Approaches to Synthesis and Isolation of Enantiomerically Pure Biologically Active Atropisomers.

Atropisomerism is a stereochemical phenomenon exhibited by molecules containing a rotationally restricted σ bond. Contrary to classical point chirality, the two atropisomeric stereoisomers exist as a dynamic mixture and can be interconverted without the requirement of breaking and reforming a bond. Although this feature increases structural complexity, atropisomers have become frequent targets in medicinal chemistry projects. Their axial chirality, e.g., from axially chiral biaryl motifs, gives access to unique 3D structures. It is often desirable to have access to both enantiomers of the atropisomers via a nonselective reaction during the early discovery phase as it allows the medicinal chemistry team to probe the structure activity relationship in both directions. However, once a single atropisomer is selected, it presents several problems. First, the pure single atropisomer may interconvert to the undesired stereoisomer under certain conditions. Second, separation of atropisomers is nontrivial and often requires expensive chiral stationary phases using chromatography or additives if a salt resolution approach is chosen. Other options can be kinetic resolution using enzymes or chiral catalysts. However, apart from the high cost often associated with the two latter methods, a maximum yield of only 50% of the desired atropisomer can be obtained. The ideal approach is to install the chiral atropisomeric axis enantioselectively or employing a dynamic kinetic resolution approach. In theory, both approaches have the potential to provide a single atropisomer in quantitative yield. This Account will discuss the successes/failures and challenges we have experienced in developing methods for resolution/separation and asymmetric synthesis of atropisomeric drug candidates in one of our early phase drug development projects. Suitability for the different methods at various stages of the drug development phase is discussed. Depending on the scale and time available, a separation of a mixture of atropisomers by chromatography was sometimes preferred, whereas asymmetric- or resolution approaches were desired for long-term supply. With the use of chromatography, the impact on separation efficiency and solvent consumption, depending on the nature of the substrate, is discussed. We hope that with this Account the readers will get a better view on the challenges medicinal and process chemists meet when designing new atropisomeric drug candidates and developing processes for manufacture of a single atropisomer.

Copper-Catalyzed Racemization-Recycle of a Quaternary Center and Optimization Using a Combined Kinetics-DoE/MLR Modeling Approach.

The copper-catalyzed racemization of a complex, quaternary center of a key intermediate on route to lanabecestat has been identified. Optimization and mechanistic understanding were achieved through the use of an efficient, combined kinetic-multiple linear regression approach to experimental design and modeling. The use of a definitive screening design with mechanistically relevant factors and a mixture of fitted kinetic descriptors and empirical measurements facilitated the generation of a model that accurately predicted complex reaction time course behavior. The synergistic model was used to minimize the formation of dimer byproducts, determine optimal conditions for batch operation, and highlight superheated conditions that could be accessed in flow, leading to a further increase in yield which was predicted by the original model.

Exploration of a Nitromethane-Carbonylation Strategy during Route Design of an Atropisomeric KRASG12C Inhibitor.

Route design and proof of concept synthesis was conducted on a synthetically challenging atropisomeric KRASG12C inhibitor to support clinical API manufacture. Improvements to the synthesis of a chiral piperazine fragment gave reduced step count and streamlined protecting group strategy via the formation and methanol ring opening of an N-carboxy-anhydride (NCA). The complex atropisomeric nitroquinoline was accessed via an early stage salt-resolution followed by a formal two-part nitromethane-carbonylation, avoiding a high temperature Gould-Jacobs cyclization that previously led to atropisomer racemization. The substrate scope of the formal nitromethane-carbonylation strategy was further explored for a range of ortho-substituted bromo/iodo unprotected anilines.

Continuous Flow Chiral Amine Racemization Applied to Continuously Recirculating Dynamic Diastereomeric Crystallizations.

