The novel doubly borylated enolate is identified as an intermediate of the double aldol reaction of acetate esters. As a precursor to the formation of the doubly borylated enolate, carbon-bound boron enolates of carboxylic esters are spectroscopically characterized for the first time. When 2,6-diisopropylphenyl acetate (10d) is treated with c-Hex(2)BOTf (1.3 equiv) and triethylamine (1.5 equiv) in CDCl(3), the corresponding mono-enolate is formed as a mixture of oxygen- (11d) and carbon-bound (12d) forms in 71% and 20% yields, respectively. The structures of these enolates have been unambiguously determined by NMR spectroscopy. Investigation of the enolization of a series of substituted aryl acetates shows that the steric factor of the acetate affects the degree of the mono-enolate (as a mixture of oxygen- and carbon-bound enolates) and the doubly borylated enolate formation. Studies also revealed that oxygen- and carbon-bound boron enolates exist as equilibrium mixtures and that a proton transfer process occurs between oxygen- and carbon-bound enolates. The doubly borylated enolate formation is general for a variety of carbonyl compounds. Besides acetate esters, carbonyl containing compounds, such as acetic acid, dimethylacetamide, methoxyacetone, and 3-acetyl-2-oxazolidinone, also produce the doubly borylated enolates when treated with c-Hex(2)BOTf (2.5 equiv) and triethylamine (3.0 equiv). A plausible pathway of the double aldol reaction involving a carbon-bound boron enolate as a key intermediate is proposed.

A zwitterionic hapten 4 featuring both positively and negatively charged functional groups was designed and synthesized with the goal of generating catalytic antibodies for the hydrolysis of ester 6 and amide 7. Of the 36 monoclonal antibodies specific to BSA-4 (bovine serum albumin) that were isolated, six accelerated the hydrolysis of 6. Two catalytic antibodies with distinctively different and representative kinetic behaviors were selected for detailed kinetic studies. Whereas H8-2-6F11 showed burst kinetic behavior, which can be attributed to the formation of an acyl intermediate, H8-1-2D5 did not, but it did exhibit high multiple turnover activity. The rate of hydrolysis of 6 catalyzed by H8-1-2D5 followed Michaelis-Menten kinetics; the apparent values of the Michaelis-Menten constant Km and the catalytic constant kcat were 488 microM and 3.5 min(-1), respectively. The catalytic rate enhancement (kcat/kun) observed for H8-1-2D5 was 1.3 x 10(5), which is approximately two orders of magnitude greater than those for monofunctional haptens. Thus H8-1-2D5 compares well in catalytic activity with antibodies isolated by a related approach called heterologous immunization.

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTThe First Doubly Borylated Enolate as an Intermediate of the Double Aldol ReactionAtsushi Abiko, Tadashi Inoue, Hiroshi Furuno, Harald Schwalbe, Christin Fieres, and Satoru MasamuneView Author Information Venture Laboratory, Kyoto Institute of Technology Matsugasaki, Sakyo-ku, Kyoto, 606-8585 Japan Department of Chemistry Massachusetts Institute of Technology Cambridge, Massachusetts, 02139 Cite this: J. Am. Chem. Soc. 2001, 123, 19, 4605–4606Publication Date (Web):April 18, 2001Publication History Received27 September 2000Published online18 April 2001Published inissue 1 May 2001https://doi.org/10.1021/ja003507iCopyright © 2001 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views577Altmetric-Citations14LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (43 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Aldol reactions,Boron,Carbon,Enolates,Organic compounds Get e-Alerts

