Epoxide-opening and group-transfer reactions mediated by monomeric zirconium imido complexes

N-tert-Butylimidozirconocene (Cp2Zr=Nt-Bu) and its analogue (ebthi)Zr=NAr (ebthi = ethylenebis(tetrahydroindenyl), Ar = 2,6-dimethylphenyl) undergo overall cycloaddition reactions at ambient temperature with epoxides that lack accessible beta-hydrogens. The process results in addition of the Zr=N bond to an epoxide C-O bond, leading to azazirconacyclopentanes. The regio- and stereochemistry of the products implicate a stepwise mechanism, with the intermediacy of zwitterions having substantial carbocation character at the substituted carbon of the ring-opened epoxide-derived fragment. The azametallacycles undergo facile cleavage to beta-amino alcohols upon addition of mild acid.     

Synthesis of proposed oxidation-cyclization-methylation intermediates of the coumarin antibiotic biosynthetic pathway

[structure: see text] A chemoenzymatic synthesis was described to prepare proposed oxidation-cyclization-methylation intermediates of the coumarin antibiotic biosynthetic pathway. The successful synthesis of these fragile molecules relies heavily on mild enzymatic deprotection and efficient enzymatic kinetic resolution to minimize epimerization, decomposition, multiple orthogonal protections, and retro aldol reactions often encountered in their chemical synthesis.     

Highly enantioselective catalytic phenylation of ketones with a constrained geometry titanium catalyst

[reaction: see text] The catalytic asymmetric addition of phenyl groups from diphenylzinc to ketones is reported. The catalyst, generated from a dihydroxy bis(sulfonamide) ligand and titanium tetraisopropoxide, gives good to excellent enantioselectivities with a range of substrates.     

Irreducibility testing over local fields

The purpose of this paper is to describe a method to determine whether a bivariate polynomial with rational coefficients is irreducible when regarded as an element in , the ring of polynomials with coefficients from the field of Laurent series in with rational coefficients. This is achieved by computing certain associated Puiseux expansions, and as a result, a polynomial-time complexity bound for the number of bit operations required to perform this irreducibility test is computed.

     

Inherent Conformational Preferences of Ac‐Gln‐Gln‐NHBn: Sidechain Hydrogen Bonding Supports a β‐Turn in the Gas Phase

Gas‐phase single‐conformation spectroscopy is used to study Ac‐Gln‐Gln‐NHBn in order to probe the interplay between sidechain hydrogen bonding and backbone conformational preferences. This small, amide‐rich peptide offers many possibilities for backbone–backbone, sidechain–backbone, and sidechain–sidechain interactions. The major conformer observed experimentally features a type‐I β‐turn with a canonical 10‐membered ring C=O—H?N hydrogen bond between backbone amide groups. In addition, the C=O group of each Gln sidechain participates in a seven‐membered ring hydrogen bond with the backbone NH of the same residue. Thus, sidechain hydrogen‐bonding potential is satisfied in a manner that is consistent with and stabilizes the β‐turn secondary structure. This turn‐forming propensity may be relevant to pathogenic amyloid formation by polyglutamine segments in human proteins.     

Rationalization of anomalous nonlinear effects in the alkylation of substituted benzaldehydes

Anomalous nonlinear effects in the alkylation of substituted benzaldehydes with diethylzinc using aminoalcohol catalysts are rationalized in terms of a simple extension of the Noyori model to allow for nonthermodynamically controlled partitioning of the catalyst between monomeric and dimeric species. This work highlights the fact that catalyst composition in such systems may be influenced by substrate properties.     

Homoallyl-Substituted Vinylcyclopropanes from α,β-Unsaturated Ketones and Allylindium Derivatives

Simply reversing the order of addition of aqueous acid and Et₂O to the Barbier intermediate 1 of the known indium-mediated allylation leads to unprecedented deoxygenative rearrangements [Eq. (a)].