A new, dynamic diastereomeric crystallization method has been developed, in which the mother liquors are continuously separated, racemized over a fixed-bed catalyst, and recirculated to the crystallizer in a resolution-racemization-recycle (R3) process. Separating the racemization from crystallization overcomes problems of using catalysts in situ, that suffer conflicting sets of conditions, inhibition, and separation. Continuous racemization has been achieved through the covalent attachment of [IrCp*I2]2 SCRAM catalyst to Wang resin solid support to give a fixed-bed catalyst. One tertiary and a variety of secondary optically enriched amines have been racemized efficiently, with residence times compatible with the crystallization (2.25-30 min). The catalyst demonstrates lower turnover (TOF) than the homogeneous analogue but with reuse shows a long lifetime (e.g., 40 recycles, 190 h) giving acceptable turnover number (TON) (up to 4907). The slow release of methylamine during racemization of N-methyl amines was found to inactivate the catalyst, which could be partially reactivated using hydroiodic acid. Dynamic crystallization is achieved in the R3 process through the continual removal of the more soluble diastereomer and supply of the less soluble one. The solubility of the diastereomers was determined, and the difference correlates to the rate of resolution but is also affected by the rates of racemization, crystal growth, and dissolution. A variety of cyclic and acyclic amine salts were resolved using mandelic acid (MA) and ditoluoyl tartaric acid (DTTA) with higher resolvability (S = yield × d.e.) than the simple diastereomeric crystallization alone. Comparing resolvabilities, resolutions were 1.6-44 times more effective with the R3 process than batch, though one case was worse. Further investigation of this revealed an unusual thermodynamic switching behavior: rac-N-methylphenethylamine was initially resolved as an (S,S)-bis-alkylammonium tartrate crystal but over time became the equivalent (R,S) salt. Thermal, mixing, concentration, stoichiometry, and seeding conditions were all found to affect the onset of the switching behavior which is only associated with difunctional resolving reagents.

Enantioselective Aza-Heck Cyclizations of N-(Tosyloxy)carbamates: Synthesis of Pyrrolidines and Piperidines.

Pd(0)-systems modified with SPINOL-derived phosphoramidate ligands promote highly enantioselective aza-Heck cyclizations of alkenyl N-(tosyloxy)carbamates. The method provides versatile access to challenging N-heterocycles and represents the broadest scope enantioselective aza-Heck protocol developed to date.

Antibiotic dispensation rates among participants in community-driven health research projects in Arctic Canada.

BACKGROUND: Community-driven projects that aim to address public concerns about health risks from H. pylori infection in Indigenous Arctic communities (estimated H. pylori prevalence = 64%) show frequent failure of treatment to eliminate the bacterium. Among project participants, treatment effectiveness is reduced by antibiotic resistance of infecting H. pylori strains, which in turn, is associated with frequent exposure to antibiotics used to treat other infections. This analysis compares antibiotic dispensation rates in Canadian Arctic communities to rates in urban and rural populations in Alberta, a southern Canadian province. METHODS: Project staff collected antibiotic exposure histories for 297 participants enrolled during 2007-2012 in Aklavik, Tuktoyaktuk, and Fort McPherson in the Northwest Territories, and Old Crow, Yukon. Medical chart reviews collected data on systemic antibiotic dispensations for the 5-year period before enrolment for each participant. Antibiotic dispensation data for urban Edmonton, Alberta (average population ~ 860,000) and rural northern Alberta (average population ~ 450,000) during 2010-2013 were obtained from the Alberta Government Interactive Health Data Application. RESULTS: Antibiotic dispensation rates, estimated as dispensations/person-years (95% confidence interval) were: in Arctic communities, 0.89 (0.84, 0.94); in Edmonton, 0.55 (0.55, 0.56); in rural northern Alberta, 0.63 (0.62, 0.63). Antibiotic dispensation rates were higher in women and older age groups in all regions. In all regions, the highest dispensation rates occurred for ß-lactam and macrolide antibiotic classes. CONCLUSIONS: These results show more frequent antibiotic dispensation in Arctic communities relative to an urban and rural southern Canadian population.

Burden of disease from Helicobacter pylori infection in western Canadian Arctic communities.

