A cycloaddition

The alkene that adds to the diene is called the dienophile Because the Diels-Alder reaction leads to the formation of a ring it is termed a cycloaddition reaction The prod uct contains a cyclohexene ring as a structural unit... 

Epoxidation of alkenes (Sec tion 6 18) IS another example of a cycloaddition... 

Contrast the Diels-Alder reaction with a cycloaddition reaction that looks superfl cially similar the combination of two ethylene molecules to give cyclobutane... 

Endo adducts are usually favored by iateractions between the double bonds of the diene and the carbonyl groups of the dienophile. As was mentioned ia the section on alkylation, the reaction of pyrrole compounds and maleic anhydride results ia a substitution at the 2-position of the pyrrole ring (34,44). Thiophene [110-02-1] forms a cycloaddition adduct with maleic anhydride but only under severe pressures and around 100°C (45). Addition of electron-withdrawiag substituents about the double bond of maleic anhydride increases rates of cycloaddition. Both a-(carbomethoxy)maleic anhydride [69327-00-0] and a-(phenylsulfonyl) maleic anhydride [120789-76-6] react with 1,3-dienes, styrenes, and vinyl ethers much faster than tetracyanoethylene [670-54-2] (46). 

Methacrylates have also found use in diastereoselective -ene reactions. Although not a cycloaddition reaction, this reaction is mechanistically related to the Diels-Alder reaction (37). 

Polymerization by Gycloaddition. Bisimides and oligoimides capped with reactive unsaturations such as maleimide, acetylene, and xylylene groups, can be chain-extended by a cycloaddition reaction with proper bisdienes. 

Some ketones undergo a cycloaddition reaction with alkenes to form oxetanes ... 

There are also reactions which show stereoselectivity primarily because of mechanism rather than spatial bias of substrate. For instance, the conversion of an olefin to a 1,2-diol by osmium tetroxide mechanistically is a cycloaddition process which is strictly suprafacial. The hydroxylation transform has elements of both substrate and mechanism control, as illustrated by the retrosynthetic conversion of 146 to 147. The validity of the retrosynthetic removal of both... 

Bis(tnfluoromethyl)-4,5-dihydrooxazin-6-ones [28] and their O-acetylated dcnvatives [96] are formed on treatment of acyl imines with acetyl chloride-hiethylamine at room temperature. The reaction was interpreted as a cycloaddition reaction involving a ketene [28] However, the periselectivity and regiochemistry of this reactwn-are not in agreement with results obtained from the reaction of... 

The first example of a cycloaddition reaction of a multiple bond to a diene was reported in 1917 Surprisingly, it was found that benzal azine adds to 2 equivalents of several unsaturated systems, when offered in excess, to yield bicyclie compounds. This reaction was named criss-cross" cycloaddition [190], Exploitation of the preparative potential of criss-cross cycloaddition began only in the early 1970s, when hexafluoroacetone azine became available on a larger scale [191,192] The study of this reaction proved to be an impetus tor the development of azine chemistry [183, 193]... 

The dimeric tellurium diimide 10.7 undergoes a cycloaddition reaction with BuNCO to generate the Ai,iV -ureatotellurium imide 10.11, which is converted to the corresponding telluroxide 10.12 by reaction with excess BuNCO. " By contrast, BuN=S=N Bu undergoes exchange reactions with isocyanates. 

In the case of enamines derived from aldehydes a cycloaddition to give a cyclobutane occurs (48-50). Thus the enamine (16) reacted with methyl acrylate in acetonitrile to give a 91 % yield of methyl 2-dimethylamino-3,3-dimethylcyclobutane carboxylate (56). Similarly, treatment of (16) with diethylmaleate at 170° gave a 70% yield of diethyl 4-dimethylamino-3,3-dimethyl-l,2-cyclobutanedicarboxylate (57), and 16 and acrylonitrile gave a 65% yield of 2-dimethylamino-3,3-dimethylcyclobutanecarbonitrile (58). 

Most of the reactions described in this chapter would fall under Huisgen s definition of a cycloaddition reaction (/, la). However some of the reactions described would not be considered cycloaddition reactions according to this restrictive definition. Therefore, the more liberal definition given by Baldwin will be used as a guideline, namely, Cycloadditions are chemical transformations giving at least one product having at least two new bonds as... 

Substituted TMM complexes also cycloadd to aldehydes in the presence of a tin cocatalyst such as MesSnOAc and MesSnOTs [31]. Reaction of 2-heptenal with methyl precursor (6) gave a mixture of methylenetetrahydrofurans (68) and (69). This regioselectivity is reversed with 10-undecenal and methyl precursor (5), where adduct (70) now predominates over (71). As in the carbocyclic system, the phenylthio group also functions as a regiocontrol element in reaction with cyclohexyl aldehyde. The initially formed adduct (72) eliminates the element of thio-phenol on attempted allyl rearrangement, and the overall process becomes a cycloaddition approach to furans (Scheme 2.21) [20]. 

