Stereoselective Opening Research Articles

ChemInform Abstract: Regio‐ and Stereoselective Ring Opening of Enantiomerically Enriched 2‐Aryl Oxetanes and 2‐Aryl Azetidines with Aryl Borates.

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 of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Syndiotactic-Enriched Poly(3-hydroxybutyrate)s via Stereoselective Ring-Opening Polymerization of Racemic β-Butyrolactone with Discrete Yttrium Catalysts

The mechanism of the ring-opening polymerization of rac-β-butyrolactone using yttrium complexes supported by dianionic aminoalkoxybis(phenolate) ligands as initiators has been investigated by NMR and shown to occur via a coordination-insertion pathway. The microstructure of the resulting syndiotactic-enriched poly(3-hydroxybutyrate)s (PHBs, Pr up to 0.94) has been studied by 13C NMR spectroscopy, enabling a detailed assignment of resonances at the diad and triad levels. On this basis, a statistical Bernoullian analysis has been performed which evidenced that syndioselectivity originates from a chain-end control. Some thermal properties of these PHBs have been studied by WAXD and thermoanalytical techniques and shown to be markedly affected by the syndiotacticity degree. This is especially the case for the melting temperature which raises up to 183 °C for Pr = 0.94, a temperature higher than that of pure isotactic PHB (ca. 180 °C).

Stable Aziridinium Salts as Versatile Intermediates: Isolation and Regio- and Stereoselective Ring-Opening and Rearrangement

Rock trapping and exploration: Aziridinium bromide salts were discovered serendipitously during bromination of N,N-dicarboxymethylated beta-amino alcohols. Regiospecific ring-opening and rearrangement of the isolated, surprisingly stable aziridinium salts produces useful molecules including C-functionalized oxomorpholines and alpha,beta-unsaturated amines.

Regio- and Stereoselective Ring-Opening Reaction of Chiral Aziridines: A Facile Synthesis of Chiral β-Amino Alcohols

The reaction of chiral 2-alkylsubstituted aziridines (1a-d) with acetic acid afforded β-amino alcohols with an (S)-chiral center at the β position (with respect to oxygen). In contrast, a reaction of the same chiral aziridines with acetyl chloride followed by treatment with water gave β-amino alcohols that have an (S)-chiral center at the a position. In addition, the reaction of the same chiral aziridines with benzyl bromide followed by treatment with 2% sulfuric acid gave the β-amino alcohols with an (R)-chiral center at the a position. Thus, we could control the regioselectivity and stereoselectivity of the ring-opening reaction of chiral aziridines (1a-d).

Convenient Synthesis of Bicyclic Carbohydrate 1,2‐Lactones and Their Stereoselective Opening to 1‐Functionalized Glucose Derivatives

Closed and re-opened for business: C-2 branched carbohydrates 1 cyclize under conditions of decarboxylation to the hitherto unknown carbohydrate 1,2-lactones 2 in high yields. The gluco isomer can be opened at the anomeric position with various nuceophiles in the presence of Sc(OTf)3, which allows the stereoselective synthesis of 1-functionalized glucose derivatives 3. Thus, 1,2-bis-C-branched saccharides become available in only a few steps starting from glycals.

Synthesis of Chiral Organocatalysts derived from Aziridines: Application in Asymmetric Aldol Reaction

We report the synthesis of a new series of highly efficient chiral organocatalysts derived via the regio- and stereoselective ring opening of chiral aziridines with azide anions. The catalysts have proved to be very efficient for a direct asymmetric aldol reaction, both with cyclic as well as acyclic ketones in brine with 2 mol % of catalyst loading, and afforded the products in excellent yields (up to 99%) and enantioselectivities (up to >99%). The chiral aldol adduct obtained has further been converted to a chiral azetidine ring via a convenient pathway.

