Recent advances in synthesizing and utilizing nitrogen-containing heterocycles

The use of organocatalysts and a pot economy has strengthened recent organic syntheses. Synthetic methodologies may be applicable in laboratory preparation or in the industrial production of valuable organic compounds. In most cases, synthetic challenges are overcome by highly efficient and environmentally benign organocatalysts in a pot-economical manner. This is exemplified by the recent synthesis of tetrahydropyridine-containing (-)-quinine.

Regioselective ring opening of aziridine for synthesizing azaheterocycle

Aziridine had different regioselective ring openings depending on the functional group of its alkyl substituent. In the case of the alkyl group bearing γ-ketone at the C2 substituent of aziridine, the ring opening by the hydroxy nucleophile from H2O occurred by attacking the aziridine carbon at the C2 position. This reaction proceeded efficiently in the presence of CF3CO2H. Interestingly, the same starting aziridine ring bearing the alkyl substituent at the C2 position with the γ-silylated hydroxy group instead of γ-ketone led to the ring-opening reaction by the same oxygen nucleophile at the unsubstituted C3 position, with the breakage of the bond between aziridine N1 nitrogen and carbon at C3. These reaction products were cyclized to afford substituted pyrrolidine and piperidine rings with representative examples of congeners of pseudoconhydrine and monomorine.

Atom Economical Multi-Substituted Pyrrole Synthesis from Aziridine

Multi-substituted pyrroles are synthesized from regiospecific aziridine ring-opening and subsequent intramolecular cyclization with a carbonyl group at the γ-position in the presence of Lewis acid or protic acid. This method is highly atom economical where all the atoms of the reactants are incorporated into the final product with the removal of water. This new protocol is applied to the synthesis of various pyrroles, including natural products.

Preparation of Enantiopure Non-Activated Aziridines and Synthesis of Biemamide B, D, and epiallo-Isomuscarine

Nitrogen-containing heterocycle aziridines are synthetically very valuable for the preparation of azacyclic and acyclic molecules. However, it is very difficult and laborious to make aziridines in optically pure forms on a large scale to apply asymmetric synthesis of aza compounds. Fortunately, we successfully achieved both enantiomers (2R)- and (2S)-aziridine-2-carboxylates with the electron-donating α-methylbenzyl group at the ring nitrogen as non-activated aziridines. These starting aziridines have two distinct functional groups-highly reactive three-membered ring and versatile carboxylate. They are applicable in ring-opening or ring-transformation with aziridine and in functional group transformation to others from carboxylate. Both of these enantiomers were utilized in the preparation of biologically important amino acyclic and/or aza-heterocyclic compounds in an asymmetric manner. Specifically, this report describes the first expedient asymmetric synthesis of both enantiomers of 5, 6-dihydrouracil-type marine natural products biemamide B and D as potential TGF-β inhibitors. This synthesis consisted of regio- and the stereoselective ring-opening reaction of aziridine-2-carboxylate and subsequent formation of 4-aminoteterahydropyrimidine-2,4-dione. One more example in this protocol dealt a highly stereoselective Mukaiyama reaction of aziridine-2-carboxylate and silyl enol ether, following intramolecular aziridine ring-opening to provide easy and facile access to (-)-epiallo-isomuscarine.

Functionalisation of esters via 1,3-chelation using NaOtBu: mechanistic investigations and synthetic applications

Unusual nucleophilic behavior of a metal t-butoxide in a transesterification reaction was demonstrated by NMR and real-time IR spectroscopies and deuterium-labeling experiments.

Lewis acid-mediated synthesis of mono- and tris-indole adducts from chiral aziridines

Lewis acid-mediated regio- and stereoselective nucleophilic addition of 2- or 3-substituted indoles to non-activated aziridine-2-carboxaldehydes in dioxane afforded 2-(indol-3-ylhydroxymethyl)aziridines whose ring opening with various nucleophiles rendered multi-substituted tryptamine derivatives. The reaction of the same aziridine-2-carboxaldehyde with three moles of indole in dichloromethane yielded tris-indole adducts β-(3,3'-bisindolyl)methyl (BIM) tryptamines from sequential steps including nucleophilic addition to aldehyde, Michael type Friedel-Crafts alkylation of the mono-adduct followed by regio- and stereoselective ring-opening of the aziridine ring.

