Showing papers by "Armando Córdova published in 2005"

Direct amino acid catalyzed asymmetric synthesis of polyketide sugars.

Abstract: The ability of amino acids to form nucleophilic enamines with aldehydes and ketones has been used in the development of asymmetric α-oxidation reactions with electrophilic oxidizing agents. Singlet molecular oxygen has for the first time been asymmetrically incorporated into aldehydes and ketones, and the products were isolated as their corresponding diols in good yields and ee’s. Organocatalytic α-oxidations of cyclic ketones with iodosobenzene and N-sulfonyloxaziridine were also possible and furnished after reduction the product diols in generally low yields and in low to good ee’s. Amino acids have also been shown to catalyze the formation of carbohydrates by sequential aldol reactions. For example, proline and hydroxy proline mediate a highly selective trimerisation of α-benzyloxyacetaldehyde into allose, which was obtained in >99 % ee. Non linear effect studies of this reaction revealed the largest permanent nonlinear effect observed in a proline-catalyzed reaction to date. Moreover, polyketides were also assembled in a similar fashion by an amino acid-catalyzed one-pot reaction, and was successful for the trimerisation of propionaldehyde, however the sequential cross aldol reactions suffered from lower selectivities. This problem was overcome by the development of a two-step synthesis that enabled the formation of a range of polyketides with excellent selectivities from a variety of aldehydes. The method furnishes the polyketides via the shortest route reported and in comparable product yields to most multi-step synthesis. All polyketides were isolated as single diastereomers with >99 % ee. Based on the observed amino acid-catalysis, amino acids are thought to have taken part in the prebiotic formation of tetroses and hexoses.

Acyclic chiral amines and amino acids as inexpensive and readily tunable catalysts for the direct asymmetric three-component Mannich reaction.

Abstract: The direct three-component asymmetric Mannich reaction catalyzed by acyclic chiral amines or amino acids is presented. Simple acyclic chiral amines and amino acids--such as alanine-tetrazole (9), alanine, valine, and serine-catalyzed the three-component asymmetric Mannich reactions between unmodified ketones, p-anisidine, and aldehydes with high chemo- and stereoselectivity, furnishing the corresponding Mannich bases with up to >99 % ee. This study demonstrates that the whole range of amino acids in nature, as well as nonproteogenic amino acid derivatives, can be considered in the design and tuning of novel, inexpensive organocatalysts for the direct asymmetric Mannich reaction.

Direct Organocatalytic Polymerization from Cellulose Fibers

Abstract: We have developed the first direct, organocatalytic, bulk ring-opening polymerization (ROP) of e-caprolactone (e-CL) with solid cotton and paper cellulose as the initiators. The mild ROPs were performed without solvent, and are operationally simple, inexpensive and environmentally benign. Organic-acid-catalyzed heterogeneous derivatization of cellulose provides a novel route to valuable cellulose-based biocompatible nanomaterials. The furnished polymerization products were characterized by FTIR, 1 H and 13 C NMR spectroscopy, MALDI-TOF mass spectrometry and electron microscopy.

The Origin of Stereoselectivity in Primary Amino Acid Catalyzed Intermolecular Aldol Reactions

Abstract: The origin of stereoselectivity in primary amino acid catalyzed intermolecular Aldol reactions

Amino acid catalyzed neogenesis of carbohydrates: a plausible ancient transformation.

Abstract: Hexose sugars play a fundamental role in vital biochemical processes and their biosynthesis is achieved through enzyme-catalyzed pathways. Herein we disclose the ability of amino acids to catalyze the asymmetric neogenesis of carbohydrates by sequential cross-aldol reactions. The amino acids mediate the asymmetric de novo synthesis of natural L- and D-hexoses and their analogues with excellent stereoselectivity in organic solvents. In some cases, the four new stereocenters are assembled with almost absolute stereocontrol. The unique feature of these results is that, when an amino acid is employed as the catalyst, a single reaction sequence can convert a protected glycol aldehyde into a hexose in one step. For example, proline and its derivatives catalyze the asymmetric neogenesis of allose with >99 % ee in one chemical manipulation. Furthermore, all amino acids tested catalyzed the asymmetric formation of natural sugars under prebiotic conditions, with alanine being the smallest catalyst. The inherent simplicity of this catalytic process suggests that a catalytic prebiotic “gluconeogenesis” may occur, in which amino acids transfer their stereochemical information to sugars. In addition, the amino acid catalyzed stereoselective sequential cross-aldol reactions were performed as a two-step procedure with different aldehydes as acceptors and nucleophiles. The employment of two different amino acids as catalysts for the iterative direct aldol reactions enabled the asymmetric synthesis of deoxysugars with >99 % ee. In addition, the direct amino acid catalyzed C2+C2+C2 methodology is a new entry for the short, highly enantioselective de novo synthesis of carbohydrate derivatives, isotope-labeled sugars, and polyketide natural products. The one-pot asymmetric de novo syntheses of deoxy and polyketide carbohydrates involved a novel dynamic kinetic asymmetric transformation (DYKAT) mediated by an amino acid.

