New glycomimetics: anomeric sulfonates, sulfenamides, and sulfonamides

The synthesis of a variety of new 1-thio-D-glucopyranose derivatives oxidized at the sulfur atom is described, including seven 1-C-sulfonic acids, three sulfonate esters, three sulfinate esters, an S,S'-diglycosyl thiolsulfonate and thiolsulfinate, four S-glycosyl sulfenamides, an S-glycosyl sulfinamide, and two S-glycosyl sulfonamides. These compounds possess unusual anomeric functionality that might be resistant or even inhibitory to normal enzymatic carbohydrate processing, and therefore, they may be of future use in studies of enzyme inhibition, structure, mechanism, and function.     

The frequently overlooked importance of solvent in free radical syntheses

This tutorial review is designed to dispel the myth, still believed by many synthetic organic chemists, that radical-based syntheses are free from significant solvent effects. However, many synthetically valuable radical reactions do exhibit large kinetic solvent effects. It is therefore important to select the solvent for any proposed radical synthesis with considerable care if good product yields are to be achieved.     

Fragmentation of aromatic sulfonamides in electrospray ionization mass spectrometry: elimination of SO(2) via rearrangement

Arylsulfonamides are attractive pharmacophores for drug candidates. Fragmentation behaviors of selected aromatic sulfonamides were investigated using electrospray ionization mass spectrometry in the positive ion mode. Some of the sulfonamides afforded unique loss of 64 (loss of SO(2)) ions upon collision-induced dissociation followed by intramolecular rearrangements in the gas phase. This SO(2) elimination-rearrangement pathway leading to the generation of [M + H - SO(2)](+) ions appeared to be susceptible to substitutions on the aromatic (Ar) ring that would affect the Ar--sulfur bond strength and the stability of the partially positive charge developed at the ipso position upon bond dissociation. Electron withdrawing groups such as chlorine attached to the aromatic ring at ortho position seem to promote the SO(2) extrusion. Although this fragmentation pathway in atmospheric pressure ionization MS is less predictable than in electron impact MS, it is a frequently encountered reaction. The absence of this fragmentation pathway in some of the arylsulfonamides indicates that other factors such as nucleophilicity of the nitrogen may also play a role in the process. With respect to the site of attachment of the migrating NR'R', ipso-substitution on the aromatic ring is evident since this fragmentation mechanism is operative in the fully ortho-substituted arylsulfonamides.     

Beckman rearrangement of oxime sulfonates by grignard reagents

The Beckmann rearrangement of oxime sulfonates by Grignard reagents provides an efficient and general entry to α-alkyl- and α,α-dialkylamines in good yields.     

Stereocontrolled syntheses of kainoid amino acids from 7-azabicyclo[2.2.1]heptadienes using tandem radical addition-homoallylic radical rearrangement

[reaction; see text] N-Boc syn-7-(2-hydroxyethyl)-4-(alkyl or aryl)sulfonyl-2-azabicyclo[2.2.1]hept-5-enes serve as precursors in syntheses of the neuroexcitants 3-(carboxymethyl)pyrrolidine-2,4-dicarboxylic acid 43, alpha-kainic acid 12, alpha-isokainic acid 14, and alpha-dihydroallokainic acid 77. The key step in these syntheses is the intermolecular radical addition of 2-iodoethanol to a N-Boc 2-(alkyl or aryl)sulfonyl-7-azabicyclo[2.2.1]heptadiene 7 to induce nitrogen-directed homoallylic radical rearrangement. Oxidative cleavage of the resulting 2-azabicyclo[2.2.1]hept-5-enes provide straightforward access to polysubstituted pyrrolidines and, in particular, an efficient entry to the kainoid amino acids.     

Diastereoselective free radical halogenation,azidation, and rearrangement of beta-silyl Barton esters

[reaction: see text] Barton esters of beta-silylcarboxylic acids were decomposed by photolysis alone in organic solvents or in the presence of ethanesulfonyl azide or bromotrichloromethane. Products of the reaction, beta-silylthiopyridyl ethers, beta-silyl azides, or alkenes, were formed with significant control of stereochemistry.     