The reaction course of the cycloaddition reaction can also be dependent on the Lewis acid complex used as the catalyst. When the substrate contains an allylic C-H bond, both a cycloaddition and an ene reaction can occur. In the reaction of glyoxylate 4 with 2,3-dimethyl-l,3-butadiene 5 both the cycloaddition product 6... 

This methodology has been used for the synthesis of the C3-C14 segment 24 of the antitumor agent laulimalide 23 (Scheme 4.22) [35]. The constrained chiral BOX ligand 21c in combination with Cu(OTf)2 afforded dihydropyrane 6f by a cycloaddition reaction in good yield and ee this was converted to the C3-C14 segment 24 via a Ferrier-type rearrangement in several steps. 

Five-tnembered heterocycles through a cycloaddition reaction... 

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 ... 

By a photochemically induced elimination of CO, a chromium carbene complex with a free coordination site is generated. That species can coordinate to an alkyne, to give the alkyne-chromium carbonyl complex 4. The next step is likely to be a cycloaddition reaction leading to a four-membered ring compound 5. A subsequent electrocyclic ring opening and the insertion of CO leads to the vinylketene complex 6 ... 

This process, named the Diels-Alder cycloaddition reaction after its discoverers, is extremely useful in organic synthesis because it forms two carbon-carbon bonds in a single step and is one of the few genera) methods available for making cyclic molecules. (As the name implies, a cycloaddition reaction is one in which two reactants add together to give a cyclic product.) The... 

A cycloaddition reaction is one in which two unsaturated molecules add to one another, yielding a cyclic product. As with electrocyclic reactions, cycloadditions are controlled by the orbital symmetry of the reactants. Symmetry-allowed... 

How can we predict whether a given cycloaddition reaction will occur with suprafacial or with antarafacial geometry According to frontier orbital theory, a cycloaddition reaction takes place when a bonding interaction occurs between the HOMO of one reactant and the LUMO of the other. An intuitive explanation of this rule is to imagine that one reactant donates electrons to the other. As with elec-trocyclic reactions, it s the electrons in the HOMO of the first reactant that are least tightly held and most likely to be donated. But when the second reactant accepts those electrons, they must go into a vacant, unoccupied orbital—the LUMO. 

The following reaction takes place in two steps, one of which is a cycloaddition and the other of which is a reverse cycloaddition. Identify the two peri-cyclic reactions, and show how they occur. 

The conformationally locked racemic enamine, 4-(4-rOT-butyl-1-cyclohexenyl)morpholine, reacts with (l-nitroethenyl)benzene to give a mixture of diastereomeric 1,2-oxazine 2-oxides 1 and 2 (ratio 1/2 75 25). Whether these arise via an ionic or a cycloaddition mechanism is unclear. Hydrolysis of 1 and 2 with dilute acid gave a 80 20 mixture of trans- and cis-ketones, 3 and 414. 

We have also used poly(propynoic acid) in our studies of the photochemical interaction of PCSs with dienophiles, such as maleic anhydride, tetracyanoethylene, and styrene. This photochemical reaction of Diels-Alder type is accompanied by the breakdown of the conjugation system and the formation of slightly colored adducts266. Together with the cycloaddition reaction, photodegradation of PPA and its adducts takes place. A cycloaddition reaction is always preceded by the formation of a donor-acceptor complex of a PCS with a dienophile. 

Since the first demonstration of a cycloaddition reaction of a, /f-unsaturated sulfones in 1938 by Alder and coworkers85, a variety of a, /3-unsaturated sulfones have been prepared and used as dienophiles. For example, when a mixture of p-tolyl vinyl sulfone and 2,3-dimethylbutadiene in benzene is heated at 145-150 °C for 10 h in a sealed tube, crystals of the cycloadduct (134) are obtained (equation 102). Other examples of this intermolecular cycloaddition reaction are given in Table 12. 

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. 

Secondary orbital interactions (SOI) (Fig. 2) [5] between the non-reacting centers have been proposed to determine selectivities. For example, cyclopentadiene undergoes a cycloaddition reaction with acrolein 1 at 25 °C to give a norbomene derivative (Fig. 2a) [6]. The endo adduct (74.4%) was preferred over the exo adduct (25.6%). This endo selectivity has been interpreted in terms of the in-phase relation between the HOMO of the diene at the 2-position and the LUMO at the carbonyl carbon in the case of the endo approach (Fig. 2c). An unfavorable SOI (Fig. 2d) has also been reported for the cycloaddition of cyclopentadiene and acetylenic aldehyde 2 and its derivatives (Fig. 2b) [7-9]. The exo-TS has been proposed to be favored over the endo- IS. 

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