Intramolecular cycloadditions 1,4-hydrogen shift

The evidence obtained clearly indicates that the above photorearrangements proceed by a mechanism involving a nitrile ylide intermediate since cycloadducts could be isolated when the irradiations were carried out in the presence of trapping agents. Intramolecular cycloaddition of the nitrile ylide followed by a 1,3-sigmatropic hydrogen shift of the initially formed five-membered ring readily accounts for the formation of the final product. 

An intramolecular version of an azide cycloaddition of 221 and 222 provided cyclopropylimines 224 and 225 via formation of triazoline 223 followed by extrusion of nitrogen with concomitant 1,2-hydrogen shift (Scheme 36) [58], The cyclization was found to be solvent dependent polar solvents such as DMF gave the best yields, whereas benzene gave several side products. 

The preparation of the annulated y-lactam (48) (Table 6) has been described from (209) which in turn has been obtained from A-allyldiazoacetamides containing a phosphinyl group at the a-position through intramolecular 1,3-cycloaddition followed by a 1,3-hydrogen shift <93BCJ1496>. Treatment of 1,3,3-trimethyl-2-cyanomethyleneindole (210) with o-phenylenediamine in phosphoric... 

Additions involving hydrogen shifts, and isomerisations through intramolecular cycloaddition will not be covered. 

The activation volume is more negative for the intramolecular cycloaddition, so at high pressure this reaction is favored by a factor of four over the 1,5-hydrogen shift. 

It is reported that unless special precautions are taken for the vacuum distillation of crude -ionylideneacetaldehyde extensive rearrangement is observed and the tricyclic ketone (327) may be isolated in about 50% yield. " The formation of (327) can be rationalized by a scheme involving successive E-Z isomerization, [1, 5] shift of aldehyde hydrogen, and intramolecular cycloaddition of the unsaturated ketene so formed to the cyclohexene part of the molecule. A second product is formed in the course of the reaction and the compound (328) is a tentative suggestion for its structure since a second mode for the ketene-cyclohexene addition is possible. 

The ease of adduct formation depends largely on the electron density on the N atom of the imine and the electrophilicity of the center carbon atom of the isocyanate. Most reactive are persubstituted guanidines and amidines on one side and aryl isocyanates with electron withdrawing substituents on the other side. The initial attack occurs on the more nucleophilic center. Delocalization of the developing charges favors intermolecular [2-I-2-I-2] cycloaddition over intramolecular [2-1-2] cycloaddition or the exchange reaction. When a hydrogen shift can occur, the intramolecular isocyanate induced enurea reaction is faster than the intermolecular [2-I-2-I-2] cycloaddition reaction. Thermodynamically controlled equilibria are established above 100 °C and the thermodynamically more stable reaction product is isolated. 

The 3//-azepines obtained by cycloaddition of azirines to cyclopentadienones (see Section 3.1.1.1.2.) are thought to arise from the initially formed 2/7-azepines by [1,5]-H suprafacial sigmatropic shifts.31-108 In contrast, 1/Z-azepine 9 results from the thermal rearrangement of the nonisolable 2//-azepine-2-carboxylate 8.13 Presumably, the 1 //-azepine is stabilized, relative to the 3//-isomer, by intramolecular hydrogen bonding between the NH and the adjacent ester group. 

The lowest-lying excited state of ketones most often corresponds to a o 7t c=o transition. The maximum of this band is around 280 nm with simple aldehydes or ketones and is shifted to the red for conjugated or aryl derivatives. As hinted above, the unpaired electron on the hq orbital gives to these states electrophilic properties similar to those of alkoxy radicals, and indeed the observed chemistry is similar in the two cases. Typical reactions are a-fragmentation, inter- or intramolecular (from the easily accessible y position) hydrogen abstraction and attack of alkenes (finally resulting in a formal 2h-2 cycloaddition to give an oxetane, the Paterno-Btichi reaction). 

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