Reversal of regioselection in the sharpless asymmetric aminohydroxylation of aryl ester substrates

Ligand Controlled Ir-Catalyzed Regiodivergent Oxyamination of Unactivated Alkenes

An intramolecular Ir(III)-catalyzed regiodivergent oxyamination of unactivated alkenes provides valuable γ-lactams, γ-lactones and δ-lactams. The regioselectivity is controlled by the electronically tunable cyclopentadienyl Ir(III)-complexes enabling oxyamination via either 5-exo or 6-endo pathways. With respect to the mechanism, we propose a highly reactive [3.1.0] bicycle intermediate derived from Ir(V) nitrene-mediated aziridination to be a key intermediate toward the synthesis of γ-lactams.

Synthesis and Medicinal Chemistry of Muraymycins, Nucleoside Antibiotics

Muraymycins, isolated from a culture broth of Streptomyces sp., are members of a class of naturally occurring nucleoside antibiotics. They are strong inhibitors of the phospho-MurNAc-pentapeptide translocase (MraY), which is responsible for the peptidoglycan biosynthesis. Since MraY is an essential enzyme among bacteria, muraymycins are expected to be a novel antibacterial agent. In this review, our efforts to synthesize muraymycin D2, simplify the chemical structure, improve antibacterial spectrum, and solve the X-ray crystal structure of the muraymycin D2/MraY complex are described.

Capturing Biological Activity in Natural Product Fragments by Chemical Synthesis

Natural products have had an immense influence on science and have directly led to the introduction of many drugs. Organic chemistry, and its unique ability to tailor natural products through synthesis, provides an extraordinary approach to unlock the full potential of natural products. In this Review, an approach based on natural product derived fragments is presented that can successfully address some of the current challenges in drug discovery. These fragments often display significantly reduced molecular weights, reduced structural complexity, a reduced number of synthetic steps, while retaining or even improving key biological parameters such as potency or selectivity. Examples from various stages of the drug development process up to the clinic are presented. In addition, this process can be leveraged by recent developments such as genome mining, antibody–drug conjugates, and computational approaches. All these concepts have the potential to identify the next generation of drug candidates inspired by natural products.

Catalytic Asymmetric Direct Aldol Reaction of α-Alkyl Azlactones and Aliphatic Aldehydes

An unprecedented highly diastereoselective and enantioselective aldol reaction of α-alkyl azlactones and aliphatic aldehydes was achieved with cinchona alkaloid catalysts. To our knowledge, this reaction provides the first useful catalytic asymmetric access toward β-hydroxy-α-amino acids bearing alkyl substituents, which are structural motifs embedded in many natural products.

Stereocontrolled synthesis of syn-β-Hydroxy-α-amino acids by direct aldolization of pseudoephenamine glycinamide

β-Hydroxy-α-amino acids figure prominently as chiral building blocks in chemical synthesis and serve as precursors to numerous important medicines. Reported herein is a method for the synthesis of β-hydroxy-α-amino acid derivatives by aldolization of pseudoephenamine glycinamide, which can be prepared from pseudoephenamine in a one-flask protocol. Enolization of (R,R)- or (S,S)-pseudoephenamine glycinamide with lithium hexamethyldisilazide in the presence of LiCl followed by addition of an aldehyde or ketone substrate affords aldol addition products that are stereochemically homologous with L- or D-threonine, respectively. These products, which are typically solids, can be obtained in stereoisomerically pure form in yields of 55-98 %, and are readily transformed into β-hydroxy-α-amino acids by mild hydrolysis or into 2-amino-1,3-diols by reduction with sodium borohydride. This new chemistry greatly facilitates the construction of novel antibiotics of several different classes.

Regioselective base-free intermolecular aminohydroxylations of hindered and functionalized alkenes

Regioselective base-free intermolecular aminohydroxylations of functionalized trisubstituted and 1,1-disubstituted alkenes employing benzoyloxycarbamate 3a and catalytic OsO(4) are described. In all cases, the more substituted alcohol isomer is favored. Sluggish reactions could be promoted by gentle heating, the use of amine ligands, or increased catalyst loadings. A competitive rearrangement was observed with a secondary allylic alcohol substrate. The adducts serve as useful precursors to dehydroamino acids.