Polycyclic aromatic compounds via radical cyclizations of benzannulated enyne-allenes derived from Ireland-Claisen rearrangement

A new synthetic sequence involving the use of Ireland-Claisen rearrangement of propargylic acetates to form the corresponding benzannulated enyne-allenes followed by Schmittel cyclization to generate benzofulvene biradicals for radical cyclizations leading to polycyclic aromatic compounds was established. Treatment of 9-fluorenone (8) with the lithium acetylide 9 followed by acetic anhydride produced the propargylic acetate 10. A sequence of reactions occurred after 10 was converted to the corresponding silyl ketene acetal 11. An initial Ireland-Claisen rearrangement produced the benzannulated enyne-allene 12, which then underwent a Schmittel cyclization reaction to generate the benzofulvene biradical 13. A subsequent intramolecular radical-radical coupling then produced the formal Diels-Alder adduct 14, which in turn underwent a prototropic rearrangement to give the silyl ester 15 and, after hydrolysis, the carboxylic acid 16 in 57% overall yield from 10 in a single operation. An intramolecular acylation reaction of 16 produced the ketone 17. The carboxylic acids 24-26 were likewise prepared from the diaryl ketones 18-20, respectively. However, the intramolecular [2 + 2] cycloaddition reaction of the benzannulated enyne-allene 33 having a tert-butyl group at the allenic terminus occurred preferentially, producing the 1H-cyclobut[a]indenyl acetic acid 35 as the predominant product.     

Synthesis of fluorinated fused benzofurans and benzothiophenes: Smiles-type rearrangement and cyclisation of perfluoro(het)aryl ethers and sulfides

Lithium-bromine exchange in 2-bromophenyl perfluoroaryl ethers or sulfides affords fused fluorinated benzofurans or benzothiophenes respectively by S(N)Ar substitution of the adjacent fluorine in the perfluoroaryl substituent. The structures of the new compounds were confirmed by NMR spectroscopy and single crystal X-ray diffraction analysis. In the case of 2-bromophenyl tetrafluoropyrid-4-yl ether, lithiation promoted a Smiles-type rearrangement which led to formation of 4-(2-hydroxyphenyl)tetrafluoropyridine, for which the structure was confirmed by X-ray crystallography.     

Facile one-pot synthesis of aromatic and heteroaromatic sulfonamides

A series of arene and heteroarene sulfonamides were prepared in one vessel from aryl and heteroaryl bromides via conversion into the corresponding Grignard reagents using either magnesium or isopropylmagnesium chloride and subsequent reaction with sulfur dioxide, sulfuryl chloride, and an amine.     

The aromatic cope rearrangement: Activation parameters

The activation parameters (ΔH^≠ = 26.7 ± 4.0 kcal/mol and ΔS^≠ = 17.5 ± 9.8 eu) have been measured for the Cope rearrangement of $\text{cis}$-1-($\text{m}$-hydroxyphenyl)-2-vinylcyclopropane.     

New aromatic azo compounds: syntheses and liquid-crystalline properties

The chemical syntheses of 12 new azo dyes are reported. The optically active compounds were prepared by the Mitsunobu reaction, amide formation, and esterification with optically active alcohols or acids. Several new compounds showed liquid-crystalline properties, and their phase transition behavior was investigated.     

Separation of aromatic hydrophobic sulfonates by micellar electrokinetic chromatography

Two different buffer systems for the separation of 12 aromatic hydrophobic sulfonates by micellar electrokinetic chromatography (MEKC) were developed. The following buffer systems were used: aqueous phosphate buffers containing either cetyltrimethylammonium bromide (CTAB) or sodium dodecyl sulfate (SDS). Eleven aromatic sulfonates were simultaneously separated in less than 35 min employing 20 mM phosphate buffer, pH 7.0 containing 50 mM SDS and 10% of acetonitrile.     

Synchronous reactions of free radical formation upon low-temperature chlorination of aromatic and olefinic hydrocarbons

The free radical formation in reactions of low-temperature (77–150 K) chlorination of monomers and phenols was studied by ESR and calorimetry. The schemes were suggested for multicentered synchronous reactions of radical formation in polymolecular complexes, whose initial composition was directly determined for the first time from the structure of the radicals formed and thermodynamic parameters of the total exothermic effect.     

Expedient total syntheses of rhein and diacerhein via Fries rearrangement

Short and practical total syntheses of rhein (1) and diacerhein (2) have been achieved via a Fries rearrangement and bis-carbonylation strategy followed by cyclization in molten salt, starting from dibromoester 7.     

Oxidation of aromatic and aliphatic triisopropylsilanylsulfanyls to sulfonyl chlorides: preparation of sulfonamides

A series of aromatic and aliphatic triisopropylsilanylsulfanyls were prepared and oxidized to the sulfonyl chlorides with KNO₃/SO₂Cl₂. The sulfonyl chlorides were characterized via their conversion to sulfonamides.