Stereoselective synthesis of 2,6-disubstituted piperidine alkaloids

Among the large number of structurally diverse alkaloids, 2,6-disubstituted piperidine and its analogs have often been targeted when exploiting new synthetic techniques perhaps because of their strong pharmacological properties. This review outlines synthetic strategies to build the 2,6-disubstituted piperidine structural motif with a focus on stereochemical control of two substituents at C2 and C6. The key reactions in this process are then classified on the basis of how the piperidine rings were built with specific examples of natural products that control the stereochemical outcomes and their transition states.

Metal-free aza-Claisen type ring expansion of vinyl aziridines: an expeditious synthesis of seven membered N-heterocycles

A metal-free approach for the synthesis of 7-membered aza-heterocycles has been developed by the intermolecular [5 + 2] cycloaddition of non-activated vinylaziridines and alkynes. This method has a broad substrate scope under mild reaction conditions to afford structurally diverse 7-membered N-heterocycles in high yield up to 92%.

Alkylative Ring-Opening of Bicyclic Aziridinium Ion and Its Application for Alkaloid Synthesis

Alkylative ring-opening of bicyclic aziridinium ion generated from 4-hydroxybutylaziridine with organocopper reagent was achieved successfully to afford 2-alkylsubstituted piperidine in high or moderate yield. This method allowed carbon-carbon bond formation of "non-activated" aziridine via aziridinium ion ring-opening in regio- and stereo-selective manner for the first time. This newly developed reaction was applied for an efficient synthesis of alkaloid with the representative example of conine and epiquinamide.

Deployment of Aziridines for the Synthesis of Alkaloids and Their Derivatives

Various (activated and non-activated) aziridines with diverse substitution patterns have been deployed successfully as starting materials for the synthesis of a wide variety of alkaloids via suitable functionalization and aziridine ring transformation. Alternatively, the preparation and interception of reactive aziridine intermediates has also been shown to constitute a valid approach toward alkaloid synthesis. This review summarizes aziridine-mediated syntheses of alkaloids, in which the aziridine is mobilized as either a substrate or an advanced synthetic intermediate.

1 Introduction

2 Alkaloids Synthesis from Aziridine Starting Materials

2.1 (2R)- and (2S)-Hydroxymethyl-N-(1-phenylethyl)aziridines

2.2 N-Benzylaziridine-2-carboxylates

2.3 2-Substituted N-Tosyl- or N-Tritylaziridines

2.4 2,3-Disubstituted N-Cbz- or N-Tosylaziridines

2.5 N-DMB-aziridines

3 Alkaloids Synthesis from Aziridines as Key Advanced Synthetic Intermediates

3.1 Alkylative Aziridine Ring Opening

3.2 Arylative Aziridine Ring Opening

3.3 Ring Expansion

3.4 Oxidative Aziridine Ring Opening

3.5 Heteroatomic Nucleophilic Aziridine Ring Opening

3.6 Reductive Aziridine Ring Opening

4 Conclusion

Theoretical insight into the regioselective ring-expansions of bicyclic aziridinium ions

Transient bicyclic aziridinium ions are known to undergo ring-expansion reactions, paving the way to functionalized nitrogen-containing heterocycles. In this study, the regioselectivity observed in the ring-expansion reactions of 1-azoniabicyclo[n.1.0]alkanes was investigated from a computational viewpoint to study the ring-expansion pathways of two bicyclic systems with different ring sizes. Moreover, several nucleophiles leading to different experimental results were investigated. The effect of solvation was taken into account using both explicit and implicit solvent models. This theoretical rationalization provides valuable insight into the observed regioselectivity and may be used as a predictive tool in future studies.

Asymmetric Synthesis of cis-5-(Aminomethyl)-3-(4-methoxy­phenyl)dihydrofuran-2(3H)-one

The asymmetric synthesis of (3R,5S)-5-(aminomethyl)-3-(4-methoxyphenyl)dihydrofuran-2(3H)-one, as the most potent selective inactivator of monoamine B, was successfully achieved by applying a newly developed synthetic method toward the key γ-aminomethyl-γ-lactone via intramolecular aziridine ring opening in 63% overall yield from a commercial starting material.

