Chiral π-Conjugated Double Helical Aminyl Diradical with the Triplet Ground State.

We report a neutral high-spin diradical of chiral C2-symmetric bis[5]diazahelicene with ΔEST ≈ 0.4 kcal mol-1, as determined by EPR spectroscopy/SQUID magnetometry. The diradical is the most persistent among all high-spin aminyl radicals reported to date by a factor of 20, with a half-life of up to 6 days in 2-MeTHF at room temperature. Its triplet ground state and excellent persistence may be associated with the unique spin density distribution within the dihydrophenazine moiety, which characterizes two effective 3-electron C-N bonds analogous to the N-O bond of a nitroxide radical. The enantiomerically enriched (ee ≥ 94%) (MM)- and (PP)-enantiomers of the precursors to the diradicals are obtained by either preparative chiral supercritical fluid chromatography or resolution via functionalization with the chiral auxiliary of the C2-symmetric racemic tetraamine. The barrier for the racemization of the solid tetraamine is ΔG‡ = 43 ± 0.01 kcal mol-1 in the 483-523 K range. The experimentally estimated lower limit of the barrier for the racemization of a diradical, ΔG‡ ≥ 26 kcal mol-1 in 2-MeTHF at 293 K, is comparable to the DFT-determined barrier of ΔG‡ = 31 kcal mol-1 in the gas phase at 298 K. While the enantiomerically pure tetraamine displays strong chiroptical properties, with anisotropy factor |g| = |Δε|/ε = 0.036 at 376 nm, |g| ≈ 0.005 at 548 nm of the high-spin diradical is comparable to that recently reported triplet ground-state diradical dication. Notably, the radical anion intermediate in the generation of diradical exhibits a large SOMO-HOMO inversion, SHI = 35 kcal mol-1.

Information-Rich, Dual-Function 13C/2H-Isotopic Crosstalk NMR Assay for Human Serine Racemase (hSR) Provides a PLP-Enzyme "Partitioning Fingerprint" and Reveals Disparate Chemotypes for hSR Inhibition.

The first dual-function assay for human serine racemase (hSR), the only bona fide racemase in human biology, is reported. The hSR racemization function is essential for neuronal signaling, as the product, d-serine (d-Ser), is a potent N-methyl d-aspartate (NMDA) coagonist, important for learning and memory, with dysfunctional d-Ser-signaling being observed in some neuronal disorders. The second hSR function is ß-elimination and gives pyruvate; this activity is elevated in colorectal cancer. This new NMR-based assay allows one to monitor both α-proton-exchange chemistry and ß-elimination using only the native l-Ser substrate and hSR and is the most sensitive such assay. The assay judiciously employs segregated dual 13C-labeling and 13C/2H crosstalk, exploiting both the splitting and shielding effects of deuterium. The assay is deployed to screen a 1020-compound library and identifies an indolo-chroman-2,4-dione inhibitor family that displays allosteric site binding behavior (noncompetitive inhibition vs l-Ser substrate; competitive inhibition vs adenosine 5'-triphosphate (ATP)). This assay also reveals important mechanistic information for hSR; namely, that H/D exchange is ∼13-fold faster than racemization, implying that K56 protonates the carbanionic intermediate on the si-face much faster than does S84 on the re-face. Moreover, the 13C NMR peak pattern seen is suggestive of internal return, pointing to K56 as the likely enamine-protonating residue for ß-elimination. The 13C/2H-isotopic crosstalk assay has also been applied to the enzyme tryptophan synthase and reveals a dramatically different partition ratio in this active site (ß-replacement: si-face protonation ∼6:1 vs ß-elimination: si-face protonation ∼1:3.6 for hSR), highlighting the value of this approach for fingerprinting the pyridoxal phosphate (PLP) enzyme mechanism.

Postsynthetic Modification of the Nonanuclear Node in a Zirconium Metal-Organic Framework for Photocatalytic Oxidation of Hydrocarbons.

