Chemical Tagging of Bioactive Amides by Cooperative Catalysis: Applications in the Syntheses of Drug Conjugates

We disclose a practical catalytic method for arming bioactive amide-based natural products and other small-molecule drugs with various functional handles for the synthesis of drug conjugates. We demonstrate that a set of readily available Sc-based Lewis acids and N-based Brønsted bases can function cooperatively to deprotonate amide N-H bonds in polyfunctional drug molecules. An aza-Michael reaction between the resulting amidate and α,β-unsaturated compounds produces an array of drug analogues that are equipped with an alkyne, azide, maleimide, tetrazine, or diazirine moiety under redox and pH-neutral conditions. The utility of this chemical tagging strategy is showcased through the production of drug conjugates by the click reaction between the alkyne-tagged drug derivatives and an azide-containing green fluorescent protein, nanobody, or antibody.

Enantioselective Organocopper-Catalyzed Hetero Diels-Alder Reaction through in Situ Oxidation of Ethers into Enol Ethers

We disclose a catalytic method for the enantio- and diastereoselective union of alkyl ethers and heterodienes. We demonstrate that a chiral Cu-BOX complex catalyzes the efficient oxidation of ethers into enol ethers in the presence of trityl acetate. Then, the organocopper promotes stereoselective hetero Diels-Alder reaction between the in situ generated enol ethers and β,γ-unsaturated ketoesters, allowing for rapid access to an array of dihydropyran derivatives possessing three vicinal stereogenic centers.

Enantioselective Synthesis of N-Alkylamines through β-Amino C-H Functionalization Promoted by Cooperative Actions of B(C6F5)3 and a Chiral Lewis Acid Co-Catalyst

We disclose a catalytic method for β-C(sp³)-H functionalization of N-alkylamines for the synthesis of enantiomerically enriched β-substituted amines, entities prevalent in pharmaceutical compounds and used to generate different families of chiral catalysts. We demonstrate that a catalyst system comprising of seemingly competitive Lewis acids, B(C₆F₅)₃, and a chiral Mg- or Sc-based complex, promotes the highly enantioselective union of N-alkylamines and α,β-unsaturated compounds. An array of δ-amino carbonyl compounds was synthesized under redox-neutral conditions by enantioselective reaction of a N-alkylamine-derived enamine and an electrophile activated by the chiral Lewis acid co-catalyst. The utility of the approach is highlighted by late-stage β-C-H functionalization of bioactive amines. Investigations in regard to the mechanistic nuances of the catalytic processes are described.

Sequential Conia-ene-type cyclization and Negishi coupling by cooperative functions of B(C6F5)3, ZnI2, Pd(PPh3)4 and an amine

We disclose a method for sequential Conia-ene-type cyclization/Negishi coupling for the union of alkynyl ketones and aryl iodides. This process is promoted through cooperative actions of Lewis acidic B(C6F5)3, ZnI2, Pd-based complex, and a Brønsted basic amine. The three Lewis acid catalysts with potential overlapping functions play their independent roles as activators of carbonyl group, alkyne moiety, and alkenyl zinc intermediate, respectively. A variety of 1,2,3-substituted cyclopentenes can be synthesized with high efficiency.

Direct Conversion of N-Alkylamines to N-Propargylamines through C-H Activation Promoted by Lewis Acid/Organocopper Catalysis: Application to Late-Stage Functionalization of Bioactive Molecules

An efficient catalytic method to convert an α-C-H bond of N-alkylamines into an α-C-alkynyl bond was developed. In the past, such transformations were carried out under oxidative conditions, and the enantioselective variants were confined to tetrahydroisoquinoline derivatives. Here, we disclose a method for the union of N-alkylamines and trimethylsilyl alkynes, without the presence of an external oxidant and promoted through cooperative actions of two Lewis acids, B(C6F5)3 and a Cu-based complex. A variety of propargylamines can be synthesized in high diastereo- and enantioselectivity. The utility of the approach is demonstrated by the late-stage site-selective modification of bioactive amines. Kinetic investigations that shed light on various mechanistic nuances of the catalytic process are presented.

