Other cycloaddition reactions

Other Cycloaddition Reactions.—Benzo[3,4]cyclobuta[l,2- /]tropone has been 

An interesting new synthesis of azulenes has been developed that involves the [6 + 4] cycloaddition of NiV-diethyl-l-butadienylamine to fulvenes. The Diels-Alder product of dimethyl acetylenedicarboxylate addition to the azulene formed from dimethylfulvene possesses structure (234), whose n.m.r. spectrum indicated it to be a mixture of conformational isomers. The high barrier (AG ss 18—20 kcal 

As discussed in Section 22.5, the [2 + 2] cycloaddition is photochemically allowed. The yields are often only mediocre, but this reaction is still useful because there are few good methods to prepare four-membered rings. As illustrated in the following equations, the cycloaddition can be used to dimerize two identical alkenes or to cyclize different alkenes  

Intramolecular [2 + 2] cycloadditions often give good yields of the adduct, as shown in the following example  

The CO double bond of an aldehyde or a ketone can act as one component in a [2 + 2] cycloaddition with an alkene. The product, a four-membered ring containing an oxygen, is called an oxetane. 

This reaction is an [8 + 2] cycloaddition and is thermally allowed. In terms of the pi systems, a 10-membered ring is formed, an entropically unfavorable process. However, the nitrogen holds the ends of the eight-electron pi system close together, so the two-electron component can easily reach them. Counting via the nitrogen, a 5-membered ring is formed, a process that is much more favorable. 

Explain whether you would use heat or light to accomplish this cycloaddition reaction  

It has been reported that a tetra-terf-butylsilene reacts with 1,3-DIBF to give a [7r2 + 772]cycloadduct (88JOM-C12). Oxyallyl systems 129 (86T4611) 

Similar reactions have been reported for other annulated furans (79CL43). For the reaction of diphenylcyclopropenethione with 1,3-DIBF see Ando and Furuhata (87NKK1293). o-Benzoquinonediimines react with 1,3-diarylisobenzofurans to give [n4 + 7r4]cycloadducts. These compounds can adopt two stable conformations [e.g., 131, 132 81H1009, 81ZN(B)632]. 

Photolysis of a mixture of furan and benzene gives mainly the [4 + 4] cycloadduct (198) a substantial amount of the adduct (199) derived by addition of carbons 2 and 5 of furan and 1 and 3 of 

N-Methylisoindole and isobenzofuran give [8 + 8] photodimers. The [8 + 8] dimer (200) obtained from isobenzofuran at — 60°C has anti stereochemistry (78HCA444,82CC1195). 

Cycloaddition reactions of C-vinyl derivatives of five-membered heterocycles are discussed in Section 3.3.3.3.2. 

Reaction of isobenzofuran with 6,6-dimethylfulvene occurs at room temperature and unexpectedly affords the [6 -I- 4] endo-adduct (268). The non-linear nature of the 6 component is thought to be responsible for the usual gndo-stereoselectivity. The reaction with 6,6-diphenylfulvene, however, occurs only at elevated temperatures 

Tabushi, K. Yamamura, Z.-i. Yoshida, and A. Togashi, Bull Chem. Soc. Japan, 1975,48, 2922. 

---

How do orbital symmetry requirements relate to [4tc - - 2tc] and other cycloaddition reactions Let us constmct a correlation diagram for the addition of butadiene and ethylene to give cyclohexene. For concerted addition to occur, the diene must adopt an s-cis conformation. Because the electrons that are involved are the n electrons in both the diene and dienophile, it is expected that the reaction must occur via a face-to-face rather than edge-to-edge orientation. When this orientation of the reacting complex and transition state is adopted, it can be seen that a plane of symmetry perpendicular to the planes of the... 

Isosydnones (146) react with alkynes to give pyrazoles (150). For example, 4,5-diphenylisosydnone (146, R = R = Ph) and ethyl phenyl propiolate gives 4-ethoxycarbonyl-l,3,5-triphenylpyrazole (150, R = R = R = Ph, R = CO Et) identical with the product from 4,5-diphenylsydnone (1, R = R = Ph). The rate of 1,3-cycloaddition for isosydnones (146) is relatively slow in comparison with sydnones (1).2o, 04 number of other cycloaddition reactions of isosydnones with alkenes, alkynes, and carbonyl compounds have been reported. ... 

Ring Synthesis via 1,3-Dipolar and Other Cycloaddition Reactions... 

Expansion of heterocyclic rings upon treatment with carbenes 5-52 Other cycloaddition reactions... 

The thermal reaction produced 5 and 6 in a 3 2 ratio but the Lewis acid-catalyzed reaction gave a 1 1 ratio. We could not separate the two cycloadducts, but based the indicated constitution on analysis of the mass spectrum and lH NMR spectrum of the mixture of the two compounds. The stereochemistry was assigned on the basis that the Diels-Alder reaction is normally suprafacial-suprafacial18 and that for other cycloaddition reactions of levoglucosenone (2) the addition anti to the 1,6-anhydro bridge is favored.13 15... 

The analysis performed on the basis of the global and local electrophilicity of reagents involved in DA and 13-DC cycloadditions has been extended to other cycloaddition reactions as [4 + 3]100,101 and [2+ 2]102 cycloadditions. 

Apart from acting as effective Lewis base catalysts, the quinuclidine structure of cinchona alkaloids can also participate in the other cycloaddition reaction by a different catalytic mechanism. Calter et al. described an interesting asymmetric interrupted Feist-Benary reaction between ethyl bromopyruvates and cyclohexadione. They proposed that the protonated cinchona alkaloid would perform as a Bronsted acid to form hydrogen-bonding interaction with a-ketoester moiety, rendering it more electrophilic toward attack by either the enol or enolate of cydohexandione. Then intramolecular alkylation would afford the formal [3 + 2] cycloadduct (Scheme 10.12) [16]. 

Scheme 7.2 Some reaction types which are characterized by large, negative volumes of activation (a) Diels-Alder and other cycloaddition reactions, including heterocyclic and intramolecular examples (b) ionogenic reactions, such as Menshutkin quaterniza-tions (c) additions to double bonds, for example, aldol reactions, Wittig reactions and, chiral-borane reductions (in the last type of reaction, pressure both accelerates the reduction and retards decomposition of the Alpine borane to achiral 9-BBN) (d) sequential...

A new synthesis of 2,4,4,6-tetramethyl-4//-l,3-oxazine (155) simply involves a reductive cycloaddition of 4-methylpent-3-en-2-one and acetonitrile in the presence of trimethylsilyl chloride and sodium iodide (Scheme 42) <89TL4741>. Other cycloaddition reactions have been used previously to synthesize 4//-l,3-oxazines and this methodology has been extended to include cycloadditions between alkynes and l-oxa-3-azabuta-1,3-dienes. For example, phenylethyne and the A-benz-oylimine (156) afford 4,4-bis(trifluoromethyl)-2,6-diphenyl-4//-l,3-oxazine (157). The reaction proceeds through a Michael-type addition between the alkyne and the heterodiene giving an adduct which when heated to 80-90°C cyclizes to the oxazine (Scheme 43) <83CC945,89ZN(B)1298>. 

---