Modification of amines and alcohols

Abstract: A process for the modification of amines and alcohols, comprising (i) providing a substrate having amino groups or alcohol groups, wherein said substrate is, e.g., a polysaccharide; (ii) providing a modifying agent which is a lactone, an ester, a polyester, a carbonate, a polycarbonate, a lactide, a glycolide, an anhydride, an acid, a thioester or a carbamate; (iii) providing a catalyst which is, e.g., a amino acid or an organic acid; and (iv) reacting the substrate with the modifying agent in the presence of the catalyst.

Acyclic Amino Acid-Catalyzed Direct Asymmetric Aldol Reactions: Alanine, the Simplest Stereoselective Organocatalyst.

Abstract: The linear amino acid-catalyzed direct asymmetric intermolecular aldol reaction is presented; simple amino acids such as alanine, valine, isoleucine, aspartate, alanine tetrazole 3 and serine catalyzed the direct catalytic asymmetric intermolecular aldol reactions between unmodified ketones and aldehydes with excellent stereocontrol and furnished the corresponding aldol products in up to 98% yield and with up to > 99% ee.

Amino Acid Catalyzed Direct Enantioselective Formation of Carbohydrates: One‐Step de Novo Synthesis of Ketoses.

Abstract: The amino acid-catalyzed direct enantioselective one-step de novo synthesis of carbohydrates using dihydroxyacetone phosphate mimetics as donors and aldehydes or in situ generated imines as acceptors is presented. The addition of water significantly accelerates as well as improves the enantioselectivity of the biomimetic aldol and Mannich reactions. The C3+Cn methodology presented herein is a direct entry to orthogonally protected C-5 and C-6 ketoses (e.g., ribulose, tagatose and piscose) and deoxy- and aminosugars such as 4-amino-4-deoxy-fructose.

Direct Organocatalytic Asymmetric α-Oxidation of Ketones with Iodosobenzene and N-Sulfonyloxaziridines.

Abstract: The novel, direct amino acid-catalyzed α-oxidation of ketones with iodosobenzene and N-sulfonyloxaziridines is presented. A screen of several synthetically common oxidants revealed that iodosobenzene and N-sulfonyloxaziridines act as electrophiles in the direct organocatalytic asymmetric α-hydroxylation of ketones. The direct proline-catalyzed asymmetric α-oxidation of ketones with iodosobenzene yielded the corresponding α-hydroxylated ketones with up to 77% ee. Furthermore, several amino acid derivatives catalyze the stereoselective α-oxidation of ketones with N-sulfonyloxaziridines. For example, the direct diamine-catalyzed enantioselective α-hydroxylation of ketones with N-sulfonyloxaziridines furnished the corresponding α-hydroxylated products in moderate yield with up to 63% ee.

Novel Organic Catalysts for the Direct Enantioselective α-Oxidation of Carbonyl Compounds.

Abstract: The proline-derived N-sulfonylcarboxamide-catalyzed direct enantioselective α-oxidation of ketones and aldehydes with nitrosobenzene is presented. The reactions proceed smoothly furnishing the corresponding α-aminoxylated compounds in good yields with up to >99% ee. The proline-derived N-sulfonylcarboxamides were also found to be excellent catalysts for the direct enantioselective nitroso Diels–Alder-type reaction between nitrosobenzene and α,β-unsaturated cyclic ketones yielding the corresponding bicyclic Diels–Alder adduct products with up to >99% ee. The proline-derived N-sulfonylcarboxamides represent a readily available and highly modular novel type of organic catalyst.

