Intramolecular electrocyclization

Thiazole derivatives can be obtained in good yields (60-95%) by intramolecular electrocyclization of thiocarboximides. The mechanism involving a two-electron process is shown in Scheme 38 [56]. After the second-electron transfer, the thiocarbonyl group attacks the cationic site located in the ortho position of the aromatic nucleus. 

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... 

Intramolecular electrocyclization of vinylketenes generated in this way also occurs with pendant vinyl groups (Eqn (4.70)) and with alkynyl groups (Eqn (4.71))7 In the electrocyclization with alkynyl groups attack at either alkyne carbon can occur, giving two isomeric diradical intermediates, which lead to the respective final products (Eqn (4.71))/ ... 

Transforming a Helix-Coil into a Helix-Helix Transition and Eliminating Intramolecular Electrocyclization... 

As demonstrated in the lecture given during the lUPAC of Macromolecules in Tashkent in 1978 [94], in its contributing publications, and in more recent publications [94—103], the c/s-transoidal and c/s-cisoidal polyphenylacetylenes exhibit helical conformations that undergo an intramolecular electrocyclization during their transition to the coil conformation. A limited extent of cis-trans isomerization takes place during this process (Fig. 10). 

Encapsulation of any of these c/s-conformers into libraries of columnar supra-molecular dendrimers eliminates the intramolecular electrocyclization and replaces the hehx-coil transition with an unprecedented helix-helix transition and a reversible transition from cw-transoidal to cis-cisoidal. When the repeat unit of the dendronized polymer also contains a stereocenter, this reversible process can be monitored by circular dichroism (CD) and visualized by different methods [104-111]. This concept was used to elaborate molecular machines that were interfaced for the first time with the real world to lift heavy objects [111]. 

Fig. 10 The irreversible intramolecular electrocyclization of cw-transoidal polyphenylacetylene (a) taking place during the helix-coil transition of the polymer (b), its elimination by encapsulation of the polymer in a cylindrical supiamolecular polymer and the transformation of the helix-coil into a helix-helix transition (c)...

Conjugated phosphazenes have been widely used for the preparation of azadienes. An important extension of the aza-Wittig/intramolecular electrocyclic ring closure (AW-IEC) methodology has been used in the construction of j8-carboline alkaloids, which contain a... 

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. 

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