Diels-Alder type reactions

A large number of Diels-Alder-type reactions, involving both aromatic and sulfonyl isocyanates, have been reported. Heterodienes having high electron density ate found to add to isocyanates to form sis membered heterocycles as shown in Figure 2 (48—50). 

Fig. 2. Diels-Alder-type reactions of aromatic and sulfonyl isocyanates.

Reaction between oxygen and butadiene in the Hquid phase produces polymeric peroxides that can be explosive and shock-sensitive when concentrated. Ir(I) and Rh(I) complexes have been shown to cataly2e this polymerisation at 55°C (92). These peroxides, which are formed via 1,2- and 1,4-addition, can be hydrogenated to produce the corresponding 1,2- or 1,4-butanediol [110-63-4] (93). Butadiene can also react with singlet oxygen in a Diels-Alder type reaction to produce a cycHc peroxide that can be hydrogenated to 1,4-butanediol. 

C-21 dicarboxyhc acids are produced by Westvaco Corporation in Charleston, South Carolina in multimillion kg quantities. The process involves reaction of tall oil fatty acids (TOFA) (containing about 50% oleic acid and 50% hnoleic acid) with acryhc acid [79-10-7] and iodine at 220—250°C for about 2 hours (90). A yield of C-21 as high as 42% was reported. The function of the iodine is apparendy to conjugate the double bond in linoleic acid, after which the acryhc acid adds via a Diels-Alder type reaction to form the cycHc reaction product. Other catalysts have been described and include clay (91), palladium, and sulfur dioxide (92). After the reaction is complete, the unreacted oleic acid is removed by distillation, and the cmde C-21 diacid can be further purified by thin film distillation or molecular distillation. 

Cycloaddition Reactions (Diels-Alder Type Reactions). 271... 

Catalyst 163 also catalyses the intermolecular [4+2] Diels-Alder-type reaction between alkynes 164 and dienes 165 (Scheme 5.43) [49b], Products 166 were obtained in an excellent regioselectivity and moderate to excellent yields. 

Scheme 5.43 Intermolecular [4+2] Diels-Alder-type reaction between alkynes and dienes...

P, y-Unsaturated sulfoxides (e.g. 3-thiene oxides) can be prepared by trapping of the in Situ-generated 349 with dienes in a Diels-Alder-type reaction (equation 127) . 

Recent study of silenes has clearly established their ability to act as dienophiles in [2 + 4] Diels-Alder-type reactions involving 1,3-dienes. However, it has also been clearly demonstrated that products of the ene reaction commonly accompany the [2 + 4] cycloadducts and may on occasion be the major products. In addition, unlike those in carbon chemistry, [2 + 2] cycloadditions are often observed to occur in competition with the above processes—not only from reactions of silenes with dienes... 

In addition, it has been discovered that there are naturally occurring enzymes that facilitate Diels-Alder type reactions within certain metabolic pathways and that enzymes are also instrumental in forming polyketides, isoprenoids, phenylpropanoids, and alkaloids (de Araujo et al., 2006). Agresti et al. (2005) identified ribozymes from RNA oligo libraries that catalyzed multiple-turnover Diels-Alder cycloaddition reactions. 

It is interesting to note that Diels-Alder-type reactions (example 23, Table I) occur in a different and stereoselective way in the presence of a nickel catalyst (55). This fact strongly supports the hypothesis of metallacycle intermediates. 

When aldehyde containing an electron-withdrawing group is employed, or when a Lewis acid promoter is present, C=0 double bonds can readily undergo Diels-Alder-type reactions. This process is referred to as the oxo Diels-Alder reaction, and it has been explored by Danishefsky and DeNinno35 for the synthesis of a wide range of saccharide derivatives. 

Domino Heck-Diels-Alder type reaction has been demonstrated by the reaction of l,3-dicydopropyl-l,2-propadiene with iodobenzene in the presence of dimethyl maleate under palladium catalysis (Scheme 16.28) [33]. 

Other structural concepts evolved, one of them being triptycenes (55). Here, additional wings are introduced into the molecules by Diels-Alder type reactions. The glass-forming properties are improved by the propeller-like rigid structure [89]. We end our overview of the different classes of molecular glasses with these materials and note that the number of published structures is growing every year. More details on the different materials can be found in one of the recent chemical reviews [102-104]. 

An ab initio study of the l-azabicyclo[1.1.0]butyl cation (97) and its isomers shows that (98) and (99) are much less stable than (97), and that the transition states between (97), (98), and (99) are too high in energy to allow (99) to form. The 3-halobicyclo[l.l.l]pent-l-yl cation (101) has been shown to be an intermediate in the addition of halogens to (100). The only product observed was (102) no rearranged products were detected. The Diels-Alder-type reaction of (103) to give (104) is said to involve several carbenium ion intermediates. 

In 2006, two groups independently developed an asymmetric Brpnsted acid-catalyzed aza-Diels-Alder-type reaction of iV-aryl aldimines 86 with cyclohexenone 101 to provide isoquinuclidines 102 in good yields (51-84%), endo-diastereoselec-tivities (3 1-9 1), and enantioselectivities (76-88% ee) (Scheme 39). 

Scheme 39 Aza-Diels-Alder-type reaction of cyclohexenone...

Scheme 41 Mechanism of the aza-Diels-Alder-type reaction of cyclohexenone 2.3.12 1,3-Dipolar Cycloaddition...

