Other Dienophiles

Steliou et al. have described a method for generation of what may be S2 from silyl and germanyl trisulfides [Eq. (24)].  

In the presence of a 1,3-diene this species produces [4 + 2] cycloadducts. The exact nature of this reactive intermediate has not yet been determined. It was found that a boron imide undergoes a Diels-Alder cycloaddition with cyclopentadiene under mild conditions [Eq. (25)].  

Previous reviews a. Hamer, J, (ed.) (1967). 1,4-Cycloaddition Reactions, the Diels-Alder Reaction in Heterocyclic Syntheses. Academic Press, New York. b. Weinreb, S. M., and Staib, R. R. (1982). Tetrahedron 38,3087. c. Wollweber, H. (1972). Diels-Alder Reaktion. G. Thieme, Stuttgart. 

For several examples of coball-mediated intramolecular cyclizations of nitnles to pyri- 

1-Oxabutadiene Systems Bearing C-2, C-3, or C-4 Electron-Withdrawing Substituents 1-Oxabutadiene Systems Bearing C-3 or C-4 Electron-Donating Substituents 

---

The conclusion is that there are no known examples of behaviour of type (a) for pyrazoles. The same negative conclusion could have been extended to the other reaction pathways of Figure 25 until very recently when an example of type (e) was described (Scheme 31 (82PC40400). Other dienophiles do not take part in this reaction. 

The Asymmetric Diels-Alder Reaction of other Dienophiles... 

Reactive unsaturated nitroso compounds such as 1112 can also be readily prepared from a-halooximes such as 1111 on treatment with powdered Na2C03 in diethyl ether to give, in the presence of enoltrimethylsilyl ether 1113 or strained olefins such as norbomene and other dienophiles, hetero-Diels-Alder adducts such as 1114 and 1115 in moderate yields [150-155] (Scheme 7.47). 

Among the unique features of Sc(03SCF3)3 is its ability to function as a catalyst in hydroxylic solvents. Other dienophiles, including (V-acryloyloxazolidinones, also are subject to catalysis by Sc(03SCF3)3. Indium trichloride is another Lewis acid that can act as a catalyst in aqueous solution.40... 

Dailey and coworkers [6] extended the studies of Prinzbach using 4-1 as substrate. These authors found that, by employing dicyanoacetylene 4-15 in the reaction with 4-1 the domino adduct 4-16 but not 4-17, as expected, is the main product (Scheme 4.4). In the formation of 4-16 one of the two 1,3-butadiene moieties in 4-1 has reacted with the dienophile 4-14 from the inside, followed by a second [4+2] cycloaddition of the formed dicyanoethene moiety. 4-17 is observed as a side product here, in the first step, the dienophile reacts from the outside, while in the second step the other formed dienophile moiety undergoes a cycloaddition with the second 1,3-bu-tadiene moiety. This mode of action is actually favored in the reaction of all other dienophiles employed, due to their larger size when compared to 4-15. 

The generation and trapping of 5,6-bis(trimethylsilyl)benzo[c]furan 126 was reported by Wong utilizing Warrener s s-tetrazine methodology. The trapping of the silylated isobenzofuran with A-phenylmaleimide is illustrated below. A number of other dienophiles such as dimethyl acetylenedicarboxylate, benzoquinone, naphthoquinone and anthra-l,4-quinone have also been used <00TL5957>. 

An example of such n2 + tz2 + n2 addition is the cycloaddition of tetracyanoethylene and other dienophile to nor bomadiene. 

The reaction of indolizines with dialkyl acetylenedicarboxylates in the presence of a dehydrogenating catalyst leads to 1,2-dicarbalkoxycycl-[3,2,2]azines.22 23 Methyl phenylpropiolate may be used instead, although attempts to effect reaction between indolizine and certain other dienophiles including diphenylacetylene, diethyl azodicarboxylate, and 1,3-cyclohexadiene were unsuccessful. Hydrolysis of the diesters yielded the corresponding acids. Subsequent decarboxylation proceeded in high yield using copper chromite in quinoline [Eq. (5)]. 

The reaction of other dienophiles with (343) has also been investigated. Use of the less reactive alkene 1,4-butenediol (348) did furnish pyridoxine directly, albeit in a low yield (6.75%). 

Isobenzimidazoles and their mono and di-JV-oxides show no tendency to react in a Diels-Alder fashion with tetracyanoethylene or a wide range of other dienophiles. However, with DMAD in cold benzene, the di-jV-oxide 421 gave a 1 2 adduct (422) whose structure was assigned on the basis of X-ray data.227... 

Benzo[6]acridine, which, like anthracene, adds maleic anhydride and other dienophiles across the 6,11-positions of a carbocyclic ring, with DMAD in methanol gives a compound corresponding to 465.438... 

