Yang-type cyclizations

There are four distinct processes initiated by y-hydrogen abstraction in excited carbonyl compounds Norrish type II photoelimination, Yang photo-cyclization (cyclobutanol formation), Yang photoenolization (o-xylylenol formation), and (3-cleavage of radicals from carbons adjacent to the radical sites of the 1,4-biradicals. Some of these require unique structures and generate distinct products. 

Yang, J., Viswanathan, G. S., Li, C.-J. Highly effective synthesis of 4-halotetrahydropyrans via a highly diastereoselective in situ Prins-type cyclization reaction. Tetrahedron Lett. 1999,40,1627-1630. 

Wang F, Zhang YJ, Yang G, Zhang W. Highly enantioselec-tive Pd(II)-catalyzed Wacker-type cyclization of 2-allyl-phenols by use of bisoxazoline ligands with axis-unfixed biphenyl backbone. Tetrahedron Lett. 2007 48 4179-4182. 

Some structures of this type of compound were established by X-ray diffraction analysis. For example, the structure of compound 84 resulting from the enantioselective Norrish-Yang cyclization of prochiral imidazolidinone was... 

The synthesis of l,3-oxazin-4-ones of type 464 is the first example of the formation of a C-O bond in the course of the Norrish-Yang reaction. Upon treatment with 1-hydroxy-l-phenyl-A -iodanyl mesylate, /3-keto amide 460 was converted to the corresponding a-mesyloxy-/3-keto amide 461 in excellent yield. On ultraviolet (UV) irradiation (A>300nm) of 461, 5-hydrogen transfer to the excited carbonyl group occurred and the diradical 462 thus formed underwent MsOH elimination to enolate diradical 463, cyclization of which resulted in formation of 3-methyl-6-phenyl-3,4-dihydro-277-l,3-oxazin-4-one 464 (Scheme 89) <2001S1258>. 

Rather than fragmentation of Norrish Type IT biradicals, cyclization can occur. This is called the Yang Cyclization. ... 

Scheme 5.15 No Irish type II fragmentation and Yang cyclization of isoleucine derivatives, illustrating the influence of conformational equilibria.

The first three chapters by Wagner, Wessig, and Griesbeck deal with typical carbonyl chemistry Norrish type II reactions followed by Yang-cyclization, homologous Norrish type II reactions (i.e. hydrogen abstractions from non y-positions), and Paterno-Buchi [2+2]-photocycloadditions. The enantiomerically pure (S-am ido-cycl o butanol 1 is formed from a chiral... 

From a synthetic point of view, the extension of the concept, described in Sec. 3.4.2.1 (type Ih), on y-ketoamides, does not only provide an attractive method but also a molecular system that is very valuable for the improvement of the mechanistic understanding of the Norrish-Yang reaction. Thus, on irradiation y-ketoamides 50 cyclize to 5-lactams 51 in good yields [48]. It should be noted that the reaction outcome strongly depends on the solvent used. Whereas the irradiation in dichloromethane gives the 5-lactams 51, the only products formed in diethylether were pinacols [49]. The formation of cyclopropanes from 50 [50] described earlier is probably based on a misinterpretation of the spectral data [49]. 

Several absolute asymmetric Norrish type II cyclizations have been reported in the solid state. Achiral a-(3-methyladamantyl)-/ -chloroacetophenone 114 formed a chiral crystal in space group P2i2i2i. Irradiation of these crystals caused Norrish/Yang cyclization to afford the cyclobutenol-type photoproduct 115 (six chiral centers) with respectable enantiomeric excess at low conversion (Scheme 27) [101]. Photolysis in solution phase led to mixtures of four of the six possible cyclobutanols, and no trace of optical activity could be detected in the mixtures. 

Recently, a variant of this type of process was reported by Barth and O-Yang(76) who reported the free-radical cyclization of unsaturated a-fluoro-a-iodoesters and amides. 

An intramolecular Pd-mediated Heck-type approach to 3-pyrrolin-2-ones has been disclosed by Yang and Shea (1995MI1 Scheme 30). Treatment of N-allyl-2-bromoacetamide 152 with palladium acetate and triphenylphos-phine in acetonitrile gives 4-methyl-3-pyrrolin-2-one 153 by a 5-exo-trig cyclization onto a terminal alkene, reductive elimination, and isomerization... 

In this reaction, the tethered alkene functionality of the aniline derivatives form a Tr-allyl complex with the Pd(II) catalyst, and then the cyclization takes place via intramolecular nucleophilic attack by the N to form another Pd-complex, which reacts with CO as the electrophilic carbon source in the presence of MeOH to afford substituted bicy-clic pyrrolidine derivatives. However, these products were not stereoselective in nature. A similar type of Pd-catalyzed nonstereoselective cyclizations were also reported by Weinreb et al. and by Larock and Yang for the construction of bicyclic and bridged aza-cycles." ... 

An enantioselective version of the similar type of cyclization strategy has been developed by Yang et al., where they used properly designed substituted aniline derivatives in the presence of catalytic Pd(trifluoroacetic acid [TFA])2 and chiral (—)-sparteine ligand to afford... 

The synthesis of cyclobutanes is the classical area of application of the Norrish/Yang reaction. It has been discussed comprehensively elsewhere. > > Here, only two special aspects are addressed. The main drawback of Yang cyclization from a practical point of view is the Norrish type 11 cleavage, which almost always competes with the cyclization and is often the only observed process. Nevertheless, the cleavage can be suppressed if the overlap of both radical orbitals with the breaking C-C bond is disturbed by... 

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