Synthesis of 3-aryl-2-phosphinoimidazo[1,2-a]pyridine ligands for use in palladium-catalyzed cross-coupling reactions

3-Aryl-2-phosphinoimidazo[1,2-a]pyridine ligands were synthesized from 2-aminopyridine via two complementary routes. The first synthetic route involves the copper-catalyzed iodine-mediated cyclizations of 2-aminopyridine with arylacetylenes followed by palladium-catalyzed cross-coupling reactions with phosphines. The second synthetic route requires the preparation of 2,3-diiodoimidazo[1,2-a]pyridine or 2-iodo-3-bromoimidazo[1,2-a]pyridine from 2-aminopyridine followed by palladium-catalyzed Suzuki/phosphination or a phosphination/Suzuki cross-coupling reactions sequence, respectively. Preliminary model studies on the Suzuki synthesis of sterically-hindered biaryl and Buchwald–Hartwig amination compounds are presented with these ligands.

3-Aryl-2-phosphoimidazo[1,2-a]pyridine ligands were prepared via two complimentary synthetic routes and were evaluated in the Suzuki–Miyaura and Buchwald–Hartwig amination cross-coupling reactions.

Palladium-catalyzed cross-coupling reactions have revolutionized the formation of C–C and C–X bond formation in the academic and industrial synthetic organic chemistry sectors.^1,2 Applications such as synthesis of natural products,³ active pharmaceutical ingredients (API),⁴ agrochemicals,⁵ and materials for electronic applications⁶ are showcased. Snieckus described in his 2010 Nobel Prize review that privileged ligand scaffolds represented the “third wave” in the cross-coupling reactions where the “first wave” was the investigation of the metal catalyst-the rise of palladium and the “second wave” was the exploration of the organometallic coupling partner.¹ In the last twenty years, it was recognized that the choice of ligand facilitated the oxidative addition and reductive-elimination steps of the catalytic cycle of transition metal-catalyzed cross-coupling reactions, increasing the overall rate of the reaction. For example, bulky trialkylphosphines facilitated the oxidative addition processes of electron-rich, unactivated substrates such as aryl chlorides.^7,8 Sterically demanding ligands also provided enhanced rates of reductive elimination from [(L)_nPd(aryl)(R), R = aryl, amido, phenoxo, etc.] species by alleviation of steric congestion.⁹ Privileged ligands such as Buchwald''s biarylphosphines,^10,11 Fu''s trialkylphosphines,^7,8,12 Nolan–Hermann''s N-heterocyclic carbenes (NHC),^13–15 Hartwig''s ferrocenes,^16,17 Beller''s bis(adamantyl)phosphines^18,19 and N-aryl(benz)imidazolyl or N-pyrrolylphosphines,^20,21 Zhang''s ClickPhos ligands,^22,23 and Stradiotto''s biaryl P–N phosphines,^24,25 to mention a few, have found wide-spread use in Suzuki–Miyaura, Corriu–Kumada, Heck, Negishi, Sonogashira, C–X (X = S, O, P) cross-coupling and Buchwald–Hartwig amination reactions (Fig. 1). Preformed catalysts with these ligands attached to the palladium metal center are also recognized as well-defined entities in cross-coupling reactions.²⁶Open in a separate windowFig. 1Privileged ligands for palladium-catalyzed cross-coupling reactions.The term privileged structure was first coined by Evans et al. in 1988 and was defined as “a single molecular framework able to provide ligands for diverse receptors”.²⁷ In the last three decades, it is clear that privileged structures are exploited as opportunities in drug discovery programs.^28–31 For example, imidazo[1,2-a]pyridines are privileged structures in medicinal chemistry programs (Fig. 2).³² Imidazo[1,2-a]pyridines are a represented motif in several drugs on the market such as zolpidem, marketed as Ambien™ for the treatment of insomnia,³³ minodronic acid, marketed as Bonoteo™ for oral treatment of osteoporosis,³⁴ and olprinone, sold as Coretec™ as a cardiotonic agent.³⁵Open in a separate windowFig. 2Imidazo[1,2-a]pyridines as privileged structures in medicinal chemistry and in our cross-coupling reactions approach.Our group is interested in a long-term research program directed at the use of key privileged structures that are employed in drug discovery programs as potential phosphorus ligands for cross-coupling reactions. In our entry into the use of privileged structures from the medicinal chemistry literature for our investigation into new phosphorus ligands, we have developed two complementary synthetic routes for the preparation of 3-aryl-2-phosphinoimidazo[1,2-a]pyridine ligands from 2-aminopyridine as our initial substrate.Our first synthetic route for the preparation of 3-aryl-2-phosphinoimidazo[1,2-a]pyridine ligands 3a–3l required the copper(ii) acetate iodine-mediated double oxidative C–H amination of 2-aminopyridine (1) with arylacetylenes under an oxygen atmosphere to give 3-aryl-2-iodoimidazo[1,2-a]pyridines 2a–2d (Scheme 1).^36,37Open in a separate windowScheme 1Preparation of 3-aryl-2-phosphinoimidazo[1,2-a]pyridine ligands 3a–3l from 2-aminopyridine via copper-catalyzed arylacetylene cyclizations/palladium-catalyzed phosphination reactions sequences.Phenylacetylene and 2-/3-/4-methoxyphenylacetylenes were commercially available reagents. With intermediates 2a–d in hand, we explored several cross-coupling phosphination reactions and we found that palladium-catalyzed phosphination with DIPPF ligand in the presence of cesium carbonate as the base in 1,4-dioxane under reflux provided twelve new ligands 3a–3l as shown in 38 Moderate to good yields were obtained under these cross-coupling conditions. There are few commercially available dimethoxyphenylacetylenes, and most are prohibitively expensive, and so an alternative synthetic strategy was explored.Palladium-catalyzed phosphination of 3-aryl-2-iodoimidazo[1,2-a]pyridines 2a–2d^a

