Osmium-Catalyzed Vicinal Oxyamination of Alkenes by N-(4-Toluenesulfonyloxy)carbamates

Dirhodium-Catalyzed Transannulation of N-Sulfonyl-1,2,3-triazoles to 2,3-Dehydropiperazines

The dirhodium(II)-catalyzed synthesis of a range of C2-substituted 2,3-dehydropiperazines using 1-mesyl-1,2,3-triazoles and β-haloalkylcarbamates is reported. The reaction is proposed to proceed through an α-imino rhodium carbene 1,3-insertion into N-H followed by a base-mediated cyclization. C-Substituted dehydropiperazines can also be conducted directly from terminal alkynes in a three-step, one-pot operation, forming the triazole in situ. This methodology has also been expanded to afford several 2,5-disubstituted 2,3-dehydropiperazines as well as a larger 4,5,6,7-tetrahydro-1H-1,4-diazepine derivative.

From dithiocarbamates to branched dithiocarbazates: Compounds with potent antischistosomal activity

Schistosomiasis or bilharzia is caused by blood flukes of the genus Schistosoma and represents a considerable health and economic burden in tropical and subtropical regions. The treatment of this infectious disease relies on one single drug: praziquantel (PZQ). Therefore, new and potent antischistosomal compounds need to be developed. In our previous work, starting with the drug disulfiram, we developed dithiocarbamates with in vitro antischistosomal activities in the low micromolar range. Based on these results, we report in this study on the synthesis and biological testing of the structurally related dithiocarbazates against Schistosoma mansoni, one of the major species of schistosomes. In total, three series of dithiocarbazate derivatives were examined, and we found that the antischistosomal activity of N-unbranched dithiocarbazates increased by further N-substitution. Comparable tetra-substituted dithiocarbazates were rarely described in the literature, thus a synthesis route was established. Due to the elaborate synthesis, the branched dithiocarbazates (containing an N-aminopiperazine) were simplified, but the resulting branched dithiocarbamates (containing a 4-aminopiperidine) were considerably less active. Taken together, dithiocarbazate-containing compounds with an in vitro antischistosomal activity of 5 µM were obtained.

Synthesis and Studies of Bulky Cycloalkyl α,β-Dehydroamino Acids that Enhance Proteolytic Stability

Three new bulky cycloalkyl α,β-dehydroamino acids (ΔAAs) have been designed and synthesized. Each residue enhances the rigidity of model peptides and their stability to proteolysis, with larger ring sizes exhibiting greater effects. Peptides containing bulky cycloalkyl ΔAAs are inert to conjugate addition by a nucleophilic thiol. The results suggest that these residues will be effective tools for improving the proteolytic stability of bioactive peptides.

α-Nitrosostyrenes as Three-Atom Units for the (3+1) Cyclization Reaction: Facile Access to 2,3-Dihydrodiazete N-Oxides and Their Diversified Synthetic Conversions

An unprecedented (3+1) cyclization of α-nitrosostyrenes, generated in situ from α-bromooximes, and N-tosyloxycarbamates was developed, which enables the synthesis of a range of structurally unique and hitherto unexplored 2,3-dihydrodiazete N-oxides in moderate to high yields. The products possess a highly strained four-membered ring structure containing two nitrogen atoms. The synthetic applicability of the products was also demonstrated by many important conversions to diverse nitrogen-containing compounds.

N-Aroyloxycarbamates as switchable nitrogen and oxygen precursor: Ir/Cu controlled divergent C–H functionalization of heteroarenes

Chemodivergent control in the functionalization of nitrogen-directed aromatic C–H bonds has been achieved by a switchable catalyst.

Peculiarities of promiscuous L-threonine transaldolases for enantioselective synthesis of β-hydroxy-α-amino acids

The introduction of β-hydroxy-α-amino acids (βHAAs) into organic molecules has received considerable attention as these molecules have often found widespread applications in bioorganic chemistry, medicinal chemistry and biomaterial science. Despite innovation of asymmetric synthesis of βHAAs, stereoselective synthesis to control the two chiral centres at C_α and C_β positions is still challenging, with poor atomic economy and multi protection and deprotection steps. These syntheses are often operated under harsh conditions. Therefore, a biotransformation approach using biocatalysts is needed to selectively introduce these two chiral centres into structurally diverse molecules. Yet, there are few ways that enable one-step synthesis of βHAAs. One is to extend the substrate scope of the existing enzyme inventory. Threonine aldolases have been explored to produce βHAAs. However, the enzymes have poor controlled installation at C_β position, often resulting in a mixture of diastereoisomers which are difficult to be separated. In this respect, L-threonine transaldolases (LTTAs) offer an excellent potential as the enzymes often provide controlled stereochemistry at C_α and C_β positions. Another is to mine LTTA homologues and engineer the enzymes using directed evolution with the aim of finding engineered biocatalysts to accept broad substrates with enhanced conversion and stereoselectivity. Here, we review the development of LTTAs that incorporate various aldehyde acceptors to generate structurally diverse βHAAs and highlight areas for future developments. KEY POINTS: • The general mechanism of the transaldolation reaction catalysed by LTTAs • Recent advances in LTTAs from different biosynthetic pathways • Applications of LTTAs as biocatalysts for production of βHAAs.

