Xanthates derived from 1,3-dithiane and its monosulfoxide; one-carbon radical equivalents

Facile synthesis of dibenzothiophene S-oxides from sulfinate esters

An efficient method to prepare dibenzothiophene S-oxides is disclosed. Suzuki-Miyaura cross-coupling of 2-bromoaryl sulfinate esters with arylboronic acids selectively at the bromo group followed by electrophilic cyclization of the resulting sulfinate ester moiety provides diverse dibenzothiophene S-oxides. Further transformations including Pummerer-type C-H propargylation and aryne reactions realize to synthesize highly functionalized dibenzothiophene derivatives.

The Xanthate Route to Ketones: When the Radical Is Better than the Enolate

The alkylation of enolates is one of the backbones of ketone chemistry, yet in practice it suffers from numerous limitations due to problems of regiochemistry (including O- versus C-alkylation), multiple alkylations, self-condensation, competing elimination, and incompatibility with many polar groups that have to be protected. Over the years, various solutions have been devised to overcome these difficulties, such as the employment of auxiliary ester or sulfone groups to modify the p K_a of the enolizable hydrogens, the passage by the corresponding hydrazones, the use of transition-metal-catalyzed redox systems to formally alkylate ketones with alcohols, etc. Most of these hurdles disappear upon switching to α-ketonyl radicals. Radicals are tolerant of most polar functions, and radical additions to flat sp² centers are generally easier to accomplish than enolate substitution at tetrahedral sp³ carbons. The main stumbling block, however, has been a lack of generally applicable methods for the generation and intermolecular capture of α-ketonyl radicals. We have found over the past years that the degenerative exchange of xanthates represents in many ways an ideal solution to this problem. It overcomes essentially all of the difficulties faced by other radical processes because of its unique ability to reversibly store reactive radicals in a dormant, nonreactive form. The lifetime of the radicals can therefore be significantly enhanced, even in the concentrated medium needed for bimolecular additions, while at the same time regulating their absolute and relative concentrations. The ability to perform intermolecular additions to highly functionalized alkene partners opens up numerous possibilities for rapid and convergent access to complex structures. Of particular importance is the elaboration of ketones that are prone to self-condensation, such trifluoroacetone, and of base-sensitive ketones, such as chloro- and dichloroacetone, since the products can be used for the synthesis of a myriad fluorinated and heteroaromatic compounds of relevance to medicinal chemistry and agrochemistry. The formal distal dialkylation of ketones, also of utmost synthetic interest, is readily accomplished, allowing convenient access to a wide array of useful ketone building blocks. Cascade processes can be implemented and, in alliance with powerful classical reactions (aldol, alkylative Birch reductions, etc.), furnish a quick route to complex polycyclic scaffolds. Furthermore, the presence of the xanthate group in the adducts can be exploited to obtain a variety of arenes and heteroarenes, such as pyrroles, thiophenes, naphthalenes, and pyridines, as well as enones, dienes, and cyclopropanes. Last but not least, the reagents and most of the starting materials are exceedingly cheap, and the reactions are safe and easy to scale up.

Use of a benzyl ether as a traceless hydrogen donor in the anti-Markovnikov hydrofunctionalization of alkenes with xanthates

A new protocol for the anti-Markovnikov hydrofunctionalization of alkenyl alcohol O-Bn ethers was developed using xanthates as functionalizing agents in the presence of lauroyl peroxide as a radical initiator and a stoichiometric oxidant. The benzyl group serves as a traceless hydrogen donor in the remote radical hydrogen atom transfer event during the process.

Photoredox radical conjugate addition of dithiane-2-carboxylate promoted by an iridium(iii) phenyl-tetrazole complex: a formal radical methylation of Michael acceptors

A readily accessible iridium(iii) phenyl-tetrazole complex ([Ir(ptrz)2(tBu-bpy)]+, 2; Hptrz = 2-methyl-5-phenyl-tetrazole; tBu-bpy = 4,4'-di-tert-butyl-2,2'-bipyridine) is shown to be a versatile catalyst for a new photocatalytic Michael reaction. Under light irradiation in the presence of 2, a dithiane 2-carboxylic acid, obtained by simple hydrolysis of a commercially available ethyl ester, generates a 1,3-dithiane radical capable of performing addition to a variety of Michael acceptors (e.g., unsaturated ketones, esters, amides and malonates). This broad scope reaction with high yields is a formal photo-redox addition of the elusive methyl radical and the adducts obtained can be starting materials for a variety of functionalized products. The excited-state oxidation potential of catalyst 2 allows selective formation of radicals only from α-heterosubstituted carboxylates. Chemical modification of this metal complex can tune the electrochemical properties, opening a route to new highly selective catalytic photo-oxidation reactions.

