Nitrile oxides, reaction with

S-Nucleophiles are very reactive in 1,3-addition reactions with nitrile oxides. A series of a-glucosinolates 27 (R = CR1=NOH R1 =Ph, CH2PI1, CH2CH2PI1, ( )-CH=CHPh, 3-indolylmethyl) was prepared by addition reactions of thiol 27 (R = H) with nitrile oxides (123). The indolyl-substituted glucosinolate was then converted to a-glucobrassicin 28. 

Heterocycles Both non-aromatic unsaturated heterocycles and heteroaromatic compounds are able to play the role of ethene dipolarophiles in reactions with nitrile oxides. 1,3-Dipolar cycloadditions of various unsaturated oxygen heterocycles are well documented. Thus, 2-furonitrile oxide and its 5-substituted derivatives give isoxazoline adducts, for example, 90, with 2,3- and 2,5-dihydro-furan, 2,3-dihydropyran, l,3-dioxep-5-ene, its 2-methyl- and 2-phenyl-substituted derivatives, 5,6-bis(methoxycarbonyl)-7-oxabicyclo[2.2.1]hept-2-ene, and 1,4-epoxy-l,4-dihydronaphthalene. Regio- and endo-exo stereoselectivities have also been determined (259). 

Dipolar cycloaddition of 2,4-(trimethylsilyl)- and 2,4-(trimethylgermyl)-substituted thiophene-1,1-dioxides as well as silylated 2,2 -bithiophene-1,1-dioxides was investigated. It was shown that only the C(4)=C(5) double bond of 2,4-disubstituted thiophene-1,1-dioxides interacts with acetonitrile oxide to give thienoisoxazoline dioxides. Bithiophene derivatives were inactive or their reaction with nitrile oxide was accompanied by desilylation. Cycloaddition of benzonitrile oxide with all mentioned sulfones did not occur. The molecular structure of 3a-methyl-5.6a-bis(trimethylgermyl)-3a,6a-dihydrothieno 2.3-c/ isoxazole 4,4-dioxide was established by X-ray diffraction (263). ... 

I.3.4.2. Intermolecular Cycloaddition at C=X or X=Y Bonds Cycloaddition reactions of nitrile oxides to double bonds containing heteroatoms are well documented. In particular, there are several reviews concerning problems both of general (289) and individual aspects. They cover reactions of nitrile oxides with cumulene structures (290), stereo- and regiocontrol of 1,3-dipolar cycloadditions of imines and nitrile oxides by metal ions (291), cycloaddition reactions of o-benzoquinones (292, 293) and aromatic seleno aldehydes as dipolarophiles in reactions with nitrile oxides (294). 

Cyclic imidate esters, 2-ethoxypyrrolin-5-one and 2-ethoxy-1II -indol-3-one, undergo 1,3-dipolar cycloaddition reactions with nitrile oxides, the reaction site being at the pyrroline C=N bond (317). Rigid and sterically congested pyrroline spiro compounds 148 demonstrate complete diastereofacial selection in site and regiospecific cycloaddition reactions with nitrile oxides to give products 149 (318). 

Iodoacetylene (prepared in situ from ethynylmagnesium bromide or tributyl (ethynyl)tin with iodine) was used as a dipolarophile in the 1,3-dipolar cycloaddition reactions with nitrile oxides to produce 2-(5-iodoisoxazol-3-yl)pyridine and 3-(4-fluorophenyl)-5-iodoisoxazole in good yield (70%-90%). Subsequently,... 

Syntheses of l-aryl-3,3,3-trilluoro-l-propynes and their reactions with nitrile oxides to give 3,5-diaryl-4-tri(luoromethylisoxazoles have been carried out. In particular, l-(4-chlorophenyl)-3,3,3-trilluoro-l-propyne, on reaction with 4-chlorobenzohydroximoyl chloride in the presence of Et3N in PhMe, give a 45% mixture of 211 (R = R1 = 4-CIC6H4) and the regioisomer 212 in the ratio of 97 3, respectively (370). 