B(C6F5)3-Catalyzed α-Deuteration of Bioactive Carbonyl Compounds with D2O

An efficient deuteration process of α-C-H bonds in various carbonyl-based pharmaceutical compounds has been developed. Catalytic reactions are initiated by the action of Lewis acidic B(C₆F₅)₃ and D₂O, converting a drug molecule into the corresponding boron-enolate. Ensuing deuteration of the enolate by in situ-generated D₂O⁺-H then results in the formation of α-deuterated bioactive carbonyl compounds with up to >98% deuterium incorporation.

Catalytic Deuterium Incorporation within Metabolically Stable β-Amino C-H Bonds of Drug Molecules

An efficient deuteration process of β-amino C-H bonds in various N-alkylamine-based pharmaceutical compounds has been developed. Catalytic reactions begin with the action of Lewis acidic B(C₆F₅)₃ and Brønsted basic N-alkylamine, converting a drug molecule into the corresponding enamine. The acid/base catalysts also promote the dedeuteration of acetone-d₆ to afford a deuterated ammonium ion. Ensuing deuteration of the enamine then leads to the formation of β-deuterated bioactive amines with up to 99% deuterium incorporation.

Enantioselective Synthesis of α-Allyl Amino Esters via Hydrogen-Bond-Donor Catalysis

We report a chiral-squaramide-catalyzed enantio- and diastereoselective synthesis of α-allyl amino esters. The optimized protocol provides access to N-carbamoyl-protected amino esters via nucleophilic allylation of readily accessible α-chloro glycinates. A variety of useful α-allyl amino esters were prepared, including crotylated products bearing vicinal stereocenters that are inaccessible through enolate alkylation, with high enantioselectivity (up to 97% ee) and diastereoselectivity (>10:1). The reactions display first-order kinetic dependence on both the α-chloro glycinate and the nucleophile, consistent with rate-limiting C-C bond formation. Computational analysis of the uncatalyzed reaction predicts an energetically inaccessible iminium intermediate, and a lower energy concerted SN2 mechanism.

Enantioselective Conia-Ene-Type Cyclizations of Alkynyl Ketones through Cooperative Action of B(C6F5)3, N-Alkylamine and a Zn-Based Catalyst

An efficient and highly enantioselective Conia-ene-type process has been developed. Reactions are catalyzed by a combination of B(C₆F₅)₃, an N-alkylamine and a BOX-ZnI₂ complex. Specifically, through cooperative action of B(C₆F₅)₃ and amine, ketones with poorly acidic α-C-H bonds can be converted in situ to the corresponding enolates. Subsequent enantioselective cyclization involving a BOX-ZnI₂-activated alkyne leads to the formation of various cyclopentenes in up to 99% yield and 99:1 er.

B(C6F5)3-Catalyzed C-H Alkylation of N-Alkylamines Using Silicon Enolates without External Oxidant

An efficient method for the coupling of N-alkylamines with silicon enolates to generate β-amino carbonyl compounds is disclosed. These reactions proceed by activation of α-amino C-H bonds by B(C₆F₅)₃, which likely generates a "frustrated" acid/base complex in the presence of large N-alkylamines. The transformation requires no external oxidant and releases hydrosilane as a byproduct. The utility of this method is demonstrated in the late-stage functionalization of bioactive molecules such as citalopram, atomoxetine, and fluoxetine.

C-H Functionalization of Amines via Alkene-Derived Nucleophiles through Cooperative Action of Chiral and Achiral Lewis Acid Catalysts: Applications in Enantioselective Synthesis

Catalytic transformations of α-amino C-H bonds to afford valuable enantiomerically enriched α-substituted amines, entities that are prevalent in pharmaceuticals and bioactive natural products, have been developed. Typically, such processes are carried out under oxidative conditions and require precious metal-based catalysts. Here, we disclose a strategy for an enantioselective union of N-alkylamines and α,β-unsaturated compounds, performed under redox-neutral conditions, and promoted through concerted action of seemingly competitive Lewis acids, B(C₆F₅)₃, and a chiral Mg-PyBOX complex. Thus, a wide variety of β-amino carbonyl compounds may be synthesized, with complete atom economy, through stereoselective reaction of an in situ-generated enantiomerically enriched Mg-enolate and an appropriate electrophile.

