An Organocatalytic Domino Thia-Michael/Aldol Condensation Reaction: Highly Enantioselective Synthesis of Functionalized Dihydrothiophenes

Construction of thioglycoside bonds via an asymmetric organocatalyzed sulfa-Michael/aldol reaction: access to 4'-thionucleosides

Thioglycoside bond formation via an asymmetric sulfa-Michael/aldol reaction of (E)-β-nucleobase acrylketones and 1,4-dithiane-2,5-diol has been achieved with a cinchona alkaloid-derived bifunctional squaramide chiral catalyst. Diverse purine, benzimidazole, and imidazole substrates are well tolerated and generate 4'-thionucleoside derivatives containing three contiguous stereogenic centers with excellent results (30 examples, up to 97% yield, >20 : 1 dr and up to 99% ee). Moreover, the novel strategy provides an efficient and convenient synthetic route to construct chiral 4'-thionucleosides.

Phosphonium Ion-Tethered Secondary Amines for Chemospecific 5-Enolexo Aldol Condensations of 6-Ketoaldehydes

A novel and highly selective 5-enolexo-exo-trig aldol condensation of 6-ketoaldehydes is presented using a proline-based alkylphosphonium ion catalyst. Bulky and oxophilic phosphonium ion plays a vital role in facilitating kinetic aldenamine formation and activating keto groups for aldol addition. This innovative approach exclusively targets five-membered carbo- and heterocyclic aldehydes, involving unusual aldehydes as donors and ketones as acceptors. Especially, enolizable aryl keto aldehydes and heteroatom-embedded ketoaldehydes exclusively produced cyclized products with our new catalyst, while other catalysts provided predominantly self-aldol or decomposed products. The scope and diversity of the method demonstrated by synthesizing different carboxaldehydes, including cyclopentene, indene, dihydrofuran, benzofuran, dihydropyrrole, indole, thiofuran, dihydrothiofuran, and benzothiofurans.

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.

Redox-Divergent Construction of (Dihydro)thiophenes with DMSO

Thiophene-based rings are one of the most widely used building blocks for the synthesis of sulfur-containing molecules. Inspired by the redox diversity of these features in nature, we demonstrate herein a redox-divergent construction of dihydrothiophenes, thiophenes, and bromothiophenes from the respective readily available allylic alcohols, dimethyl sulfoxide (DMSO), and HBr. The redox-divergent selectivity could be manipulated mainly by controlling the dosage of DMSO and HBr. Mechanistic studies suggest that DMSO simultaneously acts as an oxidant and a sulfur donor. The synthetic potentials of the products as platform molecules were also demonstrated by various derivatizations, including the preparation of bioactive and functional molecules.

Asymmetric Synthesis of 3-Amine-tetrahydrothiophenes with a Quaternary Stereocenter via Nickel(II)/Trisoxazoline-Catalyzed Sulfa-Michael/Aldol Cascade Reaction: Divergent Access to Chiral Thionucleosides

A generally useful Ni(II)/trisoxazoline-catalyzed asymmetric sulfa-Michael/Aldol cascade reaction is introduced to access chiral 3-amine-tetrahydrothiophene derivatives containing a quaternary stereocenter (32 examples, up to 93% yield, > 20:1 dr and 92% ee). Moreover, the novel strategy offers an efficient and convenient approach to construct chiral thionucleoside analogues.

Organocatalytic sulfa-Michael/aldol cascade: constructing functionalized 2,5-dihydrothiophenes bearing a quaternary carbon stereocenter

A practical sulfa-Michael/aldol cascade reaction of 1,4-dithiane-2,5-diol and α-aryl-β-nitroacrylates has been developed, which allows efficient access to functionalized 2,5-dihydrothiophenes bearing a quaternary carbon stereocenter in moderate to good yields with high enantioselectivities.

Organocatalytic asymmetric [3 + 2] annulation of 1,4-dithiane-2,5-diol with azlactones: access to chiral dihydrothiophen-2(3H)-one derivatives

Herein, the first organocatalyst-mediated enantioselective [3 + 2] annulations of 1,4-dithiane-2,5-diol with various azlactones utilizing C4 and C5 reactivity using squaramide as the catalyst in a ring-opening manner have been reported.

