Conjugated diene Diels-Alder reactions

Diels-Alder reaction is the 1,4-addition of an alkene or alkyne (dienophile) across a conjugated diene. An example is the addition of pro-penal to buta-l,3-diene to give A -tetrahy-... 

CH = CH — CH = CH — are said to have conjugated double bonds and react somewhat differently from the other diolefins. For instance, bromine or hydrogen is often added so that a product of the type -CHBr-CH=CH-CHBr- is formed. Also, these hydrocarbons participate in the Diels-Alder reaction see diene reactions). They show a tendency to form rubber-like polymers. Hydrocarbons not falling into these two classes are said to have isolated double... 

The Diels-Alder Reaction consists in the direct combination of a compound containing a conjugated diene system u ith a reagent which possesses a double or triple bond activated bj suitable adjacent groups. Examples of such reagents are maleic anhydride, p-benzoquinone, acraldehyde and acetylene dicarboxylic esters. Combination always occurs at the 1,4 positions of the diene system ... 

The Diels-Alder Reaction usually occurs readily it is of great value (a) for diagnosing the presence of a conjugated diene grouping, (6) for synthetic purposes in the preparation of cyclic systems. 

Compounds containing a double or triple bond, usually activated by additional unsaturation (carbonyl, cyano, nitro, phenyl, etc.) In the ap position, add to the I 4-positions of a conjugated (buta-1 3-diene) system with the formation of a ax-membered ring. The ethylenic or acetylenic compound is known as the dieTwphile and the second reactant as the diene the product is the adduct. The addition is generally termed the Diels-Alder reaction or the diene synthesis. The product in the case of an ethylenic dienophile is a cyctohexene and in that of an acetylenic dienophile is a cyctohexa-1 4-diene. The active unsaturated portion of the dienophile, or that of the diene, or those in both, may be involved in rings the adduct is then polycyclic. 

A particular kind of conjugate addition reaction earned the Nobel Prize in chemistry for Otto Diels and Kurt Alder of the University of Kiel (Germany) m 1950 The Diels-Alder reaction is the conjugate addition of an alkene to a diene Using 1 3 buta diene as a typical diene the Diels-Alder reaction may be represented by the general equation... 

Section 10 12 Conjugate addition of an alkene (the dienophile) to a conjugated diene gives a cyclohexene derivative in a process called the Diels-Alder reaction It is concerted and stereospecific substituents that are cis to each other on the dienophile remain cis m the product... 

Diels-Alder reaction (Section 10 12) Conjugate addition of an alkene to a conjugated diene to give a cyclohexene denva tive Diels-Alder reactions are extremely useful in synthesis... 

Myrcene with its conjugated diene system readily undergoes Diels-Alder reactions with a number of dienophiles. For example, reaction with 3-meth.5i-3-pentene-2-one with a catalytic amount of AlCl gives an intermediate monocyclic ketone, which when cyclized with 85% phosphoric acid produces the bicycHc ketone known as Iso E Super [54464-57-2] (49). The product is useful in providing sandalwood-like and cedarwood-like fragrance ingredients (91). 

The most notable chemistry of the biscylopen-tadienyls results from the aromaticity of the cyclopentadienyl rings. This is now far too extensively documented to be described in full but an outline of some of its manifestations is in Fig. 25.14. Ferrocene resists catalytic hydrogenation and does not undergo the typical reactions of conjugated dienes, such as the Diels-Alder reaction. Nor are direct nitration and halogenation possible because of oxidation to the ferricinium ion. However, Friedel-Crafts acylation as well as alkylation and metallation reactions, are readily effected. Indeed, electrophilic substitution of ferrocene occurs with such facility compared to, say, benzene (3 x 10 faster) that some explanation is called for. It has been suggested that. 

When Diels and Alder published their famous paper in 1928, Diels had been working with related reactions for several years [6]. In 1925, Diels reported the reaction of azodicarboxylic ester (Et0C(0)2CN=NCC(0)0Et) with compounds containing a conjugated diene system. He found that addition of the azodicarboxylic ester occurs at the 1,4-position of the conjugated system as with cyclopentadiene and with butadiene. This work probably led to the famous Diels-Alder reaction. In 1927, Diels and his student Alder published a paper on the reaction of azodicarboxylic ester with styrene. 

The Diels-Alder reaction,is a cycloaddition reaction of a conjugated diene with a double or triple bond (the dienophile) it is one of the most important reactions in organic chemistry. For instance an electron-rich diene 1 reacts with an electron-poor dienophile 2 (e.g. an alkene bearing an electron-withdrawing substituent Z) to yield the unsaturated six-membered ring product 3. An illustrative example is the reaction of butadiene 1 with maleic anhydride 4 ... 

Based on the facile formation and reactivity of323, and the retro Diels-Alder reaction of 325306,310, a simple procedure has been developed for the stereoselective synthesis of functionalized conjugated dienes as well as vinylallenes311 (see equation 119). 