Alkyl- and aryl-thioalkylation of olefins with organotrifluoroborates by photoredox catalysis

A new photocatalytic alkyl- and aryl-thioalkylation of electron-deficient olefins using organotrifluoroborates has been developed.

Photocatalytic direct intermolecular addition of 1,3-dithianes

A direct intermolecular radical addition of 1,3-dithianes to alkenes has been accomplished via visible-light mediated photocatalysis.

Highly diastereoselective ionic/radical domino reactions: single electron transfer induced cyclization of bis-sulfoxides

SET oxidation of bis-sulfinyl anions has enabled the uses of bis-sulfinyl radical as a synthetic equivalent of chiral acyl and methylene radicals involved in tandem reactions leading to the enantioselective construction of various carbo- and heterocyclic derivatives.

Synthesis, antimycobacterial and antitumor activities of new (1,1-dioxido-3-oxo-1,2-benzisothiazol-2(3H)-yl)methyl N,N-disubstituted dithiocarbamate/O-alkyldithiocarbonate derivatives

Reaction of 2-chloromethylsaccharin with substituted potassium dithiocarbamates and substituted potassium dithiocarbonates furnished (1,1-dioxido-3-oxo-1,2-benzisothiazol-2(3H)-yl)methyl N,N-disubstituted dithiocarbamates (4-15) and (1,1-dioxido-3-oxo-1,2-benzisothiazol-2(3H)-yl)methyl O-alkyldithiocarbonates (16-20). The new derivatives were evaluated for in vitro antimycobacterial activity against Mycobacterium tuberculosis H37Rv. Compounds 4-13, 15, and 16-20 described herein showed moderate to good inhibitory activity. In particular, seven analogs 4, 5, 6, 13, and 7, 8, and 12 exhibited excellent MIC values of 1.56 and 0.78 microg/mL, respectively. Compounds 4, 5, 10, 12, 13, and 16 were selected and screened for antitumor activity. Among the tested compounds, 4 and 5 were found to be cytotoxic, especially against leukemia cell lines CCRF-CEM, HL-60(TB), RPMI-8226, and SR with log10GI50 values lower than -6.69, and against non-small cell lung cancer NCI-H522 cell line with log10GI50 values lower than -6.31. Compound 10 was cytotoxic against leukemia cell line HL-60(TB), whereas 16 displayed favorable cytotoxicity against ovarian cancer cell line OVCAR-3 with log10GI50 values of -6.31 and -7.45, respectively.

Powerful CarbonCarbon Bond Forming Reactions Based on a Novel Radical Exchange Process

Xanthates and related derivatives have proved to be extremely useful for both inter- and intramolecular radical additions. The broad applicability of the intermolecular addition to un-activated olefins opens tremendous opportunities for synthesis, since various functional groups can be brought together under mild conditions and complex structures can be rapidly assembled. The presence of the xanthate in the product is also a powerful asset for further modifications, by both radical and non-radical pathways. Of special importance is the access to highly substituted aromatic and heteroaromatic derivatives and the synthesis of block polymers through a controlled radical polymerisation mediated by various thiocarbonylthio group containing agents (RAFT and MADIX processes).

Cyano(ethoxycarbonothioylthio)methyl Benzoate: A Novel One-Carbon Radical Equivalent

[reaction: see text] Cyano(ethoxycarbonothioylthio)methyl benzoate 3 has been prepared and shown to be an excellent one-carbon radical equivalent that can be applied for the introduction of an acyl unit via xanthate transfer radical addition to olefins. The corresponding adducts can be further elaborated. A rare 1,5-nitrile translocation was also observed during the study.