A3-Thiadiazolines (112) are unstable, losing nitrogen to yield thio-carbonyl ylids (113), which have been trapped with DMAD and other dienophiles.582 Hence, by allowing 112 to warm to room temperature in the presence of DMAD, 20% of 114 was obtained. Although... 

A.2.6. Asymmetric Diels-Alder Reactions with Other Dienophiles... 

DMAD), and a 1 1 mixture of the diesters 2a (endocyclic product) and 4a (exo-endo product) was obtained when the reaction was carried out at room temperature. The overall yield (10%) was very low due to polymerization of the 2-vinylfuran. In contrast with reactions with other dienophiles (see Section II,B,1), in which the exo-endo diene of 2-vinylfurans proved to be more reactive than the furan system itself, the reaction with DMAD resulted in reduced selectivity, probably due to equilibration of the more soluble adducts in this case (68TL4589 73AJC1059). 

Trapping of l-azetin-4-one as a dienophile was attempted by using some polymers functionalized with dienic groups, such as the 2-(polymeric carb-oxymethyl)-3-methyl-1,3-butadiene of the polymeric ester of 2-furoic acid, which have been good trapping agents for other dienophilic systems. Under several experimental conditions, both dienic polymers were recovered unchanged. 

The Diels-Alder reaction with /V-phenylmaleimidc has frequently been used for the separation, purification, and structure determination of ortho photocycloadducts [12,47,86,90,108,116,126,132,133,138], Other dienophiles that have been successfully employed in Diels-Alder reactions with ortho adducts are A-(para-bromophenyl)maleimide [116,120], maleimide [116,118,127], maleic anhydride [127,191], tetracyanoethylene [11], and dimethyl acetylenedicarboxy-late [73,127], The Diels-Alder product of A-(para-bromophenyl)maleimide with the exo-ortho adduct formed from 1,4-dioxene and benzene [120] and the Diels-Alder product of maleimide with the endo-ortho adduct from cis-cy-clooctene and benzene [118] were obtained in crystalline form and their structures could be determined by means of x-ray diffraction. 

The reaction of 85 with other dienophiles, however, proved to be less effective /V-mcthacryloyloxazolidinonc 104 gave no reaction whereas methacrolein gave product 105 in lower yield and ee (Scheme 28). 

The reaction of methacrolein with cyclopentadiene catalyzed by a chiral menthoxyaluminum complex gives adducts with ee s of up to 72%, but with other dienophiles little, if any, induction was noted.9495 A chiral cyclic amido aluminum complex 2 catalyzes the cycloaddition of cyclopentadiene with the fran.v-crotyl derivative 3 in good yield and enantioselectivity (Scheme 26.2).47 This chiral catalyst can also be easily recovered. 

There is a paucity of functional group reactions associated with the pyrrolopyrazines. A few examples may be found within the references describing their syntheses. However, a reaction has been reported which describes the [4 + 2] cycloaddition of pyrrolopyrazines. Ring expansion of 2-phenyl-2//-pyrrolo[3,4-/ ]pyrazine (32) occurs through cyclocondensation of the trimethylsilyl ether (33) with, amongst other dienophiles, methyl acrylate, and a mixture of quinoxalines (34) is obtained in 38% yield (Scheme 2) <86JHC1641>. 

Furo[3,4-(/]pyrimidines are ideally suited for participation in Diels-Alder reactions. The reaction of the derivatives (47) with dienophiles leads to tricyclic compounds of the type (48) <9iJOC245>. There is some selectivity, since methyl acrylate forms only two regioisomeric endo products whilst other dienophiles yield both endo and exo adducts (Equation (13)). 

Now for some real examples. Most ene reactions with simple alkenes are with maleic anhydride. Other dienophiles—or enophiles as we should call them in this context—do not work very well. However, with one particular alkene, the natural terpene P-pinene from pine trees, reaction does occur with enophiles such as acrylates. 

Other dienophiles, e.g., l,3,4-thiadiazole-2,5-dione 2, 4,4-dialkyl-4//-pyrazole-3,5-diones 3, 3H-pyrazol-3-ones 4 and azoquinones 5 and 6 are prepared in situ by oxidation of the dihydro precursor with lead tetraacetate or fcrt-butyl hypochlorite (see Section 7.2.10.3.). 

Precomplexation of a,/ -unsaturated ketone 163 with ATPH in CH2CI2 at -78 °C, then cyclization with cyclopentadiene, resulted in the stereochemical reversal to furnish exo-164 as a major product, as shown in Sch. 128. Similarly, the Diels-Alder reaction with other dienophiles complexed with ATPH resulted in exo selectivity [168],... 

---