Ketones, methyl vinyl Lewis acid catalysis

Cycloaddidons of 2-triethylsilyl- and 2-tributylstannyl-13-butadienes (172) to ethyl acrylate and methyl vinyl ketone also proceeded with moderate regioselectivities which could significantly enhanced by Lewis acid catalysis in favor of products (174) (Scheme 41, Table 2). The resulting cyclo-hexenylstannanes (174 M = SnBua) show potential for C—C coiq>ling reactions. 

The reaction did not require dry solvents or inert atmosphere and afforded the desired adducts in 42-98% yield. As expected, the more reactive acrolein and methyl vinyl ketone gave very good results with both acyclic and cyclic p-ketoesters and p-diketones and, surprisingly, acrylonitrile and methyl acrylate, reported to be totally inactive under Lewis acid catalysis, " afforded the corresponding adducts with a-acetylbutyrolactone in high yield (77 and 98% respectively). In the reaction with crotonaldehyde, a 1 1 mixture of... 

A detailed study of the cycloaddition of trifluoromethylethene has appeared as has work on a-siloxy-a -unsaturated ketones as dienophiles under Lewis-acid catalysis. Amongst the new dienophiles that have been reported are the two novel ketene equivalents methyl methoxypropiolate (129) and methyl phenyl-thiopropiolate (130). A series of 3-(acyloxy)but-3-en-2-ones (131) has been prepared and used as dienophiles in a route to anthracylinones, and, as part of a route to highly functionalized cyclohexenones, the vinyl sulphone (132) has been used in a Diels-Alder reaction with various dienes. ... 

It is believed that clay minerals promote organic reactions via an acid catalysis [2a]. They are often activated by doping with transition metals to enrich the number of Lewis-acid sites by cationic exchange [4]. Alternative radical pathways have also been proposed [5] in agreement with the observation that clay-catalyzed Diels-Alder reactions are accelerated in the presence of radical sources [6], Montmorillonite K-10 doped with Fe(III) efficiently catalyzes the Diels-Alder reaction of cyclopentadiene (1) with methyl vinyl ketone at room temperature [7] (Table 4.1). In water the diastereoselectivity is higher than in organic media in the absence of clay the cycloaddition proceeds at a much slower rate. 

The activation of various reactions by Lewis acids is now an everyday practice in synthetic organic chemistry. In contrast, solvent effects on Lewis acid catalysed Diels-Alder reactions have received much less attention. A change in the solvent can affect the association step leading to the transition structure. Ab initio calculations on the Diels-Alder reaction of cyclopentadiene and methyl vinyl ketone in aqueous media showed that there is a complex of the reactants which also involves one water molecule". In an extreme case solvents can even impede catalysis 12. The use of inert solvents such as dichloromethane and chloroform for synthetic applications of Lewis acid catalysed Diels-Alder reactions is thus well justified. General solvent effects, in particular those of water, will be discussed in the following section. 

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