With the aim of developing polymeric gels sensitive to external stimuli and able to reversibly adsorb and release divalent ions, copolymer gels of N-isopropylacrylamide (NIPA) and methacrylic (MAA) monomers were prepared. We chose calcium as a target divalent ion. Two MAAs form a complex with a calcium ion, and the NIPA component allows the polymers to swell and shrink reversibly in response to temperature. The adsorbing site develops an affinity to target ions when the adsorbing molecules come into proximity, but when they are separated, the affinity diminishes. To enhance the affinity to calcium, an imprinting technique was applied using Ca2+ and Pb2+ ions as templates in methylsulfoxide and dioxane media, respectively. The adsorption capacity of the imprinted gels was compared with that of the nonimprinted gels, and the effects of the templates, the solvents, and the amount of methacrylic monomers used in the synthesis and the medium temperature over the Ca2+ adsorption capacity of the gels from aqueous solutions were evaluated. The analysis of the adsorption revealed that (a) the adsorption can be described by the Langmuir isotherms; (b) there is an approximately linear relationship between saturation and methacrylic monomer concentration; (c) the affinity depends on the degree of gel swelling or shrinkage that can be switched on and off by temperature; (d) in the shrunken state, the affinity depends approximately linearly on the MAA concentration in the imprinted gels, whereas in the nonimprinted gels it is proportional to the square of MAA concentration; (e) the imprinted gels adsorb more than the nonimprinted gels when MAA concentration is less than that of permanent cross linkers. The success of imprinting of CaMAA2 and PbMAA2 complex is evidence for memory of such complex onto the weakly cross-linked gel.

We report an experimental realization of a gel system in which frustrations exist and can be minimized, thus meeting two crucial criteria predicted to enable memory of conformations in polymers. The gels consist of a thermosensitive major monomer component and two minor components. One minor component is positively charged and will form complexes around negatively charged target molecules placed in solution. The complexes can be imprinted into the gel by then cross-linking the second minor component, which will form cross-links additional to those in the major polymer matrix. The complexes are destroyed and reformed upon swelling and reshrinking of the gels, showing that memorization has been achieved.

Weakly cross-linked heteropolymer gels that memorize molecular pairs have been designed and synthesized. The polymer consists of a main monomer component responsible for volume phase transition, methacrylic acid that adsorbs one divalent ion as a pair, and cross-links; The memory of pairing of methacrylic acids within the gels was encoded in the primary sequence of main monomers, methacrylic acids and cross-links within the gels, which was achieved by "imprinting", namely, by synthesizing gels while methacrylic monomers were paired prior to polymerization. The control gels, where methacrylic monomers were randomly distributed, showed frustration in forming pairs, whereas such frustration was completely diminished in the imprinted gels allowing the memory of pair formation.

We report development of a polymer gel with a catalytic activity that can be switched on and off when the solvent composition is changed. The gel consists of two species of monomers. The major component, N-isopropylacrylamide, makes the gel swell and shrink in response to a change in composition of ethanol/water mixtures. The minor component, vinylimidazole, which is capable of catalysis, is copolymerized into the gel network. The reaction rate for catalytic hydrolysis of p-nitrophenyl caprylate was small when the gel was swollen. In contrast, when the gel was shrunken, the reaction rate increased 5 times. The activity changes discontinuously as a function of solvent composition, thus the catalysis can be switched on and off by an infinitesimal change in solvent composition. The kinetics of catalysis by the gel in the shrunken state is well described by the Michaelis-Menten formula, indicating that the absorption of the substrate by the hydrophobic environment created by the N-isopropylacrylamide polymer in the shrunken gel is responsible for enhancement of catalytic activity, In the swollen state, the rate vs. active site concentration is linear, indicating that the substrate absorption is not a primary factor determining the kinetics, Catalytic activity of the gel is studied for substrates with various alkyl chain lengths; of those studied the switching effect is most pronounced for p-nitrophenyl caprylate.

A general approach is presented for creating polymer gels that can recognize and capture a target molecule by multiple-point interaction and that can reversibly change their affinity to the target by more than one order of magnitude. The polymers consist of majority monomers that make the gel reversibly swell and shrink and minority monomers that constitute multiple-point adsorption centers for the target molecule. Multiple-point interaction is experimentally proven by power laws found between the affinity and the concentration of the adsorbing monomers within the gels.

The C1-C8 and C9-C24 fragments of (−)-discodermolide, the antipode of the marine natural product (+)-discodermolide, have been synthesized with excellent stereoselectivities. These syntheses feature the utilization of the isoxazolidine-mediated asymmetric alkylation methodology and fragment-fragment coupling aldol reactions.