Photosynthetic reaction center variants made via genetic code expansion show Tyr at M210 tunes the initial electron transfer mechanism

Photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides were engineered to vary the electronic properties of a key tyrosine (M210) close to an essential electron transfer component via its replacement with site-specific, genetically encoded noncanonical amino acid tyrosine analogs. High fidelity of noncanonical amino acid incorporation was verified with mass spectrometry and X-ray crystallography and demonstrated that RC variants exhibit no significant structural alterations relative to wild type (WT). Ultrafast transient absorption spectroscopy indicates the excited primary electron donor, P*, decays via a ∼4-ps and a ∼20-ps population to produce the charge-separated state P⁺H_A⁻ in all variants. Global analysis indicates that in the ∼4-ps population, P⁺H_A⁻ forms through a two-step process, P*→ P⁺B_A⁻→ P⁺H_A⁻, while in the ∼20-ps population, it forms via a one-step P* → P⁺H_A⁻ superexchange mechanism. The percentage of the P* population that decays via the superexchange route varies from ∼25 to ∼45% among variants, while in WT, this percentage is ∼15%. Increases in the P* population that decays via superexchange correlate with increases in the free energy of the P⁺B_A⁻ intermediate caused by a given M210 tyrosine analog. This was experimentally estimated through resonance Stark spectroscopy, redox titrations, and near-infrared absorption measurements. As the most energetically perturbative variant, 3-nitrotyrosine at M210 creates an ∼110-meV increase in the free energy of P⁺B_A⁻ along with a dramatic diminution of the 1,030-nm transient absorption band indicative of P⁺B_A^– formation. Collectively, this work indicates the tyrosine at M210 tunes the mechanism of primary electron transfer in the RC.

