Yang cyclization

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... 

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.

Scheme 5.16 Yang cyclization leading to cyclobutanol derivatives and p-lactams.

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... 

To examine the viability of CIM a number of photoreactions (electrocyclic reactions, Zimmerman (di-n) reaction, oxa-di-7i-methane rearrangement, Yang cyclization, geometric isomerization of 1,2-diphenyl-cyclopropane derivatives, and Schenk-ene reaction) which yield racemic products even in presence of chiral inductors in solution have been explored (Sch. 40) [187,189-200]. Highly encouraging enantiomeric excesses (ee) on two photoreactions within NaY have been obtained photocyclization of tropolone ethylphenyl ether (Eq. (1), Sch. 40) and Yang cyclization of phenyl benzonorbornyl ketone (Eq. (3), Sch. 40). The ability of zeolites to drive a photoreaction that gives racemic products in solution to ee >60% provides... 

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. 

In the context of chiral induction in photochemical reactions, the Norrish-Yang cyclization, which has been extensively studied in isotropic and anisotropic media, has been examined. Both cis and trans cyclobutanols exist as pairs of enantiomers (Scheme 36). These reactions are good candidates for photoreactions inside zeo-... 

Ketone 51 undergoes Norrish-Yang cyclization, giving cyclobutanols as products (Eq. (2), Scheme 37). Unlike 49, which gave only one cyclobutanol... 

Saphier S, Hu Y, Sinha SC, Houk KN, Keinan E. Origin of selectivity in the antibody 20F10-catalyzed Yang cyclization. J. Am. Chem. Soc. 2005 127(1) 132-145. 

The cyclization reaction of a biradical intermediate to form cyclobutanol (304 see also Yang cyclization Section 6.3.5) and the Diels Alder cycloaddition of the photoenols with dienophiles are common subsequent processes (Scheme 6.134).981,982 Both processes are often stereospecific, typically involving a single, long-lived ( )-photoenol. 

This reaction has been extended by Yang to the condition known as Yang cyclization. 

An alternative reaction for 76 is bond formation between the two radical centers, resulting in the formation of a cyclobutanol (79, equation 12.58). This reaction was first reported by Yang, so the process is often called the Yang cyclization or the Norrish-Yang cyclization. Zewail and co-workers found that singlet biradical closure occurs 400-700 fs after... 

Cariying out photochemical reaction in crystals offers unique synthetic possibihties, particularly for control of stereochemistry. For example, in solution the irradiation of a 2,4,6-triisopropylbenzophenone bearing the (S)-phenylethylamide group (162) gives approximately 1 1 mixture of the R,S (163) and S,S (164) diastereomers of the Norrish-Yang cyclization product (equation 12.96). However, irradiation of microcrystals of 162 led exclusively to the R,S) diastereomer (equation 12.97). This stereoselectivity was attributed to steric forces present in the chiral crystal. 

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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