Lewis Acid Catalysed [4+2] Heterocycloadditions between Ketone Enol Ethers and β-Ethylenic α-Oxo Esters

• Published in: European journal of organic chemistry Vol. 2002; no. 3; pp. 514 - 525
• Main Authors: Martel, Arnaud, Leconte, Stéphane, Dujardin, Gilles, Brown, Eric, Maisonneuve, Vincent, Retoux, Richard
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag GmbH 01.02.2002; WILEY‐VCH Verlag GmbH; Wiley; Wiley-VCH Verlag
• Subjects: Catalysis; Chemical Sciences; Chemistry; Chemistry, Organic; Cycloaddition; Enols; Ketones; Lewis acids; Organic chemistry; Physical Sciences; Science & Technology; cycloaddition; DIELS-ALDER REACTIONS; Lewis acids; catalysis; SILYL; ketones; enols; Cycloaddition; Catalysis; Ketones; Enols
• ISSN: 1434-193X; 1099-0690
• DOI: 10.1002/1099-0690(20022)2002:3<514::AID-EJOC514>3.0.CO;2-Z

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 SnCl4 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 SnCl4, respectively. These assignments 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 SnCl4 was the catalyst.