Formal total synthesis of (–)- and (+)-balanol: two complementary enantiodivergent routes from vinyloxiranes and vinylaziridines

Formal total syntheses of both enantiomers of balanol have been achieved by the preparation of the protected hexahydroazepine core 2. Two complementary routes have been investigated. The first relied on the regioselective opening of 1,2-epoxycyclohex-3-ene with a chiral-auxiliary version of the Burgess reagent to provide a diastereomeric pair of cis-fused cyclic sulfamidates. The sulfamidates were transformed to trans-amino benzoates with ammonium benzoate and, after separation, converted to (−)- 2 and (+)- 2 by oxidative cleavage and reductive amination. The second approach utilized vinylaziridines derived from 1-bromo-2,3-dihydroxycyclohexa-4,6-diene obtained by the whole-cell fermentation of bromobenzene with Escherichia coli JM109(pDTG601). Stereoselective opening of the aziridines generated the requisite trans-amino alcohol derivatives, which after saturation of the vinyl bromide moieties were converted to (−)- 2 and (+)- 2 by oxidative cleavage of the cis-diol and reductive amination. Experimental and spectral data are provided for all new compounds.

Regio- and Stereoselective Ring Opening of Enantiomerically Enriched 2-Aryl Oxetanes and 2-Aryl Azetidines with Aryl Borates

The regioselective ring opening of 2-aryl-substituted four-membered heterocyclic rings with phenols, including catechol, was achieved by the use of aryl borates in mild and neutral reaction conditions without the aid of any transition metal catalysts. While N-alkyl azetidines were found not to be reactive, optically active N-tosyl azetidines gave the corresponding beta-aryloxy amines in a racemic form, thus indicating the considerable carbocationic character of the transition state. The introduction of a hydroxyl group in the azetidine ring (i.e., an azetidinol), able to anchor the aryl borate and to direct the subsequent nucleophilic delivery, was shown to determine the ring-opening process with predominant inversion of configuration. When enantiomerically enriched 2-aryl oxetanes were used, the reduced extent of racemization observed (up to 93:7 er) was rationalized by an intramolecular delivery through a six-membered transition state, giving beta-aryloxy alcohols with a predominant retention of configuration (i.e., a syn-stereoselective ring opening). The aryloxy alcohols obtained, endowed with suitable functionalities, can be cyclized to give access to enantiomerically enriched 2-aryl-1,5-benzodioxepins.

A Stereoselective Approach for the Total Synthesis of Clonostachydiol

A stereoselective synthesis of clonostachydiol is accomplished using readily available (±)-epichlorohydrin as a precursor. The synthesis involves direct and straightforward reactions such as Sharpless asymmetric epoxidation, iodination, stereoselective opening of epoxide with allylmagnesium chloride and Sharpless asymmetric dihydroxylation.

18F-labelling of A-ring substituted 16α-fluoro-estradiols as potential radiopharmaceuticals for PET imaging

The 2-methoxy derivative of estradiol is currently in Phase II clinical trial as an anticancer agent while the 4-methyl derivative has been shown to interact with cytoplasmic and nuclear estrogen receptors in rat pituitary gland and hypothalamus. We hypothesize that the 16α- 18F-analogs of these estrogens could be suitable radiotracers to evaluate action mechanisms of the parent compounds. In this study we report the synthesis of the 16α- 18F and 16α- 19F-analogs of the A-ring substituted estradiols in high yield via stereoselective opening of the intermediate 16β,17β-O-cyclic sulfones with [ 18F]F − or F − followed by deprotection.

Total synthesis of a xylo-Puromycin analog

N(6)-bis-demethylated xylo-Puromycin analog 2 was synthesized in 56% over 6 steps from adenosine 3, involving a Mattocks bromo acetylation, a regio- and stereo-selective ribo-epoxide ring opening with sodium azide and an efficient Staudinger-Vilarrasa coupling reaction for which the conditions have been optimized.

ChemInform Abstract: Regio‐ and Stereoselective Ring Opening of Epoxides with Sodium Phenylselenide under Phase Transfer Conditions.

