Diastereo- and Enantioselective Synthesis of Orthogonally Protected 2,4-Diaminocyclopentanecarboxylates: A Flip from β-Amino- to β,γ-Diaminocarboxylates

Recent Progress in the Selective Fluorinations of Some Functionalized Cycloalkenes

Organofluorine compounds have had an increasing impact in synthetic organic chemistry and pharmaceutical research over the past two decades. Their syntheses and the development of novel synthetic approaches towards versatile fluorinated small molecules have received great interest. Our research team has designed various selective and stereocontrolled methods for the construction of fluorine-containing small molecular entities, involving the transformation of various functionalized cycloalkenes across their ring olefin bond. The synthetic methodologies developed to access various pharmacologically interesting fluorinated derivatives with multiple chiral centers might be valuable protocols for the preparation of other classes of organic compounds as well.

Stereoselective Synthesis and Antiproliferative Activity of Steviol-Based Diterpen Aminodiols

A library of steviol-based trifunctional chiral ligands was developed from commercially available natural stevisoide and applied as chiral catalysts in the addition of diethylzinc to benzaldehyde. The key intermediate steviol methyl ester was prepared according to literature procedure. Depending on the epoxidation process, both cis- and trans-epoxyalcohols were obtained. Subsequent oxirane ring opening with primary and secondary amines afforded 3-amino-1,2-diols. The ring opening with sodium azide followed by a “click” reaction with alkynes resulted in dihydroxytriazoles. The regioselective ring closure of N-substituted aminodiols with formaldehyde was also investigated. The resulting steviol-type aminodiols were tested against a panel of human adherent cancer cell lines (A2780, SiHa, HeLa, and MDA-MB-231). It was consistently found that the N-benzyl substituent is an essential part within the molecule and the ring closure towards N-benzyl substituted oxazolidine ring system increased the antiproliferative activity to a level comparable with that of cisplatine. In addition, structure–activity relationships were examined by assessing substituent effects on the aminodiol systems.

Stereoselective Synthesis of Highly Functionalized 5- and 6-Membered Aminocyclitols Starting with a Readily Available 2-Azetidinone

Stereoselective transformations of 4-vinyl-2-azetidinone derivative 4 into a variety of highly functionalized 6- and 5-membered carbocyclic compounds 7 and 9 were carried out using sequences involving sequential C1-N bond cleavage and Ru-catalyzed ring-closing metathesis. The derived carbocycles were further transformed into polyhydroxylated 6- and 5-membered aminocyclitols.

Alicyclic β- and γ-Amino Acids: Useful Scaffolds for the Stereocontrolled Access to Amino Acid-Based Carbocyclic Nucleoside Analogs

Stereocontrolled synthesis of some amino acid-based carbocyclic nucleoside analogs containing ring C=C bond has been performed on β- and γ-lactam basis. Key steps were N-arylation of readily available β- or γ-lactam-derived amino ester isomers and amino alcohols with 5-amino-4,6-dichloropyrimidine; ring closure of the formed adduct with HC(OMe)₃ and nucleophilic displacement of chlorine with various N-nucleophiles in the resulting 6-chloropurine moiety.

Stereocontrolled synthesis of polyhydroxylated bicyclic azetidines as a new class of iminosugars

We report herein the development of a stereodivergent route towards polyhydroxylated bicyclic azetidine scaffolds, namely 6-azabicyclo[3.2.0]heptane derivatives. The strategy hinges on a common bicyclic β-lactam precursor, which is forged by way of a rare example of a cationic Dieckmann-type reaction, followed by IBX-mediated desaturation. Substrate-controlled diastereoselective oxidations then allow the divergent preparation of novel iminosugar mimics.

Selective Synthesis of Fluorine-Containing Cyclic β-Amino Acid Scaffolds

Fluorine-containing organic molecules have generated increasing impact in drug research over the past decade. Their preparation and development of novel synthetic methods towards new types of fluorinated molecules among them of β-amino acid derivatives has received large interest. Our research group have designed various highly selective and stereocontrolled methods for the construction of fluorine-containing cyclic β-amino acid derivatives. The synthetic approaches developed for the synthesis of various pharmacologically interesting cyclic β-amino acid derivatives as monomers with multiple stereogenic centers might be valuable protocols for the access of other classes of organic compounds.

Highly functionalized cyclic β-amino acid moieties as promising scaffolds in peptide research and drug design

Peptide-based drug research has received high attention in the field of medicinal chemistry over the past decade. For drug design, to improve proteolytic stability, it is desirable to include unnatural building blocks, such as conformationally restricted β-amino acid moieties, into the peptide sequence. Accordingly, the synthesis and incorporation of such conformationally rigid systems into novel type of peptides has gained large interest. Our research group has designed highly efficient methods for the construction of potential antimicrobial peptides. Moreover, a number of synthetic approaches have been developed for the synthesis of various pharmacologically interesting cyclic β-amino acid derivatives as monomers with multiple stereogenic centers.

