Norrish-Yang Photocyclization

Figure 42 Results on the enantioselective Norrish-Yang photocyclization of trans-A-terf-butyl-1-methy Icy clo hexyl aryl ketone. Note that the same isomer of the chiral inductor favors opposite isomers within NaY and NaX zeolites.

Ramamurthy, Scheffer and coworkers and other investigators reported the enantiodifferentiating Norrish/Yang photocyclization [49-53] of aryl cis-4-tert-buty -cyclohexyl ketones 1 to cyclobutanols 2 (Scheme 1) in chirally modified zeolite supercages [49]. 

Chiral crystals formed from chiral molecules can undergo highly diastereoselec-tive photoreactions, while diastereodifferentiation in solution is usually low. Here three types of diastereoselective photoreactions in the crystalline state are presented. Highly diastereoselective Norrish/Yang photocyclization of adamantane [36] and p-lactam formation from oxoamides [37] have been also reported. 

Scheme 11 Enantioselective Norrish/Yang photocyclization by using an ionic chiral auxiliary or a chiraliy modified zeolite.

Highly enantioselective Norrish/Yang photocyclization of cw-4-terf-butyl-1-benzoylcyclohexane 47 was also achieved in salt crystals using chiral amines like R)- + )-l-phenylethylamine or (—)-norephedrine [61,63]. Furthermore, cis-9-decalyl aryl ketones 49 in their salt crystals with chiral amines such as (/ )-( + )-bomyl amine or ( )-( — )-1 -phenylethylamine underwent highly regio-, dia-stereo- and enantioselective Norrish/Yang photocyclizations to give the cyclobutanol 50 alone in > 98% ee [64]. 

Scheme 27 Absolute asymmetric Norrish/Yang photocyclization of adamantane derivative 114.

Solid-state photolysis of a salt crystal formed between a prochiral, photo-chemically reactive keto-acid of 2-benzoyladamantane 44 and a nonabsorbing optically active amine leads to enantioselective Norrish/Yang photocyclization giving the optically active cyclobutanol 45 (Scheme 11) [59-61]. Irradiation of a total of 17 salts gave moderate to near-quantitative enantiomeric excesses. One of the best results was obtained using the prolinol salts 44 formed dimorphic salt crystals with with (5)-( + )-prolinol, of which the needle-shaped crystals gave... 

Norrish-Yang photocyclization has been widely used to generate regio- and stereocontrolled C-C bonds under mild conditions, especially when constructing quaternary stereocenters. 

Herrera et al. [97,98] have reported the usage of Norrish-Yang photocyclization for the synthesis of new spirocyclic monosaccharide derivatives of types 243 and 244 via a hydrogen atom transfer (HAT) reaction promoted by a 1,2-diketone 241, in its excited state, followed by C-C tetrasubstituted bond formation in a diastereo-selective manner (Fig. 8.59). Of special interest is the study of the tendency to inversion at C5 (for examples of epimerization of anomeric and pseudoanomeric radicals, see [99, 100]), probably triggered by conformational changes that the 1,4-diradical intermediate 242 undergoes in its triplet state, within its lifetime (for discussions of the lifetime of diradicals in solution, see [101, 102]) before the intersystem crossing (ISC) occurs. 

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