Base Labile Sulfones

Crosslinkers that have an internal sulfone group in their cross-bridge may be cleaved using base. 

Thioesters may be broken with the production of an amide and a sulfhydryl group on either side of the crosslinker fragments. 

Fluorescent labels, by contrast, can provide tremendous sensitivity due to their property of discrete emission of light upon excitation. Proteins, nucleic acids, and other molecules can be labeled with fluorescent probes to provide highly receptive reagents for numerous in vitro assay procedures. For instance, fluorescently tagged antibodies can be used to probe cells and tissues for the presence of particular antigens, and then detected through the use of fluorescence microscopy techniques. Since each probe has its own fluorescence emission character, more 

Suggested references on fluorescent spectroscopy and the use of fluorescent probes include Lakowicz (1991, 1999) Bright (1988) Dewey (1991) McGown and Warner (1990) Ploem and Tanke (1987) Darzynkiewicz and Crissman (1990) Haugland (1991) and Waggoner (1990). 

The following sections describe the most important fluorescein derivatives having reactive groups commonly used to label biomolecules. 

The following sections describe the principal reagents available for producing tagged molecules using fluorescent probes, radiolabels, and biotinylation techniques. In many cases, suggested protocols are included (or other sections referenced) for the use of these probes. 

AMCA group does not behave as it characteristically does in the underivatized state. Upon photolysis, however, the phenyl azide either gets coupled to a target molecule or breaks down to an amine. In either case, the presence of an amine (or its derivative) provides enough ring-activating properties to restore AMCAs fluorescent character. 

Fluorescent compounds are sensitive to changes in their chemical environment. Alterations in media pH, buffer components, solvent polarity, or dissolved oxygen can affect and quench the quantum yield of a fluorescent probe (Bright, 1988). The presence of absorbing components in solution that absorb light at or near the excitation wavelength of the fluorophore will have the effect of decreasing luminescence. In addition, noncovalent interactions of the probe with other components in solution can inhibit rotational freedom and quench fluorescence. 

The following sections describe some of the most popular fluorescent probes for use in labeling biomolecules. These fluorophores are available from a number of manufac- 

Suggested references on fluorescent spectroscopy and the use of fluorescent probes include Lakowicz (1991) Bright (1988) Dewey (1991) McGown and Warner 

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Few methods exist for the protection of sulfonic acids. Imidazolides and phenolic esters are too base labile to be useful in most cases. Simple sulfonate esters often cannot be used because these are obviously quite susceptible to nucleophilic reagents. 

Vinyl sulfones such as 262 are smoothly converted to a,) -unsaturated nitriles such as 263 on treatment with KCN in the presence of dicyclohexyl-18-crown-6 in refluxing t-butyl alcohol (equation 155)148. The reaction conditions are compatible with base-labile functionalities such as a methoxycarbonyl group (equation 156)148. This method can be used in the preparation of the sesquiterpene aldehyde nuciferal from allyl phenyl sulfones. 

A variety of cleavage conditions have been reported for the release of amines from a solid support. Triazene linker 52 prepared from Merrifield resin in three steps was used for the solid-phase synthesis of aliphatic amines (Scheme 22) [61]. The triazenes were stable to basic conditions and the amino products were released in high yields upon treatment with mild acids. Alternatively, base labile linker 53 synthesized from a-bromo-p-toluic acid in two steps was used to anchor amino functions (Scheme 23) [62]. Cleavage was accomplished by oxidation of the thioether to the sulfone with m-chloroperbenzoic acid followed by 13-elimination with a 10% solution of NH4OH in 2,2,2-trifluoroethanol. A linker based on l-(4,4 -dimethyl-2,6-dioxocyclohexylidene)ethyl (Dde) primary amine protecting group was developed for attaching amino functions (Scheme 24) [65]. Linker 54 was stable to both acidic and basic conditions and the final products were cleaved from the resin by treatment with hydrazine or transamination with ra-propylamine. 

Many examples of equilibrations at C-C double bonds [mediated by Pd(ll) or via radical intermediates], at base-labile positions (e.g., a to carbonyl, cyano or sulfone groups, see the examples mentioned in Section 4.3.4.2.2.2.), of alcohols (via the corresponding carbonyl compound). at nitrogen, at sulfur (in sulfoxides), are known287. 

Sulfonates with a sulfur-bound C,H group can also eliminate alkoxide when treated with a strong base [82]. Phenylmethanesulfonic esters, for instance, are sufficiently base-labile to be useful protective groups for alcohols (Scheme 4.17). 

Our final example is a base-labile 4-(phenylsulfonyl)methyl-l,3-dioxolane protecting group for aldehydes and ketones.4 Protection is carried out by the reaction of diol 17,1 (obtained by dihydroxylation of ally phenyl sulfone) with a carbonyl compound in the presence of pyridinium p-toluene sulfonate [Scheme 2.17], Cleavage is accomplished by treatment with DBU. /erf-Butyldimethylsilyl ethers, p-toluenesulfonate esters, tetrahydropyranyl ethers, carboxylic esters and benzoates are well tolerated. A disadvantage to the use of 17.1 is the introduc-... 

Treatment of the linker with diazomethane or bromoacetonitrile gives base-labile N-alkylsul-fone amide. The N-cyanomethyl-sulfone amide linker is also cleaved by sterically hindered amines. 

A soln. of startg. thioglycoside in TFIF treated with 2 eqs. 1 M lithium naphthalenide in the same solvent at — 78°, stirred for a further 30 min after the disappearance of startg. m., and neutralized with 4 1 TFlF/acetic acid at —78° - product. Y 92%. The method is compatible with both acid- and base-labile protective groups. F.e. and from glycosyl sulfones s. A.Fernandez-Mayoralas et al.. Tetrahedron Letters 30, 2537 0 (1989). 

The synthetic penicillin sulfone [sulbactam, (42) has been shown to act as a p-lactamase inhibitor. The inhibition is based on a similar mechanism as proposed above 62). A prodrug, sultamicillin 43), which combines sulbactam and amoxacillin by labile... 

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