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Aitor Landa

Publications and source records attributed to Aitor Landa.

11 recordsLinked to original sources
PubMed2007

Bisoxazoline-Lewis acid-catalyzed direct-electron demand oxo-hetero-Diels-Alder reactions of N-oxy-pyridine aldehyde and ketone derivatives.

A general catalytic oxo-hetero-Diels-Alder reaction for pro-chiral aldehyde and ketone N-oxy-pyridines is presented. The catalytic and asymmetric oxo-hetero-Diels-Alder reaction of electron-rich dienes with N-oxy-pyridine-2-carbaldehyde and ketone derivatives, catalyzed by chiral copper(II)-bisoxazoline complexes, gives optically active six-membered oxygen heterocycles in moderate to good yields and with excellent enantioselectivities.

Acids↗
PubMed2006

2D supramolecular assemblies of benzene-1,3,5-triyl-tribenzoic acid: temperature-induced phase transformations and hierarchical organization with macrocyclic molecules.

Two-dimensional supramolecular honeycomb networks with cavities of an internal diameter of 2.95 nm were formed by the self-assembly of 4,4',4' '-benzene-1,3,5-triyl-tribenzoic acid (BTA) on a Ag(111) surface at room temperature. Annealing to higher temperatures resulted in two sequential phase transformations into closer-packed supramolecular arrangements. The phase transformations are associated with stepwise deprotonation of the carboxylic acid groups. The voids of the honeycomb network of BTA have a suitable size for the construction of hierarchical structures with guest molecules. Single molecules of the macrocyclic compound mt-33 were successfully confined inside 2D nanocavities of the honeycomb networks and released when the phase was transformed to the close-packed structure.

Journal Article↗
PubMed2006

A simple asymmetric organocatalytic approach to optically active cyclohexenones.

Optically active 2,5-disubstituted-cyclohexen-2-one derivatives have been prepared in a one-pot process consisting of five reaction steps: an organocatalytic asymmetric conjugated addition of beta-ketoesters to alpha,beta-unsaturated aldehydes that proceeds in aqueous solutions or under solvent-free conditions has been implemented in a multi-step process.

Aldehydes↗
PubMed2006

Surface-assisted coordination chemistry and self-assembly.

This article discusses different approaches to build up supramolecular nanoarchitectures on surfaces, which were simultaneously investigated by scanning tunneling microscopy (STM) on the single-molecule level. Following this general road map, first, the hydrogen-bonding guided self-assembly of two different, structural-equivalent molecular building blocks, azobenzene dicarboxylic acid and stilbene dicarboxylic acid, was studied. Secondly, the coordination chemistry of the same building blocks, now acting as ligands in metal coordination reactions, towards co-sublimed Fe atoms was studied under near surface-conditions. Extended two-dimensional tetragonal network formation with unusual Fe2L(4/2)-dimers at the crossing points was observed on copper surfaces. Complementary to the first two experiments, a two-step approach based on the solution-based self-assembly of square-like tetranuclear complexes of the M4L4-type with subsequent deposition on graphite surfaces was investigated. One- and two-dimensional arrangements as well as single molecules of the M4L4-complexes could be observed. Moreover, the local electronic properties of a single M4L4-complexes could be probed with submolecular resolution by means of scanning tunnelling spectroscopy (STS).

Journal Article↗
PubMed2006

One-pot organocatalytic domino Michael-aldol and intramolecular SN2 reactions. Asymmetric synthesis of highly functionalized epoxycyclohexanone derivatives.

The development of the organocatalytic asymmetric one-pot Michael-Darzens condensation giving highly functionalized complex epoxycyclohexanone derivatives with up to four chiral centers is presented. Depending on the reaction conditions, either the polysubstituted 7-oxa-bicyclo[4.1.0]heptan-2-one ring system or 2-chloro-cyclohex-2-enone derivatives can be formed. For the former class of compounds a high diversity in substitution pattern is demonstrated, and the optically active products are obtained in excellent diastereo- and enantioselectivities. The potential synthetic applications of the products have been demonstrated by performing a series of highly diastereoselective transformations leading to optically active products useful for the life-science industry. Furthermore, mechanistic investigations on the formation of the chiral centers in the optically active epoxycyclohexanone are presented.