Heterogeneous catalysis plays an indispensable role in chemical production and energy conversion. Incorporation of transition metals into metal oxides and zeolites is a common strategy to fine-tune the activity and selectivity of the resulting solid catalysts, as either the active center or promotor. Studying the underlying mechanism is however challenging. Decorating the metal-oxo clusters with transition metals in metal-organic frameworks (MOFs) via postsynthetic modification offers a rational approach to construct well-defined structural models for better understanding of the reaction mechanism. Therefore, it is important to expand the materials scope beyond the currently widely studied zirconium MOFs consisting of Zr6 nodes. In this work, we report the design and synthesis of a new (4,12)-connected Zr-MOF with ith topology that consists of rare Zr9 nodes. FeIII was further incorporated onto the Zr9 nodes of the framework, and the resulting MOF material exhibits significantly enhanced activity and selectivity toward the photocatalytic oxidation of toluene. This work demonstrates a delicate ligand design strategy to control the nuclearity of Zr-oxo clusters, which further dictates the number and binding sites of transition metals and the overall photocatalytic activity toward C-H activation. Our work paves the way for future exploration of the structure-activity study of catalysts using MOFs as the model system.

Human Serum Alters the Metabolism and Antibiotic Susceptibility of Staphylococcus aureus.

Staphylococcus aureus is a common source of hospital-acquired bacterial infections, where the emergence of antibiotic resistance is a serious human health concern. Most investigations into S. aureus virulence and antibiotic resistance have relied on in vitro cultivation conditions and optimized media formulations. However, S. aureus can survive and adapt to a hostile host environment or antibiotic treatments by rapidly adjusting its metabolic activity. To assess this metabolic response, S. aureus strains susceptible and nonsusceptible to daptomycin were cultivated in medium supplemented with 55% serum to more closely approximate in vivo conditions. Growth analyses, MIC testing, and NMR-based metabolomics determined that serum decreased daptomycin susceptibility and altered metabolism in S. aureus. Both S. aureus strains exhibited altered amino acid biosynthesis and catabolism, enhanced fermentation, and a modified salt tolerance response. The observation that growth conditions defined an adaptive metabolic response to antibiotics by S. aureus may be a critical consideration for designing an effective drug discovery study.

Leveraging the HMBC to Facilitate Metabolite Identification.

The accuracy and ease of metabolite assignments from a complex mixture are expected to be facilitated by employing a multispectral approach. The two-dimensional (2D) 1H-13C heteronuclear single quantum coherence (HSQC) and 2D 1H-1H-total correlation spectroscopy (TOCSY) are the experiments commonly used for metabolite assignments. The 2D 1H-13C HSQC-TOCSY and 2D 1H-13C heteronuclear multiple-bond correlation (HMBC) are routinely used by natural products chemists but have seen minimal usage in metabolomics despite the unique information, the nearly complete 1H-1H and 1H-13C and spin systems provided by these experiments that may improve the accuracy and reliability of metabolite assignments. The use of a 13C-labeled feedstock such as glucose is a routine practice in metabolomics to improve sensitivity and to emphasize the detection of specific metabolites but causes severe artifacts and an increase in spectral complexity in the HMBC experiment. To address this issue, the standard HMBC pulse sequence was modified to include carbon decoupling. Nonuniform sampling was also employed for rapid data collection. A dataset of reference 2D 1H-13C HMBC spectra was collected for 94 common metabolites. 13C-13C spin connectivity was then obtained by generating a covariance pseudo-spectrum from the carbon-decoupled HMBC and the 1H-13C HSQC-TOCSY spectra. The resulting 13C-13C pseudo-spectrum provides a connectivity map of the entire carbon backbone that uniquely describes each metabolite and would enable automated metabolite identification.

Evidence for Proline Catabolic Enzymes in the Metabolism of Thiazolidine Carboxylates.

Thiazolidine carboxylates such as thiazolidine-4-carboxylate (T4C) and thiazolidine-2-carboxylate (T2C) are naturally occurring sulfur analogues of proline. These compounds have been observed to have both beneficial and toxic effects in cells. Given that proline dehydrogenase has been proposed to be a key enzyme in the oxidative metabolism of thioprolines, we characterized T4C and T2C as substrates of proline catabolic enzymes using proline utilization A (PutA), which is a bifunctional enzyme with proline dehydrogenase (PRODH) and l-glutamate-γ-semialdehyde dehydrogenase (GSALDH) activities. PutA is shown here to catalyze the FAD-dependent PRODH oxidation of both T4C and T2C with catalytic efficiencies significantly higher than with proline. Stopped-flow experiments also demonstrate that l-T4C and l-T2C reduce PutA-bound FAD at rates faster than proline. Unlike proline, however, oxidation of T4C and T2C does not generate a substrate for NAD+-dependent GSALDH. Instead, PutA/PRODH oxidation of T4C leads to cysteine formation, whereas oxidation of T2C generates an apparently stable Δ4-thiazoline-2-carboxylate species. Our results provide new insights into the metabolism of T2C and T4C.