Aminolactonization of Unactivated Alkenes Catalyzed by Aryl Iodine

A one-step protocol of the aryl iodine-catalyzed aminolactonization of unactivated alkenes under oxidation conditions was first reported to efficiently construct diverse amino lactones in a short time using HNTs₂ as the compatible nitrogen source. In addition, we investigated the influence of the reaction rate based on the structure of the iodoarene precatalyst, which revealed the selective adjustment effect on aminolactonization and oxylactonization. Finally, preliminary experiments verified the feasibility of asymmetric aminolactonization catalyzed by a chiral iodoarene precatalyst.

Convergent Total Synthesis of Yaku'amide A

Total synthesis of the anticancer peptide natural product yaku'amide A is reported. Its β-tert-hydroxy amino acids were prepared by regioselective aminohydroxylation involving a chiral mesyloxycarbamate reagent. Stereospecific construction of the E- and Z-ΔIle residues was accomplished through a one-pot reaction featuring anti dehydration, azide reduction, and O→N acyl transfer. Alkene isomerization was negligible during this process. These methods enabled a highly convergent and efficient synthetic route to the natural product.

Studies toward the Synthesis of Chartelline C

Chartelline C, a marine alkaloid, possesses a unique core scaffold including indolenine β-lactam and imidazole moieties linked by an unsaturated 10-membered ring. A new synthetic approach for the construction of the indolenine β-lactam was planned, based on the inherent reactivity of chartelline A with NaOMe, triggered by bromination of bromoenamide, and proceeding through an N-acyl imine. However, the N-acyl imine intermediate was not observed. Instead, the corresponding bromoindolenine was obtained, which led to the desired indolenine β-lactam in 92% yield.

Alkene Syn- and Anti-Oxyamination with Malonoyl Peroxides

Malonoyl peroxide 6 is an effective reagent for the syn- or anti-oxyamination of alkenes. Reaction of 6 and an alkene in the presence of O-tert-butyl-N-tosylcarbamate (R³ = CO₂^tBu) leads to the anti-oxyaminated product in up to 99% yield. Use of O-methyl-N-tosyl carbamate (R³ = CO₂Me) as the nitrogen nucleophile followed by treatment of the product with trifluoroacetic acid leads to the syn-oxyaminated product in up to 77% yield. Mechanisms consistent with the observed selectivities are proposed.

Recent progress of 1,1-difunctionalization of olefins

Olefins are a very important class of compounds and broadly used in the construction of various synthetic building blocks and practical industrial production. The difunctionalization of olefins provides one of the most powerful methods for the C-C or C-X bond formation with a rapid increase of the molecular complexity and synthetic value economically and effectively. Compared with the vigorous growth and abundant achievements of 1,2-difunctionalization of olefins, 1,1-difunctionalization is a relatively emerging and inadequately exploited research direction, despite being tremendously attractive from synthetic perspectives. In this minireview, we provide a brief overview of the advancements of 1,1-difunctionalization of olefins in the past twenty years, and prospects of future developments.

Insights into base-free OsO4-catalyzed aminohydroxylations employing chiral ligands

Attempts to perform the OsO4-catalyzed enantioselective base-free aminohydroxylation of β,β-disubstituted enoates are described. Low yields and racemic products were obtained in the presence of standard chiral ligands, suggesting the occurrence of a "Second Cycle" process due to slow hydrolysis of the amino alcohol product from the Os metal center. Support for this hypothesis was provided by the slightly improved enantioselectivity (60:40 er) obtained with an amino alcohol ligand. Based on density functional theory calculations, it is proposed that the lack of significant enantioselectivity is due to a low-energy (3 + 2) oxo/imido cycloaddition transition state without the chiral ligand in the Second Cycle that outcompetes protonolysis in the First Cycle.

Enantioselective synthesis of anti-3-alkenyl-2-amido-3-hydroxy esters: application to the total synthesis of (+)-alexine

A straightforward synthesis of anti-3-alkenyl-2-amido-3-hydroxy esters from the corresponding racemic α-amino-β-keto esters by using a ATH/DKR protocol has been developed. This method gives moderate to excellent yields with high chemo-, diastereo- and enantioselectivities for a broad range of substrates. In order to highlight the versatility of the methodology it was applied in an efficient asymmetric synthesis of the polyhydroxylated pyrrolizidine alkaloid (+)-alexine.

