Stereospecific Introduction Research Articles

Mechanism and Origins of Stereoinduction in Natural Cinchona Alkaloid Catalyzed Asymmetric Electrophilic Trifluoromethylthiolation of β-Keto Esters with N-Trifluoromethylthiophthalimide as Electrophilic SCF3 Source

The trifluoromethylthio (SCF3) group enjoys a privileged role in the field of drug discovery because its incorporation into a drug molecule often leads to significantly improved pharmacokinetics and efficacy. In spite of its prime importance in drug discovery, the stereospecific introduction of the SCF3 group into target molecules has remained an unmet challenge. A major breakthrough was made in 2013 when Rueping and Shen simultaneously and independently disclosed natural Cinchona alkaloid catalyzed asymmetric electrophilic trifluoromethylthiolation of β-keto esters. However, two key issues remain obscure. (a) What is the preferred mode of catalysis? (b) How is asymmetric induction accomplished? Here we report an in-depth computational exploration into the mechanism and origin of stereoinduction in Cinchona alkaloid catalyzed trifluoromethylthiolation of β-keto esters with N-trifluoromethylthiophthalimide as electrophilic SCF3 source. Three mechanistic possibilities, i.e., (a) the transfer-trifluoromethyl...

Synthesis of a 3'-Fluoro-3'-deoxytetrose Adenine Phosphonate.

A new synthetic route to a 3'-fluoro-3'-deoxytetrose adenine phosphonate has been developed. The synthesis starts from l-xylose and key steps include the stereospecific introduction of the phosphonomethoxy group and adenine. In addition, a regioselective fluorination reaction allows access to the desired 3'-fluoro-3'-deoxytetrose moiety. This methodology allows the straightforward synthesis of a 3'-fluoro-3'-deoxytetrose adenine phosphonate and can be expanded toward the synthesis of other types of 3'-fluoro nucleoside phosphonates.

Diastereoselective Total Synthesis of (-)-Galiellalactone.

An enantioselective total synthesis of (-)-galiellalactone has been accomplished. The key features of the synthesis involve the highly stereoselective construction of the cis-trisubstituted cyclopentane intermediate by a Pd(0)-catalyzed cyclization, the stereospecific introduction of an angular hydroxyl group by Riley oxidation, and the efficient construction of the tricyclic system of (-)-galiellalactone via a combination of diastereoselective Hosomi-Sakurai crotylation and ring-closing metathesis (RCM).

Synthesis of the Aminocyclitol Units of (−)-Hygromycin A and Methoxyhygromycin from myo-Inositol

Concise and efficient syntheses of the aminocyclitol cores of hygromycin A (HMA) and methoxyhygromycin (MHM) have been achieved starting from readily available myo-inositol. Reductive cleavage of myo-inositol orthoformate to the corresponding 1,3-acetal, stereospecific introduction of the amino group via the azide, and resolution of a racemic cyclitol derivative as its diastereomeric mandelate esters are the key steps in the synthesis. Synthesis of the aminocyclitol core of hygromycin A involved chromatography in half of the total number of steps, and the aminocyclitol core of methoxyhygromycin involved only one chromatography.

ChemInform Abstract: The Nitrosocarbonyl Hetero‐Diels—Alder Reaction as a Useful Tool for Organic Syntheses

AbstractReview: [217 refs.]

The Nitrosocarbonyl Hetero‐Diels–Alder Reaction as a Useful Tool for Organic Syntheses

Organic transformations that result in the formation of multiple covalent bonds within the same reaction are some of the most powerful tools in synthetic organic chemistry. Nitrosocarbonyl hetero-Diels-Alder (HDA) reactions allow for the simultaneous stereospecific introduction of carbon-nitrogen and carbon-oxygen bonds in one synthetic step, and provide direct access to 3,6-dihydro-1,2-oxazines. This Review describes the development of the nitrosocarbonyl HDA reaction and the utility of the resulting oxazine ring in the synthesis of a variety of important, biologically active molecules.