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 of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Synthesis and elaboration of trans 2,3-diaryloxiranes

Diaryloxiranes represent challenging intermediates for both their preparation and synthetic elaboration. Provided their synthesis in enantiopure form, they can be considered as suitable starting material for the synthesis of functionalized 1,2-diarylethanols, which are easily found in chiral auxiliaries, ligands and organocatalysts. The most efficient asymmetric methods for their preparation and synthetic elaboration are herein described. Regio- and stereoselective ring opening reactions, as reduction, nucleophilic opening with halides and oxygen nucleophiles have allowed straightforward access to versatile chiral intermediates such as aniline alcohols, halohydrins and 1,3-dioxolanes, their transformation affords chiral bases and bi- or tridentate ligands to use in asymmetric synthesis.

Synthesis of Acyl Aminocyclopentenes with Acyl Anion Nucleophiles

The stereoselective ring-opening of diazabicycle 1 with acyl anions yielded only the anti diastereomer of 3. Under the mild reaction conditions, no isomerization of the alkene occurs in the final product. Previous Rh-catalyzed methods of carbonlyation of alkenes involved either high pressures of CO, and/or high temperatures. As a proof of concept, the procedure demonstrated that acyl anion equivalents can be generated under very mild and practical conditions. The methodology reported opens up the door to further study of the Rh-acylanion complex behavior.

Enantioselective Synthesis of P-Chirogenic Phosphorus Compounds via the Ephedrine-Borane Complex Methodology

The stereoselective synthesis of P-chirogenic organophosphorus compounds using the versatile ephedrine-methodology is reported. The principle of the method is based on the diastereoselective preparation of 1,3,2-oxazaphospholidine borane 3 from ephedrine 2 and regio- and stereoselective ring-opening of the heterocycle leading to an aminophosphine borane 4. Finally, these ring-opening products 4 are easily transformed into P-chirogenic organophosphorus compounds, by subsequent highly stereoselective reactions involving electrophilic or nucleophilic strategies.

Ultrasound promoted aminolysis of epoxides in aqueous media: A rapid procedure with no pH adjustment for additive-free synthesis of β-aminoalcohols

An efficient and environmentally friendly procedure promoted by ultrasound irradiation is developed for stereoselective ring opening of various epoxides with aromatic and aliphatic amines under aqueous conditions in the presence of no catalyst or additive. Chemoselectivity of the protocol is shown by competition of piperidine and aniline to react with different epoxides resulting in exclusive formation of the respective products of piperidine.

Regio-and Stereoselective Ring Opening of Epoxides with Sodium Phenylselenide under Phase Transfer Conditions

The ring opening reaction of a wide range of mono-, di- and trisubstituted epoxides with the phenylselenide anion under PTC conditions provides a simple, mild and efficient method for preparation of β-hydroxy phenylselenides with high regio- and stereoselectivity in yields from 67 to 98%. The reactions are carried out in aqueous NaOH/ THF using aminoiminomethanesulfinic acid (thiourea dioxide, TDO) to generate sodium phenylselenide from diphenyldiselenide.

パラジウム触媒を用いたH-MAC-TBSによるアルケニルアジリジンの高立体選択的開環反応

パラジウム触媒を用いたH-MAC-TBSによるアルケニルアジリジンの高立体選択的開環反応

The Development of Scalemic Multidentate Niobium Complexes as Catalysts for the Highly Stereoselective Ring Opening of meso‐Epoxides and meso‐Aziridines.

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.

Stereoselective Synthesis of Methyl 7-Dihydro-trioxacarcinoside B

A stereoselective synthesis of 7-dihydro-triocacarcinose B, a branched octose from the quinocyclines, has been achieved. The biocatalytic resolution of a Baylis-Hillman adduct and a subsequent ring-closing metathesis were used to assemble the molecular framework. Subsequent key steps were a highly stereoselective epoxidation and a regio- and stereoselective opening of the epoxide by allyl alcohol and HClO4 to introduce the C(3)-OH group in protected form. The 7-dihydro-triocacarcinose B could be converted into the corresponding 1,7-anhydrosugar.