Cautionary Tale of Using Tris(alkyl)phosphine Reducing Agents with NAD+-Dependent Enzymes.

Protein biochemistry protocols typically include disulfide bond reducing agents to guard against unwanted thiol oxidation and protein aggregation. Commonly used disulfide bond reducing agents include dithiothreitol, ß-mercaptoethanol, glutathione, and the tris(alkyl)phosphine compounds tris(2-carboxyethyl)phosphine (TCEP) and tris(3-hydroxypropyl)phosphine (THPP). While studying the catalytic activity of the NAD(P)H-dependent enzyme Δ1-pyrroline-5-carboxylate reductase, we unexpectedly observed a rapid non-enzymatic chemical reaction between NAD+ and the reducing agents TCEP and THPP. The product of the reaction exhibits a maximum ultraviolet absorbance peak at 334 nm and forms with an apparent association rate constant of 231-491 M-1 s-1. The reaction is reversible, and nuclear magnetic resonance characterization (1H, 13C, and 31P) of the product revealed a covalent adduct between the phosphorus of the tris(alkyl)phosphine reducing agent and the C4 atom of the nicotinamide ring of NAD+. We also report a 1.45 Å resolution crystal structure of short-chain dehydrogenase/reductase with the NADP+-TCEP reaction product bound in the cofactor binding site, which shows that the adduct can potentially inhibit enzymes. These findings serve to caution researchers when using TCEP or THPP in experimental protocols with NAD(P)+. Because NAD(P)+-dependent oxidoreductases are widespread in nature, our results may be broadly relevant.

Phosphorus NMR and Its Application to Metabolomics.

Stable isotopes are routinely employed by NMR metabolomics to highlight specific metabolic processes and to monitor pathway flux. 13C-carbon and 15N-nitrogen labeled nutrients are convenient sources of isotope tracers and are commonly added as supplements to a variety of biological systems ranging from cell cultures to animal models. Unlike 13C and 15N, 31P-phosphorus is a naturally abundant and NMR active isotope that does not require an external supplemental source. To date, 31P NMR has seen limited usage in metabolomics because of a lack of reference spectra, difficulties in sample preparation, and an absence of two-dimensional (2D) NMR experiments, but 31P NMR has the potential of expanding the coverage of the metabolome by detecting phosphorus-containing metabolites. Phosphorylated metabolites regulate key cellular processes, serve as a surrogate for intracellular pH conditions, and provide a measure of a cell's metabolic energy and redox state, among other processes. Thus, incorporating 31P NMR into a metabolomics investigation will enable the detection of these key cellular processes. To facilitate the application of 31P NMR in metabolomics, we present a unified protocol that allows for the simultaneous and efficient detection of 1H-, 13C-, 15N-, and 31P-labeled metabolites. The protocol includes the application of a 2D 1H-31P HSQC-TOCSY experiment to detect 31P-labeled metabolites from heterogeneous biological mixtures, methods for sample preparation to detect 1H-, 13C-, 15N-, and 31P-labeled metabolites from a single NMR sample, and a data set of one-dimensional (1D) 31P NMR and 2D 1H-31P HSQC-TOCSY spectra of 38 common phosphorus-containing metabolites to assist in metabolite assignments.

Preparation of Some Homologous TEMPO Nitroxides and Oxoammonium Salts; Notes on the NMR Spectroscopy of Nitroxide Free Radicals; Observed Radical Nature of Oxoammonium Salt Solutions Containing Trace Amounts of Corresponding Nitroxides in an Equilibrium Relationship.

Three new homologous TEMPO oxoammonium salts and three homologous nitroxide radicals have been prepared and characterized. The oxidation properties of the salts have been explored. The direct 13C NMR and EPR spectra of the nitroxide free radicals and the oxoammonium salts, along with TEMPO and its oxoammonium salt, have been successfully measured with little peak broadening of the NMR signals. In the spectra of all ten compounds (nitroxides and corresponding oxoammonium salts), the carbons in the 2,2,6,6-tetramethylpiperidine core do not appear, implying paramagnetic properties. This unpredicted overall paramagnetism in the oxoammonium salt solutions is explained by a redox equilibrium as shown between oxoammonium salts and trace amounts of corresponding nitroxide. This equilibrium is confirmed by electron interchange reactions between nitroxides with an N-acetyl substituent and oxoammonium salts with longer acyl side chains.

