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Task-Specific Ionic Liquids
Published in Pedro Lozano, Sustainable Catalysis in Ionic Liquids, 2018
The use of hypervalent iodine organo-reagents for highly selective oxidation of alcohols to carbonyl compounds is not uncommon. The pentavalent iodine reagents like Dess-Martin periodane and o-iodoxybenzoic acid are often used as oxidizing agents to achieve efficient and selective oxidation of alcohols to carbonyl compounds in dimethyl sulfoxide, CH2Cl2, and acetone. However, the use of these iodine (V) oxidants is not safer because they can explode. Many reaction protocols have been reported. Qian et al. incorporated the hypervalent iodine reagent to ionic liquid, Figure 1.25, and have reported some additional advantages over the routine procedure [49].
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]