Yang cyclisation

Explain how intramolecular hydrogen abstraction in carbonyl compounds can lead either to cleavage (Norrish type 2 reaction) or to the formation of cyclic compounds (Yang cyclisation). 

The 1,4 biradical can undergo cyclisation (Yang cyclisation) to give a four-membered cyclic alcohol ... 

Bach T, Aechtner T, Neumtiller B (2002b) Enantioselective Norrish-Yang cyclisation reactions of A-( )-oxo-a>-phenylaIkyl)-substituted imidazolidin-ones in solution and in the solid state. Chem Eur J 8 2464-2475 Bach T, Grosch B, Strassner T, Herdtweck E (2003) Enantioselective [6jt]-photocyclisation reaction of an acrylanilide mediated by a chiral host. Interplay between enantioselective ring closure and enantioselective protonation. J Org Chem 68 1107-1116... 

Yang, C.-C., and Fang. J.-M., Free-radical cyclisations of 2-aminoalka-2,5-dienenitriles, J. Chem. Soc., Perkin Trans. 1. 879, 1995. 

If both the radical source and the radical trap are in the same molecule, then an intramolecular atom-transfer radical cyclisation reaction occurs. In 2001, Yang et al. reported the oxazoline 39d-magnesium-catalysed bromine atom-transfer radical cyclisation reaction of unsaturated p-keto ester 49 for... 

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