Cheleotropic reaction

Cheleotropic reactions are cycloadditions in which one of the components interacts through a single atom, as in Equation 11.22. Group transfers are character-... 

The pericyclic theory predicts that cheleotropic reactions of four-electron with two-electron components may occur by the linear path (p. 628), that is, supra-facially on the one-atom component, if the diene enters also suprafacially (six-electron interaction diagram 28). A number of examples are known, most of... 

Difluorocarbene can be generated by thermolysis of a trifluoromethylmercurial in the presence of sodium iodide, and its cycloaddition to norbornadiene has been investigated. A mixture of two products (79) and (80) is obtained, the latter evidently arising by a linear cheleotropic reaction. Similar primary products are obtained from the addition of chlorofluorocarbene to norbornadiene, but here the reaction is complicated by secondary cyclopropyl-allyl rearrangements. 

Thiophenes are less reactive than furans and therefore react with very reactive dienophiles. They behave somewhat differently from furans and in many cases the intermediate addition products are unstable and undergo cheleotropic extrusion of sulfur [30]. Thiophenes 30 undergo cycloaddition reactions with DMAD (Equation 2.11) to afford bicyclic cycloadducts which lead to phthal-ates by sulfur extrusion, thus offering a one-pot synthesis of dimethylphthalates [31]. 

As a result of our previous work on the scope and mechanism of tertiary amine nitrosation (X), we became interested in the behavior of N-alkylaziridines toward nitrous acid. Possible modes of reaction are illustrated in Scheme 1. The operation of either path A or C would be consistent with our previous studies of oxidative dealkylation of tertiary amines (1 ), while pathway B would be akin to the observed cheleotropic transformation of N-nitroso-aziridines (2). 

Thermal extrusion of a sulfur atom is the most common thermal reaction of a thiepin. The mechanism of this thermal process involves two orbital symmetry controlled reactions (69CC1167). The initial concerted step involving a reversible disrotatory electrocyclic rearrangement is followed by a concerted cheleotropic elimination of sulfur (Scheme 29). Similar aromatization reactions occur with thiepin 1-oxides and thiepin 1,1-dioxides, accompanied by the extrusion of sulfur monoxide and sulfur dioxide respectively. Since only a summary of the major factors influencing the thermal stability of thiepins was given in Section... 

The simplest example of a 2 + 2 addition about which there is substantial information would appear to be the addition of carbenes and nitrenes to olefins (Equations 12.4 and 12.5). Woodward and Hoffmann have designated reactions of this kind, in which one component is a single atom, as cheleotropic.5... 

The examples shown in Equations 12.8-12.11 illustrate a few of the other known reverse 2 + 2 cheleotropic cycloadditions. Of these reactions, the first two... 

Cheleotropic fragmentation a fragmentation reaction in which two a bonds to a single atom are broken Dehydrohalogenation an elimination reaction in which the elements of a hydrogen halide molecule are eliminated from adjacent atoms leaving a n bond... 

Thiophene 1-oxide is unusually reactive both as a diene and as a dienophile in the Diels-Alder reaction and thiophene S,N-ylids apparently exhibit similar reactivity. When electron-rich alkenes, (e.g., acenaphthalene) are treated with 60 (R = C02Et) (Scheme 10), a rapid reaction ensues to yield the product derived from a [4 -f- 2]-cycloaddition followed by cheleotropic elimination of a thionitroso compound. This reaction appears to be fairly general for nonhindered alkenes, and even relatively unreactive systems, such as thiophene itself, give low yields of 61. This is an unusual reaction... 

The latter compound is similar to 60 (R = C02Et) in reactivity toward acenaphthylene, undergoing [4 + 2]-cycloaddition/cheleotropic elimination to the known sulhnylamine Et02C—N=S=0, which was sufficiently stable to be isolated but underwent facile cycloaddition with 2,3-dimethyl-1,3-butadiene to give the thiazine sulfoxide 65. Although 64 is a fairly reactive molecule, the reactivity in cycloaddition reactions is less than that of 60, which is in keeping with the known differences in reactivity of thiophene 1-oxide and thiophene 1,1-dioxide. 

In light of the known reactivity of the thiophene S,N-ylids, a detailed examination of the reaction of thiophene with ethyl azidoformate has been undertaken. A careful product analysis revealed the presence of previously unreported products, such as 68, 70, and 75, which can be explained satisfactorily only in terms of the intermediacy of the thiophene S,N-ylid 66 (Scheme 12) (86TL1105). Thus, if the initial reaction of thiophene with the ethoxycarbonylnitrene generates 66, the products 68-70 may be rationalized in terms of a Diels-Alder dimerization to 67. Cheleotropic elimination of 69 from 67, followed by aromatization, would result in the formation of 68. Alternatively, 66 could undergo rearrangement by way of a bicyclic transition state (71) (cf. 33, Scheme 7) to a dipolar intermediate 72 (analogous to 35, Scheme 7). Proton transfer in 72 would then furnish 73 analogous to the 2-... 

The reversible reaction of 1,3-dienes with sulfur dioxide [59] provides another means to mask dienes for use in cycloaddition reactions [60]. An early example of this strategy in an intramolecular context was reported by Nicolaou, leading to a total synthesis of the estratrienone derivative 100 (Equation 7) [61]. Cheleotropic extrusion of SO2 from 98 formed 99, which participated in a diastereoselective Diels-Alder cycloaddition to give 100 in 85 % isolated yield. 

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