BACKGROUND: Indigenous communities across the circumpolar north have elevated H. pylori (Hp) prevalence and stomach cancer incidence. We aimed to describe the Hp-associated disease burden among western Canadian Arctic participants in community-driven projects that address concerns about health risks from Hp infection. METHODS: During 2008-2013, participants underwent Hp screening by urea breath test and gastroscopy with gastric biopsies. We estimated Hp prevalence and prevalence by Hp status of endoscopic and histopathologic diagnoses. RESULTS: Among 878 participants with Hp status data, Hp prevalence was: 62% overall; 66% in 740 Indigenous participants; 22% in 77 non-Indigenous participants (61 participants did not disclose ethnicity); 45% at 0-14 years old, 69% at 15-34 years old, and 61% at 35-96 years old. Among 309 participants examined endoscopically, visible mucosal lesions were more frequent in the stomach than the duodenum: the gastric to duodenal ratio was 2 for inflammation, 8 for erosions, and 3 for ulcers. Pathological examination in 308 participants with gastric biopsies revealed normal gastric mucosa in 1 of 224 Hp-positive participants and 77% (65/84) of Hp-negative participants with sharp contrasts in the prevalence of abnormalities between Hp-positive and Hp-negative participants, respectively: moderate-severe active gastritis in 50 and 0%; moderate-severe chronic gastritis in 91 and 1%; atrophic gastritis in 43 and 0%; intestinal metaplasia in 17 and 5%. CONCLUSIONS: The observed pattern of disease is consistent with increased risk of stomach cancer and reflects substantial inequity in the Hp-associated disease burden in western Arctic Canadian hamlets relative to most North American settings. This research adds to evidence that demonstrates the need for interventions aimed at reducing health risks from Hp infection in Indigenous Arctic communities.

Picolinamides as effective ligands for copper-catalysed aryl ether formation: structure-activity relationships, substrate scope and mechanistic investigations.

The use of picolinic acid amide derivatives as an effective family of bidentate ligands for copper-catalysed aryl ether synthesis is reported. A fluorine-substituted ligand gave good results in the synthesis of a wide range of aryl ethers. Even bulky phenols, known to be very challenging substrates, were shown to react with aryl iodides with excellent yields using these ligands. At the end of the reaction, the first examples of end-of-life Cu species were isolated and identified as Cu(II) complexes with several of the anionic ligands tested. A preliminary mechanistic investigation is reported that suggests that the substituents on the ligands might have a crucial role in determining the redox properties of the metal centre and, consequently, its efficacy in the coupling process. An understanding of these effects is important for the development of new efficient and tunable ligands for copper-based chemistry.

Changes in Gastric Pathology after H. pylori Treatment in Community-Driven Research Aimed at Gastric Cancer Prevention.

Community-driven projects have characterized Helicobacter pylori (Hp) infection in Indigenous communities in the Northwest Territories (NT) and Yukon (YT), Canada. These projects address concerns about the frequent diagnosis of Hp infection among community members and its relation to gastric cancer deaths, perceived to occur with alarming frequency in this region. Projects included breath-test screening for Hp infection, gastroscopy with gastric biopsies, and treatment to eliminate Hp infection. Previous project results showed a high prevalence of stomach pathologies associated with increased cancer risk among Hp-positive participants at baseline. This analysis describes changes in precancerous gastric pathologies in project participants who had gastroscopy before baseline treatment during 2008-2013 and again in 2017. Throughout the study period, the same pathologist graded Hp density, active gastritis, chronic gastritis, atrophic gastritis, and intestinal metaplasia using the updated Sydney System. Of 310 participants from three communities with baseline pathology data, 69 had follow-up pathology data. Relative to baseline, the prevalence of Hp infection and precancerous gastric pathology was substantially lower at follow-up; most participants who were Hp-positive at baseline and Hp-negative at follow-up had reduced severity of active, chronic, and/or atrophic gastritis at follow-up. Though follow-up numbers are small, these results yield evidence that successful Hp treatment has the potential to reduce the risk of gastric cancer in Arctic Indigenous communities.

E- and Z-stereoselectivity in the preparation of enamides from glycidyl sulfonamides and carbamates.

Treatment of glycidyl sulfonamides with LDA delivers the corresponding enesulfonamide with good selectivity for the E-isomer, whereas the corresponding carbamates exhibit selectivity for the Z-enecarbamate. An E1cB elimination mechanism proceeding from a substrate-base chelate complex is advanced as rationalisation of the latter set of Z-selective outcomes.

Continuous-flow Synthesis of Aryl Aldehydes by Pd-catalyzed Formylation of Aryl Bromides Using Carbon Monoxide and Hydrogen.