A straightforward synthesis of anti-3-alkenyl-2-amido-3-hydroxy esters from the corresponding racemic α-amino-β-keto esters by using a ATH/DKR protocol has been developed.

Oxidative functionalization of C–H compounds induced by the extremely efficient osmium catalysts (a review)

In recent years, osmium complexes have found applications not only in thecis-hydroxylation of olefins but also very efficient in the oxygenation of C–H compounds (saturated and aromatic hydrocarbons and alcohols) by hydrogen peroxide as well as organic peroxides.

Practical Synthetic Procedures for the Iron-Catalyzed Intermolecular Olefin Aminohydroxylation Using Functionalized Hydroxylamines

A set of practical synthetic procedures for the iron-catalyzed intermolecular olefin aminohydroxylation reactions in gram scale is reported. In these transformations, a bench-stable functionalized hydroxylamine is applied as the amination reagent. This method is compatible with a broad range of synthetically valuable olefins including those that are incompatible with the existing aminohydroxylation methods. It also provides valuable amino alcohol building blocks with regio- and stereo-chemical arrays that are complementary to known methods.

Rapid chemoenzymatic route to glutamate transporter inhibitorl-TFB-TBOA and related amino acids

Rapid route tol-TFB-TBOA: Using only nine chemical transformations, we have managed to construct the complex amino acidl-TFB-TBOA starting from commercially available dimethyl acetylenedicarboxylate.

Direct Catalytic Synthesis of Unprotected 2-Amino-1-Phenylethanols from Alkenes by Using Iron(II) Phthalocyanine

Aryl-substituted amino alcohols are privileged scaffolds in medicinal chemistry and natural products. Herein, we report that an exceptionally simple and inexpensive Fe(II) complex efficiently catalyzes the direct transformation of simple alkenes into unprotected amino alcohols in good yield and perfect regioselectivity. This new catalytic method was applied in the expedient synthesis of bioactive molecules and could be extended to aminoetherification.

Acylation or phosphorylation of hydroxyurea unexpectedly takes place on N rather than on O, leading to the formation of amides instead of the expected esters

Attempted acylation of the anticancer agent hydroxyurea (HU) with acyl chlorides or anhydrides led to acylation on the NH group rather than on the OH.

Diastereoselective osmium-catalyzed vicinal oxyamination of acyclic allylic alcohol derivatives

The osmium-catalyzed oxyamination of chiral acyclic allylic alcohol derivatives bearing mono- and 1,1-di-substituted double bonds with benzyl N-(4-tosyloxy)carbamate proceeds with high regioselectivity and moderate levels of diastereoselectivity favoring the anti product. The observed stereoselectivity shows a clear and systematic trend with anti:syn ratios increasing in line with the size of substituent at both the allylic stereocenter and double bond α-carbon. The stereoinduction is in accord with the sense of diastereoselectivity predicted by Kishi's empirical rule and a previously reported transition state model for the osmium-catalyzed dihydroxylation of allylic alcohol derivatives. In contrast, allylic alcohol derivatives bearing trisubstituted double bonds show low or no reactivity in the oxyamination reaction affording the syn product in low yield in the cases examined.

Iron(II)-catalyzed intermolecular amino-oxygenation of olefins through the N-O bond cleavage of functionalized hydroxylamines

An iron-catalyzed diastereoselective intermolecular olefin amino-oxygenation reaction is reported, which proceeds via an iron-nitrenoid generated by the N-O bond cleavage of a functionalized hydroxylamine. In this reaction, a bench-stable hydroxylamine derivative is used as the amination reagent and oxidant. This method tolerates a range of synthetically valuable substrates that have been all incompatible with existing amino-oxygenation methods. It can also provide amino alcohol derivatives with regio- and stereochemical arrays complementary to known amino-oxygenation methods.

Oxidation of hydrocarbons with H2O2/O2 catalyzed by osmium complexes containing p-cymene ligands in acetonitrile

Osmium π-complexes containing p-cymene ligands efficiently catalyze (TONs up to 200 000) the oxidation of alkanes to alkyl hydroperoxides with H₂O₂.

Direct synthesis of β-hydroxy-α-amino acids via diastereoselective decarboxylative aldol reaction

A straightforward metal-free synthesis of anti-β-hydroxy-α-amino acids is described. The organic base-mediated decarboxylative aldol reaction of cheap, readily available α-amidohemimalonates with various aldehydes afforded under very mild conditions anti-β-hydroxy-α-amido esters in high yields and complete diastereoselectivity. Simple one-pot subsequent transformations enabled the corresponding anti-β-hydroxy-α-amino acids or in a few examples their syn diastereomers to be obtained directly using epimerization conditions.