Configurational Assignment of Cyclic Peroxy Metabolites Provides an Insight into Their Biosynthesis: Isolation of Plakortolides, seco-Plakortolides, and Plakortones from the Australian Marine Sponge Plakinastrella clathrata

Sixteen new compounds, comprising nine new plakortolides K-S (1-9), four seco-plakortolides (10-13), and three plakortones (14-16), were isolated from the Australian sponge Plakinastrella clathrata. Structural elucidation, including relative configurational assignment, was based on extensive spectroscopic analysis, while the absolute configurations of 1-4 were deduced from (1)H NMR analyses on MPA esters derived from Zn/AcOH reduction products. Diastereomeric sets of plakortolides, e.g., K and L, or M and N, differed in configuration at C-3/C-4 rather than at C-6, a stereochemical result that compromises a biosynthetic pathway involving Diels-Alder cycloaddition of molecular oxygen to a Δ(3,5)-diunsaturated fatty acid precursor. The biosynthesis may plausibly involve cyclization of a 6-hydroperoxydienoic acid intermediate following stereospecific introduction of the hydroperoxy group into a polyketide-derived precursor. Isolated seco-plakortolides converted under mild conditions into plakortones with full retention of configuration, suggesting C-6 hydroxy attack on an α,β-unsaturated lactone intermediate. The NMR data reported for the compound named plakortolide E are inconsistent with the current literature structure and are those of the corresponding seco-plakortolide (19). The reported conversion of the metabolite into a plakortone ether on storage is consistent with this structural revision.

Total Synthesis of (−)-Tetrodotoxin from d-Glucose: A New Route to Multi-Functionalized Cyclitol Employing the Ferrier(II) Reaction toward (−)-Tetrodotoxin

Abstract Total synthesis of (−)-tetrodotoxin (TTX) from d-glucose is described. As a critical transformation step for synthesizing TTX, a key multi-functionalized cyclitol was prepared from d-glucose employing the Ferrier(II) reaction. Stereospecific introduction of three functionalized branched chains were achieved via Peterson olefination and spiro α-chloroepoxidation of the corresponding carbonyl derivatives.

Novel Stereoselective Synthesis of 4-Acetoxyazetidinone from Methyl 6,6-Dibromopenicillanate: Key Intermediate for the Preparation of Carbapenem Antibiotics

A novel and practical process for the completely stereoselective synthesis of carbapenem key intermediate (3R,4R)-4-acetoxy-3-[(R)-1-((t-butyldimethylsilyl)oxy)ethyl]-2-azetidinone has been developed by starting from methyl 6,6-dibromopenicillanate. Aldol condensation of the magnesium enolate derived from the β-sulfoxide with acetaldehyde allows for the stereospecific introduction of a 1-R-hydroxyethyl substituent as C-6. The hydroxyethylated product was reduced with Zn-NH4OAc in tetrahydrofuran (THF) efficiently to give methyl 6-(1-hydroxyethyl)-penicillanate-1β-oxide. Protection of the hydroxy group followed by treatment with 2-mercaptobenzothiazole afforded the dithioazetidinone, which was easily reduced and methylated to give 4-methylthioazetidinone. Then this compound was oxidized with permanganate to selectively remove the side chain at the N-position to afford the 4-methylthioazetidinone derivative. A facile conversion of the methylthio group to acetoxy for a practical synthesis of 4-acetoxyazetidinone was also reported. This method provided an efficient and cost-effective process with good overall yield and excellent stereoselectivity.

Osmium‐Mediated Oxidative Cyclizations: A Study into the Range of Initiators That Facilitate Cyclization

A general route to prepare substituted, saturated five-membered heterocycles has been developed. The application of a wide range of starting materials to the osmium-catalyzed oxidative cyclization reaction is described. Diols, hydroxy-amides, hydroxy-sulfonamides, and carbamates all cyclize in moderate to excellent yields to give cis-tetrahydrofurans and pyrrolidines, depending upon the position of the heteroatoms in the starting materials. These cyclizations all proceed with near total selectivity for the cis-heterocycles, and with stereospecific introduction of a hydroxy group adjacent to the ring. Moreover, routes to enantiopure starting materials are described, which give enantiopure products upon cyclization. Catalyst loadings of as low as one mol percent have been successfully employed for this transformation.