Thiophene-Based Double Helices: Syntheses, X-ray Structures, and Chiroptical Properties.

We demonstrate facile and efficient construction of conjugated double helical ladder oligomers from the saddle-shaped cyclooctatetrathiophene (COTh) building blocks. The key step involves deprotonation of tetra[3,4]thienylene (ß,ß-COTh) with n-BuLi which displays remarkably high ipsilateral selectivity. Three racemic double helical ladder oligomers, rac-DH-1, rac-DH-2, and rac-DH-3, containing two, three, and five COTh annelated moieties are efficiently synthesized by diastereoselective coupling of the racemic precursors. The X-ray crystallographic studies of rac-DH-1, rac-DH-2 and rac-DH-3 unambiguously revealed that each double helical scaffold has two single helices intertwined with each other via the C-C single bonds. Following removal of TMS groups, double helical ladder oligomer rac-DH-1-D had sufficient solubility to be resolved via chiral HPLC, thus enabling determination of its chirooptical properties such as CD spectra and optical rotation. (+)-DH-1-D has a large barrier for racemization, with lower limit of ΔG(‡) > 48 kcal mol(-1), which may be compared to DFT-computed barrier of 51 kcal mol(-1). The enantiomers of DH-1-D show 1 order of magnitude stronger chirooptical properties than the carbon-sulfur [7]helicene, as determined by the anisotropy factor g = Δε/ε = -0.039, based on Δεmax = -11 and ε = 2.8 × 10(2) L mol(-1) cm(-1) in cyclohexane at 327 nm.

In Vitro Fermentation of Animal and Plant Protein Isolates by the Human Gut Microbiota Under High and Low Carbohydrate Conditions.

SCOPE: There is a lack of research comparing how different protein isolates influence the microbiome, especially when carbohydrate (CHO) availability is varied. The objective is to determine changes in gut microbiota composition and function during fermentation of digested protein isolates under high and low CHO conditions. METHODS AND RESULTS: Protein isolates from beef, egg white, milk, pea, and soy are subjected to in vitro digestion and fermentation with human fecal microbiota. Under low CHO conditions, the microbiota is primarily proteolytic with decreased concentrations of peptides and increased variance among microbial taxa and production of ammonia and branched chain fatty acids by the microbiota. Milk protein not only results in the highest production of butyrate and p-hydroxyphenylacetate but also has high concentrations of deleterious fermentation metabolites. Amino acid composition of the protein isolates is significantly correlated with abundances of many microbial taxa and metabolites, but the correlations are stronger in the low CHO medium. CONCLUSION: This study shows that low CHO conditions increase proteolytic fermentation and result in increased differences in microbiota response to protein isolates. It also showed that amino acid composition is highly associated with microbiota composition and function especially under low CHO conditions.

Stability of N-glycosidic bond of (5'S)-8,5'-cyclo-2'-deoxyguanosine.

8,5'-Cyclopurine deoxynucleosides are unique tandem lesions containing an additional covalent bond between the base and the sugar. These mutagenic and genotoxic lesions are repaired only by nucleotide excision repair. The N-glycosidic (or C1'-N9) bond of 2'-deoxyguanosine (dG) derivatives is usually susceptible to acid hydrolysis, but even after cleavage of this bond of the cyclopurine lesions, the base would remain attached to the sugar. Here, the stability of the N-glycosidic bond and the products formed by formic acid hydrolysis of (5'S)-8,5'-cyclo-2'-deoxyguanosine (S-cdG) were investigated. For comparison, the stability of the N-glycosidic bond of 8,5'-cyclo-2',5'-dideoxyguanosine (ddcdG), 8-methyl-2'-deoxyguanosine (8-Me-dG), 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-Oxo-dG), and dG was also studied. In various acid conditions, S-cdG and ddcdG exhibited similar stability to hydrolysis. Likewise, 8-Me-dG and dG showed comparable stability, but the half-lives of the cyclic dG lesions were at least 5-fold higher than those of dG or 8-Me-dG. NMR studies were carried out to investigate the products formed after the cleavage of the C1'-N9 bond. 2-Deoxyribose generated α and ß anomers of deoxyribopyranose and deoxyribopyranose oligomers following acid treatment. S-cdG gave α- and ß-deoxyribopyranose linked guanine as the major products, but α and ß anomers of deoxyribofuranose linked guanine and other products were also detected. The N-glycosidic bond of 8-Oxo-dG was found exceptionally stable in acid. Computational studies determined that both the protonation of the N7 atom and the rate constant in the bond breaking step control the overall kinetics of hydrolysis, but both varied for the molecules studied indicating a delicate balance between the two steps. Nevertheless, the computational approach successfully predicted the trend observed experimentally. For 8-Oxo-dG, the low pK(a) of O(8) and N3 prevented appreciable protonation, making the free energy for N-glycosidic bond cleavage in the subsequent step very high.