Photosynthetic reaction centers (RCs) are the integral membrane protein assemblies responsible for nearly all the solar energy conversion maintaining our biosphere. In this study, we focus on the initial electron transfer (ET) steps in bacterial RCs from Rhodobacter sphaeroides, a three-subunit (H, L, and M) ∼100-kDa integral membrane protein complex. RCs in R. sphaeroides possess two branches of chromophores, the A and B (or L and M) branches (Fig. 1A), and each possesses nearly identical chromophore composition, orientation, and distances. The protein secondary structure is pseudo-C2 symmetric, and the symmetry-related amino acids that differ are often structurally similar (Fig. 1A) (1). Despite this high structural symmetry, ET proceeds rapidly down only the A branch of chromophores with near-unity quantum yield (2, 3). Additionally, RC ET is remarkably robust, as few structurally verified single mutations that maintain RC chromophore composition and positioning significantly impact ET kinetics or yield (1, 4–11).Open in a separate windowFig. 1.RC chromophore arrangement and energetics. (A) Chromophore arrangement in WT RCs (PDB ID: 2J8C; accessory carotenoid, chromophore phytyl tails, and quinone isoprenoid tails are removed here for clarity). Tyr at M210, the target in this work, and its symmetry-related residue Phe at L181 are shown in purple. The blue arrow indicates unidirectionality of ET down the A branch. (B) Schematic free-energy diagram of different charge-separated states in WT RCs, where P* is 1.40 eV above ground state and P⁺H_A^– is 0.25 eV below P*. The dashed magenta line and double-headed arrow next to P⁺B_A^– indicates the expected major effect of ncAA incorporation at M210 on the free energy of this state.To understand RC ET asymmetry or unidirectionality and factors underlying its robust nature, a thorough understanding of the mechanism of ET is required. In the model largely accepted in the current literature (1, 12–14), ET is initiated by excitation of the excitonically coupled bacteriochlorophyll pair P. The lowest singlet excited state P* transfers an electron to the bacteriochlorophyll B_A with a time constant of ∼3 ps to form P⁺B_A^–. B_A^– subsequently transfers an electron to the bacteriopheophytin H_A with a time constant of 1 ps, thus forming P⁺H_A^– in a two-step primary ET process, P* → P⁺B_A^– → P⁺H_A^– (1, 12, 13). An alternative model for ET has been proposed in which P* transfers an electron to H_A directly through a superexchange mechanism, as defined by Parson et al. (1). Here, the B_A chromophore mediates the electronic coupling between P* and H_A, and experimental evidence for superexchange ET must be inferred spectroscopically from the absence of P⁺B_A^– formation during transient absorption (TA) measurements and generally slower ET. In wild-type (WT) RCs, evidence favors two-step ET at room temperature (1, 12, 13, 15, 16). It has been previously proposed that minor degrees of superexchange occur in WT RCs, likely arising from or enhanced by the inherent distribution in the energies of P*, P⁺B_A^–, and P⁺H_A^– caused by protein populations with slight variations in amino acid nuclear coordinates around chromophores (5, 17–19), but this has been difficult to study experimentally (1, 12).The ET mechanisms in RCs likely have their origins in the different energies of the various charge-separated states for the two branches relative to P* and each other (Fig. 1B) (20–22), but it is difficult to determine these energetics either experimentally or theoretically (1, 23). Contributions of individual symmetry-breaking amino acid have been thoroughly studied (1, 15, 24–28), and while the importance of certain amino acids has been ascertained, the roles of local protein–chromophore interactions are not always fully understood (8, 24, 25). One highly examined residue has been the tyrosine at site M210 (RC residue numbers are preceded by the protein subunit designation: H, L, or M) because it is a clear deviation in symmetry between A and B branches (Fig. 1A), it is close to B_A, and it is the only one of 27 tyrosines that lacks a hydrogen bond acceptor. Theoretical studies indicate that the magnitude and orientation of the hydroxyl dipole of tyrosine M210 may play an important role in energetically stabilizing P⁺B_A^– (29, 30). Indeed, previous efforts to change the orientation of this tyrosine’s hydroxyl dipole significantly slowed ET (31). It is difficult, however, to subtly vary the electrostatic nature of this tyrosine using canonical mutagenesis without entirely removing the phenolic hydroxyl.To perturb the effects of the tyrosine at M210 in WT protein, we used amber stop codon suppression (32–34) to site-specifically replace it with five noncanonical amino acids (ncAAs), each a tyrosine with a single electron-donating or electron-withdrawing meta-substituent (at the 3 position). We will refer to RC protein variants by acronyms for the amino acid incorporated at M210: 3-methyltyrosine (MeY), 3-nitrotyrosine (NO₂Y), 3-chlorotyrosine (ClY), 3-bromotyrosine (BrY), and 3-iodotyrosine (IY) (Fig. 2 and SI Appendix, Fig. S1 and Table S1). In this way, we engineered a series of RC variants with more systematic electrostatic ET perturbation at this important tyrosine while minimally affecting other RC features.Open in a separate windowFig. 2.RC variants made and structurally characterized in this study, in which truncated P_A and B_A chromophores are depicted for each variant (in teal and magenta, respectively) and electron density maps from solved X-ray structures are shown (2F_o-F_c contoured at 1 σ) for tyrosine analogs at M210. Halogen variants required two different tyrosine ring conformers to model halogen substituent orientations with the contribution of each indicated, one with the halogen oriented toward P (only P_A depicted above) and the other with halogen toward B_A. The resolution for each crystal structure is denoted in black next to the P_A of each RC variant (PDB IDs for NO₂Y, MeY, ClY, BrY, and IY RCs are 7MH9, 7MH8, 7MH3, 7MH4, and 7MH5, respectively; SI Appendix, Table S2).     

