Methyl isocyanate 2+2 cycloaddition reactions

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Vinyl ethers undergo many cycloaddition reactions similar to those which take place with enamines. In general, however, these cycloaddition reactions with vinyl ethers take place less readily than those with enamines. These reactions include cycloaddition of vinyl ethers with ketene (200-205), phenyl isocyanate (206), sulfene (207,208), methyl acrylate (209), diethyl acetylenedicarboxylate (210), and diphenylnitrilimine (183). 

The pyrido[l,2-tf][l,3,5]triazine-2,4(3//)-dione derivative 89 was obtained in a cycloaddition reaction of diphenyl-methyl isocyanate 90 with 2-pyridyl isocyanate 91 derived from the corresponding acyl azide via Curtius rearrangement <2002ARK438>. Compound 89 was also synthesized by the reaction of diphenylacetyl chloride 118 and picolinyl azide 116a in the presence of triethylamine (Scheme 11) <2002ARK438>. ... 

One of the earliest reported photochemical reactions of t-1 is the formation of the -lactam 84 in 45% yield upon irradiation in neat phenyl isocyanate (137). Because isomerization of t-1 is more rapid than cycloaddition, - -t is the presumed intermediate in this reaction. Methyl isocyanate fails to react with It. ... 

Cycloaddition reactions have been carried out with corresponding bases of Vilsmeier salts of pyrroles. Thus reaction of 153 with dimethyl acetylenedicar-boxylate gave pyrrolizines 154 and 3a-azaazulenes 155 in a nonsynchronous reaction. The structure of 3H-pyrrolizine 154a was proved by X-ray analysis.96 In a closely related reaction, pyrrolizinones 156 and 157 were obtained from the corresponding Vilsmeier salts with phenyl isocyanate.97 Reaction of 2-p-nitrobenzoylindolin-3-one (158) with methyl acetoacetate gave the benzopyrrolizinone (159).5 7... 

Azine approach. Pyridine TV-oxides undergo 1,3-dipolar cycloaddition reactions with aryl isocyanates. The first formed 1,2-dihydropyridines are unstable and rearrange at once to the 2,3-dihydropyridines (287). From 3-picoline 1-oxide both the 6-methyl and 7a-methyl isomers are formed. The adduct (288) from 3,5-dibromopyridine 1-oxide reacts further to eliminate HBr, thereby giving the fully aromatic system (79CPB2261). 

Several additional examples of the intramolecular cycloaddition of unactivated acetylenes with isomiinchnones were reported by Maier [30]. This cycloaddition approach represents an efficient method for providing rapid access to annulated furans present in several sesqui- and diterpenes, such as the panicu-lides [51],furanonaphthoquinones [52],furodysin,andfurodysinin [53,54].The decomposition of acyclic acetylenic diazoimides 102 and 103 with Rh2(OAc)4 resulted in cycloaddition and retro-Diels-Alder extrusion of methyl isocyanate to give annulated furans 104 and 105 in good yield. The overall transformation is closely related to the intramolecular Diels-Alder reactions of acetylenic oxa-zoles extensively studied by Jacobi and coworkers [55]. 

Carbodiimides also play a role as building blocks of numerous N-heterocycles. Examples are the cycloaddition reactions of carbodiimides. In the formation of [2-1-2] cycloadducts N-methyl-N -t-butylcarbodiimide is used as a marker to determine the mode of addition of isocyanates or isothiocyanates to carbodiimides. In these reactions addition across the C=N or the C=0 bond of the isocyanate or across the C=N or the C==S bond of the isothiocyanate can occur. Addition to the N-methyl-N -t-butylcarbodiimide always proceeds across the N-methyl double bond. Also, theretroreaction of the [2-1-2] cycloaddition reaction of carbodiimides can afford a different set of heterocumulenes. ... 

