Palladium(II) complexes of bowls, pinwheels, cages, and N,C,N-pincers of starburst ligands 1,3,5-Tris(di-2-pyridylamino)benzene and 2,4,6-Tris(di-2-pyridylamino)-1,3,5-triazene

The reactions of Pd(II) ions with starburst ligands 1,3,5-tris(di-2-pyridylamino)benzene (tdab) and 2,4,6-tris(di-2-pyridylamino)-1,3,5-triazene (tdat) have been investigated. Complexes with the Pd:tdab (or tdat) ratio being 1:1 and 3:1 have been isolated and characterized. The structures of five new Pd(II) complexes containing the starburst ligands have been determined by X-ray diffraction analyses, which include chelate compounds [PdCl(2)(tdab)], 1, [(PdCl(2))(3)(tdab)], 2, [(Pd(OAc)(2))(3)(tdab)], 4, and [(Pd(OAc)(2))(3)(tdat)], 5, and a cyclometalated compound [Pd(OAc)(NCN-tdab)], 3. The Pd(II) ion in the 1:1 compound 1 is chelated by two pyridyl groups. Similarly, each Pd(II) center in the 3:1 compounds 2, 4, and 5 is chelated by two pyridyl groups. However, these three compounds display distinct structural features: 2 adopts a "bowl-shaped" structure, 4 has a "pinwheel"-like structure, and 5 has a "up-and-down" structure. Compounds 4 and 5 were examined in solution by variable-temperature (1)H NMR, which revealed that both compounds retain the "pinwheel" and the "up-and-down" structure, respectively. The observed structural preference by 4 and 5 is attributed to both electronic and steric factors.     

Mechanism and conditions for highly enantioselective epoxidation of alpha,beta-enones using charge-accelerated catalysis by a rigid quaternary ammonium salt

[formula: see text] Highly enantioselective (up to 130:1) epoxidation of a variety of alpha,beta-enones to form alpha,beta-epoxy ketones is described along with a rational analysis of the mechanistic basis for this strong absolute stereochemical control by the chiral catalyst 2.     

Sensitive high resolution inverse detection NMR spectroscopy of proteins in the solid state

A new indirect detection scheme for obtaining (15)N/(1)H shift correlation spectra in crystalline proteins is described. Excellent water suppression is achieved without the need for pulsed field gradients, and using only a 2-step phase cycle. Careful attention to overall NMR instrument stability was found critical for obtaining the best resolution and sensitivity. Magnetic dilution by deuteration of the protein in combination with high-speed magic angle spinning produces (1)H resonances averaging only 0.22 ppm in width, and in some cases lines as narrow as 0.17 ppm are obtained. In application to two different polymorphs of ubiquitin, structure dependent differences in both (15)N and (1)H amide chemical shifts are observed. In one case, distinct shifts for different molecules in the asymmetric unit are seen, and all differ substantially from solution NMR shifts. A gain of 7 in sensitivity makes the method competitive with solution NMR as long as nanocrystalline samples are available.     

A new chiral Rh(II) catalyst for enantioselective [2 + 1]-cycloaddition. mechanistic implications and applications

A novel chiral Rh(II) catalyst (1) is introduced for the [2 + 1]-cycloaddition of ethyl diazoacetate to terminal acetylenes and olefins with high enantioselectivity. The catalyst 1 consists of one acetate bridging group and three mono-N-triflyldiphenylimidazoline-2-one bidentate ligands (DPTI) spanning the Rh(II)-Rh(II) metallic center in a structure that was determined by single-crystal X-ray diffraction analysis. A rational mechanism is advanced that provides a straightforward explanation for the enantioselectivity and absolute stereochemical course of the [2 + 1]-cycloaddition reactions. A key element in this explanation is the cleavage of one of the Rh-O bonds of the bridging acetate group in the intermediate Rh-carbene complex to form a new pentacoordinate Rh carbene complex (formally 1.5 valent Rh) that can undergo [2 + 2]-cycloaddition with the C-C pi-bond of the acetylenic or olefinic substrate. Reductive elimination of the resulting adduct affords the cyclopropene or cyclopropane product. The C2-symmetry of the two DPTI ligands orthogonal to the bridging acetate also contributes to the high observed enantioselectivity and mechanistic clarity. The catalyst 1, which functions effectively at 0.5 mol %, can be recovered efficiently for reuse. Its ready availability, robustness, and effectiveness suggest it as a useful addition to the list of practical chiral Rh(II) catalysts for synthesis.     

Dimethylzinc-mediated additions of alkenylzirconocenes to aldimines. New methodologies for allylic amine and C-cyclopropylalkylamine syntheses

Hydrozirconation of alkynes with zirconocene hydrochloride followed by in situ transmetalation to dimethylzinc provides access to reactive alkenyl organometallic reagents from readily available precursors. Upon addition of imines, 1,2-attack leads to synthetically useful allylic amine building blocks. In the presence of CH(2)I(2) or CH(2)Cl(2), the N-metalated allylic amide intermediate is cyclopropanated and C-cyclopropylalkylamines are formed in high yield and excellent diastereoselectivities favoring the anti products. The use of enynes as starting materials for this domino reaction provides conjugated biscyclopropanes and thus allows the stereoselective formation of five new carbon-carbon bonds. A transition state that explains the need for both zirconocene complex and alkyl zinc in the cyclopropanation reaction is proposed.     

Total synthesis of slow reacting substances (SRS's): 6-epi-leukotriene C and 6-epi-leukotriene D

6-$\text{Epi}$-leukotrienes C and D (3 and 4) have been synthesized unambiguously via the 5($\text{S}$), 6($\text{R}$)-epoxide (5,6-$\text{cis}$) which is isomeric with leukotriene A. These 6-$\text{epi}$-leukotrienes are less active (especially 4) than leukotrienes C and D (1 and 2) and have not been found in substantial quantity in natural SRS sources.     

TiCl(OPr)3 and NaBH(OAc)3: an efficient reagent combination for the reductive amination of aldehydes by electron-deficient amines

Sodium triacetoxyborohydride, NaBH(OAc)₃ with tri-isopropoxytitanium chloride, TiCl(OⁱPr)₃ is a useful reagent combination for reductive amination. Electron-deficient amines and heteroaromatic amines such as 2-aminopyrimidine and 2-aminothiazole can be reductively alkylated at room temperature to afford the corresponding secondary amines in good yields.     

A Scalable Biomimetic Synthesis of Resveratrol Dimers and Systematic Evaluation of their Antioxidant Activities

An efficient synthetic route to the resveratrol oligomers quadrangularin A and pallidol is reported. It features a scalable biomimetic oxidative dimerization that proceeds in excellent yield and with complete regioselectivity. A systematic evaluation of the natural products and their synthetic precursors as radical‐trapping antioxidants has revealed that, contrary to popular belief, this mode of action is unlikely to account for their observed biological activity.     

A study of oligoprenyl coupling reactions with allylic stannanes

All-E or E,Z,E-oligoprenols may be synthesized from allylic stannanes by reaction with prenyl aldehydes. [reaction: see text]