3-aryl-3- diazirines

Scheme 4 Possible reactions of the intermediates formed after photolysis of 3-aryl-3//-diazirines. Inset shows olefin formation from carbene derivative of unsubstituted 3-alkyl-3//-diazirines via a hydride shift...

Surface-introduced photoreactive group, aryl diazirine, can generate carbenes by 365 nm UV exposure. However, gold-sulfiir bonds, which support photolinker on gold, can be easily broken by UV light below 300 nm. Therefore, UV transmission filter is essential in UV irradiation. 

Like diazo compounds and aryl azides, diazirines give rise in most instances to unwanted photolysis products besides the desired carbenes, including long-lived electrophilic intermediates. The most serious problem is the generation of diazo compounds (e.g. Fig. 3.13). For example, 3-H-3-aryl-diazirines form diazo isomers to the extent of 30 to 70% when irradiated. These isomers are themselves photolysed to form carbenes, but relatively slowly at wavelengths at which the diazirines absorb (Smith and Knowles, 1973, 1975). 

Brunner et al. (1980) have reported on the properties of a new class of aryl diazirines, the 3-aryl-3-trifluoromethyldiazirines. Besides forming a class of the most attractive photoactivatable reagents available, these molecules may be made in far higher yield than the corresponding 3-H-diazirines. The route used, which incidentally fails with the 3-H-com-pounds (Smith and Knowles, 1975), is shown in Fig. 3.24. Two drawbacks are the large number of steps involved and the difficulty of preparing... 

Y. Chevolot, Surface Photoimmobilization of Aryl Diazirine Containing Carbohydrates — Tools toward Surface Glycoengineering (PhD Thesis EPFL, 1999), no. 1995. 

Figure 11.3 Identification of protein targets using photo-cross-linking beads, (a) Upon UV Irradiation, aryl-diazirine groups covalently Introduced onto a solid matrix (e.g., agarose beads) are transformed into highly reactive carbenes, which in turn bind to or insert irreversibly into a proximal small molecule in a functionality independent manner, (b) Pull-down assays are performed by mixing...

In the following discussion, It Is Intended to compare the Immobilization of two photoactivatable carbohydrates on polystyrene (PS) and to Illustrate how their biological activity was tested. The same galactose aryl diazirine as the one described for diamond modification (see Scheme 1) was used and compared to the also newly synthesized lactose aryl diazirine displayed in Scheme 2. 

Scheme 2. Chemical structure of lactose aryl diazirine (R=H)...

When comparing samples A, B and C, simple signatures of the molecules such as fluorine, CF3 among others, should allow one to test easily the efficiency of their immobilization on PS with XPS and ToF-SIMS. Table 2 illustrates the case of lactose aryl diazirine. The residual level of molecules observed on sample B by ToF-SIMS was below the XPS instrument detection limit. It confirms that the washing procedure was able to remove physlsorbed molecules. For sample C, after washing, covalently attached molecules were expected to remain bound to the surface, in contrast with sample B [94,96). XPS fluorine atomic percentage, as well as CF3 and F normalized intensities (Table 2), illustrate in the same way the successful immobilization of the molecule at the surface of polystyrene. These results are... 

Table 2. XPS and ToF-SIMS analysis of lactose aryl diazirine grafted PS. XPS atomic percentages are displayed for samples A, B and C. Sample A corresponds to as received polystyrene. Sample B corresponds to polystyrene on which a methanoUc solution of lactose aryl diazirine was deposited, and no light activation was performed before the final washing. This surface should be similar to sample A. Sample C corresponds to polystyrene on which a methanolic solution of lactose aryl diazirine was deposited and light activation was performed before the final washing. The molecule should be present at the surface of the material. Three areas were analyzed per sample, after [105]...

In addition, XPS demonstrated that the surface concentrations of the two molecules (MAD-gal, lactose aryl diazirine) were almost similar (see Table 2). Figure 21 Indicates quantitative XPS and normalized ToF-SIMS data of MAD-Gal... 