Palladium-Catalyzed Transformations of Alkyl C-H Bonds

This Review summarizes the advancements in Pd-catalyzed C(sp³)-H activation via various redox manifolds, including Pd(0)/Pd(II), Pd(II)/Pd(IV), and Pd(II)/Pd(0). While few examples have been reported in the activation of alkane C-H bonds, many C(sp³)-H activation/C-C and C-heteroatom bond forming reactions have been developed by the use of directing group strategies to control regioselectivity and build structural patterns for synthetic chemistry. A number of mono- and bidentate ligands have also proven to be effective for accelerating C(sp³)-H activation directed by weakly coordinating auxiliaries, which provides great opportunities to control reactivity and selectivity (including enantioselectivity) in Pd-catalyzed C-H functionalization reactions.

Enantioselective synthesis of tertiary α-chloro esters by non-covalent catalysis

We report an enantioselective approach to tertiary α-chloro esters through the reaction of silyl ketene acetals and N-chlorosuccinimide. The reaction is promoted by a chiral squaramide catalyst, which is proposed to engage both reagents exclusively through non-covalent interactions. Application of the tertiary chloride products in stereospecific substitution reactions is demonstrated.

Asymmetric Mannich synthesis of α-amino esters by anion-binding catalysis

We report a scalable, one-pot Mannich route to enantioenriched α-amino esters by direct reaction of α-chloroglycine ester as a practical imino ester surrogate. The reaction is promoted by a chiral aminothiourea, which is proposed to operate cooperatively by generating an iminium ion by chloride abstraction and an enolate by deprotonation, followed by highly stereoselective C-C bond formation between both reactive intermediates associated non-covalently within the catalyst framework.

Palladium(II)-catalyzed enantioselective C(sp³)-H activation using a chiral hydroxamic acid ligand

An enantioselective method for Pd(II)-catalyzed cross-coupling of methylene β-C(sp(3))-H bonds in cyclobutanecarboxylic acid derivatives with arylboron reagents is described. High yields and enantioselectivities were achieved through the development of chiral mono-N-protected α-amino-O-methylhydroxamic acid (MPAHA) ligands, which form a chiral complex with the Pd(II) center. This reaction provides an alternative approach to the enantioselective synthesis of cyclobutanecarboxylates containing α-chiral quaternary stereocenters. This new class of chiral catalysts also show promises for enantioselective β-C(sp(3))-H activation of acyclic amides.

Palladium(0)-catalyzed alkynylation of C(sp3)-H bonds

The alkynylation of β-C(sp(3))-H bonds in aliphatic amides with alkynyl halides has been enabled using Pd(0)/N-heterocyclic carbene (NHC) and Pd(0)/phosphine (PR3) catalysts. This is the first example of utilizing [AlkynylPd(II)L(n)] complexes to activate C(sp(3))-H bonds.

Ligand-enabled methylene C(sp3)-H bond activation with a Pd(II) catalyst

Pd(II) insertion into β-methylene C(sp(3))-H bonds was enabled by a mutually repulsive and electron-rich quinoline ligand. Ligand tuning led to the development of a method that allows for installation of an aryl group on a range of acyclic and cyclic amides containing β-methylene C(sp(3))-H bonds.

Pd(II)-catalyzed ortho trifluoromethylation of arenes and insights into the coordination mode of acidic amide directing groups

A Pd(II)-catalyzed trifluoromethylation of ortho C-H bonds with an array of N-arylbenzamides derived from benzoic acids is reported. N-Methylformamide has been identified as a crucial promoter of C-CF(3) bond formation from the Pd center. X-ray characterization of the C-H insertion intermediate has revealed a rare coordination mode of acidic amides as directing groups and the origin of their capacity in directing C-H activation.

