Electrocyclizations intramolecular reactions

The corresponding reaction of 2-alkenyl-3-alkadienyM-nitropyrrole 1156 also leads to 3-nitroindoles 1158 (21-90%) via intermediate 1157, however, now together with 3-nitrotetrahydroindole derivatives 1159 (Scheme 225) <1998T1913>. The latter compounds are formed by a tandem 6jt-electrocyclization-intramolecular Diels-Alder reaction, and are the predominant (or only) products when nitrobenzene is replaced by triglyme (70-78% yield). 

An electrocyclic reaction is an intramolecular reaction in which a new a (sigma) bond is formed between the ends of a conjugated tt (pi) system. This reaction is easy to recognize—the product is a cyclic compound that has one more ring and one fewer TT bond than the reactant. 

Notice that electrocyclic reactions and sigmatropic rearrangements occur within a single 7T system—they are intramolecular reactions. In contrast, cycloaddition reactions involve the interaction of two different tt systems—they are usually iniermolecular reactions. The three kinds of pericyclic reactions share the following common features ... 

An electrocyclic reaction is an intramolecular reaction in which the rearrangement of tt electrons leads to a cyclic product that has one fewer tt bond than the reactant. An electrocyclic reaction is completely stereoselective—it preferentially forms one stereoisomer. For example, when (2 , 4Z,6 )-octatriene undergoes an electrocyclic reaction under thermal conditions, only the cis product is formed when (2 , 4 6Z)-octatriene undergoes an electrocyclic reaction under thermal conditions, only the trans product is formed. Recall that E means the high-priority groups are on opposite sides of the double bond, and Z means the high-priority groups are on the same side of the double bond (Section 3.5). 

The types of reaction that have been observed for heterocyclic o-quinodimethanes are outlined below. Some of these, such as dimerisation and electrocyclic ring closure, involve only the r)-quinodimethanes whereas the majority involve the participation of another compound. The methods of generation of ri-quinodimethanes are discussed in Section 2.4 however, it is our experience that one of the commonly used methods, flash vacuum pyrolysis (FVP). tends to favour intramolecular reactions and dimerisation over reactions with other species. It is best to use one of the solution methods of generation when intermolecular reactions are required. 

An electrocycUc reaction is an intramolecular reaction in which the formation of a a bond between the ends of the tt system and the rearrangement of the ir electrons leads to a cyclic product that has one less tt bond than the reactant. An electrocyclic reaction is completely stereoselective it is also stereospecific. 

An electrocyclic reaction is an intramolecular reaction in which a new a bond is formed between the ends of a conjugated tt system. 

The intramolecular reaction of N-aryl imine 210 under phase-transfer conditions in the presence of cinchonidinium chloride 211 furnishes the chiral indoUne 212 in quantitative yield with high enantiomeric excess (Scheme 28.27). An intramolecular Mannich reaction under kinetic control formally constitutes a 5-(molexo)-endo trig reaction, which is stereo-electronicaUy demanding as delineated by the Baldwin rules therefore, this transformation includes a cation-directed Gtt electrocyclization as depicted in the reaction model 213 [106]. 

It is common in the intramolecular reaction of arene-ethene nonconjugated bichromophores that, if the molecular architecture allows, the first formed ort/jo-cycloaddition product ring opens thermally to the cyclooctatriene, which then undergoes photochemical electrocyclization to give either or both of the... 

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]... 

Electrocyclization of 1,4-dienes is an efficient process when a heteroatom with a lone pair of electrons is placed in the 3-position, as in 77 (Scheme 20)38. Photoexcitation of these systems generally results in efficient formation of a C—C bond via 6e conrotatory cyclization to afford the ylide 78. These reactive intermediates can undergo a variety of processes, including H-transfer (via a suprafacial 1,4-H transfer) to 79 or oxidation to 80. In a spectacular example of reaction, and the potential it holds for complex molecule synthesis, Dittami and coworkers found that the zwitterion formed by photolysis of divinyl ether 81 could be efficiently trapped in an intramolecular [3 + 2] cycloaddition by the... 

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