Total Synthesis and Absolute Stereochemical Assignment of Microgrewiapine A and Its Stereoisomers

Total synthesis of both enantiomers of (-)-(2 S,3 R,6 S)- and (+)-(2 R,3 S,6 R)-microgrewiapine A along with (+)-microcosamine A and (-)-6- epi-microgrewiapine A from chiral 1-(α-methylbenzyl)-aziridine-2-carboxylate was accomplished for the first time. Key steps involved in this synthesis include one-pot reductive ring-opening of aziridine, debenzylation, intramolecular N-alkylation to obtain the key piperidine ring, and Julia-Kociensky olefination. The absolute configuration of natural microgrewiapine A is assigned as (+)-(2 R,3 S,6 R), which is opposite to the originally proposed structure by comparing optical rotation data of both synthetic enantiomers.

Morita-Baylis-Hillman reaction of a chiral aziridine aldehyde

The Morita-Baylis-Hillman reaction of (R)-1-((R)-1-phenylethyl)aziridine-2-carbaldehyde with alkyl acrylate was carried out under various conditions by changing solvents, bases, and alcohol additives. The reaction at room temperature under neat conditions (no solvent) with quinuclidine as an amine nucleophile, in the presence of benzyl alcohol as an additive, afforded a product, γ-(aziridin-2-yl)-β-hydroxy-α-methylene butanoate, in 97% yield with a diastereomeric ratio of anti and syn as 86 : 14.

Preparation of Stable Bicyclic Aziridinium Ions and Their Ring-Opening for the Synthesis of Azaheterocycles

Bicyclic aziridinium ions were generated by the removal of an appropriate leaving group through internal nucleophilic attack by nitrogen atom in the aziridine ring. The utility of bicyclic aziridinium ions, specifically 1-azoniabicyclo[3.1.0]hexane and 1-azoniabicyclo[4.1.0]heptane tosylate highlighted in the aziridine ring openings by the nucleophile with the release of the ring strain to yield the corresponding ring-expanded azaheterocycles such as pyrrolidine, piperidine and azepane with diverse substituents on the ring in regio- and stereospecific manner. Herein, we report a simple and convenient method for the preparation of the stable 1-azabicyclo[4.1.0]heptane tosylate followed by selective ring opening via a nucleophilic attack either at the bridge or at the bridgehead carbon to yield piperidine and azepane rings, respectively. This synthetic strategy allowed us to prepare biologically active natural products containing piperidine and azepane motif including sedamine, allosedamine, fagomine and balanol in highly efficient manner.

In vivo growth and genomic characterization of rickettsia-like organisms isolated from farmed Chinook salmon (Oncorhynchus tshawytscha) in New Zealand

A rickettsia-like organism, designated NZ-RLO2, was isolated from Chinook salmon (Oncorhynchus tshawytscha) farmed in the South Island, New Zealand. In vivo growth showed NZ-RLO2 was able to grow in CHSE-214, EPC, BHK-21, C6/36 and Sf21 cell lines, while Piscirickettsia salmonis LF-89T grew in all but BHK-21 and Sf21. NZ-RLO2 grew optimally in EPC at 15°C, CHSE-214 and EPC at 18°C. The growth of LF-89 T was optimal at 15°C, 18°C and 22°C in CHSE-24, but appeared less efficient in EPC cells at all temperatures. Pan-genome comparison of predicted proteomes shows that available Chilean strains of P. salmonis grouped into two clusters (p-value = 94%). NZ-RLO2 was genetically different from previously described NZ-RLO1, and both strains grouped separately from the Chilean strains in one of the two clusters (p-value = 88%), but were closely related to each other. TaqMan and Sybr Green real-time PCR targeting RNA polymerase (rpoB) and DNA primase (dnaG), respectively, were developed to detect NZ-RLO2. This study indicates that the New Zealand strains showed a closer genetic relationship to one of the Chilean P. salmonis clusters; however, more Piscirickettsia genomes from wider geographical regions and diverse hosts are needed to better understand the classification within this genus.