Aryl halodiazarines 119 when reduced with potassium ethyl xanthate are reported to give 3-aryl-5-ethoxy-1,2,4-thiadiazoles 120 (13%) along with the expected product benzonitrile (87%) (Equation 31). A mechanism involving fragmentation of the diazirine 119 is proposed <1999TL29>. 

Keywords Signal transduction, Photoaffinity labeling, G-Protein coupled receptors, Protein kinase, Tyrosine kinase, Aryl azide, Benzophenone, Diazirine... 

Several comparative studies are available in the literature, which clearly shows that, according to the latest results, tetrafluoro-pheny/ azides (AZ), trifluoro-methyl-phenyl diazirines (DZR), and particularly benzophenone (BP) are the best choice (Scheme 3.). Diazocarbonyl compounds, which played a historically role in the evolution of PAL, can also be considered together with aryl-diazonium salts [6] (not shown). For aryl azides the more common unsubstituted photophore is presented. It should be noted that tetrafluoro-phenyl azides have an increased reactivity towards CH-groups and they do not rearrange. Other substituted phenyl azides can readily alter the excitation wavelength allowing the application of milder or differential photoactivation. 

The most frequent synthetic approaches, summarized in Scheme 4, are towards the primary photophores. The preparation of aryl azide derivatives follows the typical retro-synthetic pathway in the majority of the reported cases (Scheme 4 A), and, practically, diazotation is the most commonly used procedure [24 - 29]. In the case of diazirines only one major synthetic sequence is repeated ammonolysis of oximes followed by dehydrogenation (Scheme 4B) [30-32]. There are various ways of preparing diazo- or diazocarbonyl-compounds most frequently the Forster and Bamford-Stevens reactions (Scheme 4C) are employed [33-37]. 

Thermolysis of aryl chloro diazirine (18) in the presence of acetone and a trapping agent such as A -phenylmaleimide gave rise to cycloadducts such as 41. The unstable adduct hydrolyzed during purification resulting in synthesis of bicyclic hemiacetals 42 and 43 as a mixture of endo and exo adducts in 37 and 8% yield, respectively. The exclusive generation of the singlet carbene was confirmed by low-temperature electron spin resonance (ESR) study of the irradiated diazirine. 

Diazo compounds are usually easy to synthesize and handle if the N2 group is next to a carbonyl or an aryl group. In other cases, especially when electronreleasing substituents such as halogen or oxygen atoms are adjacent to the diazo moiety, diazirines prove to be more practical. Sometimes, diazirines undergo photorearrangement to diazo compounds prior to decomposition (or vice versa, as in the case of phenyldiazomethane" ), but this does not impede the formation of the desired carbene. 

Table 4 Photoreactive Peptides Modified with 4-[3-(Trifluoromethyl)-3//-diazirin-3-yl]aryl-Based...

Fluoroalkylatcd diazirines are an interesting class of compounds that are prepared from the corresponding diaziridincs. Thus, the oxidation of 3-aryl-3-(lrifluoromethyl)diaziridine (t) to diazirine 2 is carried out with a mixture of dimethyl sulfoxide/oxalyl chloride in good yields.256 This gentle oxidation does not affect the sulfur in 2-thienyl aziridines. Similarly, the oxidation of diaziridine 3 to bis(Lrifluoromethyl)diazirine 4 is accomplished using lead(IV) acetate.244 This diazirine can be stored in a steel cylinder at 25r C and shows no tendency to detonate. 

In Chapter 2 an account was given of the various classes of molecules that have been used as photoactivatable reagents. This chapter concentrates on the three classes, aryl azides, diazo compounds and diazirines, which can be considered as proven and established. 

Aryl diazomethanes vary in stability. They are formed when diazirines are irradiated and will be be discussed below. 

Like aryl azides the useful diazirines are quite bulky but again they may often be built into molecules with structures similar to naturally occurring compounds (Fig. 3.12 Huang et al., 1982). 

Smith and Knowles (1973) made several 3-H-3-aryldiazirines and suggested that they had several advantages as photoaffinity labeling reagents over the aryl azides and diazo compounds that were then current. These diazirines did not become popular, however, because they are inconvenient to prepare. The situation today is unchanged. 

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