A Stereocontrolled Protocol to Highly Functionalized Fluorinated Scaffolds through a Fluoride Opening of Oxiranes

A novel selective and substrate-dependent synthetic protocol has been developed towards the synthesis of various fluorine-containing, highly functionalized cycloalkane derivatives. The method involves the stereoselective epoxidation of some unsaturated cyclic β-amino acid derivatives as model compounds, followed by a regioselective fluoride opening of oxiranes under various conditions with Deoxofluor and XtalFluor-E reagents, thereby offering an insight into this new epoxide opening methodology with fluoride.

Chemoselective, Substrate-directed Fluorination of Functionalized Cyclopentane β-Amino Acids

This work describes a substrate-directed fluorination of some highly functionalized cyclopentane derivatives. The cyclic products incorporating CH₂ F or CHF₂ moieties in their structure have been synthesized from diexo- or diendo-norbornene β-amino acids following a stereocontrolled strategy. The synthetic study was based on an oxidative transformation of the ring carbon-carbon double bond of the norbornene β-amino acids, followed by transformation of the resulted "all cis" and "trans" diformyl intermediates by fluorination with "chemodifferentiation".

Stereo- and Regiocontrolled Syntheses of Exomethylenic Cyclohexane β-Amino Acid Derivatives

Cyclohexane analogues of the antifungal icofungipen [(1R,2S)-2-amino-4-methylenecyclopentanecarboxylic acid] were selectively synthesized from unsaturated bicyclic β-lactams by transformation of the ring olefinic bond through three different regio- and stereocontrolled hydroxylation techniques, followed by hydroxy group oxidation and oxo-methylene interconversion with a phosphorane. Starting from an enantiomerically pure bicyclic β-lactam obtained by enzymatic resolution of the racemic compound, an enantiodivergent procedure led to the preparation of both dextro- and levorotatory cyclohexane analogues of icofungipen.

High-performance liquid chromatographic separation of paclitaxel intermediate phenylisoserine derivatives on macrocyclic glycopeptide and cyclofructan-based chiral stationary phases

High-performance liquid chromatographic methods were developed for the separation of enantiomers of four unnatural paclitaxel precursor phenylisoserine analogs on chiral stationary phases containing macrocyclic glycopeptides and cyclofructans as chiral selectors. The effects of the mobile phase composition, the nature and concentration of different mobile phase additives (alcohols, amines and acids) in different chromatographic modes, temperature and the structures of the analytes on the separations were investigated. Separations were carried out at constant mobile phase compositions in the temperature range 10-50°C on macrocyclic antibiotic-based and 5-35°C on cyclofructan-based columns and the changes in enthalpy, Δ(ΔH°), entropy, Δ(ΔS°), and free energy, Δ(ΔG°), were calculated. The elution sequence was determined in most cases; no general rule could be observed.

An insight into the synthesis of novel aryl-substituted alicyclic β-amino acid derivatives through substrate-directed palladium-catalysed regio- and stereoselective cross-coupling

Novel aryl-substituted alicyclic β-amino acid derivatives were synthesized through substrate-determined palladium-catalysed cross-coupling of aryl iodides with five- or six-membered cycloalkene β-amino esters.

Alkyne-azide cycloadditions with copper powder in a high-pressure continuous-flow reactor: high-temperature conditions versus the role of additives

A safe and efficient flow-chemistry-based procedure is presented for 1,3-dipolar cycloaddition reactions between organic azides and acetylenes. This simple and inexpensive technique eliminates the need for costly special apparatus and utilizes Cu powder as a plausible Cu(I) source. To maximize the reaction rates, high-pressure/high-temperature conditions are utilized; alternatively, the harsh reaction conditions can be moderated at room temperature by the joint application of basic and acidic additives. A comparison of the performance of these two approaches in a series of model reactions has resulted in the formation of useful 1,4-disubstituted 1,2,3-triazoles in excellent yields. The risks that are associated with the handling of azides are lowered, thanks to the benefits of flow processing, and gram-scale production has been safely implemented. The synthetic capability of this continuous-flow technique is demonstrated by the efficient syntheses of some highly functionalized derivatives of the antifungal cispentacin.

Regio- and diastereoselective fluorination of alicyclic β-amino acids

A regio- and stereoselective approach to fluorinated β-aminocyclohexene or cyclohexane esters has been developed, starting from a bicyclic β-lactam (1). The procedure involves six or seven steps, based on regio- and stereoselective iodolactonization, lactone opening and hydroxy-fluorine exchange. The method has been extended to the synthesis of fluorinated amino ester enantiomers.

Direct enzymatic route for the preparation of novel enantiomerically enriched hydroxylated beta-amino ester stereoisomers

Enantiomerically enriched hydroxy-substituted beta-amino esters have been synthesized through CAL-B-catalyzed enantioselective hydrolysis in organic media. Moderate to good enantiomeric excess values (ee > or = 52%) were obtained when the CAL-B-catalyzed reactions were performed in t-BuOMe, at 60 degrees C with 0.5 equiv. of added H(2)O as nucleophile.