The classical Diels-Alder reaction is a cycloaddition between a conjugated diene and a second component, called dienophile, which has at least a n bond (Equation 1.1). When one or more heteroatoms are present in the diene and/or dienophile framework, the cycloaddition is called a hetero-Diels-Alder reaction. 

Keywords hefera-Diels-Alder reaction of ethylglyoxylate with conjugated dienes... 

Lopez J. C., Lukacs G. Pyranose-Derived Dienes and Conjugated Enals. Preparation and Diels-Alder Cycloaddition Reactions ACS Symp. Ser. 1992 494 33-49 Keywords carbohydrate, befera-Diels-Alder reactions, stereoselectivity... 

In the Diels-Alder reaction a double bond adds 1,4 to a conjugated diene (a 2 + 4 cycloaddition), so the product is always a six-membered ring. The double-bond compound is called a dienophile. The reaction is easy and rapid and of very broad scope and reactivity of dienes and dienophiles can be predicted based on analysis of the HOMOs and LUMOs of these species. Ethylene and simple alkenes make poor dienophiles, although the reaction has been carried out with these compounds. 

As applied to cycloaddition reactions the rule is that reactions are allowed only when all overlaps between the HOMO of one reactant and the LUMO of the other are such that a positive lobe overlaps only with another positive lobe and a negative lobe only with another negative lobe. We may recall that monoalkenes have two n molecular orbitals (p. 9) and that conjugated dienes have four (p. 36), as shown in Figure 15.1. A concerted cyclization of two monoalkenes (a 2 -f- 2 reaction) is not allowed because it would require that a positive lobe overlap with a negative lobe (Fig. 15.2). On the other hand, the Diels-Alder reaction (a 2 -f 4 reaction) is allowed, whether considered from either direction (Fig. 15.3). 

The compounds p2C=CX2 (X = P or Cl), especially p2C=Cp2, form cyclobutanes with many aikenes. Compounds of this type even react with conjugated dienes to give four-membered rings rather than undergoing normal Diels-Alder reactions. 

The use of chiral bis(oxazoline) copper catalysts has also been often reported as an efficient and economic way to perform asymmetric hetero-Diels-Alder reactions of carbonyl compounds and imines with conjugated dienes [81], with the main focus on the application of this methodology towards the preparation of biologically valuable synthons [82]. Only some representative examples are listed below. For example, the copper complex 54 (Scheme 26) has been successfully involved in the catalytic hetero Diels-Alder reaction of a substituted cyclohexadiene with ethyl glyoxylate [83], a key step in the total synthesis of (i )-dihydroactinidiolide (Scheme 30). 

In its simplest form the Diels-Alder reaction is a cycloaddition reaction between a conjugated diene and alkene (dienophile). 

The Diels-Alder reaction is an organic chemical reaction (specifically, a cycloaddition) between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene system. The reaction can also proceed if the alkene is replaced by an alkyne moiety or even if some of the atoms... 

Similar reactions were applied to the syntheses of rf/-ep/-pyroangolen-solide, i/-pyroangoensolide (Eq. 12.29)88 and the formal synthesis of the Inhoffen-Lythgoe diol (Eq. 12.30).89 The key step in the formal synthesis of the Inhoffen-Lythgoe diol is the aqueous Diels-Alder reaction between the sodium salt of the diene and methacrolein to form the cycloadduct, which then undergoes subsequent reactions to form the known hydrin-dan. Sodium (E)-4, 6, 7-octatrienoate reacted smoothly with a variety of dienophiles to give conjugated diene products.90... 

The aqueous Diels-Alder reaction has also been used for bioconjugate studies. A Diels-Alder reaction of diene oligonucleotides with maleimide dieneophiles was used to prepare oligonucleotide conjugates in aqueous media under mild conditions (Eq. 12.39).102 A Diels-Alder-type cycloaddition of an electronically matched pair of saccharide-linked conjugated dienes and a dienophile-equipped protein was the... 

It is of interest to investigate the usefulness of this theory to the chemical change involving the interaction between the conjugated systems 56,62,145). Such a-n interactions are frequently stereoselective. The addition to olefinic double bonds and the a, -elimination are liable to take place with the fraMS-mode 146h The Diels-Alder reaction occurs with the cis-fashion with respect to both diene and dienophile. 

Cycloaddition of 2-cyanoalk-2-enones with several conjugated dienes proceeded under zinc chloride catalysis.636 Zinc halides have also shown reactivity with phenylacetylenes.637 Zinc chloride is an effective Lewis acid catalyst in the Diels Alder reactions of the keto esters and the effects on stereochemistry of catalysts used have been examined.638... 

The natural product panepophenanthrin (6/1-170), isolated in 2002 from the fermented broth of the mushroom strain Panus radus IFO 8994 [90], is the first example of an inhibitor of the ubiquitin-activating enzyme [91]. Retrosynthetic analysis based on a biomimetic analysis led to the conjugated diene 6/1-172 by a retro-Diels-Alder reaction via the hemiacetal 6/1-171. Further disconnections of 6/1-172 produces the vinyl stannane 6/1-173 and the vinyl bromide 6/1-174 [92]. 

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