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Five-Membered Fused Polyheterocycles
Published in Navjeet Kaur, Metals and Non-Metals, 2020
Gold-catalyzed allene cycloisomerization with enantioenriched allene substrates yields (–)-isochrysotricine and (–)-isocyclocapitelline enantioselectively. Similarly, the A-D rings of azaspiracid containing trioxadispiroketal core structure are prepared employing a gold-catalyzed nucleophilic bis-spiroketalization (Scheme 14) [47a–d]. Epoxyalcohol, when treated with Dess-Martin periodinane (or with IBX, 88% yield), results in the formation of a configurationally stable ketone. Subsequent Grignard addition and SN2-substitution of the tertiary alcohol to allene occurs without any loss of stereochemical information (> 98% enantiomeric excess) and with excellent yield. The axis-to-center chirality transfer following the above addition and substitution proceeds through gold-catalyzed cycloisomerization (0.05 mol% gold(III) chloride in tetrahydrofuran) of allene to provide a key intermediate with high stereochemical purity (96% de, > 98% enantiomeric excess) in excellent yields (97%).
Recent Advances in Nanostructured Enzyme Catalysis for Chemical Synthesis in Organic Solvents
Published in Grunwald Peter, Biocatalysis and Nanotechnology, 2017
Zheng Liu, Jun Ge, Diannan Lu, Guoqiang Jiang, Jianzhong Wu
To improve enzyme performance in common organic solvents, we synthesized a new class of enzyme-polymer nanoconjugates (Zhu et al., 2013). The nano-conjugates consist of enzymes and Pluronic, a copolymer with a hydrophobic backbone (poly (propylene oxide)) flanked by two hydrophilic side chains (poly(ethylene oxide)). The first step is to oxidize Pluronic F-127 (POH) with Dess–Martin periodinane to convert its hydroxyl end-groups into aldehyde functionalities (PCHO). The PCHO is then conjugated to the lysines of protein by forming a Schiff base followed by a reduction with NaCNBH3. The resulted nanoconjugates are readily dissolvable in organic solvents at high temperature and show greatly enhanced catalytic activities compared with their native counterparts. For example, the enzyme activity gains a 67-fold increase for lipase and a 670-fold increase for cytochrome c (Fig. 11.5).
Methods for the direct synthesis of thioesters from aldehydes: a focus review
Published in Journal of Sulfur Chemistry, 2020
Noor H. Jabarullah, Kittisak Jermsittiparsert, Pavel A. Melnikov, Andino Maseleno, Akram Hosseinian, Esmail Vessally
In 2007, Bandgar and co-workers found that aldehydes 9 were readily converted into the corresponding thioesters 11 by treatment with aromatic thiols 10 in the presence of 6 equiv. of Dess–Martin periodinane and 6.5 equiv. of NaN3 in DCM at room temperature (Scheme 4) [41]. Various aromatic, heteroaromatic, and aliphatic aldehydes were used to establish the general applicability of this metal-free cross-coupling reaction. However, this procedure was not applicable for the synthesis of α,β-unsaturated thioesters from the corresponding alkenyl aldehydes. The results proved that the electron-rich aldehydes afforded better yields compared to the electron-deficient ones. It should be mentioned that these mild reaction conditions were also successfully applied for the synthesis of the same products via the reaction of the corresponding aldehydes with disulfides. Unfortunately, the authors did not propose a viable mechanism for this dehydrogenative cross-coupling reaction.
Copper-assisted synthesis of five-membered O-heterocycles
Published in Inorganic and Nano-Metal Chemistry, 2020
Navjeet Kaur, Yamini Verma, Neha Ahlawat, Pooja Grewal, Pranshu Bhardwaj, Nirmala Kumari Jangid
The ester[78] was transformed into enynoate by a reduction–olefination one-pot sequence[79] (Scheme 22). The secondary alcohol was formed by standard reduction–oxidation–Grignard addition. The enyne was an excellent precursor for a kinetic resolution by Katsuki–Sharpless epoxidation;[80–82] with L-(+)-diethyl tartrate, both the unreacted starting compound (R)-secondary alcohol (43% yield) and the epoxide ent-oxirane (42% yield, not shown) were obtained with >98% ee. The enyne (R)-secondary alcohol, by a matched Katsuki–Sharpless epoxidation using D-(−)-diethyl tartrate, was transformed into oxirane. The key steps involved in the preparation were the anti-selective Cu-mediated SN2-substitution of propargyl oxirane, employing a triphenylphosphite and methylmagnesium–cyanocuprate as ligand to Cu,[83] and the Au-catalyzed cycloisomerization of dihydroxyallene. This center-to-axis-to-center chirality transfer occurred in good yield and with excellent stereoselectivity. The cycloisomerization of dihydroxyallene was the most efficient in homogeneous Au catalysis in the presence of a catalyst loading of 0.05 mol% gold(III) chloride in tetrahydrofuran. On the other hand, the transformation of secondary alcohol into the tertiary alcohol was tricky. The oxidation of secondary alcohol to ketone occurred with Dess–Martin periodinane in dichloromethane (91% yield) or with 2-iodoxybenzoic acid (IBX) in dimethylsulfoxide (79% yield). Unfortunately, this ketone underwent epimerization (through enol), so that the subsequent Grignard addition formed alcohol as a 60:40 mixture of diastereomers. This epimerization occurred easily because the ketone bearing a benzyloxymethyl in place of benzyloxyethyl side chain was configurationally stable under the same conditions.[84]