Preparation of Chiral Contiguous Epoxyaziridines and Their Regioselective Ring-Opening for Drug Syntheses

Synthetically valuable chiral (aziridin-2-yl)oxirane-3-carbaldehydes bearing three consecutive functional groups including aziridine, epoxide, and aldehyde were prepared from the stereoselective epoxidation of (aziridin-2-yl)acrylaldehydes with H₂ O₂ using organocatalyst (2R)- or (2S)-[diphenyl(trimethylsilyloxy)methyl]pyrrolidine as organocatalyst. The regioselective ring opening of aziridines and epoxides enabled us to achieve the highly efficient asymmetric synthesis of the antibiotic edeine D fragment 3-hydroxy-4,5-diaminopenatanoic acid, an intermediate for the formal synthesis of non-proteinogenic amino acid (-)-galantinic acid, and for potent antifungal agent (+)-preussin, and the medicinally important framework 3-hydroxy-2-hydroxymethylpyrrolidine.

Atom-Economical and Metal-Free Synthesis of Multisubstituted Furans from Intramolecular Aziridine Ring Opening

Multisubstituted furans were prepared from dialkyl 2-(aziridin-2-ylmethylene)malonate and/or 1,3-dione through aziridine ring opening by the internal carbonyl oxygen with the assistance of BF3·OEt2, followed by aromatization. This synthetic method is free from any metal and is atom-economical with all of the atoms in the starting material retained in the final product.

Mixed Monosilyl Acetals and Catalyst-Dependent Chemoselective Mukaiyama Aldol Reactions

We report a direct, simple, and straightforward approach for the construction of a mixed monosilyl acetal as a new and synthetically valuable functional group by mixing an aldehyde, sodium tert-butoxide, and trimethylsilyl azide. We also demonstrate a catalyst-dependent chemoselective reaction between mixed monosilyl acetals and silyl ketene acetals through Mukaiyama aldol reactions to give different structures of O-protected β-hydroxy esters in excellent yields with high chemoselectivities. This study provided the existence of an oxonium ion intermediate and of its kinetically controlled reaction with the pre-equilibrated silyl enol ether obtained from (E)- and (Z)-isomerization.

Monitoring Theileria orientalis (Ikeda)-associated bovine anaemia in affected cattle over time

The aim of the study was to observe changes in haematocrit (HCT) over time in a New Zealand South Island dairy herd affected by an outbreak of Theileria-associated bovine anaemia (TABA; Ikeda). A secondary aim was to relate individual cow HCTs to the amount of Theileria orientalis Ikeda DNA present in the blood, as measured by cycle threshold values, using a quantitative PCR (qPCR). Over a 6 month period, blood samples from 19 randomly selected cattle were monitored from a herd of 600 dairy cows. The sampling interval was approximately fortnightly for the first six weeks, followed by sampling at between four and six week intervals. At the initial report of the outbreak, two from six cattle were anaemic (HCT<0.25L/L). Blood collected from 14 cattle 11 days later showed that 57% (95% CI 33-77%) of the cattle sampled were anaemic. Of the 19 cattle that went on to be monitored, 12 (63% 95% CI=41-81%) developed anaemia at some point during the period of monitoring. One of the anaemic animals did not meet the case definition for TABA Ikeda. For individual cattle, the average number of days between when cattle were first detected as anaemic and when HCT returned to normal was 53days (median=47 days, range=6-92 days). At the point of notification the amount of T. orientalis Ikeda DNA in the blood of the six cattle tested was low (Cq median=36), but 11days later the amount of DNA in blood of 14 additional cows tested was relatively high (Cq median=24). Levels of all 19 cows monitored continued to remain moderately high through the period of testing (Cq median=29). This was despite a general improvement in the HCT of affected cattle. In four of the 15 cattle positive to T. orientalis Ikeda where blood fractions (plasma and whole blood) were tested, it appeared that T. orientalis Ikeda (as measured by qPCR) dropped more rapidly in plasma fractions than in whole blood at the point that HCT started to return to normal levels. Despite the assumption that tick populations were low in the Canterbury region of the South Island the impact of TABA (proportion of herd affected and the average period that animals remained anaemic) on the case herd was still relatively high.