meso-Tetrakis(pentafluorophenyl)porphyrin-derived chromene-annulated chlorins.

The synthesis of mono- and bis-chromene-annulated meso-(pentafluorophenyl)chlorins from meso-tetrakis(pentafluorophenyl)porphyrins by an OsO(4)-mediated dihydroxylation reaction, followed by an intramolecular nucleophilic aromatic substitution reaction, is described. The reaction sequence is applicable to the free base systems as well as their Zn(II), Ni(II), Pd(II), and Pt(II) complexes. The optical properties (UV-vis and fluorescence spectra) of the (metallo)chlorin-like chromophores that possess slightly red-shifted optical spectra compared to the corresponding 2,3-dihydroxychlorins are reported. Molecular modeling and (1)H-(19)F-HOESY NMR spectroscopy provide indications for the conformation of the chromene-annulated chromophores. Using (1)H-(1)H COSY and (19)F-(19)F QF-COSY NMR spectra, we interpret the (1)H and (19)F NMR spectra of the porphyrins and chlorins, thus providing a refined reference point for the use of (19)F NMR spectroscopy as a diagnostic tool in the analysis of meso-pentafluorophenyl-substituted porphyrinoids.

Toward a formal synthesis of laureatin: unexpected rearrangements involving cyclic ether nucleophiles.

Laureatin, a metabolite of the red algae Laurencia nipponica, has shown potent activity as a mosquito larvicide. The two previously published syntheses of laureatin involved an initial preparation of the 8-membered cyclic ether, followed by formation of the oxetane ring. Our strategy was the reverse, i.e., to utilize an oxetane as the framework to construct the larger ring. During this work, attempted N-bromosuccinimide (NBS)-mediated cyclization of oxetane alcohol 17, prepared from readily accessible 2-methyleneoxetane 12, yielded epoxytetrahydrofuran 19 rather than the expected laureatin core. Further derivatization of 19 yielded trans fused bis-tetrahydrofuran 32. The synthesis of 19 and 32, as well as structural and stereochemical elucidation studies, are described.

Synthesis of 3-guaninyl- and 3-adeninyl-5-hydroxymethyl-2-pyrrolidinone nucleosides.

L- and D-glutamic acids, as well as trans-4-hydroxy-L-proline, are converted to the corresponding 3-guaninyl-5-hydroxymethyl-2-pyrrolidinone (4) or 3-adeninyl-5-hydroxymethyl-2-pyrrolidinone (5) nucleoside analog. The protecting group used to block the lactam nitrogen in key intermediates has a significant effect on the diastereoselectivity of the coupling reaction with adenine or guanine.

Access to oxetane-containing psico-nucleosides from 2-methyleneoxetanes: a role for neighboring group participation?

The first psico-oxetanocin analogue of the powerful antiviral natural product, oxetanocin A, has been readily synthesized from cis-2-butene-1,4-diol. Key 2-methyleneoxetane precursors were derived from ß-lactones prepared by the carbonylation of epoxides. F(+)-mediated nucleobase incorporation provided the corresponding nucleosides in good yield but with low diastereoselectivity. Surprisingly, attempted exploitation of anchimeric assistance to increase the selectivity was not fruitful. A range of 2-methyleneoxetane and related 2-methylenetetrahydrofuran substrates was prepared to explore the basis for this. With one exception, these substrates also showed little stereoselectivity in nucleobase incorporation. Computational studies were undertaken to examine if neighboring group participation involving fused [4.2.0] or [4.3.0] intermediates is favorable.