Development of a highly beta-selective ribosylation reaction without using neighboring group participation: total synthesis of (+)-caprazol, a core structure of caprazamycins

Full details of the total synthesis of (+)-caprazol are described. The key elements of our approach include the early stage introduction of the aminoribose in a highly beta-selective manner, using the steric hindrance in the transition state and the construction of the diazepanone by a modified intramolecular reductive amination. The 5'-C-glycyluridine derivative 9, which was prepared stereoselectively via Sharpless asymmetric aminohydroxylation, was ribosylated with 2,3-O-alkylidene ribofuranosyl donors. It was revealed that increasing the size of the alkyl substituents of the acetal unit resulted in improving the stereoselectivity of the anomeric position, and the desired ribosides 21b (1' '-beta) and 22b (1' '-alpha) were obtained in 80% yield (21b/22b = 24.0/1) when the ribosyl fluoride 16 possessing a more sterically hindered 3-pentylidene group was used. The origin of the stereoselectivity of the ribosylation was also discussed. Construction of the diazepanone system was optimized with the model aldehyde 37, and the desired diazepanone 38 was obtained in 88% yield via two-step reaction sequence including catalytic hydrogenation followed by hydride reduction. Application of this method to the aldehyde 44 successfully afforded the diazepanone derivatives 45 and 46, functional group manipulation of which completed the total synthesis of (+)-caprazol.

Stereoselective Synthesis of trans-Olefins by the Copper-Mediated S(N)2' Reaction of Vinyl Oxazines with Grignard Reagents. Asymmetric Synthesis of D-threo-Sphingosines

The S(N)2' reaction of 6-vinyl-5,6-dihydro-4H-[1,3]oxazines with Grignard reagents in the presence of CuCN was studied, and high trans selectivity for the formation of double bond was observed with a variety of RMgX. The S(N)2' reaction, coupled with regioselective asymmetric aminohydroxylation reaction, provided a highly efficient route for the asymmetric synthesis of D-threo-N-acetylsphingosine.

Total synthesis of (-)-mucocin

[reaction: see text] An enantioselective total synthesis of (-)-mucocin has been completed. A combination of asymmetric glycolate aldol additions and ring closing metathesis reactions were exploited to construct the C18-C34 and C7-C17 fragments. A selective cross-metathesis reaction was employed as the key step to couple two complex fragments.

Enantioselective total synthesis of (+)-gigantecin: exploiting the asymmetric glycolate aldol reaction

The enantioselective synthesis of the potent, selective, cytotoxic, annonaceous acetogenin, (+)-gigantecin, has been completed. An asymmetric glycolate aldol serves to establish the stereocenters at C13,14 and at C21,22. A Carreira asymmetric acetylide addition is used to establish the C17 stereocenter.

Efficient Acyclic Stereocontrol Using the Tethered Aminohydroxylation Reaction

[reaction: see text] The tethered aminohydroxylation (TA) of acyclic allylic carbamates has been achieved in a stereospecific and stereoselective manner. Unusually high levels of stereocontrol were observed in the oxidation of 1,1-disubstituted substrates.

The use of pH to influence regio- and chemoselectivity in the asymmetric aminohydroxylation of styrenes

[reaction: see text] The pH-controlled Sharpless asymmetric aminohydroxylation (AA) of styrenes provides 1-aryl-2-amino ethanols (regioisomer B) with high enantio-, chemo-, and regioselectivity. As existing AA protocols typically give regioisomer A as the major reaction product when using carbamate nitrogen sources, this method is a convenient alternative for the selective production of regioisomer B.

An asymmetric aminohydroxylation approach to the azepine core of (-)-balanol

[reaction: see text]An efficient formal synthesis of the potent protein kinase C inhibitor (-)-balanol that relies on a modified asymmetric aminohydroxylation of the alpha,beta-unsaturated aryl ester (1) is reported. The aryl ester functionality and the dihydroquinyl alkaloid ligand system (DHQ)2-AQN are used to control the regio- and enantioselectivity of the process.