Ehud Keinan

Pancreatic β‐Cell Neogenesis by Direct Conversion from Mature α‐Cells

Because type 1 and type 2 diabetes are characterized by loss of β‐cells, β‐cell regeneration has garnered great interest as an approach to diabetes therapy. Here, we developed a new model of β‐cell regeneration, combining pancreatic duct ligation (PDL) with elimination of pre‐existing β‐cells with alloxan. In this model, in which virtually all β‐cells observed are neogenic, large numbers of β‐cells were generated within 2 weeks. Strikingly, the neogenic β‐cells arose primarily from α‐cells. α‐cell proliferation was prominent following PDL plus alloxan, providing a large pool of precursors, but we found that β‐cells could form from α‐cells by direct conversion with or without intervening cell division. Thus, classical asymmetric division was not a required feature of the process of α‐ to β‐cell conversion. Intermediate cells coexpressing α‐cell‐ and β‐cell‐specific markers appeared within the first week following PDL plus alloxan, declining gradually in number by 2 weeks as β‐cells with a mature phenotype, as defined by lack of glucagon and expression of MafA, became predominant. In summary, these data revealed a novel function of α‐cells as β‐cell progenitors. The high efficiency and rapidity of this process make it attractive for performing the studies required to gain the mechanistic understanding of the process of α‐ to β‐cell conversion that will be required for eventual clinical translation as a therapy for diabetes. STEM CELLS 2010; 28:1630–1638.

Enolstannanes as electrofugal groups in allylic alkylation

Abstract Enolstannanes serve as nucleophiles towards allylic acetates under the influence of palladium(O) catalyst.

Traceless Ligation of Cysteine Peptides Using Selective Deselenization

The synthesis of proteins with a fully native sequence is an ongoing challenge in protein chemistry. Native chemical ligation (NCL) approaches have proven to be generally applicable where cysteine (Cys) residues are appropriately positioned,[1,2] however, the synthesis of many proteins often require ligation at non-Cys sites in the polypeptide sequence.[3-7] Previously, we introduced a reductive strategy for ligation at Ala sites[7] based on global desulfurization of Cys[8] that has found widespread utility for the synthesis of complex proteins by NCL.[9,10] Selective desulfurization can be affected by both Rainey Ni and Pd/C/H2[7] and, more recently, by the radical initiator VA-044 in combination with the water soluble phosphine TCEP.[11] However, since these conditions result in global desulfurization of all thiols in the protein, the method requires protection and deprotection of all other Cys residues in the native sequence.[6a,12] These additional steps complicate the synthesis of larger polypeptides and limit the use of natural Cys residues for ligation.[13]

A general approach to γ-lactones via osmium-catalyzed asymmetric dihydroxylation. Synthesis of (−)- and (+)-muricatacin.

Abstract Both enantiomers of hydroxy γ-lactones have been prepared highly enantioselectively (92–99% ee) using either AD-mix-β or AD-mix-α with both β,γ- and γ,δ-unsaturated esters. The method is exemplified by the three-step synthesis of (−) and (+)-muricatacin in 74% yield and >99% ee.

Chemistry for the 21st Century

Here, numerous winners of the Wolf prize from all chemical disciplines provide an overview of the new ideas and approaches that will shape this dynamic science over the forthcoming decades and so will have a decisive influence on our living conditions.[...]

Allylstannanes as electrofugal partners in allylic alkylation

Abstract Unsymmetrical allyl-allyl couplings occur between allylstannanes and allyl acetates catalyzed by palladium(O) and a novel direct coupling of an allyl acetate in the presence of a distannane and a palladium(O) catalyst is also possible.

Organo tin nucleophiles IV. Palladium catalyzed conjugate reduction with tin hydride

Abstract Highly chemoselective conjugate reduction of α,β-unsaturated carbonyl compounds is now possible by using tributyl tin hydride with Pd(PO 3 ) 4 ; an optimization study puts forth the importance of added radical scavenger and proton source in these reductions.

Potential applications of an alcohol-aldehyde/ketone oxidoreductase from thermophilic bacteria

Practical uses of a novel alcohol dehydrogenase from Thermoanaerobium brockii have been examined in crude and purified form. Stoichiometric reduction of NADP (50 mg) was demonstrated with agarose-immobilized enzyme and 0.3 (v/v) 2-propanol solution as reductant. A coenzyme recycle number of 20000 was achieved in enzymatic reactions that employed the alcohol dehydrogenase for NADPH/NADP regeneration. Gram-scale synthesis of chiral R(+) 2-pentanol was shown in a system composed of enzyme, 2-pentanone and 2-propanol as reductant. The effect of temperature, reaction time and substrate concentration on alcohol optical purity was examined. An optical purity of 80% was achieved in the enzymatic synthesis of R(+) 2-pentanol. The enzyme was easily immobilized and stable on an enzyme electrode for analytical detection of alcohols and carbonyls. T. brockii enzyme has potential applications as a commercial alcohol dehydrogenase because of broad substrate specificity and activity at high temperature or high solvent concentration, rare carbonyl si-face stereo-specificity in hydrogen transfer, and high stability and activation of immobilized enzyme.

Chemical mimicry of viral capsid self-assembly.

Stable structures of icosahedral symmetry can serve numerous functional roles, including chemical microencapsulation and delivery of drugs and biomolecules, epitope presentation to allow for an efficient immunization process, synthesis of nanoparticles of uniform size, observation of encapsulated reactive intermediates, formation of structural elements for supramolecular constructs, and molecular computing. By examining physical models of spherical virus assembly we have arrived at a general synthetic strategy for producing chemical capsids at size scales between fullerenes and spherical viruses. Such capsids can be formed by self-assembly from a class of molecules developed from a symmetric pentagonal core. By designing chemical complementarity into the five interface edges of the molecule, we can produce self-assembling stable structures of icosahedral symmetry. We considered three different binding mechanisms: hydrogen bonding, metal binding, and formation of disulfide bonds. These structures can be designed to assemble and disassemble under controlled environmental conditions. We have conducted molecular dynamics simulation on a class of corannulene-based molecules to demonstrate the characteristics of self-assembly and to aid in the design of the molecular subunits. The edge complementarities can be of diverse structure, and they need not reflect the fivefold symmetry of the molecular core. Thus, self-assembling capsids formed from coded subunits can serve as addressable nanocontainers or custom-made structural elements.

Selective deacetylation of anomeric sugar acetates with tin alkoxides

There are three main categories of methods for synthesising glucosides and glucuronides (1 + 2 + 3), namely, (a) the Koenigs-Knorr reaction (1, where Y = halogen) and its modifications2, (6) reactions catalysed by Lewis acids3 (1, Y = OAc), and (c) reactions where HO-1 of the starting derivative is unsubstituted (1, Y = OH). Category (c) includes methods in which HO-1 is initially converted into a more reactive species 4a-g, and those which involve a Lewis acid catalyst’S4h. Direct coupling is attractive since it avoids the preparation of reactive, unstable, and readily hydrolysable derivatives of 1 with Y = Br, OCNHCC&, OCNHNHR, etc. The need for simple methods for the preparation of sugar derivatives with HO-1 unsubstituted is further exemplified by conversions into glycosyl fluorides which are useful in stereoselective glycosidations’. + R3-OH __L1