And photolytic cleavage

Carbon-carbon cleavage induced by chemical attack is rare for oxiranes [cf. thermal and photolytic cleavage (Section 5.05.3.2.1)], but a few examples are known, e.g. Scheme 64 (70TL3025). 

Another aminomethyl PS resin supported o-nitrobenzyl photolabile hnker 1.23 (80) has been employed for the synthesis of heterocycles such as thiazohdinones. Attachment to the resin is through an acid or aldehyde group and photolytic cleavage is performed in 5% dimethyl sulfoxide (DMSO)-aqueous buffer to facilitate the biological testing of the final compound. A photolabile hydroxyl linker 1.24 (81), again supported on aminomethyl PS resin, has been used to synthesize carbohydrate and peptide derivatives in SP. The easily prepared diol linker 1.25 (82) allowed the attachment of aldehydes as acetals and their photo induced release using standard conditions. 

Scheme 16.9. Modification and photolytic cleavage of a phenacyl ester-based linker.

More recently. Erase has also introduced the T2 triazene linker unit. The T2 linkers are most commonly used for immobilization of amines (and other nitrogenous compounds). As their T1 counterparts, the T2 linkers have also proven robust linkers for SPOS. For example, amines can be cleaved by treating with TFA (Table 1.8, Entry 1), while treatment with trimethylsilyl chloride is typically used when preparing (and cleaving) ureas (Table 1.8, Entry 2) or amides (Table 1.8, Entry 3). Alternatively, the T2 linker can also behave as a photolabile linker unit and photolytic cleavage (X, = 355 nm) by Enders et al. was used as a strategy to release amines (Table 1.8, Entry 4). ... 

E. B. Akerblom, Six new photolabile linkers for solid-phase synthesis. 2. Coupling of various building blocks and photolytic cleavage. Mol Diversity 1998, 4, 53-69. 

The most intriguing hydrocarbon of this molecular formula is named buUvalene, which is found in the mixture of products of the reaction given above. G. SchrOder (1963, 1964, 1967) synthesized it by a thermal dimerization presumably via diradicais of cyciooctatetraene and the photolytical cleavage of a benzene molecule from this dimer. The carbon-carbon bonds of buUvalene fluctuate extremely fast by thermal Cope rearrangements. 101/3 = 1,209,6(X) different combinations of the carbon atoms are possible. 

Heterocycles which provide the NOC or CNO component synthon Isoxazoles can be prepared by the thermal or photolytic cleavage of a number of heterocycles, such as 1,3,5-dioxazolidone, furazans, furoxans and 1,3,2,4-dioxathiazole 2-oxides, in the presence of a reactive alkene or alkyne. 

Three selective methods to remove protective groups are receiving much attention assisted, electrolytic, and photolytic removal. Four examples illustrate assisted removal of a protective group. A stable allyl group can be converted to a labile vinyl ether group (eq. 4) a /3-haloethoxy (eq. 5) or a /3-silylethoxy (eq. 6) derivative is cleaved by attack at the /3-substituent and a stable o-nitro-phenyl derivative can be reduced to the o-amino compound, which undergoes cleavage by nucleophilic displacement (eq. 7) ° ... 

Photolytic cleavage reactions (e.g., of o-nitrobenzyl, phenacyl, nitrophenylsul-fenyl derivatives) take place in high yield on irradiation of the protected compound for a few hours at 254-350 nm. For example, the o-nitrobenzyl group, used to protect alcohols, amines, and carboxylic acids,has been removed by irradiation. Protective groups that have been removed by photolysis are described at the appropriate places in this book in addition, the reader may wish to consult five review articles. 

Photolysis, 350 nm, CH3CN, H2O. The by-product from the reaction is 2-phenyl-benzo[ ]furan. Cleavage with TBAF and PhCH2SH has been demonstrated (70-94% yield). The related 3,5-dimethoxybenzoin analogue is cleaved with a rate constant of >10 sec Photolytic cleavage occurs by heterolytic bond dissociation. ... 

Equations 1-10 illustrate some mild methods that can be used to cleave amides. Equations 1 and 2 indicate the conditions that were used by Woodward and Eschenmoser, respectively, in their synthesis of vitamin B12. Butyl nitrite, nitrosyl chloride, and nitrosonium tetrafluoroborate (NO BF4 ) have also been used to cleave amides. Since only tertiary amides are cleaved by potassium -butoxide (eq. 3), this method can be used to effect selective cleavage of tertiary amides in the presence of primary or secondary amides.(Esters, however, are cleaved by similar conditions.) Photolytic cleavage of nitro amides (eq. 4) is discussed in a review. 

Acyl azides can undergo photolytic cleavage and rearrangement upon irradiation at room temperature or below. In that case acyl nitrenes 8 have been identified by trapping reactions and might be reactive intermediates in the photo Curtius rearrangement. However there is also evidence that the formation of isocyanates upon irradiation proceeds by a concerted reaction as in the case of the thermal procedure, and that the acyl nitrenes are formed by an alternative and competing pathway " ... 

With unsymmetrical ketones two different bonds are available for photolytic cleavage the actual cleavage pathway depends on the relative stability of the possible radical species R and R. ... 

A photolytic cleavage can break the molecule into two smaller molecules or into two free radicals (see p. 318). Cleavage into two ions, though known, is much rarer. Once free radicals are produced by a photolysis, they behave like free radicals produced in any other way (Chapter 5) except that they may be in excited states, and this can cause differences in behavior. 

To ascertain that aminomethylphosphonic acid ( ) but not glycine (4) is a photolysis product, the primary amino product was identified by comparison of TLC Rf values. Possible products and their formation pathways from photolytic cleavage of N-nitrosoglyphosate are shown in Figure 1 (5). The TLC Rf values on silica gel for different solvent systems are shown in Table II (5). The data clearly indicate that aminomethylphosphonic acid... 

The hexakis(methyl isocyanide) dimers, [Pt2(CNMe)6], undergo photolytic cleavage of the Pt—Pt bond to give 15-electron radicals, Pt(CNMe)3.94 Mixtures of platinum and palladium dimers give rise to heteronuclear complexes under photolytic conditions. Mixtures of normal and deuterium-labeled methyl isocyanide complexes reveal that the metal-ligand bonds undergo thermal redistribution.94... 

Scheme 5.4 Photolytic cleavage of resin-bound heptasaccharide 15 and deprotection to yield the heptasaccharide phytoalexin elicitor 16.29...

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