A metal-free cascade reaction of β-halo-α,β-unsaturated aldehydes and 1,4-dithiane-2,5-diols: synthesis of polycyclic 2-formylthiophenes

A cascade reaction of β-halo-α,β-unsaturated aldehydes and 1,4-dithiane-2,5-diols for the synthesis of 2-formylthiophenes has been described employing a polymer supported organic base.

The enantioselective construction of chiral spirooxindole-based 4-thiazolidinone via asymmetric catalytic formal [3+2] annulation using a bifunctional catalyst

A facile and efficient method based on the asymmetric [3+2] annulation of 1,4-dithiane-2,5-diol to ketimines for the construction of chiral spirooxindole-based 4-thiazolidinone was reported.

Toward the Synthesis of Nuphar Sesquiterpene Thioalkaloids: Stereodivergent Rhodium-Catalyzed Synthesis of the Thiolane Subunit

A stereodivergent approach to the central thiolane subunit of Nuphar sesquiterpene thioalkaloids has been developed. This approach features a rhodium-catalyzed Stevens-type rearrangement in conjunction with an enzyme resolution reaction. Further elaboration into a polycyclic ring system via alcohol oxidation and ring-closing metathesis is also described.

One-pot synthesis of functionalized isoxazole–thiolane hybrids via Knoevenagel condensation and domino sulfa-1,6-Michael/intramolecular vinylogous Henry reactions

One-pot synthesis of isoxazole–thiolane hybrids are reported via the Knoevenagel condensation, domino sulfa-1,6-Michael/intramolecular vinylogous Henry reactions using piperidine (30 mol%) with >95% yields in 2–2.5 h overall reaction time.

A cinchona alkaloid catalyzed enantioselective sulfa-Michael/aldol cascade reaction of isoindigos: construction of chiral bispirooxindole tetrahydrothiophenes with vicinal quaternary spirocenters

Enantioselective sulfa-Michael/aldol reaction of isoindigos has been successfully developed to afford bispirooxindole tetrahydrothiophenes with vicinal quaternary spirocenters.

Synthesis of 2,3-disubstituted thiophenes from 2-aryl-3-nitro-cyclopropane-1,1-dicarboxylates and 1,4-dithiane-2,5-diol

A two-step synthesis of 2,3-disubstituted thiophenes from trans-2-aryl-3-nitrocyclopropane-1,1-dicarboxylates and 1,4-dithiane-2,5-diol is described.

Peptide-catalyzed consecutive 1,6- and 1,4-additions of thiols to α,β,γ,δ-unsaturated aldehydes

Regio- and enantioselective addition of thiols to α,β,γ,δ-unsaturated aldehydes was performed with a resin-supported peptide catalyst. It was shown that a 1,4-adduct was generated mainly at the initial stage of the reaction, and this was eventually converted to a thermodynamically stable 1,6- and 1,4-diadduct through retro-addition/addition reactions.

Dihydrothiophenes containing quaternary stereogenic centres by sequential stereospecific rearrangements and ring-closing metathesis

Tandem stereospecific rearrangements by [3,3]-sigmatropic shift and N to C aryl migration give tertiary thiols that undergo ring closing metathesis to dihydrothiophenes.

Asymmetric synthesis of trisubstituted tetrahydrothiophenes bearing a quaternary stereocenter via double Michael reaction involving dynamic kinetic resolution

The stereoselective synthesis of highly functionalized tetrahydrothiophenes bearing three contiguous stereocenters, one of them quaternary, can be achieved by reacting trans-α-cyano-α,β-unsaturated ketones and trans-tert-butyl 4-mercapto-2-butenoate in the presence of a readily available amine thiourea. The products are obtained in high yield, good diastereoselectivity, and excellent enantioselectivity. The overall formation of tetrahydrothiophenes occurs via a cascade double Michael reaction involving a highly efficient process of dynamic kinetic resolution.