Dienic structure

The reactions of 4-nitrobenzodifuroxan 242 with a series of common dienes, such as cyclopentadiene, cyclohexa-diene, isoprene, 2,3-dimethylbutadiene, and 1-acetoxybutadiene, with ethoxymethyleneacetylacetone were found to proceed very readily to afford stable cycloadducts, which are the result of highly stereoselective normal electron-demand (NED) Diels-Alder reactions. Due to the additional activation provided by the two adjacent furoxan rings, the nitroalkene double bond of compound 242 is also prone to undergo NED reactions with less reactive dienic structures, such as the enol form of ethoxymethyleneacetylacetone and the in situ generated 2-ethoxy-4-(2-furfur-yl)buta-l,3-diene <2004TL1037, 2005T8167>. 

Crosslinking by chemical bond formation was tried with maleic anhydride. It is known that the addition of dienophiles such as maleic anhydride or acrylamide reduces color formation ( discoloration ) in acrylic fibers. A reasonable explanation for this phenomenon has been given by Marien It is based on a Diels-Alder reaction of the dienophile with cis-dienic structures created during the oligomerization of the... 

Note that such cis-dienic structures can be present only if intermolecular oligomerization of nitrile groups has taken place.) The Diels-Alder product interrupts conjugation, thus reducing color formation. In the particular case of maleic anhydride, and under the relatively rough conditions of stabilization, this type of reaction should lead to crosslinking, by cross-anhydridization ... 

Z,E)- or ( , Z)-dienic structures are rather common in insect sex pheromones. The cross-... 

Z)-dienic structure 64 (Z,E)-dienic structure 64 dienyl sulfoxides 190 diethyl 4 chloro-4 -... 

LH symbolizes another molecule of linoleic acid in micelles. 9- and IS-HPODE are the main hydroperoxides formed, possessing a conjugated dienic structure. 

Probably, in the first step the double bonds of linoleic acid isomerise to a dienic structure which leads to dimeric acid by a formal Diels-Adler reaction (reaction 17.29). 

Other types of tetramic acid derivatives are A-acyl-4-methoxy-3-pyrrolin-2-ones or 4-O-methyl ethers of A-acylated tetramic acids. The only metabolites of this type containing a dienic structure are pukeleimides A (87), G (88), B (89) and F (90). They are nontoxic compounds isolated from the marine cyanophyte Lyngbya majuscula [158], a blue-green algae. 

Use of superacids at moderate temperatures has been the subject of an interesting study [1] applied to oils bearing multiple unsaturations. Scheme 3.1 shows the mechanism of this reaction, in which the acid-catalysed partial isomerisation of the dienic structure is followed by Diels-Alder coupling with formation of a cyclic adduct. This phenomenon implies that monounsaturated fatty-acid sequences do not react alone but can be exploited if they are mixed with a dienic counterpart, as in the... 

Grafting an electron-withdrawing group onto a dienic structure has important consequences on its reactivity therefore many methods have been proposed to access dienic esters, amides, nitriles, etc. The electron-deficiency induced on a diene not only reverses its behavior in cycloaddition reaction but also makes it a possible substrate for Michael additions. This well-known phenomenon has, for instance, been put into evidence in the recently isolated bioactive diterpenoid briareolate esters L-N where an ( ,Z)-dienone motive acts as a reversible spring-loaded acceptor [52]. 

Palladium-catalyzed cross-coupling reactions have been largely employed in the synthesis of dienic structures. The very general Suzuki-Miyaura methodology has found a successful application in the stereoselective synthesis of ethyl substituted ( , )-dienoic esters and dienones [54]. This coupling involved a series of vinylboronates 58 and vinyltriflates (or nonaflates) 59, and led, in good yield, to the expected esters and ketones 60. However, a partial isomerization of the electrophilic partner 59 occurred (Scheme 34). 

Me, R = R = R = H R = R = Me, R = R =H) (90JA2432, 9IJA559, 92JAI5I). The dienic group and the sulfur atom are planar but the rhodium atom is above the plane. A localized dienic structure is observed in the six-membered metallocycle. Complex 215 (X = H) formed in the corresponding photochemical reaction with thiophene appears unstable and gradually rearranges to 241... 

The first empirical and qualitative approach to the electronic structure of thiazole appeared in 1931 in a paper entitled Aspects of the chemistry of the thiazole group (115). In this historical review. Hunter showed the technical importance of the group, especially of the benzothiazole derivatives, and correlated the observed reactivity with the mobility of the electronic system. In 1943, Jensen et al. (116) explained the low value observed for the dipole moment of thiazole (1.64D in benzene) by the small contribution of the polar-limiting structures and thus by an essentially dienic character of the v system of thiazole. The first theoretical calculation of the electronic structure of thiazole. benzothiazole, and their methyl derivatives was performed by Pullman and Metzger using the Huckel method (5, 6, 8). 

Fe—Fe bond can be assigned structures 201 or 202 based on spectral data. The other product of this reaction is 193 (R = r-Bu), however, it is produced in minor amounts. Complexes 199 (R = R = r-Bu, R = Ph, R = r-Bu) were obtained. Reaction of 146 (M = Mo, R = Ph, R = R = Ft, R = r" = Me) with (benzyli-deneacetone)iron carbonyl gives rise to the bimetallic complex 200 (M = Mo), which reacts further with the free phosphole to form the bimetallic heteronuclear sandwich 203. The preferable coordination of the molybdenum atom to the dienic system of the second phosphole nucleus is rather unusual. The molybdenum atom is believed to have a greater tendency to coordinate via the trivalent phosphorus atom than via the dienic system. 

The 4ne component in a (4tc + 2n) cycloaddition need be neither a four-atom system (as in 1,3-dienes), nor involve carbon atoms only, so long as the HOMO/LUMO symmetry requirements for a concerted pathway can be fulfilled. The most common of these non-dienic 4ne systems involve three atoms, and have one or more dipolar canonical structures, e.g. (34a), hence the term—1,3-dipolar addition. They need not, however, possess a large permanent, i.e. residual, dipole, cf. diazomethane (34a 34f>) ... 

Due to the formal analogy to the classical Diels-Alder reaction, the mechanism of cyclic peroxide formation through cycloadditions of 1,3-dienes with O2 was considered for a long time to involve a concerted suprafacial [4 4- 2]-cycloaddition of a super-dienophile, namely a singlet oxygen to 1,3-dienic system In such a case, the concerted or almost concerted cycloaddition must be c -stereospecific and the stereochemical properties of the diene must be reflected in the three-dimensional structure of cyclic peroxide according to well-defined rules. Indeed, it was found in early stereochemical... 

Epoxy syntactic foams are the best known representatives of this type of material. The brands manufactured in the USSR are EDS (with glass microspheres), EDM (with phenol microspheres). Dienic, novolac, bisphenolic, and esteric structured epoxy resins are used... 

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