Carbonyl dienophile

Scheme 2.7). The phenols were formed during isolation (chromatography on silica gel) from the corresponding cycloadducts. In the reaction with p-benzoquinone, a product was unexpectedly obtained from a hetero-T>ie s-Alder reaction with the quinone acting as a carbonyl dienophile. 

Reversible O-silylation also enhances the electrophilicity of carbonyl dienophiles. For example, 10 mol % A-trimethylsilyl triflimide catalyzes the reaction of pent-3-en-2-one with cyclopentadiene. A hindered base, such as 2,6-/p/.S - -butyl-4-mclhylpyridinc improves the yield in cases in which the catalyst causes the occurrence of reactant degradation. 

This retention of reagents by the graphite support has been shown in a series of experiments involving volatile dienes such as 2,3-dimethylbutadiene (12) and iso-prene (13) (Tab. 7.2) [30, 31]. The reaction of 12 with diethyl mesoxalate gave 14 in 75% yield after SMW irradiation at low power (30 W) for 20 x 1 min only (Tab. 7.2, entry 1). When conventional heating was used a satisfactory yield was obtained after 4 h at 135 °C in a sealed tube [32]. Ethyl glyoxylate is a weaker carbonyl dienophile... 

Zr-Catalyzed Enantioselective Cycloadditions 6.2.11.1 Cycloadditions with carbonyl dienophiles... 

The dihydropyrans resulting from an oxa Diels-Alder reaction represent valuable intermediates for the synthesis of numerous natural compounds. In particular, they exhibit many structural elements of carbohydrates. It is therefore not surprising that both the normal electron demand cycloaddition of dienes to carbonyl dienophiles as well as the reaction of 1-oxa-l,3-butadienes with electron-rich alkenes have extensively been used for the synthesis of sugar derivatives. Nevertheless, various approaches to other natural products have been worked out by means of these powerful tools. 

This methodology which bases on Oppolzer s sultams has also been employed in the synthesis of 2,6-N,N-diacetyl-D-purpurosamidine C [475,476]. Hetero Diels-Alder reactions of carbonyl dienophiles are furthermore involved in the preparation of ( )-ketoheptulosic acid [477] and in the already mentioned asymmetric approach to the C-26-C-32 tetrahydropyran subunit of swinholide A [92]. 

In the Lewis acid-catalyzed reaction of a Danishefsky diene (102 or 102 ) with a carbonyl dienophile, two different reaction paths, the Mukaiyama aldol path and the concerted DA path, are available for formation of the cychc producL depending on the Lewis acid used (Scheme 10.119) [331]. Strictly speaking, the former path is not that of DA reaction, but the stepwise reaction also is generally classified as an HDA reaction. The B(III)- and Ti(IV)-catalyzed HDA reactions probably proceed through the Mukaiyama aldol path [318, 320]. The concerted path has been positively identified in reactions catalyzed by Lewis acids based on Al(III) [319], Cr(III) [323], Co(III) [324], Zn(II) [331], Rh(II) [327], and Eu(III) [294, 317]. 

The [4 + 2] cycloaddition of chiral dienes to carbonyl dienophiles has been studied extensively. 1-Alkoxydienes l23 were chosen as model compounds, and as the chiral auxiliaries derivatives of D-hexoses D-glucose and D-galactose. Adducts thus obtained are, in fact, disaccharide precursors and can be converted into naturally occurring disaccharides24. 

Lewis acid catalyzed [4 + 2] cycloaddition of chiral dienes to carbonyl dienophiles is still an unexplored area. Such an approach has been applied to the synthesis of spectinomycin analogs. Cycloadditions of the two dienes 1 and 5 have been investigated. The reaction of diene 1 with acetaldehyde in the presence of Eu(fod)3 proceeds with 60% yield and gives a 5.7 1 ratio of adducts 3 and 4 d.r. [(2/ , 65 )/(2/ , 6/ )] 85 15. Under the same conditions, diene 5 produces adduct 6 as a single product with 50% yield55. 

A few instances of intramolecular carbonyl dienophile Diels-Alder cy-clizations are known. In the first reported example, Oppolzer found that an aldehyde benzocyclobutene on heating afforded a 25% yield of two epimeric cycloadducts [Eq. (19)].51... 

Carbonyl cyanide, 100 Carbonyl dienophile, 94-119 Carpanone, 196 CC-1065, 347 Chalcogran, 174 Chiral dihydrooxazine, 82 Chloral, 95... 

Although o- and p-quinones generally react with 1,3-dienes to give carbocycles, a few scattered examples of quinones acting as carbonyl dienophiles have been reported. For example, o-chloranil and o-bromanil react with 2,3-dimethylbutadiene to give [4 + 2] adducts [Eq. (7)].29... 

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