Enantiospecific synthesis of [2.2]paracyclophane-4-thiol and derivatives

This paper describes a simple route to enantiomerically enriched [2.2]paracyclophane-4-thiol via the stereospecific introduction of a chiral sulfoxide to the [2.2]paracyclophane skeleton. The first synthesis of an enantiomerically enriched planar chiral benzothiazole is also reported.

Total Syntheses of Racemic, Natural (−) and Unnatural (+) Glyceollin I

The first total syntheses of racemic glyceollin I and its enantiomers are described. A Wittig approach was utilized as an entry to the appropriately substituted isoflav-3-ene so that an osmium tetroxide mediated asymmetric dihydroxylation could be deployed for stereospecific introduction of the 6a-hydroxy group. While using triphenylphosphine hydrobromide, a novel method was found for gently removing MOM from protected phenolic hydroxyl groups present within sensitive systems.

A Stereospecific Entry to Functionalized cis‐1,2‐Difluoroalkenes

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Stereospecific aziridination of olefins via electrophile-induced cyclization of γ,δ-unsaturated imines and subsequent hydrolytic rearrangement

The olefinic bond of gamma,delta-unsaturated aldehydes underwent a net aziridination through electrophile-induced cyclization and subsequent rearrangement of the resulting cyclic iminium salts: this methodology allows the stereospecific introduction of aziridine moieties into cyclic systems.

Ring‐Opening Metathesis/Oxy‐Cope Rearrangement: A New Strategy for the Synthesis of Bicyclic Medium Ring‐Containing Compounds.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Ring-opening metathesis/oxy-cope rearrangement: a new strategy for the synthesis of bicyclic medium ring-containing compounds.

Ring-opening/ring-closing metathesis on cyclobutene-containing substrates with angular oxygen functionality provides a stereospecific introduction of 1,5-bis-dienes required for an anion-accelerated oxy-Cope rearrangement. The reaction sequence offers generally a stereocontrolled preparation of a variety of medium ring-containing bicyclic ring systems, while rearrangement to the bicyclo[7,3,0]dodecane (9-5) system leads to a mixture of olefin isomers.

The first enantiospecific synthesis of (−)-koumidine via the intramolecular palladium-catalyzed enolate driven cross coupling reaction. The stereospecific introduction of the 19-(Z) ethylidene side chain

Abstract The stereospecific synthesis of (−)-koumidine was realized by replacement of triphenylphosphine with tricyclohexylphosphine in the enolate driven palladium-catalyzed cross coupling process.

A facile asymmetric route to (-)-aphanorphine.

8O-methylaphanorphine was synthesized from 4-methoxyphenylacetaldehyde in 36% overall yield and in nine steps, featuring the formation of ring B via a Friedel-Crafts alkylative cyclization with the concomitant stereospecific introduction of the benzylic quaternary carbon center. The current work constitutes an efficient enantioselective formal synthesis of 3-benzazepine marine alkaloid (-)-aphanorphine.

Biomimetic Entry to the Sarpagan Family of Indole Alkaloids: Total Synthesis of (+)-Geissoschizine and (+)-N-Methylvellosimine.

For Abstract see ChemInform Abstract in Full Text.

Total synthesis of hydroxymethyl isonucleosides as potential antiviral agents

Hydroxymethyl isodideoxynucleosides, designed as potential antiviral agents, have been synthesized through development of a multi-step procedure starting from furan and cyanovinyl acetate. Key steps in the synthesis include a Diels-Alder reaction, oxidative cleavage of alkene, a Mitsunobu reaction, stereospecific introduction of amino group and nucleobase construction.