Relative reactivity, nitrile oxide cycloadditions

Cycloaddition with nitrile oxides occur with compounds of practically any type with a C=C bond alkenes and cycloalkenes, their functional derivatives, dienes and trienes with isolated, conjugated or cumulated double bonds, some aromatic compounds, unsaturated and aromatic heterocycles, and fullerenes. The content of this subsection is classified according to the mentioned types of dipolarophiles. Problems of relative reactivities of dienophiles and dipoles, regio- and stereoselectivity of nitrile oxide cycloadditions were considered in detail by Jaeger and... 

The many successful applications of nitrile oxide cycloadditions in synthesis are intimately linked with theory, both the simple FMO variety as well as the more sophisticated ab initio treatment, where the work of Sustmann and subsequently of Houk and his group has been seminal. We, the practitioners, have thus been supplied with a consistent view on the nature of 1,3-dipoles, their reactivity toward dipolarophiles, and the origin and interpretation of stereoselectivity of cycloaddition chemistry. It is of course desirable that our understanding of the relative reactivities of alkenes as well as of many 1,3-dipoles would be also improved, thereby leading to simple, extended recipes for the chemist practicing synthetics. We hope that this account will stimulate further advances in this field of cycloaddition chemistry and promote further uses of nitrile oxides in organic synthesis. 

Although monosubstituted alkenes usually show relatively high reactivity in nitrile oxide cycloadditions to give 5-substituted-2-isoxazolines in a regioselective... 

Hereinafter we wish to report the results relative to the oxidation of aldoximes by hydrogen peroxide catalyzed by W(VI) peroxopolyoxo complexes. In the presence of alkynes or alkenes it is possible to obtain isoxazolines and isoxazoles respectively in very good yields. Implication of such findings will be discussed at the light of nitrile oxides cycloaddition chemistry and on the basis of W(VI) peroxo complex reactive behavior. 

Nitrile oxides are considerably more reactive than nitrones, and nitrile oxide dimerization is a prominent side reaction that can effectively compete with dipolar cycloaddition. To overcome this obstacle, nitrile oxides are typically generated in situ under conditions that lead to their participation in 1,3-dipolar cycloaddition reactions. The resulting isoxazolines constitute a versatile class of heterocycles, which are amenable to extensive manipulation. Confalone s synthesis of biotin (16) includes clever use of an intramolecular nitrile oxide cycloaddition reaction [46] that sei-ves to install the requisite relative configuration as well as the necessary side chain (Scheme 18.4) [47]. 

The relative rate constants (fe ) do not account for the fact that approach of the nitrile oxide to the 7i-bond can occur from both olefinic diastereofaces with two regioisomeric modes of reaction (Scheme 6.14). In the case of achiral 1-alkenes, only one regioisomer is formed. With chiral dipolarophiles, preference for one of the two is usually found (diastereodifferentiation). The relative diastereofacial reactivity (fejH) is used to evaluate this effect (121). With ethylene, there are four possibilities of attack (two for each face corresponding to the different regio-isomers), and the of each is set as 0.25. In diastereodifferentiating cycloadditions, such as those with a-chiral alkenes, the major isomer generally results... 

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