Weak coordination as a powerful means for developing broadly useful C-H functionalization reactions

Reactions that convert carbon-hydrogen (C-H) bonds into carbon-carbon (C-C) or carbon-heteroatom (C-Y) bonds are attractive tools for organic chemists, potentially expediting the synthesis of target molecules through new disconnections in retrosynthetic analysis. Despite extensive inorganic and organometallic study of the insertion of homogeneous metal species into unactivated C-H bonds, practical applications of this technology in organic chemistry are still rare. Only in the past decade have metal-catalyzed C-H functionalization reactions become more widely utilized in organic synthesis. Research in the area of homogeneous transition metal-catalyzed C-H functionalization can be broadly grouped into two subfields. They reflect different approaches and goals and thus have different challenges and opportunities. One approach involves reactions of completely unfunctionalized aromatic and aliphatic hydrocarbons, which we refer to as "first functionalization". Here the substrates are nonpolar and hydrophobic and thus interact very weakly with polar metal species. To overcome this weak affinity and drive metal-mediated C-H cleavage, chemists often use hydrocarbon substrates in large excess (for example, as solvent). Because highly reactive metal species are needed in first functionalization, controlling the chemoselectivity to avoid overfunctionalization is often difficult. Additionally, because both substrates and products are comparatively low-value chemicals, developing cost-effective catalysts with exceptionally high turnover numbers that are competitive with alternatives (including heterogeneous catalysts) is challenging. Although an exciting field, first functionalization is beyond the scope of this Account. The second subfield of C-H functionalization involves substrates containing one or more pre-existing functional groups, termed "further functionalization". One advantage of this approach is that the existing functional group (or groups) can be used to chelate the metal catalyst and position it for selective C-H cleavage. Precoordination can overcome the paraffin nature of C-H bonds by increasing the effective concentration of the substrate so that it need not be used as solvent. From a synthetic perspective, it is desirable to use a functional group that is an intrinsic part of the substrate so that extra steps for installation and removal of an external directing group can be avoided. In this way, dramatic increases in molecular complexity can be accomplished in a single stroke through stereo- and site-selective introduction of a new functional group. Although reactivity is a major challenge (as with first functionalization), the philosophy in further functionalization differs; the major challenge is developing reactions that work with predictable selectivity in intricately functionalized contexts on commonly occurring structural motifs. In this Account, we focus on an emergent theme within the further functionalization literature: the use of commonly occurring functional groups to direct C-H cleavage through weak coordination. We discuss our motivation for studying Pd-catalyzed C-H functionalization assisted by weakly coordinating functional groups and chronicle our endeavors to bring reactions of this type to fruition. Through this approach, we have developed reactions with a diverse range of substrates and coupling partners, with the broad scope likely stemming from the high reactivity of the cyclopalladated intermediates, which are held together through weak interactions.

Pd(II)-catalyzed enantioselective C-H activation of cyclopropanes

Systematic ligand development has led to the identification of novel mono-N-protected amino acid ligands for Pd(II)-catalyzed enantioselective C-H activation of cyclopropanes. A diverse range of organoboron reagents can be used as coupling partners, and the reaction proceeds under mild conditions. These results provide a new retrosynthetic disconnection for the construction of enantioenriched cis-substituted cyclopropanecarboxylic acids.

Pd(II)-Catalyzed Cross-Coupling of C(sp2 )-H Bonds and Alkyl-, Aryl- and Vinyl-Boron Reagents via Pd(II)/Pd(0) Catalysis

Pd(II)-catalyzed cross-coupling of ortho-C-H bonds in benzoic acid and phenylacetic acid amides with alkyl-, aryl- and vinyl-boron reagents have been achieved via Pd(II)/Pd(0) catalysis, demonstrating the unprecedented versatility of C-H activation reactions.