Selective Ring-Opening of N-Alkyl Pyrrolidines with Chloroformates to 4-Chlorobutyl Carbamates

Our study shows that among aza-heterocycles of various ring sizes, including aziridines, azetidines, pyrrolidines, and piperidines, only N-alkyl pyrrolidines undergo competitive reaction pathways with chloroformates to yield N-dealkylated pyrrolidines and 4-chlorobutyl carbamates. The pathway taken depends on the substituent on the nitrogen, i.e., ring-opening with methyl and ethyl substituents and dealkylation with a benzyl substituent. Computational calculations support the substituent-dependent product formation by showing the energy difference between the transition states of both reaction pathways. Selective ring-opening reactions of N-methyl and N-ethyl pyrrolidine derivatives with chloroformates were utilized to prepare various 4-chlorobutyl carbamate derivatives as valuable 1,4-bifunctional compounds.

The First Detection of Ornithobacterium rhinotracheale in New Zealand

Ornithobacterium rhinotracheale (ORT) has been considered exotic to New Zealand and thus, any samples from poultry suspected of ORT infection are submitted as part of an exotic disease investigation managed by Ministry for Primary Industries (MPI) and subjected to standardized test protocols carried out in the physical containment level 3+ laboratory at MPI's Animal Health Laboratory (AHL). All previous exotic disease investigations concerning ORT produced negative results by bacterial culture and conventional PCR. Following the recent introduction of a real-time PCR for ORT at the AHL, several tracheal wash fluids from backyard chickens ( Gallus gallus domesticus ) were tested positive. This identification constituted the first detection of ORT in New Zealand poultry. As a result, a second premise was investigated with further samples testing positive for ORT by molecular assays. This paper describes the two exotic disease investigations associated with the first detection of ORT in New Zealand poultry and its implications.

First report and histological features of Chlamydia pecorum encephalitis in calves in New Zealand

CASE HISTORY

Between September and October 2013, 40 of 150 crossbred Friesian dairy calves on a farm in the Manawatu region of New Zealand developed neurological signs when between 1 and 3 months of age. Calves were grazed in multiple mobs and calves from each mob were affected. A variable response was observed to initial treatment with thiamine, fluoroquinolone antibiotics and non-steroidal anti-inflammatory drugs.

CLINICAL AND PATHOLOGICAL FINDINGS

Affected calves exhibited a range of neurological signs that included generalised depression, hind limb ataxia with a stiff gait, and knuckling of the fetlocks. In advanced cases, calves became recumbent with opisthotonous. Over a 4-week period, 13 calves died or were subject to euthanasia and a thorough necropsy was performed on three of these calves. Necropsy findings included fibrinous peritonitis, pleuritis and pericarditis, with no gross abnormalities visible in the brain or joints. Histology of the brain was possible in seven of the affected calves, with lesions ranging from lymphocytic and histiocytic vasculitis and meningoencephalitis, to extensive thrombosis and neutrophilic inflammation. Immunohistochemistry using an anti-chlamydial lipopolysaccharide antibody revealed positive immuno-staining in all seven cases, with no brain samples exhibiting immunostaining for Histophilus somni. DNA was extracted from a sample of fresh brain from one case and chlamydial DNA sequences were amplified by PCR and found to be identical to Chlamydia pecorum. PCR was also performed on formalin-fixed brain tissue from three of the other cases, but no chlamydial DNA was amplified.

DIAGNOSIS

Chlamydia pecorum meningoencephalomyelitis (sporadic bovine encephalomyelitis).

CLINICAL RELEVANCE

This is the first time that C. pecorum has been confirmed as a cause of clinical disease in New Zealand. Practitioners should be aware of this disease as a differential in calves with neurological signs, and submit samples of formalin-fixed brain as well as fresh brain to enable confirmation of suspected cases using PCR analysis. Furthermore, these cases illustrate that the histological lesions in the brains of calves with C. pecorum are more variable than previously reported, and pathologists should be aware that histological features may overlap with those traditionally ascribed to other organisms, such as H. somni.

