Nitrile oxides defined

Recently, Denmark and coworkers have developed a new strategy for the construction of complex molecules using tandem [4+2]/[3+2]cycloaddition of nitroalkenes.149 In the review by Denmark, the definition of tandem reaction is described and tandem cascade cycloadditions, tandem consecutive cycloadditions, and tandem sequential cycloadditions are also defined. The use of nitroalkenes as heterodienes leads to the development of a general, high-yielding, and stereoselective method for the synthesis of cyclic nitronates (see Section 5.2). These dipoles undergo 1,3-dipolar cycloadditions. However, synthetic applications of this process are rare in contrast to the functionally equivalent cycloadditions of nitrile oxides. This is due to the lack of general methods for the preparation of nitronates and their instability. Thus, as illustrated in Scheme 8.29, the potential for a tandem process is formulated in the combination of [4+2] cycloaddition of a donor dienophile with [3+2]cycload-... 

Most 4,5-dihydroisoxazoles are prepared by cycloaddition of alkenes and nitrile oxides. By variation of the two components, a variety of mono-, bi- and polycyclic products with defined relative configuration is obtainable36. 

Similarly, nitrile oxides react with methyl acrylate 2.42 to give the adduct 2.43 with the substituent on C-5 and terminal alkenes also react in this way to place the alkyl group on C-5. Many dipoles react well with electron-rich dipolarophiles, but not with electron-poor dipolarophiles. Other dipoles are the other way round. To make matters even more complex, the presence of substituents on the dipole can change these patterns and impart their own regioselectivity. Thus the carbonyl ylid reaction 2.45 has a well defined regiochemistry determined only by the substituents, since the core dipole is symmetrical. This reaction also illustrates the point that dipolarophiles do not have to be alkenes or alkynes—they can also have heteroatoms. 

Figures 15.45 and 15.46 illustrate impressively that the significance of 1,3-dipolar cycloadditions extends beyond the synthesis of five-membered heterocycles. In fact, these reactions can provide a valuable tool in the approach to entirely different synthetic targets. In the cases at hand, one can view the 1,3-dipolar cycloaddition of nitrile oxides to alkenes as a ring-closure reaction and more specifically, as a means of generating interestingly functionalized five- and six-membered rings in a stereochemically defined fashion.

The inside alkoxy effect is useful for predicting the stereoselectivity of nitrile oxide cycloaddition reactions with chiral lylic ethers. The hypothesis states that allylic ethers adopt the inside position and alkyl substituents prefer the sterically less-crowded anti conformation in transition states for these electrophilic cycloadditions . The terms inside and outside are defined in (17) for a hypothetical nitrile oxide cycloaddition transition state. Both ab initio (Gaussian 80 with 3-2IG basis set) and molecular mechanics calculations agree, each predicting the lowest-energy transition state to be the one described, i.e. (18 H outside) just above it lies one where the alkyl group is anti, OR outside and H inside (19 ). As illustrated, the former leads to a product wherein OR and the nitrile oxide oxygen are anti, the latter to one with them syn (Scheme 19). 

Alkenoylation of a camphor-based cyclic acylhydrazide leads to substrate-defined dipolarophiles (for nitrile oxides)." On temporary (in situ) derivatization with 75, enals are activated to undergo enantioselective cycloaddition with nitrones. ... 

Bent 1,3-dipolar systems such as ozone, nitrile imines, nitrile oxides, nitrous oxide, and bent allenoid azides are devoid of stereochemical handles at the termini nevertheless they have well-defined molecular faces, and if needed, the HED system can be applied to them as well. 

Elsewhere, x-ray crystallographic analyses have been used to define the structure of a dimeric product from A (2)-acetylation of 4-phenyl-5/f-2,3-benzodiazepine <84JCS(P1)2027>, and confirm 2-ethoxycarbonyl-5-methyl-l0-aryl-l,2,9-triaza-8-oxabicyclo[5.3.0 ]-3,5,9-decatrienes (10), as the main class of products from cycloaddition of aryl nitrile oxides and l-ethoxycarbonyl-5-methyl-l//-... 

Three generations of latices as characterized by the type of surfactant used in manufacture have been defined (53). The first generation includes latices made with conventional (/) anionic surfactants like fatty acid soaps, alkyl carboxylates, alkyl sulfates, and alkyl sulfonates (54) (2) nonionic surfactants like poly(ethylene oxide) or poly(vinyl alcohol) used to improve freeze—thaw and shear stabiUty and (J) cationic surfactants like amines, nitriles, and other nitrogen bases, rarely used because of incompatibiUty problems. Portiand cement latex modifiers are one example where cationic surfactants are used. Anionic surfactants yield smaller particles than nonionic surfactants (55). Often a combination of anionic surfactants or anionic and nonionic surfactants are used to provide improved stabiUty. The stabilizing abiUty of anionic fatty acid soaps diminishes at lower pH as the soaps revert to their acids. First-generation latices also suffer from the presence of soap on the polymer particles at the end of the polymerization. Steam and vacuum stripping methods are often used to remove the soap and unreacted monomer from the final product (56). 

Reaction constants peroxy radical of wo-propylbenzene with 4-Z-re-plased -2,6-di-fert.butylphenols (k ) defined in conditions inhibiting oxidations /50-propylbenzene by oxygen at 50°C in presence azodiisobutyro-nitrile. 

Here we have set nFAEi/2 equal to AG for the electronic promotion This is precise if we define E i/2C1 for the oxidation of the chloride anion in the presence of the reduced metal as the appropriate half-cell reaction. We will return to this point after discussing mixed nitrile halide complexes, where the electrode potentials are again experimentally inaccessible for both ligand types. 

A possible mechanism for the conversion of aryl halides 38 to aryl nitriles 40 invokes a copper(III) intermediate 41, which is reminiscent of the postulated key complex 35 in the Cadiot-Chodkiewicz mechanism outlined above. Despite the gross similarities among these three transformations, there is no clear evidence that the oxidative addition/elimination pathway and copper(III) intermediates which define the Cadiot-Chodkiewicz and Rosenmund-von Braun reaction mechanisms are operant in the typical Castro-Stephens coupling. 

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