Synthesis of fluorinated piperidine and azepane β-amino acid derivatives

Selective Synthesis of Tetrahydroisoquinoline and Piperidine Scaffolds by Oxidative Ring Opening/Ring Closing Protocols of Substituted Indenes and Cyclopentenes

Novel tetrahydroisoquinoline and piperidine derivatives were selectively synthesized from substituted indenes or cyclopentenes. The process starts with an oxidative cleavage of the ring olefin bond, which gives reactive diformyl intermediates. By a ring-closing step using chiral (R) or (S) α-methylbenzylamine under a reductive amination protocol facilitated ring formation with ring expansion of the corresponding nitrogen-containing heterocycles. The stereocontrolled methodology enabled accurate control of the stereochemistry of the final products. Additionally, the synthesized amino acid derivatives possessing an aryl moiety in their structure may be relevant building blocks for foldamer chemistry.

Photocatalyzed Hydroaminodifluoroalkylation of Alkenes

The synthesis of undescribed β-aminodifluoroethylsulfinates and their uses in the hydroaminodifluoroalkylation of alkenes is reported. This reaction is performed in the presence of a photocatalyst (4CzIPN, Ru complexes) and enables the direct incorporation of a β-difluoroamine moiety into vinylic aryls, unactivated alkenes, and electron-rich, or -deficient alkenes. The mechanism was studied, and the formation of a gem-difluoromethyl radical was observed after the selective oxidation of the sulfinate function.

N-Fluoroalkylated Morpholinos - a New Class of Nucleoside Analogues

The first concise and efficient synthesis of some fluorine-containing morpholino nucleosides has been developed. One synthetic strategy was based on the oxidative ring cleavage of the vicinal diol unit of uridine, cytidine adenosine and guanosine derivatives, followed by cyclisation of the dialdehyde intermediates by double reductive amination with fluorinated primary amines to obtain various N-fluoroalkylated morpholinos. Another approach involved cyclisation of the diformyl intermediates with ammonia source, followed by dithiocarbamate formation and desulfurization-fluorination with diethylaminosulfur trifluoride yielding the corresponding morpholine-based nucleoside analogues with a N-CF₃ element in their structure.

Diastereoselective Alkylations of Chiral Tetrazolo[1,5-a]azepines via Heterobenzylic Anion Intermediates

The alkylations of chiral seven-membered rings fused to tetrazoles are highly diastereoselective. The diastereoselectivity depended on the placement and the size of the substituent on the ring and on the electrophile. Subsequent alkylations occurred with high stereoselectivity, allowing for the construction of quaternary stereocenters. Computational studies revealed that torsional effects are responsible for the observed diastereoselectivities. Substituted products can be reduced to the corresponding secondary amines, thus providing an approach for synthesizing diastereomerically enriched azepanes.

Application of Oxidative Ring Opening/Ring Closing by Reductive Amination Protocol for the Stereocontrolled Synthesis of Functionalized Azaheterocycles

The current Account gives an insight into the synthesis of some N-heterocyclic β-amino acid derivatives and various functionalized saturated azaheterocycles accessed from substituted cycloalkenes via ring C=C bond oxidative cleavage followed by ring closing across double reductive amination. The ring-cleavage protocol has been accomplished according to two common approaches: a) Os-catalyzed dihydroxylation/NaIO4 vicinal diol oxidation and b) ozonolysis. A comparative study on these methodologies has been investigated. Due to the everincreasing relevance of organofluorine chemistry in drug research as well as of the high biological potential of β-amino acid derivatives several illustrative examples to the access of various fluorine-containing piperidine or azepane β-amino acid derivatives are also presented in the current Account.

