Cycloaddition reactions involving

Isocyanates are Hquids or soHds which are highly reactive and undergo addition reactions across the C=N double bond of the NCO group. Reactions with alcohols, carboxyUc acids, and amines have been widely exploited ia developiag a variety of commercial products. Cycloaddition reactions involving both the C=N and the C=0 double bond of the NCO group have been extensively studied and used for product development (1 9). 

When the orbitals have been classified with respect to symmetry, they can be arranged according to energy and the correlation lines can be drawn as in Fig. 11.10. From the orbital correlation diagram, it can be concluded that the thermal concerted cycloadditon reaction between butadiene and ethylene is allowed. All bonding levels of the reactants correlate with product ground-state orbitals. Extension of orbital correlation analysis to cycloaddition reactions involving other numbers of n electrons leads to the conclusion that the suprafacial-suprafacial addition is allowed for systems with 4n + 2 n electrons but forbidden for systems with 4n 7t electrons. 

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... 

Although the first metal-catalyzed asymmetric 1,3-dipolar cycloaddition reaction involved azomethine ylides, there has not been any significant activity in this area since then. The reactions that were described implied one of more equivalents of the chiral catalyst, and further development into a catalytic version has not been reported. 

The following cycloaddition reactions involve chiral auxiliaries and proceed with a good degree of diastereoselectivity. Provide a rationalization of the formation of the preferred product on the basis of a TS. 

In 1995, Boyd and co-workers <95TL7971 > covalently linked a porphyrin to fullerene Cgo through a 1,3-dipolar cycloaddition reaction involving the porphyrinic azomethine ylide 28 (Scheme 8). The ylide was generated in situ from befa-formyl-meso-tetraphenylporphyrin 27 and A -methylglycine, and provided the porphyrin-C6o diad 29 in good yield. 

Cycloaddition reactions involving dithione intermediates derived by the reductive extrusion of sulfur atoms from [1,2,3,4,51pen tathi enopino[6,7-i>]pyrroles provided access to 1,4-dithin-fused pyrroles (i.e., 89) <06OL4529>. 

Therefore, in spite of extensive investigations, it is difficult to a priori predict the stereochemical outcome of the intramolecular [3 + 2]-cycloaddition reaction involving six-membered cyclic nitronates as well as to find optimal reaction conditions. 

In cycloaddition reactions involving the acridizinium and isoquino-linium ions, the presence of a positive charge leads with great stereoselectivity to products which could not be predicted by the rule of Alder and Stein. A charge-transfer complex arising from the attraction... 

Cycloaddition reactions involving thermal/photochemical/catalytic decomposition of iodonium ylides are applicable to oxazole derivatives... 

Theoretical calculations have been an important means of rationalizing the electronic course of hetero-Diels-Alder and related pericylic reactions for the formation of 1,2-thiazines 25 and 26. MOP AC 93 PM3 calculations have been used to deduce the regioselectivity of [4-1-2] cycloaddition reactions involving thiazinylium perchlorate 27 (Scheme 1) <1999TL1505>. Due to the higher lowest unoccupied molecular orbital (LUMO) coefficient at C-6 compared to N-2, the C-6 and S-1 behave preferentially as the dienophile double bond in cycloaddition reactions of this substrate with butadienes 28. 

Despite the fact that relatively numerous examples of [4 + 2] cycloaddition reactions involving electron-rich and, to a lesser extent, electron-... 

Relatively few authenticated photoinduced concerted [w2 + w2] cycloaddition reactions involving pyrroles or indoles have been reported. The formation of many cycloadducts can be rationalized in terms of two-step thermally induced ionic mechanisms. 

The cycloaddition reactions involving group 14 metalloles have been extensively reported in the previous review123. Here we summarize the main points, and give new results. 

Wagner and Sakamoto [95] have studied the triplet decay of para-alkeny-loxyacetophenones with a substituent (X = Cl, CN, OMe, Me) at the position ortho to the alkenyloxy side chain. They propose that the cycloaddition reaction involves charge-transfer interaction between donor double-bond and acceptor triplet benzene first, followed by biradical formation (Scheme 18). 

In the presence of two equivalents of silver fluoride, N-protected bis[(trimethylsi-lyl)methyl]amines lead also to azomethine ylids which can be trapped by dipolarophiles. The mechanism of the cycloaddition reaction involves sequential electron-Me3Si+-electron transfer process from the amine to silver fluoride, which forms silver metal, ruling out a fluoride-induced desilylation process. Although silver is recovered at the end of the reaction, a cheaper oxidizing reagent is still lacking.448,449... 

The enolate ion derived from the nitrone 15 undergoes an unusual intramolecular dipolar cycloaddition reaction involving the enol double bond to yield 16 (95T6285). 

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