Yang photocyclization reaction

Scheme 8 Asymmetric induction in the Yang photocyclization reaction.

Leibovitch M, Olovsson G, Scheffer JR, Trotter J (1998) An investigation of the Yang photocyclization reaction in the solid state asymmetric induction studies and crystal strac-ture-reactivity relationship. J Am Chem Soc 120 12755-12769... 

Irradiation of the octahydroinden-2-yl derivatives of acetophenone (279) in both solution and the solid state afforded cA-hexahydro-1 //-indene (280) and a para-substituted acetophenone 281 via a Norrish type II cleavage process (Scheme 6.120).957 The Yang photocyclization reaction was not observed. 

Our studies on the adamantyl (10) and decalyl (1) systems are onlytwo of several examples of the successful application of the ionic chiral auxiliary approach to asymmetric induction in the Yang photocyclization reaction. Space does not permit a full exposition of our results in this area. The interested reader may wish to consult references 17, 25, 27, 31, 35, 38 and 44 for additional examples and further details. As a note of interest, reference 44 describes the use of anionic chiral auxiliaries in the Yang photocyclization of a cationic counterion in the crystalline state aU of our other studies have been of the reverse type. 

Single crystal-to-single crystal reactions are quite rare in solid-state organic photochemistry, and we were fortunate to discover a second example in the Yang photocyclization of the 1-phenylethylamine salt of 2-methyl-2-benzoyl-p-carboxylic acid 39 [37]. 

This nonoxidative photocyclization reaction was extended to the A-acetyl derivative 122, which was irradiated through a Vycor filter in the presence of iodine and hydriodic acid to give the product 123 in 42% yield (106). Lenz and Yang (106) have synthesized a protoberberine alkaloid, / -coralydine, by applying this type of photocyclization (Schemes 52 and 53). 

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. 

Liu L, Yang B, Katz TJ, Poindexter MK (1991) Improved methodology for photocyclization reactions. J Org Chem 56 3769-3775... 

Stereoselective Photocyclization of Ketones (Norrish-Yang Reaction) 569... 

Taking into account the wide-spread occurrence of molecules containing the tetrahydrofuran substructure, it is astonishing that there are only few examples of synthesis of these heterocycles by means of the Norrish-Yang reaction [38]. Interestingly, both aliphatic and aromatic ketones were used as exemplified by the photocyclization of ketones 17 [38b], 19 [38c], and 21 [3a] (Sch. 9). 

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... 

Structural changes produced by the Norrish-Yang reaction proceeding in two crystal forms of (12) can be monitored by X-ray structure analysis. The (i ,5)-cyclobutenol (13) is obtained as single photoproduct from the corresponding isopropylbenzophenone derivative, via diastereospecific Norrish type II photocyclization in the crystalline state. The crystal packing is retained during the photoreaction. 

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