Organic Letters

The chemoenzymatic synthesis of heparan sulfate tetrasaccharide (1) and hexasaccharide (2) with a fluorous tag attached at the reducing end is reported. The fluorous tert-butyl dicarbonate ((F)Boc) tag did not interfere with enzymatic recognition for both elongation and specific sulfation, and flash purification was performed by standard fluorous solid-phase extraction (FSPE). Based on an (F)Boc attached disaccharide as acceptor, a series of partial N-sulfated, 6-O-sulfated heparan sulfate oligosaccharides were successfully synthesized employing fluorous techniques...

The synthesis of N-aminoimidazolidin-2-one (Aid) peptidomimetics has been achieved by alkylation of the urea nitrogen of a semicarbazone residue using ethylene bromide. The Aid scaffold combines electronic and structural constraints to rigidify the peptide backbone in the equivalent of an aza variant of a Freidinger-Veber lactam. The syntheses and isolation of 25 Aid peptides, including eight GHRP-6 analogues, are reported to demonstrate the utility of this method for controlling conformation.

A strategy for the synthesis of spiroketal compounds through a tandem iodocyclization of 1-(2-ethynylphenyl)-4-hydroxybut-2-yn-1-one derivatives is presented. This reaction could proceed under very mild conditions in a short time and avoid the use of expensive and toxic metal catalysts. Moreover, the resulting halides can be further exploited by subsequent palladium-catalyzed coupling reactions, which act as the important intermediates for building other valuable compounds.

α-Heteroarylpyrrolidines have been efficiently prepared via 1,3-dipolar cycloaddition between silylimines and activated olefins. In the presence of Cu(CH3CN)4PF6/Walphos as catalytic system, high levels of enantioselectivity (up to ≥99% ee) and diastereoselectivity were achieved (major formation of C-2/C-4 trans-substituted pyrrolidines). The reaction is compatible with a broad variety of dipolarophiles including maleimides, maleates, fumarates, nitroalkenes, and vinylsulfones. The resulting cycloadducts can be transformed into bioactive pyrrolidine derivatives...

An enantioselective Reformatsky reaction has been developed for the reaction of ethyl dibromofluoroacetate (1) with an imine. This method represents the first ligand-promoted imino-Reformatsky approach to use a halofluoroacetate. The use of an amino alcohol ligand allowed for the preparation of enantioenriched α-bromo-α-fluoro-β-lactams in good yields with enantioselectivities up to 96% ee. This process also provided access to β-lactam rings bearing two stereogenic centers.

Radical biaryl coupling of iodoarenes and benzene can be effectively promoted by tetrabutylammonium cyanoborohydride with air under photoirradiation conditions using a Xe lamp. The utility of this methodology is highlighted by its functional group tolerance and chemoselectivity.

Described here is the design, synthesis, and conformational analysis of cyclic tetrapeptides (CTPs) with α3γ architecture containing a furan-based locked Z-vinylogous amino acid (Vaa). This unnatural amino acid locks into a γ-turn that induces type IαRS-turn in the CTPs. Stabilized by a 13-membered intramolecular H-bond, these CTPs show robust conformation in water and aprotic solvent irrespective of the sequence of tripeptide consisting of α-amino acids used.

High contents of curcusones A and B and trace amounts of spirocurcasone exist in the roots of Jatropha curcas. Here, a one-step semisynthesis method of spirocurcasone and pyracurcasone was built, not only resulted an increased yield of spirocurcasone but also produced pyracurcasone, which exhibited greater cytotoxicity compared to curcusones A and B. The plausible mechanism of the formation of pyracurcasone was proposed, and the proposed biogenetic origin for spirocurcasone by Taglialatela-Scafati was confirmed...

An organocatalytic asymmetric trifluoromethylthiolation reaction via in situ generation of active electrophilic trifluoromethylthio species involving trichloroisocyanuric acid and AgSCF3 as a practical and easily handled electrophilic SCF3 source for CSP(3)-SCF3 bond formation was developed. Reactions with this one-pot version strategy occurred in good yields and excellent stereoselectivities to access enantiopure oxindoles bearing a SCF3-substituted quaternary chiral center. The straightforward process described here makes use of simple starting materials and proceeds under mild conditions, which will be useful in medicinal chemistry and diversity-oriented syntheses...