While simple unactivated cyclopropanes have yet to be used for [3 + 2] cycloaddition, Tsuji and coworkers have developed a palladium-catalyzed cycloaddition reaction using electron-deficient vinylcy-clopropanes. Thus, vinylcyclopropane (43) undergoes smooth cyclization with methyl acrylate in the presence of a palladium catalyst to give vinylcyclopentane (44) as a mixture of diasteroisomers (equation 35). The cycloaddition probably proceeds through the zwitterionic ( ir-allyl)palladium intermediate (45) and its stepwise reaction with the acrylate (equation 36). Enones such as cyclopentenone and methyl vinyl ketone will also react. Reaction of the same vinylcyclopropane with phenyl isocyanate produces vi-nyllactam (46) (equation 37).Some cycloaddition reactions with (cyclopropyl)Fp complexes have also been reported. However, the substrates are limited to SO2 and TCNE and the yields have not been disclosed (equation 38). ... 

The structure confirmation of dimethyl l-(4-chlorophenyl)-2-methyl-5-oxo-2-imidazoline-4,4-dicarboxylate 91, synthesized via a 1,3-dipolar cycloaddition reaction between an azomethine ylide and an isocyanate, was based on an nOe (7.5%) between the 2-methyl group and the 2 -aromatic H, and the fact that there was one s-[ nitrogen at —72 ppm (N3) in the NMR and one sf nitrogen at —212 ppm (Nj) in the N MMR <1997T6351> (Figure 20). 

Studies detailing the thermal and photochemical reactions of these compounds are rare. The l-oxa-2,4-diazine 63 is known, from H NMR, to be in equilibrium with the spiro isomer 64, with the former predominating. Heating this mixture above 40 °C results in decomposition to acetonitrile, methyl isocyanate, and benzene (Scheme 4) <1995H(40)619>. The formation of compounds such as 63 (and 64) by cycloaddition of nitrile oxides to 8-azahepta-fulvenes is discussed in Section 9.05.10. 

Often the initially formed bonds at low temperature are not the ones that are isolated at room temperature. Also the electronic configurations play a part in product formation. For example, in the [2+2] cycloaddition reaction involving two carbodiimides the more nucleophilic carbodiimide attacks the more electrophilic carbodiimide giving rise to the formation of only one reaction product. The latter reactions proceed stepwise, while sometimes concerted reactions are observed. Sterical hindrance also plays an important role in product formation. We have utilized A-methyl-A -t-butylcarbodiimide as a probe in determining the structure of the derived cycloadducts, because the reaction always proceeds via addition across the C=N bond with the methyl substituent. For example, in the [2+2] cycloaddition reaction with benzoyl isocyanate the reaction proceeds across the C=0 bond of the isocyanate, because t-butyl isocyanate is the only product generated in the retro reaction... 

In a recent theoretical study of the reaction of 1 -aza-2-azoniaallene salts, with isocyanates it was concluded that these reactions proceed in asynchronous but concerted pathways. The presence of chloro groups in the l-aza-2-azoniaallene salts, as well as methyl groups in the isocyanates, favor the cycloaddition reactions . 

Intramolecular [4+2] cycloaddition reactions of isocyanates are sometimes observed. For example, heating of 1,3-pentadienyl isocyanate 465 in o-dichlorobenzene affords 3-methyl-2(l/f)-pyridine-2-one 466, albeit in 17 % yield" . 

Trifluoromethylcarbonyl isocyanate reacts with diarylimines via a [4+2] cycloaddition reaction to give 6-trifluoro-methyl-4-oxo-2,3-dihydro-4//-l,3,5-oxadiazines 512. ... 

While the cycloaddition reaction of phenyl isothiocyanates with nitrones proceeds across its C=N bond, substituted phenyl isothiocyanates, methyl isothiocyanate and benzoyl isothiocyanate react across their C=S bonds. However, these cycloadducts are not stable and they undergo fragmentation with formation of thioamides and isocyanates . ... 

Several syntheses of annulated uracils of biological value were recently reported. The key reaction was a microwave-assisted one-pot [4 -i- 2] cycloaddition of oxazino[4,5-d]-, pyrano-[2,3-d]-, pyrido[2,3-dj- and pyrimido[4,5-djpyrimidines, in the sohd state [134] and under solvent-free conditions [135]. The synthetic approach was based on the reaction of NJ -di-methyl-5-formylbarbituric acid 208 with maleimide in the sohd state for 5 min under microwave irradiation at 120 °C to give the pyrano[2,3-d]pyrimi-dine derivative 209 in 90% yield (Scheme 76). The reaction of 208 with phenyl isocyanate under microwave irradiation in the absence of solvent... 

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