Dienophile, in the Diels-Alder reaction,

Vinylboranes are interesting dienophiles in the Diels-Alder reaction. Alkenylboronic esters show moderate reactivity and give mixtures of exo and endo adducts with cyclopentadiene and 1,3-cyclohexadiene (441). Dichloroalkenylboranes are more reactive and dialkylalkenylboranes react even at room temperature (442—444). Dialkylalkenylboranes are omniphilic dienophiles insensitive to diene substitution (444). In situ formation of vinyl-boranes by transmetaHation of bromodialkylboranes with vinyl tri alkyl tin compounds makes possible a one-pot reaction, avoiding isolation of the intermediate vinylboranes (443). Other cycloadditions of alkenyl- and alkynylboranes are known (445). 

Dicyanoacetylene, 2-hiitynedinitri1e, is obtained from dimethyl acetylenedicarboxylate by ammonolysis to the diamide, which is dehydrated with phosphoms pentoxide (44). It bums in oxygen to give a flame with a temperature of 5260 K, the hottest flame temperature known (45). Alcohols and amines add readily to its acetylenic bond (46). It is a powerhil dienophile in the Diels-Alder reaction it adds to many dienes at room temperature, and at 180°C actually adds 1,4- to benzene to give the bicyclo adduct (7) [18341 -68-9] C QHgN2 (47). 

Oxidation of thiophene with peracid under carefully controlled conditions gives a mixture of thiophene sulfoxide and 2-hydroxythiophene sulfoxide. These compounds are trapped by addition to benzoquinone to give ultimately naphthoquinone (225) and its 5-hydroxy derivative (226) (76ACS(B)353). The further oxidation of the sulfoxide yields the sulfone, which may function as a diene or dienophile in the Diels-Alder reaction (Scheme 88). An azulene synthesis involves the addition of 6-(A,A-dimethylamino)fulvene (227) to a thiophene sulfone (77TL639, 77JA4199). 

Benzoquinone is an excellent dienophile in the Diels-Alder reaction. What product would you expect from reaction of benzoquinone with 1 equivalent of 1,3-butadiene Prom reaction with 2 equivalents of 1,3-butadiene ... 

Dihydropyrones as dienophiles in the Diels-Alder reaction application to the synthesis of 1-oxadecalones [158]... 

Avenoza A., Busto J. H., Cativiela C., Peregrina G. M. and Zurbano M. M. The use of 4-Hetaryliden- and 4-Aryliden-5(4H)-Oxazolones as Dienophiles in the Diels-Alder Reactions in Targets in Heterocyclic Systems-Chemistry and Properties vol 3. 1999, Eds. Attanasi O. A. and Spinelli D., Pb. Soc. Chim. Ital. Keywords hetero-Diels-Alder reactions... 

The magnitude of the electrical effect in the 1,3-dipolar cycloaddition reaction is roughly comparable to that of substituted dienophiles in the Diels-Alder reaction. 

Dehydration of P-nitro alcohols provides an important method for the preparation of nitroalkenes. Because lower nitroalkenes such as nitroethylene, 1-nitro-1-propene, and 2-nitro-l-propene tend to polymerize, they must be prepared carefully and used immediately after preparation. Dehydration with phthalic anhydride is the most reliable method for such lower nitroalkenes.42,43 Such lower nitroalkenes have been used as important reagents for Michael acceptors or dienophiles in the Diels-Alder reaction, which will be... 

Functionalized nitroalkenes are important dienophiles in the Diels-Alder reaction. For example, ( )-methyl P-nitroacrylate is an important reagent in organic synthesis. The nitro group can be readily eliminated the Diels-Alder reaction of P-nitroacrylate is equivalent to that of ethyl propiolate with an inverse regiochemistry (Eq. 8.4).11... 

It has been known that aromatic heterocycles such as furan, thiophene, and pyrrole undergo Diels-Alder reactions despite their aromaticity and hence expected inertness. Furans have been especially used efficiently as dienes due to their electron-rich properties. Thiophenes and pyrroles are less reactive as dienes than furans. But pyrroles with A-elecIron-withdrawing substituents are efficient dienes. There exists a limited number of examples of five-membered, aromatic heterocycles acting as dienophiles in Diels-Alder reactions. Some nitro heteroaromatics serve as dienophiles in the Diels-Alder reactions. Heating a mixture of l-(phenylsulfonyl)-3-nitropyrrole and isoprene at 175 °C followed by oxidation results in the formation of indoles (see Eq. 8.22).35a A-Tosyl-3-nitroindole undergoes high-yielding Diels-Alder reactions with... 

As mentioned above, the electrochemical oxidation of a diene yields 1,2- and 1,4-addition products when the reaction is carried out in the presence of a nucleophile such as methanol or acetic acid. When the oxidation is carried out in the absence of the nucleophile it usually yields a polymeric compound as the major product. The formation of a small amount of the Diels-Alder adduct is, however, observed when the reaction is carried out in CH2CI2 with graphite anode. One of the proposed reaction pathways is shown in equation 68, though it is not clear whether the cyclohexadienyl radical serves as a diene (as shown in equation 6) or a dienophile in the Diels-Alder reaction. 

Ethyl azodiformate is a well-known and useful dienophile in the Diels-Alder reaction.8 /-Butyl azodiformate behaves ami-... 

