Aryl azides radical cyclizations

The field of alkaloid synthesis via tandem cyclizations has favored the application of (TMS)3SiH over other radical-based reagents, due to its very low toxicity and chemoselectivity. For example, cyclization of the iodo aryl azide 12 mediated by (TMS)3 SiH under standard experimental conditions, produced the N-Si(TMS)3 protected alkaloid 13, which after washing with dilute acid afforded the amine 14 in an overall 83% yield from 12 (eq 40). The formation of the labile N-Si(TMS)3 bond was thought to arise from the reaction of the product amine 14 with the by-product (TMS)3SiI. The skeletons of ( )-horsflline, ( )-aspidospermidine and ( )-vindoline have been achieved by this route. ... 

Other carboxamidation reactions involving aromatic compounds involve the generation of isocyanates from carbonyl azides. For example, thermolysis of certain carbonyl azides substituted in the a-position by vinylidene radicals, such as 220, or aryl rings undergo cyclization to the pyridones 221 or condensed pyridones . 

Evidence for the formation of alkyl and aryl radicals in some cases following loss of SO2 (Scheme 1) has been obtained. Thus, a small amount of M-pentane was formed in the decomposition of M-pentanesulphonyl azide in mineral oil ). Thermolysis of diphenyl sulphone-2-sulphonyl azide (8) in dodecane at 150 °C gave diphenyl sulphone 9 (27%) and diphenyl sulphone-2-sulphonamide 10 (9%) which arise by hydrogen abstraction by the aryl radical and sulphonyl nitrene, respectively. When this thermolysis was carried out in Freon E-4 at 150 °C, the products were diphenylene sulphone 77 (1.3%) (Pshorr-type cyclization product of the aryl radical) and 10 (1.5%) together with tars 16h Ferro-... 

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