One-pot multiple reactions: asymmetric synthesis of 2,6-cis-disubstituted piperidine alkaloids from chiral aziridine

A divergent, new, and highly stereoselective synthesis of cis-2,6-disubstituted piperidine natural products including isosolenopsins, deoxocassine, and spectaline was achieved from chiral aziridine decorated with appropriate alkyl chains for isosolenopsins or alkynyl groups for deoxocassine and spectaline at C2. The characteristic feature of this synthesis is one-pot sequential reactions under atmospheric hydrogen including the reduction of alkyne (for deoxocassine and spectaline), reductive ring-opening of aziridine, debenzylation, and intramolecular reductive amination in high yields. The prerequisite aziridines were elaborated from commercially available (2S)-hydroxymethylaziridine through oxidation, Wittig olefination, and the Grignard reaction for isosolenopsins or substrate-controlled lithium alkynylate addition for deoxocassine and spectaline.

Investigation of bovine haemoplasmas and their association with anaemia in New Zealand cattle

CASE HISTORY AND CLINICAL FINDINGS

A dairy cow, from a herd in the Waikato region of New Zealand, was reported with regenerative anaemia on 12 September 2014. Testing of blood from the animal using PCR assays for Theileria orientalis produced a negative result for both Chitose and Ikeda types.

LABORATORY FINDINGS

Using PCR and DNA sequencing, blood from the cow was positive for Candidatus Mycoplasma haemobos. Further testing of another 12 animals from the case herd, 27 days after the affected cow was first reported, showed 11 animals were positive for Candidatus M. haemobos or Mycoplasma wenyonii in the PCR. None of these cattle were clinically anaemic or positive for T. orientalis Ikeda type using PCR. A convenience sample of 47 blood samples from cattle throughout New Zealand, submitted to the Investigation and Diagnostic Centre (Ministry for Primary Industries) for surveillance testing for T. orientalis Ikeda, was selected for further testing for bovine haemoplasmas. Of these samples, 6/47 (13%) and 13/47(28%) were positive for M. wenyonii and Candidatus M. haemobos, respectively. There was no difference in the proportion of samples positive for the bovine haemaplasmas between cattle with anaemia that were negative for T. orientalis (6/20, 33%), or without anaemia or T. orientalis (10/18, 56%), or from cattle herds experiencing anaemia and infection with T. orientalis Ikeda type (3/9, 33%).

DIAGNOSIS

Bovine haemoplasmosis.

CLINICAL RELEVANCE

The presence of bovine haemoplasmas in blood does not establish causality for anaemia in cattle. Diagnosis of anaemia associated with haemoplasmosis would require exclusion of other causes of regenerative anaemia and an association of the agent with anaemia in affected cattle herds. The data collected in this study did not provide evidence that bovine haemoplasmas were associated with a large number of outbreaks of anaemia in cattle in New Zealand.

Stereoselective Pd-catalyzed etherification and asymmetric synthesis of furanomycin and its analogues from a chiral aziridine

A chiral aziridine was utilized for the synthesis of the anti-bacterial natural amino acid L-(+)-furanomycin, and its analogues including 5'-epi-furanomycin and norfuranomycin. Key steps of this synthesis are the stereoselective Pd-catalyzed etherification for diallyl ethers and ring closing metathesis.

Simple preparation of new [(18) F]F-labeled synthetic amino acid derivatives with two click reactions in one-pot and SPE purification

New [(18) F]fluorinated 1,2,3-triazolyl amino acid derivatives were efficiently prepared from Huisgen 1,3-dipolar cycloaddition reactions, well known as click reaction. We developed two simultaneous click reactions in one-pot with a simple solid-phase extraction (SPE) purification method. [(18) F]fluoro-1-propyne was obtained at a 45% non-decay corrected radiochemical yield based on the [(18) F]fluoride ion. The one-pot and simultaneous two click reactions were performed with unprotected azido-alkyl amino acid, [(18) F]fluoro-1-propyne, and lipophilic additive alkyne to produce three synthetic amino acid derivatives, AMC-101 ([(18) F]-6a), AMC-102 ([(18) F]-6b), and AMC-103 ([(18) F]-6c) with 29%, 28%, and 24% of non-decay corrected radiochemical yields, respectively. All radiotracers indicated that radiochemical purities were >95% without any residual organic solvent. Our new method involving two click reactions in one-pot showed high radiochemical and chemical purity by easy removal of the residual precursor from the simultaneous two click reactions.