Iron(III)-Catalyzed Regioselective Synthesis of Electron-Rich Benzothiazoles from Aryl Isothiocyanates via C-H Functionalization

We report herein a direct synthetic route for the preparation of 2-arylbenzothiazoles using aryl isothiocyanates and electron-rich arenes. The synthetic route involves triflic acid promoted addition of the arenes to aryl isothiocyanates followed by FeCl₃-catalyzed C-S bond formation via C-H functionalization. The approach provides the advantage of synthesis of benzothiazoles without the conventional use of aryl aldehyde/carboxylic acid precursors employing the less expensive iron(III) catalyst.

Transformation of Amides to Thioamides Using an Efficient and Novel Thiating Reagent

A convenient protocol was developed for the transformation of N-aryl-substituted benzamides to N-aryl-substituted benzothioamides using N-isopropyldithiocarbamate isopropyl ammonium salt as a novel thiating reagent. The major advantages of this protocol are its one-pot procedure, short reaction times, mild conditions, simple work-up, high yields and pure products.

Synthesis of Isothiocyanates Using DMT/NMM/TsO- as a New Desulfurization Reagent

Thirty-three alkyl and aryl isothiocyanates, as well as isothiocyanate derivatives from esters of coded amino acids and from esters of unnatural amino acids (6-aminocaproic, 4-(aminomethyl)benzoic, and tranexamic acids), were synthesized with satisfactory or very good yields (25-97%). Synthesis was performed in a "one-pot", two-step procedure, in the presence of organic base (Et3N, DBU or NMM), and carbon disulfide via dithiocarbamates, with 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium toluene-4-sulfonate (DMT/NMM/TsO-) as a desulfurization reagent. For the synthesis of aliphatic and aromatic isothiocyanates, reactions were carried out in a microwave reactor, and selected alkyl isothiocyanates were also synthesized in aqueous medium with high yields (72-96%). Isothiocyanate derivatives of L- and D-amino acid methyl esters were synthesized, under conditions without microwave radiation assistance, with low racemization (er 99 > 1), and their absolute configuration was confirmed by circular dichroism. Isothiocyanate derivatives of natural and unnatural amino acids were evaluated for antibacterial activity on E. coli and S. aureus bacterial strains, where the most active was ITC 9e.

Selective synthesis of aryl thioamides and aryl-α-ketoamides from α-oxocarboxylic acids and tetraalkylthiuram disulfides: an unexpected chemoselectivity from aryl sulfonyl chlorides

Tosyl chloride-controlled generation of thioamides and α-ketoamides via base-promoted decarboxylative reaction of α-oxocarboxylic acids with tetraalkylthiuram disulfides has been established.

Synthesis of α-Keto Thioamides by Metal-Free C═C Bond Cleavage in Enaminones Using Elemental Sulfur

An unprecedented method for cleaving the C═C bond in N, N-disubstituted enaminones in the presence of elemental sulfur and N, N-dimethyl-4-aminopyridine is disclosed. Without using any metal catalyst or additive, the cascade functionalization of both C═C and C-H bonds takes place to enable the formation of new C═S and C-N bonds, thus providing a facile and practical method for the synthesis of N, N-disubstituted α-keto thioamides.

Palladium-Catalyzed Decarboxylative ortho-Amidation of Indole-3-carboxylic Acids with Isothiocyanates Using Carboxyl as a Deciduous Directing Group

Palladium-catalyzed ortho-amidation of indole-3-carboxylic acids with isothiocyanates by using the deciduous directing group nature of carboxyl functionality to afford indole-2-amides is demonstrated. Both C-H functionalization and decarboxylation took place in one pot, and hence, this carboxyl group served as a unique, deciduous (or traceless) directing group. This reaction offers a broad substrate scope as demonstrated for several other heterocyclic carboxylic acids like chromene-3-carboxylic acid, imidazo[1,2- a]pyridine-2-carboxylic acid, benzofuran-2-carboxylic acid, pyrrole-2-carboxylic acid, and thiophene-2-carboxylic acid. In the reaction using 2-naphthoic acid, of the two possible isomers, only one isomer of the amide was exclusively formed. The indole-2-amide product underwent palladium-catalyzed C-H functionalization to afford the diindole-fused 2-pyridones by combining two molecules of the indole moiety, with the elimination of an amide group from one of them, attached at the C3-position for the C-C/C-N bond formation. The structures of key products are confirmed by X-ray crystallography.

