Enatioselective organocatalytic synthesis of highly functionalized tetrahydrothiophenes by a Michael-aldol cascade reaction

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.

Stereoselective amine-thiourea-catalysed sulfa-Michael/nitroaldol cascade approach to 3,4,5-substituted tetrahydrothiophenes bearing a quaternary stereocenter

An investigation on the stereoselective cascade sulfa-Michael/aldol reaction of nitroalkenes and commercially available 1,4-dithiane-2,5-diol to 3,4,5-substituted tetrahydrothiophenes, bearing a quaternary stereocenter, is presented. A secondary amine thiourea derived from (R,R)-1,2-diphenylethylamine was found to be the most effective catalyst when using trans-β-methyl-β-nitrostyrenes affording the heterocyclic products in good yields and moderate stereoselectivities.

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.

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.

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.

Amphoteric amino aldehydes reroute the aza-Michael reaction

Amphoteric amino aldehydes, which exist as stable dimers, participate in an aza-Michael/aldol domino reaction with alpha,beta-unsaturated aldehydes to afford stable 1,5-aminohydroxyaldehydes in high yields and diastereoselectivies. The reaction outcome hinges upon the dimeric nature of amphoteric amino aldehydes and the orthogonality between the NH aziridine and the two aldehyde functionalities during the reaction. Through the use of reaction conditions that disfavor dimer dissociation, the aza-Michael process has been directed toward a novel 8-(enolendo)-exo-trig cyclization. The results described herein further demonstrate the potential of amphoteric molecules in reaction discovery.