Metabolic profiling of historical and modern wheat cultivars using proton nuclear magnetic resonance spectroscopy.

To determine changes in the grain components between historical and modern wheat (Triticum aestivum L.) cultivars, wholemeal flours from 19 wheat cultivars and 2 landraces released or introduced between 1870 and 2013 and grown over two crop years were extracted using hydroalcoholic solution and analyzed using one dimensional 1H NMR spectral profiling. Grain yield, grain volume weight (GVW), and grain protein concentration were also measured. Grain yield increased while protein concentration decreased by release year (p < 0.001). Increasing trends (p < 0.01) were observed for tryptophan, sum of the measured amino acids, chlorogenic acid, ferulic acid, vanillic acid, and sum of the measured phenolic acids. Grain yield, phenolic acids, and tryptophan were mainly associated with modern cultivars, whereas grain protein concentration and GVW were associated with historical cultivars. The findings from this study showed changes in concentration of grain components over a century of breeding that may have implications for grain quality and human health.

Identification of the Biosynthetic Gene Cluster for the anti-MRSA Lysocins through Gene Cluster Activation Using Strong Promoters of Housekeeping Genes and Production of New Analogs in Lysobacter sp. 3655.

The Gram-negative gliding bacteria Lysobacter represent a new and rich source for bioactive natural products. In an effort to discover new antibiotics, we found a cryptic biosynthetic gene cluster (BGC) in Lysobacter sp. 3655 that shared a high similarity with the putative lysocin BGC identified in silico previously from Lysobacter sp. RH2180-5. Lysocins are cyclic lipodepsipeptides with potent activity against MRSA (methicillin-resistant Staphylococcus aureus) using a novel mode of action, but the lysocin BGC had not been experimentally verified so far. Using an activity-guided screening, we isolated the main antibiotic compound and confirmed it to be lysocin E. However, the putative lysocin BGC was barely transcribed in the wild type, in which lysocins were produced only in specific conditions and in a negligible amount. To activate the putative lysocin BGC, we screened for strongly transcribed housekeeping genes in strain 3655 and found several powerful promoters. Upon engineering the promoters into the BGC, the lysocin gene transcription was significantly enhanced and the lysocin yield was markedly increased. With readily detectable lysocins production in the engineered strains, we showed that lysocin production was abolished in the gene deletion mutant and then restored in the complementary strain, even when grown in conditions that did not support the wild type for lysocin production. Moreover, the engineered strain produced multiple new lysocin congeners. The determination of the lysocin BGC and the Lysobacter promoters will facilitate the ongoing efforts for yield improvement and new antibiotic biosynthesis using synthetic biology strategies.

Facile access to functionalized chiral secondary benzylic boronic esters via catalytic asymmetric hydroboration.

Allylic and homoallylic phosphonates bearing an aryl or heteroaryl substituent at the γ- or δ-position undergo rhodium-catalyzed asymmetric hydroboration by pinacolborane to give functionalized chiral secondary benzylic boronic esters in yields up to 86% and enantiomer ratios up to 99 : 1. Compared to minimally-functionalized terminal and 1,1-disubstituted vinyl arenes, there are relatively few reports of efficient catalytic asymmetric hydroboration (CAHB) of more highly functionalized internal alkenes. Phosphonate substrates bearing a variety of common heterocyclic ring systems, including furan, indole, pyrrole and thiophene derivatives, as well as those bearing basic nitrogen substituents (e.g., morpholine and pyrazine) are tolerated, although donor substituents positioned in close proximity of the alkene can influence the course of the reaction. Stereoisomeric (E)- and (Z)-substrates afford the same major enantiomer of the borated product. Deuterium-labelling studies reveal that rapid (Z)- to (E)-alkene isomerization accounts for the observed (E/Z)-stereoconvergence during CAHB. The synthetic utility of the chiral boronic ester products is illustrated by stereospecific C-B bond transformations including stereoretentive electrophile promoted 1,2-B-to-C migrations, stereoinvertive SE2 reactions of boron-ate complexes with electrophiles, and stereoretentive palladium- and rhodium-catalyzed cross-coupling protocols.