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTAntibody Catalyzed Cleavage of an Amide Bond Using an External Nucleophilic CofactorOguz Ersoy, Roman Fleck, Anthony Sinskey, and Satoru MasamuneView Author Information Departments of Chemistry and Biology Massachusetts Institute of Technology Cambridge, Massachusetts 02139 Cite this: J. Am. Chem. Soc. 1998, 120, 4, 817–818Publication Date (Web):January 14, 1998Publication History Received16 September 1997Published online14 January 1998Published inissue 1 February 1998https://pubs.acs.org/doi/10.1021/ja9732542https://doi.org/10.1021/ja9732542rapid-communicationACS PublicationsCopyright © 1998 American Chemical SocietyRequest reuse permissionsArticle Views203Altmetric-Citations17LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose SUBJECTS:Amides,Aromatic compounds,Biopolymers,Immunology,Peptides and proteins Get e-Alerts

Protein folding is one of the most challenging problems in science. How can polymers memorize and fold into unique conformations? How can they recognize molecules, catalyze chemical reactions, transfer molecular information, and create motions? The principle behind these mechanisms lies in the concept of thermodynamically stable phases of heteropolymers. Recent theories predict that the collapsed phase should be further classified into three phases: freely fluctuating like liquid, frozen in degenerate conformations, and frozen in a unique conformation. This yields a clue on how one can attempt creation artificial polymers capable to mimic some of the protein properties and functions. The reversible adsorption of target molecules is suggested as a primary means to achieve this goal. Target molecules with multiple adsorption sites play a twofold role. First, they mediate specific interactions between monomers and thus serve as "gluons". Second, monitoring the adsorption provides the experimental possibility to test directly on monomer contacts, which is directly related to observation of the order parameter associated with heteropolymer freezing transition. A slight change in the backbone conformation alters the spacial arrangement of the group, allowing for reversible adsorption and release. Polymer gels are developed that can reversibly change their affinity to target molecules by orders of magnitude. The gels are made of copolymers of backbone monomers that can reorganize themselves through thermal volume phase transition and of monomers that can attract the target at multiple contact points. Further the gels "imprinted" with the target showed a marked increase in the affinity, thus mimicing a protein-like ability to memorize and recognize certain target.

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTThe Anti-Selective Boron-Mediated Asymmetric Aldol Reaction of Carboxylic EstersAtsushi Abiko, Ji-Feng Liu, and Satoru MasamuneView Author Information Institute for Fundamental Research, Kao Corporation Ichikai-machi, Haga-gun, Tochigi 321-34, Japan Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue Cambridge, Massachusetts 02139 Cite this: J. Am. Chem. Soc. 1997, 119, 10, 2586–2587Publication Date (Web):March 12, 1997Publication History Received28 October 1996Published online12 March 1997Published inissue 1 March 1997https://pubs.acs.org/doi/10.1021/ja963754fhttps://doi.org/10.1021/ja963754frapid-communicationACS PublicationsCopyright © 1997 American Chemical SocietyRequest reuse permissionsArticle Views6150Altmetric-Citations180LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-AlertscloseSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Aldehydes,Aldol reactions,Molecular structure,Organic compounds,Selectivity Get e-Alerts

The asymmetric Horner-Emmons reaction of the phosphonate derived from a chiral benzopyrano-isoxazolidine (with 4-substituted cyclohexanones) proceeded in high diastereoselectivity with the aid of KHMDS and 18-crown-6 ether. Enantiomerically pure, axially dissymmetric cyclohexylidene alcohols, aldehydes and ketones were obtained from the diastereomerically pure Horner-Emmons products in a single step.

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTConcerning the Boron-Mediated Aldol Reaction of Carboxylic EstersAtsushi Abiko, Ji-Feng Liu, and Satoru MasamuneView Author Information Institute for Fundamental Research, Kao Corporation, Ichikai-machi, Haga-gun, Tochigi 321-34, Japan Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 Cite this: J. Org. Chem. 1996, 61, 8, 2590–2591Publication Date (Web):April 19, 1996Publication History Received7 February 1996Published online19 April 1996Published inissue 1 January 1996https://pubs.acs.org/doi/10.1021/jo960252bhttps://doi.org/10.1021/jo960252brapid-communicationACS PublicationsCopyright © 1996 American Chemical SocietyRequest reuse permissionsArticle Views1458Altmetric-Citations63LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-AlertscloseSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Aldol reactions,Amines,Boron,Organic compounds,Selectivity Get e-Alerts

Asymmetric alkylation of the potassium enolates derived from N-propionyl benzopyrano[4,3-c]-isoxazolidine derivatives with chiral alkyl triflates proceeded smoothly with high diastereoselectivity. The stereochemistry of the newly formed stereogenic center was fully controlled by the facial selectivity of the enolate according to the rule of double asymmetric synthesis. The application of this methodology led to the first synthesis of (+)-siphonarienone, a marine polypropionate natural product.

