Stereoselective synthesis of hydrazinodihydrofurans via cascade Michael addition-substitution involving the reaction of curcumin and other ß-dicarbonyls with α-hydrazinonitroalkenes.

Highly diastereoselective synthesis of 2-hydrazinated 2,3-dihydrofurans in good to excellent yields involving an interrupted Feist-Bénary type reaction by treating a wide variety of 1,3-dicarbonyl compounds, including curcumins, with α-hydrazinated nitroalkenes is reported here. The first ever enantioselective reaction of α-hydrazinonitroalkenes has also been carried out with two selected 1,3-dicarbonyls, dimedone and cyclohexanone by employing an l-t-leucine derived squaramide as the chiral organocatalyst to afford the enantio-enriched 2-hydrazinodihydrofurans as single diastereomers in good yields and with good enantioselectivities.

1,3,4-Oxadiazole and Heteroaromatic-Fused 1,2,4-Triazole Synthesis using Diverted Umpolung Amide Synthesis.

Umpolung Amide Synthesis (UmAS) has emerged as a superior alternative to conventional amide synthesis methods based on carbonyl electrophiles in a range of situations, particularly when epimerization-prone couplings are prescribed. In an unanticipated development during our most recent studies, it was discovered that diacyl hydrazide products from UmAS were not formed as intermediates when using an acyl hydrazide as the amine acceptor. This resulted in a new preparation of 1,3,4-oxadiazoles from α-bromonitroalkane donors. We hypothesized that a key tetrahedral intermediate in UmAS was diverted toward a more direct pathway to the heterocycle product rather than through formation of the diacyl hydrazide, a typical oxadiazole progenitor. In studies reported here, diversion to 1,2,4-triazole products is described, a behavior hypothesized to also result from an analogous tetrahedral intermediate, but one formed from heteroaromatic hydrazine acceptors.

Brønsted Acid Catalyzed [3 + 2]-Cycloaddition of Cyclic Enamides with in Situ Generated 2-Methide-2H-indoles: Enantioselective Synthesis of Indolo[1,2-a]indoles.

An efficient formal [3 + 2]-cycloaddition toward the highly diastereo- and enantioselective synthesis of indolo[1,2-a]indoles is disclosed. A chiral BINOL-derived phosphoric acid catalyzed the highly enantioselective conjugate addition of cyclic enamides to in situ generated 2-methide-2H-indoles and subsequent aminalization to give rise to acetamide-substituted indolo[1,2-a]indoles carrying three contiguous stereogenic centers. Importantly, these products were formed as single diastereomers and with excellent yields and enantioselectivities. Mild reaction conditions at ambient temperatures, the facile removal of the acetamido group, and the possibility of a scale-up highlight the practicality of this methodology.

Enantioselective Synthesis of Quaternary α-Amino Acids via l-tert-Leucine-Derived Squaramide-Catalyzed Conjugate Addition of α-Nitrocarboxylates to Enones.

Enantioselective Michael addition of tertiary α-nitroesters to ß-unsubstituted vinyl ketones has been carried out in the presence of an l-tert-leucine-derived squaramide as organocatalyst. The products, quaternary α-nitroesters, were formed in excellent yield and moderate to good ee's in most cases. Scale-up of the reaction and synthetic applications of the products, including transformation to representative quaternary α-amino acids, have also been demonstrated.

Brønsted Acid Catalyzed [3+2]-Cycloaddition of 2-Vinylindoles with In†Situ Generated 2-Methide-2H-indoles: Highly Enantioselective Synthesis of Pyrrolo[1,2-a]indoles.

Pyrrolo[1,2-a]indoles are privileged structural elements of many natural products and pharmaceuticals. An efficient one-step process for their highly diastereo- and enantioselective synthesis, comprising a direct [3+2]-cycloaddition, has been developed. A chiral BINOL-derived phosphoric acid catalyzes the reaction of in situ-generated 2-methide-2H-indoles with 2-vinylindoles, furnishing the target products incorporating three contiguous stereogenic centers as single diastereoisomers and with excellent yields and enantioselectivities.

Quinine-derived thiourea and squaramide catalyzed conjugate addition of α-nitrophosphonates to enones: asymmetric synthesis of quaternary α-aminophosphonates.

Conjugate addition of α-nitrophosphonates to enones was carried out in the presence of two sets of organocatalysts, viz. a quinine-thiourea and a quinine-squaramide. The quinine-thiourea provided the products possessing an α-quaternary chiral center in high enantioselectivities only in the case of electron rich enones. On the other hand, the quinine-squaramide was more efficient in that a wide variety of electron rich and electron poor enones underwent Michael addition of nitrophosphonates to afford the quaternary α-nitrophosphonates in excellent yields and enantioselectivities. The hydrogen bonding donor ability of the bifunctional catalyst, as shown in the proposed transition states, appears primarily responsible for the observed selectivity. However, a favorable π-stacking between the aryl groups of thiourea/squaramide and aryl vinyl ketone also appeared favorable. The reaction was amenable to scale up, and the enantioenriched quaternary α-nitrophosphonates could be easily transformed to synthetically and biologically useful quaternary α-aminophosphonates and other multifunctional molecules.

Enantioselective synthesis of γ-tetrasubstituted nitrosulfonyl carboxylates and amides via L-tert-leucine-derived-squaramide catalyzed conjugate addition of nitrosulfones to acrylates and acrylamides.

Michael addition of α-nitrosulfones to aryl- and alkyl acrylates and acrylamides proceeds in the presence of 5-10 mol% of an amino acid derived new organocatalyst to provide γ-tetrasubstituted γ-nitro-γ-sulfonyl carboxylates and amides in excellent yields and enantioselectivities. Scale-up of the reaction to multi-grams, convenient recovery of the catalyst and its recyclability without any drop in yield and selectivity are attractive features of this methodology.

Enantioselective synthesis of α-nitro-δ-ketosulfones via a quinine-squaramide catalyzed conjugate addition of α-nitrosulfones to enones.

Conjugate addition of α-nitrosulfones to vinyl ketones in the presence of 0.2 mol% of a quinine-squaramide organocatalyst afforded α-nitro-δ-ketosulfones possessing a tetrasubstituted chiral center in excellent yield and enantioselectivity in most cases. This strategy also offers a facile and convenient entry into γ-sulfonylhydroxamates that are one carbon homologs of potent enzyme inhibitors.

Enantioselective synthesis of quaternary α-aminophosphonates via conjugate addition of α-nitrophosphonates to enones.

Enantioselective Michael addition of α-nitrophosphonates to enones for the synthesis of α-aminophosphonates is reported for the first time. The reaction proceeds in good to high yields and moderate to high selectivity in the presence of a new quinine thiourea catalyst. The quaternary nitrophosphonates were conveniently transformed to cyclic quaternary α-aminophosphonates via in situ reduction-intramolecular cyclization or Baeyer-Villiger oxidation followed by in situ reduction-intramolecular cyclization.