Chelotropic addition

Finally, it should be of interest that the rearrangements of several cyclic benzylic sulfinates have also been described in the literature by Durst46 and Hogeveen47, and seem to proceed by a special two-step mechanism retro-Diels-Alder extrusion of S02, followed by its chelotropic addition to the unstable quinodimethane intermediate (e.g. equation 10). 

Extrusion (or elimination) of sulfur from thiiranes and thiirenes is a facile process. Virtually all thiiranes and thiirenes, as well as their V-oxides and V,V-dioxides, undergo thermal extrusion of the sulfur moiety with increasing facility according to the trend S << SO < S02. Sulfur monoxide produced in this way can be utilized in cycloaddition processes, for example, from 2 in refluxing toluene with 2,3-dimethyl-l,3-butadiene in a chelotropic addition <1997JOC8366>. 

Chelotropic addition of dichlorocarbene to bornadiene gave (200 X = C1) whereas with difluorocarbene the, yyn-adduct was favoured over the anh-adduct (200 X = F) (cf. Vol. 3, p. 59). A thio-Claisen rearrangement has been reported with the allylic enethiolic ether of thiocamphor. Flash thermolysis of allyl exo-2-bornyl sulphide to thiocamphor and propene has been examined. 

Carbene certainly fulfills the requirements for a chelotropic reagent, having two unused AOs containing two electrons. On the other hand, ethylene differs from all other even systems in that it cannot for steric reasons undergo conrotatory chelotropic additions. Since disrotatory cis addition by the path of Fig. 5.39 would lead to an antiaromatic transition state isoconjugate with cyclobutadiene, the reaction has to follow a different course. If therefore reactions of this kind are classed as chelotropic, they have to be regarded as unique exceptions. 

MO calculations of the reaction of 3,4-bis(methoxycarbonyl)-l,2-dithiete (150) with alkenes to produce 2,3-dihydro-1,4-dithiins (152) confirmed that the reaction is a reverse-electron-demand hetero-Diels-Alder type between the valence isomer 1,2-bis(methoxycarbonyl)ethane-l,2-dithione (151) and the dienophiles (Scheme 58). Sulfur dioxide has been shown to promote its hetero-Diels-Alder and chelotropic additions to 1,2-dimethyUdenecyclohexane. The competition between hetero-Diels-Alder and chelotropic addition of sulfur dioxide with substituted 1,3-dienes has been investigated by studying the theoretical and experimental effects on the relative stability of sultines and sulfolenes. ... 

The canthines are a tetracyclic subclass of j8-carboline alkaloids bearing an additional D-ring. Members of the canthine family have been shown to have pharmacological activity, including antifungal, antiviral, and antitumor properties. In 2003 Lindsley and coworkers described a microwave-mediated procedure for one-pot synthesis of the basic canthine skeleton [122]. The key step is an inverse-electron demand Diels-Alder reaction and subsequent chelotropic expulsion of N2, a reaction that can be achieved after a three-component condensation of an acyl hydrazide-tethered indole with a 1,2-diketone and excess ammonium acetate to form a triazine (Scheme 10.61). 

Additions to Electron-deficient Species. Diazomethane will also add to highly electrophilic species such as sulfenes or im-minium salts to give the corresponding three-memheredring heterocycles. When the reaction is performed on sulfenes, the products are episulfones which are intermediates in the Ramherg-Backlund rearrangement, and are therefore precursors for the s)mthesis of alkenes via chelotropic extrusion of SO2. The sulfenes are typically prepared in situ by treatment of a sulfonyl chloride with a mild base, such as Triethylamine (eq 47). Similarly, the addition of diazomethane to imminium salts has been used to methyle-nate carbonyls. In this case, the intermediate aziridinium salt is treated with a strong base, such as Butyllithium, in order to induce elimination (eq 48). 

The term chelotropic reaction refers to reactions in which an odd-numbered ring is formed by addition of a monocentric reagent across the end of an even, conjugated system. The classic example is the formation of the sulfone (214) by addition of sulfur dioxide to butadiene,... 

The addition of carbene to an olefin could be regarded as the simplest possible chelotropic reaction and indeed Woodward and Hoffmann class it as such e.g.,... 

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