DFT (M06-L) calculations on the transition state for the 1,3-dipolar cycloadditions between substituted vinyl sulfones with sugar azide have been reported in conjunction with new experimental results, and the origin of reversal of regioselectivity has been revealed using a distortion/interaction model. This study provides the scientific justification for combining organic azides with two different types of vinyl sulfones for the preparation of 1,5-disubstituted 1,2,3-triazoles and 1,4-disubstituted triazolyl esters under metal-free conditions...

Enantioselective syntheses of FR901464 and spliceostatin A, potent spliceosome inhibitors, are described. The synthesis of FR901464 has been accomplished in a convergent manner in 10 linear steps (20 total steps). The A-tetrahydropyran ring was constructed from (R)-isopropylidene glyceraldehyde. The functionalized tetrahydropyran B-ring was synthesized utilizing a Corey-Bakshi-Shibata reduction, an Achmatowicz reaction, and a stereoselective Michael addition as the key steps. Coupling of A- and B-ring fragments was accomplished via cross-metathesis...

Thermally and in nonprotic media N,N-dimethyl-2-propyn-1-amine N-oxide undergoes two consecutive sigmatropic rearrangements affording propenal. Two molecular containers capable of the quantitative inclusion/encapsulation of the N-oxide are described. The N-oxide becomes kinetically stabilized when included in the containers. The relationship between the observed kinetic stabilization of the N-oxide and the thermodynamic and kinetic stability of its inclusion complexes is explained and modeled.

Efficient aerobic oxidative methyl esterification of primary alcohols has been achieved with a heterogeneous catalyst consisting of 1 mol % Pd/charcoal (5 wt %) in combination with bismuth(III) nitrate and tellurium metal. The Bi and Te additives significantly increase the reaction rate, selectivity, and overall product yields. This readily accessible catalyst system exhibits a broad substrate scope and is effective with both activated (benzylic) and unactivated (aliphatic) alcohols bearing diverse functional groups...

A new keto reductase (ArQR), identified from Agrobacterium radiobacter ECU2556, can efficiently reduce 3-quinuclidinone in excellent enantioselectivity and high space-time yield for the synthesis of (R)-3-quinuclidinol, a chiral building block of many antimuscarinic agents. This is the first time that a high yield of (R)-3-quinuclidinol up to 916 g L(-1) d(-1) using a bioreduction approach is reported.

1,4-Naphthalenediyl-bridged macrocages (2, 3, and 4) were synthesized as novel molecular gyrotops. Compound 2 (C14 chains) does not show rotation of the naphthalene ring about an axis in solution. The 1,4-naphthalenediyl moieties of compounds 3 (C16 chains) and 4 (C18 chains) show restricted and rapid rotation inside the cage in solution, respectively. Therefore, steric protective effects on the rotation of the rotor in molecular gyrotops can be controlled by changing the size of the cage.

Monosubstituted γ(4)-residues (γ(4)Leu, γ(4)Ile, and γ(4)Val) form helices even in short homooligomeric sequences. C14 helix formation is established by X-ray diffraction in homooligomeric (γ)n tetra-, hexa- and decapeptide sequences demonstrating the high propensity of γ residues, with proteinogenic side chains, to adopt locally folded conformations.

A general synthesis of dibenzoxepine lactams has been developed using a one-pot Cu-catalyzed etherification/aldol condensation cascade reaction. The reaction of 4-hydroxyisoindolin-1-one with a wide range of 2-bromobenzaldehydes in the presence of a copper catalyst provided various aristoyagonine derivatives in good yields.

A conceptually new strategy has been described for the mild, practical, and environmentally friendly preparation of naphthols and furans using a visible-light promoted photoredox neutral approach. These reactions between accessible electron-deficient bromides and commercially available alkynes could be carried out at room temperature in good-to-excellent chemical yields without any external stoichiometric oxidants.

The development and validation of copper-catalyzed, electrophilic amination of aryl and heteroaryl organolithiums with N,N-dialkyl-O-benzoylhydroxylamines have been achieved exploiting recoverable siloxane transfer agents. Given the ready availability of organolithium compounds, the mild reaction conditions, the ease of product purification, and the ready recovery of the siloxane transfer agents, this transformation comprises a useful tactic to access diverse aryl and heteroaryl amines.

An NHC-catalyzed sila-Stetter reaction between aliphatic acylsilanes and vinylketones bearing silyl ether substituents affords functionalized 1,4-diketones, which upon treatment under acidic conditions leads to the corresponding bis-spiroacetals. The two-step sequence may also be carried out in a one-pot operation leading to high yields of the desired bis-spiroacetals.