Journal Article↗
PubMed2006

Bis(oxazoline) Lewis acid catalyzed aldol reactions of pyridine N-oxide aldehydes--synthesis of optically active 2-(1-hydroxyalkyl)pyridine derivatives: development, scope, and total synthesis of an indolizine alkaloid.

A new, short, and simplified procedure for the synthesis of optically active pyridine derivatives from pro-chiral pyridine-N-oxides is presented. The catalytic and asymmetric Mukaiyama aldol reaction between ketene silyl acetals and 1-oxypyridine-2-carbaldehyde derivatives catalyzed by chiral copper(II)-bis(oxazoline) complexes gave optically active 2-(hydroxyalkyl)- and 2-(anti-1,2-dihydroxyalkyl)pyridine derivatives in good yields and diastereoselectivities, and in excellent enantioselectivities-up to 99 % enantiomeric excess. As a synthetic application of the developed method, a full account for the asymmetric total synthesis of a nonnatural indolizine alkaloid is provided.

Acids↗
PubMed2005

Programming supramolecular assembly and chirality in two-dimensional dicarboxylate networks on a Cu(100) surface.

We report a comparative study on the 2D self-assembly of two related ditopic benzoic acid species, which have similar shape and endgroups but different backbone symmetry. High-resolution scanning tunneling microscopy data reveal how the symmetry information of molecular building blocks is readily expressed in the resulting chiral or nonchiral supramolecular networks. The underlying square Cu(100) surface steers network orientation and accounts for carboxylate formation, resulting in an unusual intermolecular hydrogen bond motif. Our results demonstrate that symmetry and chiral resolution in 2D supramolecular assembly can be controlled via the design of functional molecules and choice of substrate.

Adsorption↗
PubMed2002

Design and synthesis of a novel class of sugar-peptide hybrids: C-linked glyco beta-amino acids through a stereoselective "acetate" Mannich reaction as the key strategic element.

A new type of sugar-amino acid hybrid, which is comprised of a sugar unit (gluco-, galacto-, or mannopyranose) linked through a C-glycosidic linkage to the beta-position of an alpha-unsubstituted beta-amino acid unit, is presented. It is hypothesized that these new compounds, or the oligomeric peptides derived therefrom, might possess the structural features of beta-amino acid oligomers and the chemical and enzymatic resistance of C-glycosides to hydrolysis. The synthetic strategy is based on a new Mannich-type reaction between a chiral acetate enolate equivalent and alpha-amido sulfones derived from the corresponding sugar-C-glycoside aldehydes. While the sugar-C-glycoside aldehyde partner is prepared from well-established transformations on known sugar precursors, the lithium enolate derived from (1R)-endo-2-acetylisoborneol 3 is employed as the key element. This Mannich approach proceeds with essentially perfect diasteromeric control leading to the new beta-amino carbonyl adducts in good yields. Further, cleavage of the camphor auxiliary is smoothly performed by oxidative treatment with ammonium cerium nitrate (CAN). Complementarily, direct peptide-type coupling of the beta-amino carbonyl Mannich adducts with an alpha- or beta-amino acid residue and subsequent CAN-promoted detachment of the auxiliary yields dipeptide fragments bearing a sugar-containing aliphatic side chain and is a process that can be iterated. A preliminary conformational study based on the combination of experimental NMR data and molecular mechanics and molecular dynamics (MD) of one particular adduct is also provided.

Amino Acids↗
PubMed1998

Practical Synthesis of alpha-Amino Acid N-Carboxy Anhydrides of Polyhydroxylated alpha-Amino Acids from beta-Lactam Frameworks. Model Studies toward the Synthesis of Directly Linked Peptidyl Nucleoside Antibiotics.

A straightforward method for the synthesis of polyhydroxylated alpha-amino acid N-carboxy anhydrides (NCAs) is described as the means by which short peptide segments comprised of a polyhydroxylated chain are easily affordable. The entire sequence lies in the preparation of nonracemic 3-hydroxy beta-lactams through the highly diastereoselective Staudinger reaction of hydroxyketene equivalents with chiral alpha-oxyaldehyde-derived imines, followed by TEMPO radical assisted cycloexpansion to the corresponding NCA and subsequent peptide coupling with alpha-amino acid esters. The method has been applied to the synthesis of short peptide segments derived from carbamoylpolyoxamic acid, some glycoglycines, as well as C(2) symmetric hydroxy amino acids.

Journal Article↗