Cycloaddition process

The above cycloaddition process consists of two separate [3-1-2] cycloaddition steps and represents a 1,3-2,4 addition of a multiple bond system to a hetero-1,3-diene [7S7]. The structure ot the azomethine imine intermediate has been proved unequivocally by X-ray analysis [195] Ethylene [194], acetylene [/iS2] . many alkyl- and aryl- as well sgemmal dialkyl- and diaryl-substituted alkenes [196,197, 198, 199], dienes [200], and alkynes [182, 201], certain cyclic alkenes [198, 199, 

An intramolecular variant of this cycloaddition process is combined with a Knoevenagel reaction in a total synthesis of the insectan leporin A, a pyrano[3,2-c]pyridine derivative <96JOC2839>. 

It is instructive to note that the intramolecular [2+2] cycloaddition process should benefit from the presence of the cis C1-C2 double bond in 14. Indeed, the cis C1-C2 double bond is expected to facilitate the key [2+2] cycloaddition event by bringing into proximity the reactive ketene moiety and the C5-C6 olefin and by 

Mathey et al. have described a quite unusual intramolecular [4+2] cycloaddition process. In this reaction the diene and the dienophile are part of two different ligands within the same complex. Thus, ris-(vinyl ethoxycarbene) (1-phenyl-3,4-dimethylphosphole)tetracarbonylchromium complex reacts at 

The application of 1,3-dipolar cycloaddition processes to the synthesis of substituted tetrahydrofurans has been investigated, starting from epoxides and alkenes under microwave irradiation. The epoxide 85 was rapidly converted into carbonyl ylide 86 that behaved as a 1,3-dipole toward various alkenes, leading to quantitative yields of tetrahydrofuran derivatives 87 (Scheme 30). The reactions were performed in toluene within 40 min instead of 40 h under classical conditions, without significantly altering the selectivi-ties [64]. 

The unconventional structure of fulvenes with a unique C=C bond conjugation leads to unusual cycloaddition reactions with other unsaturated systems. For example, alkenylcarbene complexes react with fulvenes leading to indanone or indene derivatives which can be considered as derived from a [6S+3C] cycloaddition process [118] (Scheme 72). The reaction pathway is well explained by an initial 1,2-addition of the fulvene to the carbene carbon followed by [1,2]-Cr(CO)5-promoted cyclisation. 

Nitrile ylides derived from the photolysis of 1-azirines have also been found to undergo a novel intramolecular 1,1-cycloaddition reaction (75JA3862). Irradiation of (65) gave a 1 1 mixture of azabicyclohexenes (67) and (68). On further irradiation (67) was quantitatively isomerized to (68). Photolysis of (65) in the presence of excess dimethyl acetylenedicar-boxylate resulted in the 1,3-dipolar trapping of the normal nitrile ylide. Under these conditions, the formation of azabicyclohexenes (67) and (68) was entirely suppressed. The photoreaction of the closely related methyl-substituted azirine (65b) gave azabicyclohexene (68b) as the primary photoproduct. The formation of the thermodynamically less favored endo isomer, i.e. (68b), corresponds to a complete inversion of stereochemistry about the TT-system in the cycloaddition process. 

Chromium cyclopropylcarbene complexes react with alkynes to provide cyclopentenone derivatives in a formal [2c+2s+lCo] cycloaddition process (see Sect. 3.2). However, tungsten and molybdenum cyclopropylcarbene complexes 

Other relevant reactions have been described for Bfx and Fx as reactants, among them cycloaddition processes, photochemical transformations, and complexation with metals. 

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 

Figure 10 12 shows the interaction between the HOMO of one ethylene molecule and the LUMO of another In particular notice that two of the carbons that are to become ct bonded to each other m the product experience an antibondmg interaction during the cycloaddition process This raises the activation energy for cycloaddition and leads the reaction to be classified as a symmetry forbidden reaction Reaction were it to occur would take place slowly and by a mechanism m which the two new ct bonds are formed m separate steps rather than by way of a concerted process involving a sm gle transition state 

The reaction of JV,iV-dimethylhydrazones (1-amino-1-azadienes) and alkenylcarbene complexes mainly produces [3C+2S] cyclopentene derivatives (see Sect. 2.6.4.5). However, a minor product in this reaction is a pyrrole derivative which can be considered as derived from a [4S+1C] cycloaddition process [75]. In this case, the reaction is initiated by the nucleophilic 1,2-addition of the nitrogen lone pair to the metal-carbon double bond followed by cyclisation and 

The photochemistry of alkenes and dienes has already been mentioned in connection with the principles of orbital symmetry control in electrocyclic and cycloaddition processes in Section 13.2. Cycloadditions are considered, from a synthetic viewpoint, in Chapter 6 of Part B. This section will emphasize unimolecular photoreactions of alkenes and dienes. 

Norbornadienes, norbornenones and their homologs have been prepared [23, 24] by cycloaddition of cyclopentadiene (21) and cyclohexadiene (22) with l-benzenesulfonyl-2-trimethylsilylacetylene (23) and l-ethoxy-2-carbomethox-yacetylene (24). Both were efficient dienophiles in the cycloaddition processes and dienophile 23 acted as an effective acetylene equivalent (Scheme 2.12). Norbornanes and their homologs can also be attained by Diels-Alder reaction 

As pericyclic reactions are largely unaffected by polar reagents, solvent changes, radical initiators, etc., the only means of influencing them is thermally or photochemically. It is a significant feature of pericyclic reactions that these two influences often effect markedly different results, either in terms of whether a reaction can be induced to proceed readily (or at all), or in terms of the stereochemical course that it then follows. Thus the Diels-Alder reaction (cf. above), an example of a cycloaddition process, can normally be induced thermally but not photochemically, while the cycloaddition of two molecules of alkene, e.g. (4) to form a cyclobutane (5), 

The reactions of Fischer carbene complexes with 1,3-dienes (carbodienes or heterodienes) lead to the formation of cyclic products with different ring sizes depending upon both the nature of the reaction partners and the reaction conditions. Between these synthetically useful transformations are found [2c+2s], [3C+2S], [4S+1C], [3S+3C], [4S+2C], [4S+3C] and [2S+1C+1C0] cycloaddition reactions which will be summarised further on, in addition to the [2S+1C] cycloaddition processes here described. 

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