Enantioselective recognition of 1,2-aminoalcohols by the binol receptor dangled with pyrrole-2-carboxamide and its analogues

Novel binol receptor with pyrrole-2-carboxamide moiety and its analogues have been designed, synthesized, and used to enantioselectively recognize 1,2-aminoalcohols via multiple hydrogen bonding. The pyrrole-based binol receptor showed the highest enantioselectivity among the four receptors as determined by the ¹H NMR. The DFT calculation strongly supports complementary hydrogen bonding between alcoholic -OH and pyrrolyl groups.     

Synthesis of hindered tertiary amines by a mild reductive amination procedure

An efficient metal-free procedure for the synthesis of tertiary amines by the reductive amination of carbonyl compounds is reported, which allows a convergent access to sterically hindered amines of the general formulas NR(R′)₂ and NRR′R″. The mild and operationally simple protocol uses the Hantzsch ester for transfer hydrogenation and catalytic amounts of thiourea for imine activation.     

A synthetic route to highly substituted 1,2,3,4-tetrahydroisoquinolines via Yb(OTf)3 -catalyzed diastereoselective ring opening of bridged oxazolidines: asymmetric synthesis of 2-azapodophyllotoxin

We herein report a robust and efficient synthetic route to highly functionalized enantiopure 1,2,3,4-tetrahydroisoquinolines (THIQs) from Garner aldehyde. We utilized the inherent chirality of Garner aldehyde through 1,2- and 1,3-/1,4-asymmetric inductions iteratively to obtain 1,2,3,4-tetrasubstitued THIQs using rigid and isolable bridged oxazolidines without any external chiral sources. All possible stereoisomers of bridged oxazoliodines were efficiently synthesized from L- and D-Garner aldehydes and transformed into fully functionalized THIQs via diastereoselective ring opening with various nucleophiles in the presence of Yb(OTf)(3). This methodology furnished four out of eight possible diastereomers of 1,2,3,4-tetrasubstituted THIQs despite the electronic nature of substituents on the aryl rings. Finally, the enantioselective synthesis of 2-azapodophyllotoxin was achieved with an overall yield of 35.4% (eight steps) from D-Garner aldehyde using this synthetic route.     

Synthesis of an enantiomerically pure 2,2,4-trisubstituted cyclobutanone building block by zirconocene-promoted deoxygenative ring contraction of structurally modified 4-vinylfuranosides

A route to an enantiopure trisubstituted cyclobutanone has been devised. The pursuit of this building block begins with d-glucose and features a zirconocene-promoted ring contraction.     

Synthesis of enantiomerically enriched β-hydroxy selenides by catalytic asymmetric ring opening of meso-epoxides with (phenylseleno)silanes

The first example of the enantioselective ring opening of meso-epoxides by (phenylseleno)silanes using salen(Cr)complexes as catalyst is described. This desymmetrization reaction constitutes a simple and convenient approach to synthetically versatile optically active β-hydroxy selenides.     

Synthesis by radical cyclization and cytotoxicity of highly potent bioreductive alicyclic ring fused [1,2-a]benzimidazolequinones

The key step in the synthesis of new five, six and seven-membered alicyclic ring [1,2-a]-fused bioreductive benzimidazolequinones was radical cyclisation. Six and seven-membered tributyltin hydride-mediated homolytic aromatic substitutions of nucleophilic N-alkyl radicals onto the benzimidazole-2-position occurred in high yields (63-70 %) when quaternising the pyridine-like 3-N of imidazole with camphorsulfonic acid and using large excesses of the azo-initiator, 1,1'-azobis(cyclohexanecarbonitrile), to supplement the non-chain reaction. Elaboration of benzimidazoles to the benzimidazolequinones occurred in excellent yields. The IC50 values for the cytotoxicity of benzimidazolequinones towards the human skin fibroblast cell line GM00637 were in the nanomolar range, as determined by using the MTT assay. The benzimidazolequinones were much more cytotoxic than indolequinone analogues. 1,2,3,4-Tetrahydropyrido[1,2-a]benzimidazole-6,9-dione was the most potent compound prepared being more than 300 times more cytotoxic than the clinically used bioreductive drug, mitomycin C. The latter benzimidazolequinone was more potent under hypoxic conditions (associated with solid tumors), being 4.4 times more cytotoxic than under aerobic conditions, while mitomycin C was 1.8 times more selective towards hypoxia. The cyclopropane fused pyrido[1,2-a]benzimidazolequinone, 1a,2,3,9b-tetrahydro-1H-cyclopropa[3,4]pyrido[1,2-a]benzimidazole-5,8-dione was less cytotoxic and selective than the five-membered ring analogue, 1,1a,8,8a-tetrahydrocyclopropa[3,4]pyrrolo[1,2-a]benzimidazole-3,6-dione. Modifying the structure of the most potent pyrido[1,2-a]benzimidazolequinone by attaching methyl substituents onto the quinone moiety increased reductive potentials and decreased cytotoxicity and selectivity towards hypoxia.     

Synthesis of anionic rhenium carbonyl hydrides by the ring opening of rhenium carbonyl heterocycles

The heterocyclic rhenium-bismuth complex Re₃(CO)₁₂(μ-H)₂(μ-BiPh₂), 1 was obtained in 62% yield from the reaction of [Bu₄N][Re₃(CO)₁₂(μ-H)₂] with BiPh₂Cl at room temperature. Compound 1 was converted to the [Bu₄N] salt of the dihydrido monoanion [{HRe(CO)₄}₂(μ-BiPh₂)], 2 by reaction with [Bu₄N][OH]. The anion of 2 contains a BiPh₂ ligand bridging two HRe(CO)₄ groups. The corresponding [Bu₄N] salt of the SbPh₂-bridged monoanion [{HRe(CO)₄}₂(μ-SbPh₂)], 4 was obtained from the reaction of Re₂(CO)₈(μ-SbPh₂)(μ-H), 3 with NaBH₄ followed by precipitation with [Bu₄N]Br. The anion of 4 was converted back to 3 by treatment with H[BF₄], but no characterizable product was obtained from the treatment of 2 with H[BF₄]. The crystal and molecular structures of compounds 1, 2, and 4 were established by single-crystal X-ray diffraction analyses.     

Regiochemical control of the ring opening of 1,2-epoxides by means of chelating processes. Part 15: Regioselectivity of the opening reactions with MeOH of remote O-substituted regio- and diastereoisomeric pyranosidic epoxides under condensed- and gas-phase operating conditions

The regiochemical behavior of pairs of regio- and diastereoisomeric epoxides derived from the 3,4,5,6-tetrahydro-2H-pyrane system, bearing an acetal group as the remote functionality, was determined in the acid methanolysis in the condensed phase (cd-phase) and in the reaction with MeOH in the gas-phase using a gaseous acid (D₃⁺), as the promoting agent. With only one exception, the results obtained in the opening process of these epoxides indicate the incursion in the gas-phase of D⁺-mediated chelated bidentate species able to modify the regiochemical result found in the methanolysis in the cd-phase.     

Regiochemical control of the ring opening of 1,2-epoxides by means of chelating processes. Part 18: Regioselectivity of the opening reactions with MeOH of 1-(benzyloxy)-2,3- and -3,4-epoxyalkanes under condensed and gas phase operating conditions

The regiochemical behavior of a series of aliphatic open chain epoxides bearing a heterofunctionality in an allylic or homoallylic relationship has been examined in the opening reaction with MeOH under both condensed and gas phase operating conditions. The results indicate that the proton (actually D⁺) possesses in the gas phase intrinsic chelating properties which are even superior than those of Li⁺ in the condensed phase.