The chiral (E)-oxazolone derived from 1,2-O-isopropyIidene-D-glyceraldehyde has also been used as a dienophile in the Diels-Alder reaction and, in this case, ( /Z) isomerization of the oxazolone can be avoided using heterogeneous catalysts that promote the synthesis of trans-adducts. ... 

The use of 4-heteroarylmethylene- and 4-aryhnethylene-5(47/)-oxazolones as dienophiles in the Diels-Alder reaction has been recently reviewed. More recently the reactivity of the exocyclic double bond of 4-(alkoxymethylene)-5(4//)-oxazolones with several dienes has been assessed. Reaction of 4-(methoxymethyl-ene)-2-phenyl-5(477)-oxazolone and 1,3-butadiene requires the presence of Et2AlCl as a catalyst and even then the Diels-Alder cycloadduct is obtained in low yield. 

Sulfur dioxide acts as a dienophile in the Diels-Alder reaction with many dienes (253,254) and this reaction is conducted on a commercial scale with butadiene. The initial adduct, sulfolene [77-79-2] is hydrogenated to a solvent, sulfolane [126-33-0], which is useful for selective extraction of aromatic hydrocarbons from... 

Arsenin, antimonin and bismin react readily with dienophiles in the Diels-Alder reaction, and the reaction with hexafluoro-2-butyne and other acetylenic dienophiles has been used to trap these heterocycles as barrelenes (115), in particular the unstable bismin (equation 21). The reactivity increases with increasing size of the heteroatom and this may be related to decreased aromatic character of the heterobenzenes with increasing size of the heteroatom. In fact, bismin and antimonin are so reactive that at low temperatures (< -10 °C and < - 50 °C respectively) both exist as Diels-Alder dimers (116 equation 20). 

The nitroaldol may be dehydrated to a nitroalkene in the case of nitroaldols arising from aromatic aldehydes this dehydration reaction occurs spontaneously as is illustrated in the preparation of co-nitrostyrene (Expt 6.136). The nitroalkenes are important dienophiles in the Diels-Alder reaction (Section 7.6, p. 1117). An example of the Michael addition reaction is illustrated in the syn-... 

Both intermediates (40) and (41) are able to act either as dienes or as dienophiles in the Diels-Alder reactions. The Diels-Alder reactions have been carried out by using polymeric monoester of acetylenedicaboxylic acid (42) as a dienophile trapping agent and polymeric ester of 2-furoic acid (23) as a dienic trapping polymer (81JA1797). 

Alkynes are poor dienophiles in the Diels- Alder reaction decomposition occurs by an attempted thermal intramolecular Diels-Alder reaction of dienynes at 160 °C. In contrast, the Ni-catalysed [4+2] cycloaddition of the dienyne 50 proceeded smoothly at room temperature using tri(hexafluoro)isopropyl phosphite to give 51, which was converted to the yohimbine skeleton 52 [15]. The same reaction is catalysed by RhCl(Ph3P)3 in trifluoroethanol [16]. Intramolecular Diels-Alder reactions of the 6,8-dieneyne 53 and the 1,3,8-triene 55, efficiently catalysed by [Rh(dppe)(CH2CH2)2]SbF6 at room temperature, gave 54 and 56 [17],... 

The enol form of dihydrofuro[2,3-, ]coumarin-9-one 116 was also employed as a dienophile in the Diels-Alder reaction with 3,6-bis(trifluoromethyl)l,2,4,5-tetrazine 117 to afford the desired tetracyclic product 118 in 54% yield (Equation 104) <2002S43>. 

Aliphatic nitroso compounds can be prepared from IV-alkylhydroxylamines by oxidation with bromine, chlorine or sodium hypochlorite in weakly acidic solution, by reaction with potassium dichromate in acetic or sulfuric acid, and by oxidation with yellow mercury(II) oxide in suspension in an organic solvent. Silver carbonate on (Telite has also been used to prepare aliphatic nitroso compounds, such as ni-trosocyclohexane, in high yield from the corresponding hydroxylamines. Aqueous sodium periodate and tetraalkylammonium periodates, which are soluble in organic solvents, are the reagents most commonly used for the oxidation of hydroxamic acids and / -acylhydroxylamines to acylnitroso compounds (40). These compounds are rarely isolated, but are useful as highly reactive dienophiles in the Diels-Alder reaction. ... 

Modification of allenes. a-Allenic ketones are excellent dienophiles in the Diels-Alder reaction. This fact coupled with the reversibility of the cycloaddition provides a method for modification of the allenic ketones. An example is cited in equation (I). ... 

A-Sulfinyl amides function as dienophiles in the Diels Alder reaction and with suitably substituted dienes yield 3,6-dihydrothiazine 1-oxides containing two stereogenic carbon atoms bonded to the sulfinyl sulfur and nitrogen93 recent studies have shown that the reaction occurs in a stepwise fashion via dipolar intermediates94. 

Bicyclic diazenes, in which the diazcnc group is not activated by acyl substituents, can act as dienophiles in the Diels-Alder reaction with cyclic dienes only in the presence of a stoichiometric amount of tetrafluoroboric acid to give. V,/V"-bicyclic hydrazines 111 . When two diastereomers can be produced, due to the nonsymmetric dienophile (i.e., when n 2), a single diastereomer is obtained ( H-NMR studies). 

Cyclopentene-l,4-dione is a very reactive dienophile in the Diels-Alder reaction and thus provides access to a variety of compounds containing the reactive /3-dicarbonyl grouping in a five-... 

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