The carbon-nitrogen triple bond of aryl thiocyanates acts as a dipolarophile in 1,3-dipolar cycloadditions. Reactions with nitrile oxides yield 5-arylthio-1,2,4-oxadiazoles 227 (X = O Y = S). Aryl selenocyanates behave similarly forming 5-arylseleno-l,2,4-oxadiazoles 227 (X = 0 Y = Se). Reactions of 5-aryl-... 

In the case of primary carbocation, reaction with nitrile oxide gives a mixture of two regioisomeric oximes (Scheme 1.41). Probably, this is a result of the attack of the nitrile oxide - BF3 complex on neutral 3-chloromethylanisole. 

Unsaturations of hydroxy-containing compounds are reduced on reaction with nitrile oxides such as tetramethyl terephthalonitrile N,N -dioxide (506) or 1,3,5-triethylbenzene-2,6-dicarbonitrile oxide (507). The reaction of a nitrile oxide with terminal unsaturation, associated with the preparation of a poly-ol from propylene oxide, reduces the mono-ol content of the poly-ol composition. Thus, stirring a solution of an ethylene oxide-propylene oxide copolymer with an OH content of 2.39% and vinyl unsaturation of 3.58% in THF with l,3,5-triethylbenzene-2,6-dicarbonitrile oxide for 1 min results in an effective removal of the terminal unsaturation. 

Dihydrodiazepines have been shown to undergo regioselective cycloaddition reactions with nitrile oxides or nitrile imines (88MI1 89JHC1619). 

The a,p-unsaturated amides 180-188a have all been used in 1,3-dipolar cycloadditions with nitrile oxides, and some of them represent the most diastereoselective reactions of nitrile oxides. The camphor derivative 180 of Chen and co-workers (294), the sultam 181 of Oppolzer et al. (295), and the two Kemp s acid derived compounds 186 (296) and 187 (297) described by Curran et al. (296) are excellent partners for diastereoselective reactions with nitrile oxides, as very high diastereos-electivities have been observed for all of them. In particular, compound 186 gave, with few exceptions, complete diastereoselection in reactions with a wide range of different nitrile oxides. Good selectivities were also observed when using compounds 183 (298) and 184 (299-301) in nitrile oxide cycloadditions, and they have the advantage that they are more readily available. Curran and co-workers also studied the 1,3-dipolar cycloaddition of 187 with silyl nitronates. However, compared to the reactions of nitrile oxides, lower selectivities of up to 86% de were obtained (302). 

If 4-(ethoxymethylene)-2-phenyl-5(47/)-oxazolone 403 is used as the dipolar-ophile, reaction with nitrile oxides yields cycloadducts 682 with reversed regio-chemistry. Further reaction of 682 can then yield 4-aminoisoxazoline-4-carboxylic acids 683 or 4-amino-3-arylisoxazoles 684 depending on the reaction conditions... 

The orange azo compound (123) may undergo a further reaction with nitrile oxides (used for the oxatriazoline formation), most likely to form a new oxatriazoline which isomerizes to (124 equation 74) (72CB2841). 

The importance of 1,3-dipolar cycloadditions (1,3-DC) in the realm of heterocyclic synthesis is widely documented and recent results concerning the intramolecular version of this methodology, including reactions with nitrile oxides and nitrones for access to isoxazole derivatives, have been reviewed <07T12247>. A quantum chemical study of the Lewis acid effect on the cycloaddition of benzonitrile oxide to propyne has been reported evidencing a small influence on the outcome of the reaction <07T5251>. 

Oxo-2-alkenylphosphonates (259) have been prepared from alkenylphos-phonates by cycloaddition reactions with nitrile oxides, reduction and deprotection (Scheme 42). The approach offers a new method for regioselective y-acylation of allylicphosphonates. 

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