Amino Acid Catalyzed Direct Enantioselective Synthesis of β‐Amino‐α‐oxyaldehydes.

Abstract: The first direct amino acid-catalyzed asymmetric syntheses of α-oxy-β-aminoaldehydes are presented. The organocatalytic Mannich reactions between unmodified α-oxyaldehydes and anilines proceeded with excellent enantioselectivities. In most cases, the corresponding α-oxy-β-aminoaldehyde adducts were isolated in high yield with excellent chemo- and enantioselectivity. For example, orthogonally protected 3-amino-tetroses and α-amino acid derivatives were isolated in up to >99% ee.

Direct Amino Acid Catalyzed Asymmetric Synthesis of Polyketide Sugars.

Abstract: The ability of amino acids to form nucleophilic enamines with aldehydes and ketones has been used in the development of asymmetric α-oxidation reactions with electrophilic oxidizing agents. Singlet molecular oxygen has for the first time been asymmetrically incorporated into aldehydes and ketones, and the products were isolated as their corresponding diols in good yields and ee’s. Organocatalytic α-oxidations of cyclic ketones with iodosobenzene and N-sulfonyloxaziridine were also possible and furnished after reduction the product diols in generally low yields and in low to good ee’s. Amino acids have also been shown to catalyze the formation of carbohydrates by sequential aldol reactions. For example, proline and hydroxy proline mediate a highly selective trimerisation of α-benzyloxyacetaldehyde into allose, which was obtained in >99 % ee. Non linear effect studies of this reaction revealed the largest permanent nonlinear effect observed in a proline-catalyzed reaction to date. Moreover, polyketides were also assembled in a similar fashion by an amino acid-catalyzed one-pot reaction, and was successful for the trimerisation of propionaldehyde, however the sequential cross aldol reactions suffered from lower selectivities. This problem was overcome by the development of a two-step synthesis that enabled the formation of a range of polyketides with excellent selectivities from a variety of aldehydes. The method furnishes the polyketides via the shortest route reported and in comparable product yields to most multi-step synthesis. All polyketides were isolated as single diastereomers with >99 % ee. Based on the observed amino acid-catalysis, amino acids are thought to have taken part in the prebiotic formation of tetroses and hexoses.

Direct Amino Acid Catalyzed Asymmetric α Oxidation of Ketones with Molecular Oxygen.

Abstract: The ability of amino acids to form nucleophilic enamines with aldehydes and ketones has been used in the development of asymmetric α-oxidation reactions with electrophilic oxidizing agents. Singlet molecular oxygen has for the first time been asymmetrically incorporated into aldehydes and ketones, and the products were isolated as their corresponding diols in good yields and ee’s. Organocatalytic α-oxidations of cyclic ketones with iodosobenzene and N-sulfonyloxaziridine were also possible and furnished after reduction the product diols in generally low yields and in low to good ee’s. Amino acids have also been shown to catalyze the formation of carbohydrates by sequential aldol reactions. For example, proline and hydroxy proline mediate a highly selective trimerisation of α-benzyloxyacetaldehyde into allose, which was obtained in >99 % ee. Non linear effect studies of this reaction revealed the largest permanent nonlinear effect observed in a proline-catalyzed reaction to date. Moreover, polyketides were also assembled in a similar fashion by an amino acid-catalyzed one-pot reaction, and was successful for the trimerisation of propionaldehyde, however the sequential cross aldol reactions suffered from lower selectivities. This problem was overcome by the development of a two-step synthesis that enabled the formation of a range of polyketides with excellent selectivities from a variety of aldehydes. The method furnishes the polyketides via the shortest route reported and in comparable product yields to most multi-step synthesis. All polyketides were isolated as single diastereomers with >99 % ee. Based on the observed amino acid-catalysis, amino acids are thought to have taken part in the prebiotic formation of tetroses and hexoses.

Plausible Origins of Homochirality in the Amino Acid Catalyzed Neogenesis of Carbohydrates

Abstract: The intrinsic ability of amino acids to catalyze the asymmetric formation of carbohydrates, which enzymes have mediated for millions of years, with significant amplification of enantiomeric excess suggests a plausible ancient catalytic process for the evolution of homochirality.