1 Introduction

2 Olefin-Bond Transformation by Oxidative Ring Cleavage

3 Synthesis of Saturated Azaheterocycles via Oxidative Ring-Opening/Ring-Closing Double Reductive Amination

3.1 Importance of Fluorine-Containing Azaheterocycles in Pharmaceutical Research

3.2 Synthesis of Azaheterocyclic Amino Acid Derivatives with a Piperidine or Azepane Framework through Oxidative Ring Opening/Reductive Amination

3.2.1 Synthesis of Piperidine β-Amino Esters

3.2.2 Synthesis of Azepane β-Amino Esters

3.2.3 Synthesis of Fluorine-Containing Piperidine γ-Amino Esters

3.3 Synthesis of Tetrahydroisoquinoline Derivatives through Oxidative Ring Opening/Reductive Amination Protocol

3.4 Synthesis of Functionalized Benzazepines through Reductive Amination

3.4.1 Synthesis of Benzo[c]azepines

3.4.2 Synthesis of Benzo[d]azepines

3.5 Synthesis of Various N-Heterocycles via Ozonolysis/Reductive Amination

3.5.1 Synthesis of Compounds with an Azepane Ring

3.5.2 Synthesis of Piperidine β-Amino Acids and Piperidine-Fused β-Lactams

3.5.3 Synthesis of γ-Lactams with a Piperidine Ring

3.5.4 Synthesis of other N-Heterocycles

4 Summary and Outlook

5 List of Abbreviations

Tightly linked morpholino-nucleoside chimeras: new, compact cationic oligonucleotide analogues

The polyanionic phosphodiester backbone of nucleic acids contributes to high nuclease sensitivity and low cellular uptake and is therefore a major obstacle to the biological application of native oligonucleotides. Backbone modifications, particularly charge alterations is a proven strategy to provide artificial oligonucleotides with improved properties. Here, we describe the synthesis of a new type of oligonucleotide analogues consisting of a morpholino and a ribo- or deoxyribonucleoside in which the 5'-amino group of the nucleoside unit provides the nitrogen of the morpholine ring. The synthetic protocol is compatible with trityl and dimethoxytrityl protecting groups and azido functionality, and was extended to the synthesis of higher oligomers. The chimeras are positively charged in aqueous medium, due to the N-alkylated tertiary amine structure of the morpholino unit.

Selective Transformation of Norbornadiene into Functionalized Azaheterocycles and β-Amino Esters with Stereo- and Regiocontrol

Novel functionalized azaheterocycles with multiple chiral centers have been accessed from readily available norbornene β-amino acids or β-lactams across a stereocontrolled synthetic route, based on ring-opening metathesis (ROM) of the staring unsaturated bicyclic amino esters, followed by selective cyclization through ring-closing metathesis (RCM). The RCM transformations have been studied under various experimental conditions to assess the scope of conversion, catalyst, yield, and substrate influence. The structure of the starting norbornene β-amino acids predetermined the structure of the new azaheterocycles, and the developed synthetic route took place with the conservation of the configuration of the chiral centers.

Synthesis of complex unnatural fluorine-containing amino acids

The area of fluorinated amino acid synthesis has seen rapid growth over the past decade. As reports of singly fluorinated natural amino acid derivatives have grown, researchers have turned their attention to develop methodology to access complex proteinogenic examples. A variety of reaction conditions have been employed in this area, exploiting new advances in the wider synthetic community such as photocatalysis and palladium cross-coupling. In addition, novel fluorinated functional groups have also been incorporated into amino acids, with SFX and perfluoro moieties now appearing with more frequency in the literature. This review focuses on synthetic methodology for accessing complex non-proteinogenic amino acids, along with amino acids containing multiple fluorine atoms such as CF3, SF5 and perfluoroaromatic groups.

Stereocontrolled Synthesis of Functionalized Azaheterocycles from Carbocycles through Oxidative Ring Opening/Reductive Ring Closing Protocols

Fluorine-containing organic scaffolds are of significant interest in medicinal chemistry. The incorporation of fluorine into biomolecules can lead to remarkable changes in their physical, chemical, and biological properties. There are already many drugs on the market, which contain at least one fluorine atom. Saturated functionalized azaheterocycles as bioactive substances have gained increasing attention in pharmaceutical chemistry. Due to the high biorelevance of organofluorine molecules and the importance of N-heterocyclic compounds, selective stereocontrolled procedures to the access of new fluorine-containing saturated N-heterocycles are considered to be a hot research topic. This account summarizes the synthesis of functionalized and fluorine-containing saturated azaheterocycles starting from functionalized cycloalkenes and based on oxidative ring cleavage of diol intermediates followed by ring expansion with reductive amination.

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.