Pd(II)-catalyzed carbonylation of C(sp3)-H bonds: a new entry to 1,4-dicarbonyl compounds

Pd(II)-catalyzed β-C(sp(3))-H carbonylation of N-arylamides under CO (1 atm) has been achieved. Following amide-directed C(sp(3))-H cleavage and insertion of CO into the resulting [Pd(II)-C(sp(3))] bond, intramolecular C-N reductive elimination gave the corresponding succinimides, which could be readily converted to 1,4-dicarbonyl compounds. This method was found to be effective with substrates containing α-hydrogen atoms and could be applied to effect methylene C(sp(3))-H carbonylation of cyclopropanes.

Cross-coupling of C(sp)-H Bonds with Organometallic Reagents via Pd(II)/Pd(0) Catalysis**

Palladium-catalyzed C-H activation/C-C bond-forming reactions have emerged as a promising class of synthetic tools in organic chemistry. Among the many different means of forging C-C bonds using Pd-mediated C-H activation, a new horizon in this field is Pd(II)-catalyzed cross-coupling of C-H bonds with organometallic reagents via a Pd(II)/Pd(0) catalytic cycle. While this type of reaction has proven to be effective for the selective functionalization of aryl C(sp(2))-H bonds, the focus of this review is on Pd(II)-catalyzed C(sp(3))-H activation/C-C cross-coupling, a topic of particular importance because reactions of this type enable fundamentally new methods for bond construction. Since our laboratory's initial report on cross-coupling of C-H bonds in 2006, this area has expanded rapidly, and the unique ability of Pd(II) catalysts to cleave and functionalize alkyl C(sp(3))-H bonds has been exploited to develop protocols for forming an array of C(sp(3))-C(sp(2)) and C(sp(3))-C(sp(3)) bonds. Furthermore, enantioselective C(sp(3))-H activation/C-C cross-coupling has been achieved through the use of chiral amino acid-derived ligands, offering a novel technique for producing enantioenriched molecules. Although this nascent field remains at an early stage of development, further investigations hold the potential to revolutionalize the way in which chiral molecules are synthesized in industrial and academic laboratories.

Amide-Directed Arylation of sp C-H Bonds using Pd(II) and Pd(0) Catalysts

Protocols to effect β-arylation of sp(3) C-H bonds via Pd(II)/(IV) and Pd(0)/(II) catalytic cycles have been achieved using a newly developed monodentate CONHC(6)F(5) directing group. These reactions provide an unprecedented means to functionalize sp(3) C-H bonds in aliphatic carboxylic acid-derived substrates.

Pd(II)-catalyzed olefination of sp3 C-H bonds

The first Pd(II)-catalyzed sp(3) C-H olefination reaction has been developed using N-arylamide directing groups. Following olefination, the resulting intermediates were found to undergo rapid 1,4-addition to give the corresponding gamma-lactams. Notably, this method was effective with substrates containing alpha-hydrogen atoms and could be applied to effect methylene C-H olefination of cyclopropane substrates.

Pd(0)/PR3-catalyzed intermolecular arylation of sp3 C-H bonds

Pd(0)-catalyzed intermolecular arylation of sp(3) C-H bonds has been achieved using PR(3)/ArI. This protocol can be used to arylate a variety of aliphatic carboxylic acid derivatives, including a number of bioactive drug molecules. The use of fluorinated aryl iodides also allows for the introduction of fluorine into a molecule of interest.

Converting gem-dimethyl groups into cyclopropanes via Pd-catalyzed sequential C-H activation and radical cyclization

A novel route to the synthesis of cyclopropane derivatives is described. 1,1-Dimethyls in 2-(1,1-dimethylalkyl)dimethyloxazolines are first converted into 1,3-diiodide derivatives via Pd-catalyzed sequential C-H activation and then radically cyclized to provide 2-(1-alkylcylclopropyl)dimethyloxazolines. The use of EtOAc as a solvent is crucial for the diiodination of the functionalized substrates. [reaction: see text]

Successful laparoscopic fundoplication in children with ventriculoperitoneal shunts

