Premium
Lewis Acid Catalysed [4+2] Heterocycloadditions between Ketone Enol Ethers and β‐Ethylenic α‐Oxo Esters
Author(s) -
Martel Arnaud,
Leconte Stéphane,
Dujardin Gilles,
Brown Eric,
Maisonneuve Vincent,
Retoux Richard
Publication year - 2002
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/1099-0690(20022)2002:3<514::aid-ejoc514>3.0.co;2-z
Subject(s) - chemistry , silyl enol ether , enol , adduct , ketone , lewis acids and bases , stereoselectivity , enol ether , catalysis , bicyclic molecule , ether , medicinal chemistry , silylation , stereochemistry , organic chemistry
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 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 SnCl 4 was the catalyst.