Nitrogen radicals, generation and cyclization

Iminyl radicals are particularly useful for the construction of nitrogen heterocycles, since the cyclization products are functionally disposed for further elaboration into a variety of useful systems. Zard and coworkers have pioneered the development of methodology for the generation and cyclization of iminyl radicals (Table 8) [5, 71]. Iminyl radicals cyclize one order of magnitude more rapidly than the related neutral aminyl radicals but react less rapidly than the aminyl radicals with hydrogen atom transfer trapping agents, and one would predict that iminyl radicals formed in chain reaction sequences could prove to be as versatile as carbon radicals [78]. 

Free-radical cyclization on to unsaturated CN bonds and also the cyclization of a range of nitrogen-centred radicals have continued to attract interest and have been reviewed. Aryl radicals, generated from BusSnH- or TTMSS-mediated homolytic cleavage of aryl-bromide bonds, have been shown to cyclize on to the nitrogen atom of imidate esters in the 5-exo mode (Scheme 9). Loss of an ethyl radical leads to the observed A-acylindolines. No cyclization in the 6-endo mode was detected. 

The Generation of Nitrogen Radicals and Their Cyclizations for the Construction of the Pyrrolidine Nucleus... 

Useful synthetic methodologies are based on the cyclization or rearrangement of the nitrogen-centered radicals generated in the reaction of the appropriate amides with (diacetoxyiodo)benzene in the presence of iodine [652-655]. Specific examples are illustrated by the synthesis of bicyclic spirolactams 622 from amides 621 [653] and preparation of the oxa-azabicyclic systems (e.g., 624) by the intramolecular hydrogen atom transfer reaction promoted by carbamoyl and phosphoramidyl radicals generated from the appropriately substituted carbohydrates 623 (Scheme 3.244) [654],... 

Fortucine 112, first isolated from the Fortune variety of narcissus by Tokhtabaeva et al. [43] in 1987, is a Lycorine alkaloid. To clarify the structure, Zard and coworkers [44] carried out the first total synthesis of ( )-fortucine using a radical domino process as the key step (Scheme 5.23). It was suggested that an amidyl-nitrogen radical is generated from 110, followed by a 5-exo-trig cyclization in sequence with... 

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