We reported successful laparoscopic fundoplication in 2 pediatric cases with VPS and discuss the safety and feasibility of the procedure. Case 1: A 13-year-old girl with VPS underwent laparoscopic fundoplication. Case 2: a 9-year-old boy with VPS underwent laparoscopic fundoplication. In both cases, laparoscopic Nissen fundoplication was performed with a standard five-port technique with a low pressure of a pneumoperitoneum. The VPS system had no effect on port layout and intraabdominal manipulation and no adverse complications were observed in either case. The effect of a pneumoperitoneum in the VPS system remains controversial, however, the author emphasized that advanced laparoscopic surgery can be performed safely with creating a low pressure of a pneumoperitoneum.

Regulation of glutamine transport by the type I insulin-like growth factor receptor in a human neuroblastoma cell line

We examined the effects of the monoclonal antibody against the type I insulin-like growth factor receptor (IGF-IR), alphaIR3, on cell growth and membrane glutamine (Gln) transport in a human neuroblastoma cell line, SK-N-SH. In the presence of alphaIR3 (2 microg/ml), the cell proliferation rate was significantly attenuated. Gln transport was decreased in the alphaIR3-treated group with the mechanism of decreasing maximum transport velocity without affecting transport affinity. The addition of alphaIR3 significantly decreased DNA and protein biosynthesis. Our results support the concept that the activation of IGF-IR partially mediates neuroblastoma cell proliferation by regulating membrane Gln transport.

Three-dimensional pressure imaging: a novel method for intraoperative manometry during laparoscopic esophagomyotomy for esophageal achalasia

With the introduction of computerized manometry, the creation of the three-dimensional (3-D) pressure image with measurement of pressure vector volume (PVV) has become available to assess the pressure structure of the high-pressure zone (HPZ) of the distal esophagus. We have applied this technique to intraoperative manometry during laparoscopic surgery for esophageal achalasia (EA). A laparoscopic esophagomyotomy with anterior fundoplication was performed in a 12-year-old boy with EA. Computerized 3-D images were obtained by a stepwise pullback of a manometric assembly with eight radial side-holes to quantitate the PVV of the HPZ of the distal esophagus in each stage of the operation for EA. Virtual abolishment of the peak pressure image and a marked reduction in the PVV after esophagomyotomy suggested appropriate decompression of the HPZ. Intraoperative manometry using computerized 3-D pressure imaging with measurement of the PVV is thus useful to assess the HPZ during laparoscopic surgery for EA.

On-off study of manganese administration to adult patients undergoing home parenteral nutrition: new indices of in vivo manganese level

BACKGROUND

Recently, there have been reports that magnetic resonance imaging (MRI) reveals high-intensity T1-weighted images (HI) in the basal ganglia (especially in the globus pallidus) of patients receiving total parenteral nutrition (TPN). This finding is presumably due to excess administration of manganese. We investigated the reversibility and reproducibility of these changes by means of an on-off manganese administration study. We also investigated the temporal relationships between the intensity of T1-weighted images (MRI intensity) and the whole-blood and plasma manganese concentrations to evaluate the potential for the MRI intensity to serve as an index of the in vivo manganese level.

METHODS

Eleven adult patients undergoing home parenteral nutrition received TPN solutions containing manganese (0 or 20 micromol/d) according to an on-off design. The whole-blood and plasma manganese concentrations were determined at the same time the brain MRI was performed.

RESULTS

Both the whole-blood manganese concentration and the MRI intensity in the globus pallidus changed in response to the administration and withdrawal of manganese. It took at least 5 months for HI to disappear when manganese was withdrawn, and this change was reversible and reproducible. The whole-blood manganese concentration showed strong correlations with both the MRI intensity and the T1 value (r = 0.7693, -0.7011). The MRI intensity and the T1 value showed a strong correlation (r = -0.9051).

CONCLUSIONS

The whole-blood manganese concentration, the MRI intensity in the globus pallidus and the T1 value, an objective index of the MRI intensity, may be useful indices of the manganese level in the body.