Benzopyrano[4,3-c]isoxazolidine derivatives, newly developed chiral auxiliaries for asymmetric alkylation, were synthesized and resolved in a unique manner. Resolution could be executed so efficiently that both enantiomers were obtained with a single resolving reagent in excellent yields.

Several intriguing transformations culminated in the first total synthesis of the title compound 1—the nonnatural ent-1 had been published previously—on coupling the fragments A, A′, B, and C. Crucial steps of the synthesis are the aldol reactions in the construction of fragment B, which solved the tricky problem of the stereoselective coupling of chiral fragments.

Three structurally related haptens 1-3 were designed and synthesized with the goal of generating antibodies for the hydrolysis of ester 4a and amide 4b. These haptens contain a 1,2-amino alcohol functionality which replaces the ester/amide moiety of the substrates. A number of catalytic antibodies were generated, and the Michaelis-Menten kinetics constants of three representative catalytic antibodies induced to each of haptens 1-3 were determined. These catalytic antibodies accelerated the hydrolysis of ester 4a with k(cat)/k(un) = approximately 3 X 10(3), and their catalytic activities were effectively inhibited by the addition of their respective haptens. To evaluate the structural influences of the hapten on antibody binding and specificity as well as catalytic activity, a total of 18 antibodies including the above catalytic antibodies and three representative noncatalytic antibodies from each group were selected, and their dissociation constants with their respective haptens, amide 4b, and products were determined. The studies have shown that (1) the structural variations among the three haptens induce no significant changes in catalytic activity of antibodies while they slightly influence the antibody binding and specificity for the substrate and products and (2) a high probability (reaching nearly 50%) of finding catalytic activity among the monoclonal antibodies raised to hapten 1 is found, suggesting that the induction of a charged complementary amino acid residue in close proximity to the reaction site may be important to generation of catalytic antibodies.

Polyhydroxyalkanoate (PHA) synthase has been expressed in Escherichia coli by reengineering the 5'-end of the wild-type (wt) gene and subsequent transformation of this gene into protease-deficient E. coli UT5600 (ompT-). Induction with IPTG results in soluble PHA synthase, which is approximately 5% of the total protein. The soluble synthase has been purified to > 90% homogeneity using FPLC chromatography on hydroxylapatite and Q-Sepharose and has a specific activity of 5 mumol min-1 mg-1. The molecular weight of the PHA product is approximately 10(6) Da based on PlGel chromatography and calibration using polystyrene molecular weight markers. The synthase in the absence of substrate appears to exist in both monomeric and dimeric forms. Incubation of the synthase with an excess of substrate converts it into a form that is now extractable into CHCl3 and sediments on sucrose density ultracentrifugation with PHA. Studies in which the ratio of substrate, 3-D-hydroxybutyrylCoA, to synthase is varied suggest that during polymerization the elongation process occurs at a rate much faster than during the initiation process. A mechanistic model has been proposed for the polymerization process [Griebel, R., Smith, Z., & Merrick, J. (1968) Biochemistry 7, 3676-3681] in which two cysteines are required for catalysis. This model is based on the well-characterized enzymes involved in fatty acid biosynthesis. To test this model, several site-directed mutants of synthase, selected based on sequence conservation among synthases, have been prepared. The C459S mutant has activity approximately 90% that of the wt protein, while the C319S and C319A synthases possess < 0.01% the activity of the wt protein. CD and antibody studies suggest that the mutant proteins are properly folded. The detection of only a single essential cysteine by mutagenesis and the requirement for posttranslational modification by phosphopantetheine to provide a second thiol in many enzymes utilizing coenzyme A thiol ester substrates made us consider the possibility that posttranslational modification was required for synthase activity as well. This hypothesis was confirmed when the plasmid containing PHA synthase (pKAS4) was transformed into E. coli SJ16, requiring beta-alanine for growth. Growth of SJ16/pKAS4 on [3H]-beta-alanine followed by Coomassie staining of the protein and autoradiography revealed that PHA synthase is overexpressed and that beta-alanine is incorporated into the protein. These results suggest PHA synthase is posttranslationally modified by phosphopantetheine.(ABSTRACT TRUNCATED AT 400 WORDS)