Norrish-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... 

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>. 

There are still two other factors influencing the reactivity of ketones in the Norrish-Yang reaction. If functional groups with a relatively low oxidation potential (amino, alkenyl or aryl groups, thioethers) are present in the reactants, the excited state may be quenched by partial or complete charge transfer from these groups to the extited carbonyl group. The effects of such processes may vary from a complete loss of reactivity to an entirely new reaction mode - cyclization reactions ini-... 

Mechanistic remarks relating to the regioselectivity may be subdivided in three parts. The first deals with concepts within the framework of the classic Norrish-Yang reaction, i.e. homolytic hydrogen transfer is the basis of the reaction. In the second part the cyclization reactions based on photoinduced electron transfer (PET) will be discussed and in the third part the spin-center-shift approach is elucidated. 

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]. 

Two other examples of the synthesis of bicyclic compounds whose skeleton consists only of carbon atoms are shown in Sch. 24, each of them is instructive in its own way. Upon irradiation of the cyclohexane derivative 65, not the expected product from a hydrogen abstraction from the allylic position, but the bicyclo[3.3.1]heptane 67 was obtained by cyclization of the less stable biradical 66-B [57]. The reason for this is the exclusive hydrogen back-transfer of the more stable biradical and it underlines the importance of this process in regioselectivity phenomena of the Norrish-Yang reaction. 

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... 

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... 

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

Norrish discovered the cleavage reactions (E of Eq. 4) of excited state ketones to methyl ketones and olefins [257]. It was later recognized by Yang and Yang [258] that the biradical intermediates (BR) can also cyclize (C) to yield cyclobutanols. 

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