerted mechanism of these acid-catalysed reactions. With methyl (E)-tert-butoxymethylenepyruvate (1d), the scope of the reaction proved to be restricted to alkyl enol ethers as the dienophiles. In this case, the stereochemical process followed a different pathway, exemplified by the X-ray structure of the adduct 4q, obtained with total stereoselectivity from 1d and 1-methoxycyclohexene (2f) when SnCl 4 was the catalyst. present a survey of these results, and Ϫ from stereochemical assignment of representative structures Ϫ to deduce a set of significant arguments concerning the concerted vs. nonconcerted mechanisms that can be ascribed to [Eu(fod) 3 ]vs. SnCl 4 -catalysed reactions.
A new and highly simple method for the synthesis of E-g-aryl-a-oxobutenoic esters is described. This unprecedented one-step procedure is based on an unexpected nucleophilic reactivity of alkyl pyruvates towards aromatic aldehydes in refluxing dichloromethane, when using catalytic amounts of Cu(OTf) 2 and in the presence of stoichiometric amounts of trimethyl orthoformate in the case of electron-rich aldehydes. Under these conditions, yields obtained are uniformly higher than those obtained by previous multistep procedures.In the past decade, a-oxo-b-unsaturated esters have emerged as novel useful synthons, 1 especially in DielsAlder reactions as heterodienes 2 or dienophiles. 3 Chiral E-g-C-substituted-a-oxo-b-unsaturated esters 4 have been employed in the asymmetric syntheses of biologically important subunits (e.g. b-unsaturated-and b-amino-a-hydroxy esters 5 ) and natural products (norlignans). 6 E-g-Aryl-a-oxo-b-unsaturated esters have been previously prepared by alkaline condensation of pyruvic acid (or ester) with aromatic aldehydes, acidification and final esterification. Sugimura's group introduced a more general three-steps procedure 7 involving O-silylation of the pyruvic acid ester, Mukayaima-aldol-type cross-condensation with an acetal (even in situ generated 8,5b ) at low temperature and subsequent b-elimination using silica gel (or florisil 2k ). In connection with our recent results concerning the solide-phase synthesis of dihydropyrans via cycloaddition of a solid-supported benzylidenepyruvic acid ester, 9 we needed a direct preparation method of title compounds 1 starting from the corresponding pyruvates, that would involve neither insoluble reagents for the elimination step nor alteration of the ester functionality.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.