A continuous-flow protocol utilizing syngas (CO and H2 ) was developed for the palladium-catalyzed reductive carbonylation of (hetero)aryl bromides to their corresponding (hetero)aryl aldehydes. The optimization of temperature, pressure, catalyst and ligand loading, and residence time resulted in process-intensified flow conditions for the transformation. In addition, a key benefit of investigating the reaction in flow is the ability to precisely control the CO-to-H2 stoichiometric ratio, which was identified as having a critical influence on yield. The protocol proceeds with low catalyst and ligand loadings: palladium acetate (1 mol % or below) and cataCXium A (3 mol % or below). A variety of (hetero)aryl bromides at a 3 mmol scale were converted to their corresponding (hetero)aryl aldehydes at 12 bar pressure (CO/H2 =1:3) and 120 °C reaction temperature within 45 min residence time to afford products mostly in good-to-excellent yields (17 examples). In particular, a successful scale-up was achieved over 415 min operation time for the reductive carbonylation of 2-bromo-6-methoxynaphthalene to synthesize 3.8 g of 6-methoxy-2-naphthaldehyde in 85 % isolated yield. Studies were conducted to understand catalyst decomposition within the reactor by using inductively coupled plasma-mass spectrometry (ICP-MS) analysis. The palladium could easily be recovered using an aqueous nitric acid wash post reaction. Mechanistic aspects and the scope of the transformation are discussed.

Palladium-catalyzed carbonylation of aryl tosylates and mesylates.

A general protocol for the palladium-catalyzed carbonylation of aryl tosylates and mesylates to form esters has been developed using a catalyst system derived from Pd(OAc)2 and the bulky, bidentate dcpp ligand. The system operates under mild conditions: atmospheric CO pressure and temperatures of 80-110 degrees C. A broad substrate scope has been demonstrated allowing carbonylation of electron-rich, electron-poor, and heterocyclic tosylates and mesylates, and the reaction shows wide functional group tolerance.

Asymmetric synthesis of 6'-hydroxyarenarol: the proposed biosynthetic precursor to popolohuanone E.

The first synthesis of (+)-6'-hydroxyarenarol 3, the proposed biogenetic precursor to popolohuanone E (1), is described. An enantioselective route to key iodide intermediate 12 has been developed allowing the asymmetric synthesis of the known cis-decalin 22. Conditions which allow the removal of the methyl ether protecting groups on the hydroxyarene leaving the exocyclic methylene moiety in tact have also been developed to complete this synthesis.

Palladium-catalyzed method for the synthesis of carbazoles via tandem C-H functionalization and C-N bond formation.

The development of a new method for the assembly of unsymmetrical carbazoles is reported. The strategy involves the selective intramolecular functionalization of an arene C-H bond and the formation of a new arene C-N bond. The substitution pattern of the carbazole product can be controlled by the design of the biaryl amide substrate, and the method is compatible with a variety of functional groups. The utility of the new protocol was demonstrated by the concise synthesis of three natural products from commercially available materials.

Helicobacter pylori infection in Canada's arctic: searching for the solutions.

The Canadian North Helicobacter pylori (CANHelp) working group is a team composed of investigators, health officials and community leaders from Alberta and the Northwest Territories. The group's initial goals are to investigate the impact of H pylori infection on Canada's Arctic communities; subsequent goals include identifying treatment strategies that are effective in this region and developing recommendations for health policy aimed at management of H pylori infection. The team's investigations have begun with the Aklavik H pylori Project in the Aboriginal community of Aklavik, Northwest Territories.

Complete Genome Sequences of Two Helicobacter pylori Strains from a Canadian Arctic Aboriginal Community.

We report here the complete genome sequences of two Amerind Helicobacter pylori strains from Aklavik, Northwest Territories, Canada. One strain contains extra iron-cofactored urease genes and ~140 rearrangements in its chromosome relative to other described strains (typically differing from one another by <10 rearrangements), suggesting that it represents a novel lineage of H. pylori.

Helicobacter pylori incidence and re-infection in the Aklavik H. pylori Project.

