Intramolecular electrocyclic reaction

The 2,6-dimethyl derivatives 353a gives the tricyclic ketone 354 resulting from an intramolecular electrocyclic reaction. However, the parent thieno-tropone (51) and its 2- (353b) or 7-methyl derivatives give [4 + 2] dimers (e.g., 355) of the head-to-head type. These dimers are formed by reaction of the 5,6-ene bond of one molecule with the 5,7-diene system of another. The dimerization seems to be the normal photochemical reaction of this group of thienotropones and to be suppressed by a 6-methyl substituent (e.g., in 353a). 

Pcricyclic reaction intramolecular electrocyclic reaction (e.g., spiropyrans, spirooxazines, Mgides, etc.), cycloaddition (2 + 2) reaction. 

Whereas the cycloaddition of arylazirines to electron-deficient olefins produces J -pyrrolines, a rearranged isomer is formed when the alkene and the azirine moieties are suitably arranged in the same molecule. This intramolecular electrocyclization reaction was first observed by Padwa and Smolanoff using 2-vinyl-substituted 2/f-azirines. Irradiation of 2i/-azirine 107 afforded a 2,3-disubstituted pyrrole (108), while thermolysis gave a 2,5-disubstituted pyrrole (109). Photolysis of azirine 110 proceeded similarly and gave 1,2-diphenylimidazole (111) as the exclusive photoproduct. This stands in marked contrast to the thermal reaction of 110 which afforded l,3-diphenylp3nrazole... 

Next, another work on a Br0nsted acid-catalyzed intramolecular electrocyclization reaction was reported by list [36]. They demonstrated that chiral Bronsted acid 104 could efficiently promote the cycloisomerization of a,P-unsaturated hydrazones 105 to give pyrazoUnes 106 in high yields and with good enantioselectivity (Scheme 36.28). This is also the first example of an organocatalytic asymmetric 671 electrocyclization reaction. 

The reaction of 4,4-bis(tnfluoromethyl)-I,3-diaza-1,3-butadienes with certain a,P-unsaturated ketones yields pyrimidine derivatives A two-step mechanism, metathesis-electrocyclic ring closure and metathesis-intramolecular ene reaction, is a plausible explanation for the experimental results (pathway 4, equa-bon 25) [259]... 

An electrocyclic reaction is defined as a concerted closure of a ring starting from a conjugated open system in which a k orbital is converted into a o orbital. It is therefore an example of intramolecular cycloaddition or retroaddition. 

Pharmaceutically important 3-substituted-[l,8]naphthyridine-2,4-diones have been prepared by the reaction of 2-methyl-477-pyrido[2,3-r/][3,l]oxazin-4-one with active methylene compounds (Scheme 66) <1997J(P1)1487, 2003JOC4567> and by the same group via an intramolecular azadiene-ketene electrocyclization reaction of amino-nicotinic acid derivatives in a related process <2001JOC4413>. 

This technique applies to many open-chain compounds, as discussed in later chapters. Pertinent here is the intramolecular cyclization of polyenes (an electrocyclic reaction). 

The only example of the generation of a monocyclic heterocycle by y-bond formation is provided by the thermally promoted conversion of the bisallenyl selenide (143) into the selenophene (144) (78TL1493). The cyclization is conveniently formulated as proceeding via an intramolecular ene reaction. Another electrocyclic process appears to be involved in the conversion of allyl phenyl selenide (145) into 2-methyl-2,3-dihydrobenzo[6]seleno-phene (146) (71TL49). The origin of the minor amount of 2-methylbenzoselenophene (147) which is also formed has not been established. Access to 3-keto-2,3-dihy-drobenzo[h]selenophenes (148) (71KGS333) and 2-arylidene-3-keto-2,3-... 

The second category is that of the electrocyclic reaction, illustrated in Equations 11.6-11.8, characterized by intramolecular opening or closing of a ring by conversion of [Pg.571]

In these cycloaddition reactions which may be intermolecular, or intramolecular, a r -bond is converted into a o-bond. The reverse occurs when ring opening takes place. The closure or opening involves the movement of electrons and atoms but no atoms are gained or lost. Such transformations have been called electrocyclic reactions and may occur thermally or photoche.nically as governed by symmetry considerations. 

In intramolecular arylations, a new bond is created between two aromatic moieties of the same molecule or between an aromatic nucleus and an atom of a side-chain. Many intramolecular arylation reactions of homocyclic and heterocyclic aromatic halides have been studied mainly in view of their synthetic applications, and it is not always clear which mechanistic pathway is followed. The reaction may start with homolytic or heterolytic dissociation of the carbon-halogen bond and proceed by attack of the aryl radical or aryl cation on another part of the molecule. Electrocyclization followed by elimination of hydrogen halide is another possibility. Especially when heteroatoms such as nitrogen, sulphur or phosphorus are involved, the initial step may be a nucleophilic attack on the carbon atom bearing the halide atom. 

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