Staudinger/aza-Wittig reaction to access Nβ-protected amino alkyl isothiocyanates

A unified approach to access Nβ-protected amino alkyl isothiocyanates using Nβ-protected amino alkyl azides through a general strategy of Staudinger/aza-Wittig reaction is described. The type of protocol used to access isothiocyanates depends on the availability of precursors and also, especially in the amino acid chemistry, on the behavior of other labile groups towards the reagents used in the protocols; fortunately, we were not concerned about both these factors as precursor-azides were prepared easily by standard protocols, and the present protocol can pave the way for accessing title compounds without affecting Boc, Cbz and Fmoc protecting groups, and benzyl and tertiary butyl groups in the side chains. The present strategy eliminates the need for the use of amines to obtain title compounds and thus, this method is step-economical; additional advantages include retention of chirality, convenient handling and easy purification. A few hitherto unreported compounds were also prepared, and all final compounds were completely characterized by IR, mass, optical rotation, and 1H and 13C NMR studies.

Electrophilic Carboxamidation of Ferrocenes with Isocyanates

Ferrocenes undergo one-step carboxamidation by reaction with isocyanates in CF₃SO₃H solution. The chemistry is most efficient in excess superacid, and it has been accomplished with aryl and aliphatic isocyanates. In conversions with ferrocene carboxylic acids, isocyanates provide imides in good yields. A mechanism for this conversion is suggested involving carbamic acid anhydride formation and subsequent intramolecular reaction at the substituted cyclopentadienyl ring.

Friedel-Crafts-type reaction of pyrene with diethyl 1-(isothiocyanato)alkylphosphonates. Efficient synthesis of highly fluorescent diethyl 1-(pyrene-1-carboxamido)alkylphosphonates and 1-(pyrene-1-carboxamido)methylphosphonic acid

Friedel–Crafts-type reaction of pyrene with diethyl 1-(isothiocyanato)alkylphosphonates promoted by trifluoromethanosulfonic acid afforded diethyl 1-(pyrene-1-carbothioamido)alkylphosphonates in 83–94% yield. These compounds were transformed, in 87–94% yield, into the corresponding diethyl 1-(pyrene-1-carboxamido)alkylphosphonates by treatment with Oxone^®. 1-(Pyrene-1-carboxamido)methylphosphonic acid was obtained in a 87% yield by treating the corresponding diethyl phosphonate with Me₃Si-Br in methanol. All of the synthesized amidophosphonates were emissive in solution and in the solid state. The presence of a phosphonato group brought about an approximately two-fold increase in solution fluorescence quantum yield in comparison with that of a model N-alkyl pyrene-1-carboxamide. This effect was tentatively explained by stiffening of the amidophosphonate lateral chain which was caused by the interaction (intramolecular hydrogen bond) of phosphonate and amide groups. The synthesized phosphonic acid was soluble in a biological aqueous buffer (PBS, 0.01 M, pH 7.35) and was strongly emissive under these conditions (λ_em = 383, 400 nm, τ = 18.7 ns, Φ_F > 0.98). Solid-state emission of diethyl 1-(pyrene-1-carboxamido)methylphosphonate (λ_max = 485 nm; Φ_F = 0.25) was assigned to π–π aggregates, the presence of which was revealed by single-crystal X-ray diffraction analysis.

Sulfenylation of β-Diketones Using C-H Functionalization Strategy

Sulfenylation of β-diketones is challenging as β-diketones undergo deacylation after sulfenylation in the reaction medium. The sulfenylation of β-diketones without deacylation under metal-free conditions at ambient temperature via a cross dehydrogenative coupling (CDC) strategy is reported. The resultant products can be further manipulated to form α,α-disubstituted β-diketones and pyrazoles.