Synthesis of Substituted Benzo[b]thiophenes via Base-Promoted Domino Condensation–Intramolecular C–S Bond Formation

Synthesis of 3,5-Disubstituted-1,2,4-thiadiazoles via NaH-DMF-Promoted Dehydrogenative Intramolecular N-S Bond Formation

A facile transition-metal-free synthesis of 3,5-bis(het)aryl/arylaminothiadiazoles has been reported. The overall protocol involves base-mediated tandem thioacylation of amidines with dithioesters or aryl isothiocyanates in DMF solvent and subsequent in situ intramolecular dehydrogenative N-S bond formation of thioacylamidine intermediates under an inert atmosphere. A probable mechanism involving a carbamoyl anion, generated by deprotonation of DMF, acting as a radical initiator has been suggested.

Regio- and Chemoselective Access to Dihydrothiophenes and Thiophenes via Halogenation/Intramolecular C(sp2)-H Thienation of α-Allyl Dithioesters at Room Temperature

An operationally simple, practical, and efficient cascade approach employing α-allyl dithioesters and NBS/NIS has been achieved to access a series of dihydrothiophenes and thiophenes containing diverse functional groups of different electronic and steric natures in good to excellent yields at room temperature in open air. The reaction proceeds via the electrophilic addition of a halogen source (NBS/NIS) to an allylic double bond, followed by intramolecular regio- and chemoselective S-cyclization. This protocol avoids potential toxicity and tedious work-up conditions, and features easy synthesis from readily available starting materials under catalyst-free conditions. Furthermore, 4,5-dihydrothiophenes were aromatized to thiophenes by treatment with KOH in DMF at room temperature. A probable mechanism for the formation of dihydrothiophenes and thiophenes from α-allyl dithioesters has been suggested. Notably, a large-scale experiment and the transformations of products indicated the potential utility of this reaction compared to competing processes for the synthesis of 4,5-dihydrothiophenes and thiophenes.

Domino Synthesis of Thiazolo-Fused Six- and Seven-Membered Nitrogen Heterocycles via Intramolecular Heteroannulation of In-Situ-Generated 2-(Het)aryl-4-amino-5-functionalized Thiazoles

Synthesis of novel 2-(het)aryl-substituted thiazolo-fused six- and seven-membered heterocycles, such as thiazolo[4,5-b]pyridin-5(4H)-ones, thiazolo[4,5-c]isoquinolin-5(4H)-ones, thiazolo[4,5-b]quinolin-9(4H)-ones, 4H-benzo[e]thiazolo[4,5-b]azepine-5,10-diones, have been developed in a single-pot operation via intramolecular heteroannulation of in-situ-generated 2-(het)aryl-4-amino-5-functionalized thiazoles. These 4-amino-5-functionalized thiazoles were readily obtained in a one-pot process by treatment of a range of (het)aryldithioesters with cyanamide in the presence of NaH, followed by in situ S-alkylation-intramolecular condensations of the resulting thioimidate salts with appropriate activated methylene halides. On the other hand, the corresponding 4H-benzo[b]thiazolo[4,5-e][1,4]diazepin-10(9H)-ones were synthesized in a two-step process, requiring prior isolation of 5-carboethoxy-4-(2-nitrophenyl)aminothiazoles and their subsequent reductive cyclization. The activated methylene halides employed in these reactions for the synthesis of various thiazolo-fused heterocycles were methyl bromocrotonate, ethyl 2-(bromomethyl)benzoate, 2-fluorophenacyl bromides, ethyl 2-(2-bromoacetyl)benzoate, and ethyl bromoacetate. Several of these thiazolo-fused heterocycles display yellow green to green fluorescence, and their absorption and emission spectra have also been examined.

Lewis Base Catalyzed Synthesis of Sulfur Heterocycles via the C1‐Pyridinium Enolate

While the addition of C1‐Lewis base enolates to carbonyls and related structures are well established, the related addition to thiocarbonyls compounds are unknown. Herein, we report a reaction cascade in which a C1‐pyridinium enolate undergos addition to dithioesters, trithiocarbonates and xanthates. The reaction provides access to a range of dihydrothiophenes and dihydrothiopyrans (28‐examples). Mechanistic investigations, including isolation of intermediates, electronic correlation, and kinetic isotope effect studies support the viability of an activated acid intermediate giving rise to the C1‐pyridinium enolate which undergoes turnover limiting cyclization. Subsequent formation of a β‐thiolactone regenerates the catalyst with loss of carbon oxysulfide providing the observed products.

Stereoselective sulfenylation of oxindole-derived propargyl alcohols to access sulfenylated-3-alkenyloxindoles

A Ca-catalyzed, tetrasubstituted alkenyl-sulfenylation was achieved using readily available aryl/alkyl thiols and easily prepared oxindole-derived propargyl alcohols under solvent-free conditions. The reaction proceeded with hydrogen bonding assisted regioselective α-thiolation and subsequent calcium catalyzed stereoselective alkenylation to yield E-alkenyl thioethers with high diastereoselectivity.

Chemoselective Ullmann Reaction of α-Trisubstituted Thioamides: Synthesis of Novel 2-Iminobenzothiolanes

New classes of unexplored benzo[b]thiolanes are synthesized from trisubstituted thioamides through copper-catalyzed intramolecular S-arylation of thioamides for the first time. This method provides good to excellent yields with fully controlled chemoselectivity. Unusually, iminobenzo[b]thiolanes are very stable under mild acidic conditions. A plausible mechanism is proposed for the chemoselective S-arylation process.

Single-Pot Preparation of 4-Amino-2-(het)aryl-5-Substituted Thiazoles Employing Functionalized Dithioesters as Thiocarbonyl Precursors

An effective, diversity oriented, one-pot reaction of 4-amino-2-(het)aryl/alkyl-5-functionalized thiazoles has been disclosed, utilizing aryl/heteroaryl/alkyl dithioesters as thiocarbonyl coupling partners in a modified Thorpe-Ziegler type cyclization. The reaction proceeds at room temperature, under mild conditions, in excellent yields, displaying broad functional group compatibility at 2 and 5 positions of thiazoles. This synthetic strategy has been further expanded for the one-pot construction of two highly potent tubulin polymerization inhibitors, i.e., 2-(het)aryl-4-amino-5-(3,4,5-trimethoxyaroyl) thiazoles, in high yields.

Unusual Behavior of Ketoximes: Reagentless Photochemical Pathway to Alkynyl Sulfides

The unique properties of ketoximes are used prominently for the synthesis of heterocycles. In contrast, their potential to absorb light and photoelectron transfer processes remains challenging. Widespread interest in controlling direct excitation of ketoxime tacticity unlocks unconventional reaction pathways, enabling photochemical intramolecular skeletal modification to constitute alkynyl sulfides that cannot be realized via traditional activation. Despite decades of advancements, the alkynyl sulfides, particularly those composed of polar functionalities and derived from renewable sources, remain unknown. These findings demonstrate the importance of decelerated ketoxime from β-oxodithioester for the identification of reaction conditions. The method uses mild reaction conditions to generate excited-state photoreductant for the functionalization of an array of alkynyl sulfides. Additionally, a fundamental understanding of elementary steps using electrochemical and spectroscopic techniques/experiments revealed a PCET pathway to this transformation, while the involved substrates and their properties with improved economical tools indicated the translational potential of this method.