The cobalt catalyzed cocyclization of alkynes with heterofunctional substrates is not limited to nitriles. cpCo-core complexes are capable of co-oligomerizing alkynes with a number of C,C, C,N or C,0 double bonds in a Diels-Alder-type reaction. Chen, in our laboratories, has observed that these cycloadditions are best performed with the help of stabilizers such as ketones or acetic esters that are weakly coordinated to the cobalt and prevent the alkynes from being cyclotrimerized at the metal center... 

In some cases, it is possible to promote the Diels-Alder-type reaction. We have found that in the presence of polar auxiliary ligands (stabilizer), the Co-mediated reaction of alkynes with activated C,C or C,0 double bonds takes place (87MI6). These findings are only understandable if there is a direct interaction between the cp-cobaltacyclopentadiene and the polar stabilizer, which may act by blocking free coordination sites at the Co [Eq.(45)]. 

The 5-tosyloximes of 2,2-dimethyl-l,3-dioxane-4,5,6-trione react with various dienes in a hetero-Diels-Alder-type reaction. The products, aza-dioxaspiro[5,5]undecenes, readily decompose with basic T-chlorosuccinimide to afford 2-carboxypyridines <1998JOC7840>. The same substrates gave with amines cyanoformamides in 43-73% yield (Scheme 22) <2000H(52)283>. 

CpC2 gives the monomer CpC. It was suggested that one of these dimers might be the result of a Diels-Alder type reaction. 

About seventy kinds of new phenolic compounds could be isolated from Japanese cultivated mulberry tree (Morus alba, M. bombycis, and M, Ihou) and Chinese crude drug Sang-Bai-Pi (the root bark of Chinese mulberry tree). Most of them are isoprenylated flavonoids. Among them, kuwanon G (1) was the first isolation of the active substance exhibiting the hypotensive effect from the Japanese Morus alba root bark [9]. Furthermore, kuwanon G (1) and its isoprenylated derivative kuwanon H (2) [10] are considered to be formed through an enzymatic Diels-Alder type reaction of a chalcone and a dehydrokuwanon C or its... 

The analogy of these dimerization processes to thermal Diels-Alder type reactions which sometimes also yield cyclobutane structures is worth noting and may be taken as one of the arguments for a diradical structure of the transition in the latter process. Also, it may be pointed out that the photoexcited state involved is presumably the same one involved in the well-known photochemical trans-cis interconversion of such olefins. 

Inverse electron demand Diels-Alder/retro-Diels-Alder-type reactions, of di- and especially poly-azines with electron-rich dienophiles, interconvert six-membered rings. 1,2,4-Triazines react with enamines and enol ethers to give pyridines (Scheme 76) (CHEC-n(5)242). 

Other reduced pyridazines and 1,2-oxazines can be also prepared by Diels-Alder-type reactions. Butadiene condenses with Et02CN = NC02Et and nitrosobenzene to yield (99) and (100), respectively. 

Diels-Alder reaction of onitrosobenzamide and 1,3-DIBF (no mp and spectral data given, only HRMS 93BSF101). Taylor and co-workers reported Diels-Alder type reactions of 1,3-DIBF and nitrosopyridines [82JOC552]. A Diels-Alder adduct of 1,3-DIBF and an azetidinone rearranges thermally to a ring opened product (92CJC2792). Whereas... 

Hi. [2+4] Cycloaddition reactions. Conlin and Namavari have demonstrated that 1-silabuta-1,3-dienes may undergo a stereospecific [4 + 2] and non-stereospecific [2 + 2] cycloaddition with alkenes54. Isomerization of 1,1-dimethylsilacyclobutene 80 in the presence of 20-fold excess of ethylene at 350 °C leads cleanly to a mixture of the isomeric silacyclohexenes 82 and 83 in nearly quantitative yields. Formation of the silacyclohex-2-ene 82 might be anticipated from a Diels-Alder-type reaction between ethylene and sila-1,3-diene 81 (equation 185). 

Both l-(butadien-l, 3 -yl)silatrane (93) and -trialkoxysilane can react by a Diels-Alder-type reaction with tetracyanoethylene (TCNE) or maleic anhydride (MA) to give the corresponding adducts (equations 129 and 130). However, a higher temperature is required for effective conversion of trialkoxysilane355. 

Photolysis of the oxadiazabicyclo[2.2.3]nonadiene 285 gave a rearranged bicyclic system 286 plus 1-ethoxycarbo-nyl-177-azepine 287 (Equation 42) < 1999H(51)141 The latter was shown to form on photolysis of 285, but it was suggested on the basis of kinetic evidence, that some may also be formed directly from 285 by what is formally a retro-Diels-Alder type reaction. 

The benzyne functions as a dienophile towards reactive diene systems. The reactivity of the 9,10-positions in anthracene is well known (Diels-Alder reaction, Section 7.6), and addition of benzyne to 9-bromoanthracene yields the interesting cage-ring alkyl halide 9-bromotriptycene (9-bromo-9,10-o-benzenoanthracene). The reaction is incomplete and some unreacted 9-bromoanthracene remains in the crude reaction products, but may be removed by virtue of its ready conversion into a maleic anhydride adduct in a further Diels-Alder-type reaction. 

Domino-Knoevenagel-hetero-Diels-Alder-type Reactions... 

Domino-Knoevenagel-hetero-Diels—Alder-type Reactions I 291... 

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