Radicals Containing a Six-Membered Heterocycle

B. Radicals Containing a Six-Membered Heterocycle 1. Pyranyl Radicals... 

An important exception to this regularity is the cyclization of aromatic alkoxides containing aromatic radical moieties. In these cases, C-0 bond formation is not observed, but C-C bond formation is achieved instead. As Galli and Gentili (1998) pointed out, this is primarily due to the unfavorable thermodynamic driving force for C-0 bond formation compared to C-C bond formation. Thus, the photostimulated reaction depicted in Scheme 7.39 results in the formation of a six-membered carbocycle rather than an octa-membered oxa-heterocycle. The carbocycle is formed in 75% yield (Barolo et al. 2006). This product is a precursor to the thalicmidine biomolecule of the alkaloid group. 

Examples of the addition of solvent molecules to nitrogen-containing heterocycles, arising via initial hydrogen abstraction from the solvent, have again been described. The mechanism of this reaction with reference to six-membered aza-aromatic compounds has been the subject of a recent review,137 and in the reaction of acridine with alcohols and ethers, a radical-pair intermediate has been detected.138 Hydrogen abstraction is also implicated in the conversion of 4-quino-linecarbonitrile (172) into the alcohol (173) on irradiation in ethanol 139 the... 

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