BACKGROUND: The Aklavik H. pylori Project (AHPP) (www.canhelpworkinggroup.ca) is a community-driven project examining Helicobacter pylori infection and its influence on health in a diverse Aboriginal community in the Northwest Territories. Initial research revealed that 58% of 333 participants who underwent a urea breath test (UBT) between 2007 and 2010 were H. pylori-positive. From 2008 to 2010, we offered treatment to H. pylori-positive participants and 113 consented to this treatment. OBJECTIVE: We estimated H. pylori incidence in AHPP participants who initially tested negative and the re-infection frequency in initially positive participants who were successfully treated to clear the infection. METHODS: Participants who were initially H. pylori-negative or negative after treatment during 2008-2010 were eligible for inclusion. From November 2011 to June 2012, participants were offered a UBT and the samples were analyzed using infrared spectroscopy (IRIS). Participants with a positive test result were classified as new cases for estimating incidence among participants testing negative at baseline and re-infection among those successfully treated for H. pylori infection. RESULTS: Among 38 initially negative participants, follow-up UBT showed that 33 remained negative, 3 were positive, and 2 had uncertain status. The estimated incidence proportion during the follow-up period was 8.3% (95% CI: 1.8-22.0%). Among 43 participants with a negative post-treatment UBT, 41 remained negative and 2 were positive. The estimated re-infection proportion during the follow-up period was 4.7% (95% CI: 0.6-16.0%). The frequency of new cases was similar in males and females. Aboriginal participants had a combined re-infection/incidence rate of 2.4% per year (95% CI: 0.8-5.9% per year). All 9 non-Aboriginal participants remained free from infection throughout the study period, as did all 23 participants aged 55 years and above. CONCLUSIONS: The AHPP has substantially reduced the burden of infection in Aklavik since 2008. Continued monitoring, treatment, community engagement and knowledge translation activities are needed to ensure a lasting benefit of the project.

A randomized controlled trial comparing sequential with triple therapy for Helicobacter pylori in an Aboriginal community in the Canadian North.

BACKGROUND: Helicobacter pylori infection occurs more frequently in Arctic Aboriginal settings than elsewhere in North America and Europe. Research aimed at reducing health risks from H pylori infection has been conducted in the Aboriginal community of Aklavik, Northwest Territories. OBJECTIVE: To compare the effectiveness of the Canadian standard therapy with an alternative therapy for eliminating H pylori infection in Aklavik. METHODS: Treatment-naive H pylori-positive individuals were randomly assigned to a 10-day regimen (oral twice-daily doses) with rabeprazole (20 mg): standard triple therapy (proton pump inhibitor, added clarithromycin [500 mg] and amoxicillin [1 g] [PPI-CA]); sequential therapy (ST) added amoxicillin (1 g) on days 1 to 5, and metronidazole (500 mg) and clarithromycin (500 mg) on days 6 to 10. Participants with clarithromycin-resistant H pylori were randomly assigned to ST or quadruple therapy. Treatment effectiveness was estimated as per cent (95% CI) with a negative urea breath test at least 10 weeks after treatment. RESULTS: Of 104 (53 PPI-CA, 51 ST) randomized participants, 89 (49 PPI-CA, 40 ST) had post-treatment results. Per-protocol treatment effectiveness was 59% (95% CI 45% to 73%) for PPI-CA and 73% (95% CI 58% to 87%) for ST. Based on intention to treat, effectiveness was 55% (95% CI 41% to 69%) for PPI-CA and 57% (95% CI 43% to 71%) for ST. Of 77 participants (43 PPI-CA, 34 ST) with 100% adherence, effectiveness was 63% (95% CI 43% to 82%) for PPI-CA and 81% (95% CI 63% to 99%) for ST. CONCLUSIONS: While additional evidence is needed to confirm that ST is more effective for Arctic Aboriginal communities than the Canadian standard H pylori treatment, these results show standard PPI-CA treatment to be inadequate for communities such as Aklavik.

Remote stereocontrol in [3,3]-sigmatropic rearrangements: application to the total synthesis of the immunosuppressant mycestericin G.

The Ireland-Claisen [3,3]-sigmatropic rearrangement has been used to access biologically important ß,ß'-dihydroxy α-amino acids. The rearrangement reported is highly stereoselective and offers excellent levels of remote stereocontrol. This strategy has been used to synthesize the natural immunosuppressant mycestericin G and ent-mycestericin G, allowing for a revision of absolute configuration of this natural product.