Effect of Cardiorespiratory Fitness on Academic Achievement is Stronger in High‐SES Elementary Schools Compared to Low

BACKGROUND

Academic achievement is influenced by factors at the student, school, and community levels. We estimated the effect of cardiorespiratory fitness performance on academic performance at the school level in Georgia elementary schools and examined effect modification by sociodemographic factors.

METHODS

This study is a repeat cross‐sectional analysis of Georgia elementary schools between 2011 and 2014 (approximately 1138 schools per year). Multivariable beta regression estimated the effect of the proportion of 4th and 5th graders meeting cardiorespiratory fitness standards on the proportion of 5th graders passing standardized tests for Reading, English and Language Arts, Mathematics, Science, and Social Studies and considered potential interaction by school‐level socioeconomic status (SES), racial composition, and urbanity.

RESULTS

There was a 0.15 higher estimated odds (OR: 1.15 (1.09, 1.22)) of passing the mathematics standardized test for every 10‐percentage‐point increase in school‐level cardiorespiratory fitness among high‐SES schools and 0.04 higher odds (OR: 1.04 (1.02, 1.05)) for low‐SES schools. This pattern was similar for other academic subjects. No effect modification by racial composition or urbanity was observed for any academic subject.

CONCLUSIONS

Promoting physical fitness may be effective in improving academic performance among high‐SES schools, but additional strategies may be needed among lower‐SES schools.

     

In situ vaccination with laser interstitial thermal therapy augments immunotherapy in malignant gliomas

Journal of Neuro-Oncology - Laser interstitial thermal therapy (LITT) remains a promising advance in the treatment of primary central nervous system malignancies. As indications for its use...     

RT-PCR Increases Detection of Submicroscopic Peritoneal Metastases in Gastric Cancer and Has Prognostic Significance

Background  

Positive peritoneal cytology confers the same prognosis as clinical stage IV disease in gastric cancer. Conventional cytology examination, however, has low sensitivity. We hypothesize that real-time polymerase chain reaction (RT-PCR) may have increased sensitivity and provide more accurate staging information.