Preparation of Optically Active Biphenyl Compounds via an Albumin- Mediated Asymmetric Nitroaldol Reaction

Abstract:

Human serum albumin (HSA) was found to catalyze the asymmetric nitroaldol reaction of biphenyl aldehydes with nitromethane to afford the corresponding optically active 2-nitro alcohols. Careful optimization of the conditions for the reaction of 4-phenylbenzaldehyde with nitromethane in water at a neutral pH improved both the reactivity and the enantioselectivity. Finally, the reaction of 4-phenylbenzaldehyde (56 mg, 0.30 mmol) in nitromethane (2.8 mL) and water (1.1 mL) using HSA (68 mg) at 5 °C for 240 h gave (R)-1-([1,1'-biphenyl]-4-yl)-2-nitroethanol in 71% yield (52 mg), with an ee up to 85% ee. Subsequent recrystallization improved the ee up to 95%. The reaction was useful in a preparative-scale operation, and the biocatalyst could be reused several times. The procedure was also applicable to other substrates with different substitution patterns. Although the nitroaldol reaction of 2-phenylbenzaldehyde with nitromethane proceeded with low enantioselectivity to afford the corresponding (R)-2-nitroalcohols (35% ee), the reactions of the substrates bearing Br, Me, OMe, or CN group at the 4'-position of the benzene ring gave the corresponding optically active compounds with high enantioselectivities (80-88% ee).

Synthesis and practical applications of 2-(2-nitroalkyl)pyrroles

Functionalization of pyrroles introducing a 2-nitroalkyl moiety allows the formation of nitro-containing compounds to be used as pivotal intermediates for the synthesis of bioactive compounds. The reaction of pyrroles with nitroalkenes under the Friedel-Crafts conditions allows a direct entry to 2-(2-nitroalkyl)pyrroles. This approach can also be used for the preparation of enantioenriched derivatives exploiting asymmetric catalysis. In a complementary fashion, the Henry reaction between 2-formylpyrroles and nitroalkanes generates the corresponding nitroaldol products which upon dehydration and reduction of the intermediate 2-pyrrolylnitroethene efficiently afford 2-(2-nitroalkyl)pyrroles. This review article summarizes the most relevant procedures for the preparation of 2-(2-nitroalkyl)pyrroles during the last two decades as well as their significant practical applications.

Photothermal conversion triggered thermal asymmetric catalysis within metal nanoparticles loaded homochiral covalent organic framework

For seeking high enantiopurity, the previously reported thermal asymmetric catalysis is usually carried out at low temperature sometimes with limited yield, that is, the high enantiomeric excess (ee) usually at the cost of high yield. Thus, the achieving both high stereoselectivity and yield is an enormous challenge. We report herein two metal nanoparticle (M NP)-loaded and porphyrin-containing homochiral covalent organic framework (CCOF)-based composite catalysts, and their application in the thermally-driven asymmetric one-pot Henry and A³-coupling reactions. All the reactions are conducted at elevated temperatures with both excellent stereoselectivity and yield which resulted from the synergy of CCOF confinement effect and M NP catalytic activation. Notably, the needed thermal energy for the asymmetric reactions herein is derived from the photothermal conversion via porphyrin-based CCOF upon irradiation with visible light. Remarkably, the CCOF confinement effect can be effectively maintained up to 100 °C for the asymmetric one-pot Henry and A³-coupling reactions herein.

Achieving both high stereoselectivity and yield is a challenge for conventional asymmetric catalysis. Here, the authors report two metal nanoparticle-loaded and porphyrin-containing homochiral covalent organic framework-based composite catalysts that exhibit high stereoselectivity and yield in the thermally-driven asymmetric reactions.

Copper-catalyzed oxidative coupling of quinoxalin-2(1H)-ones with alcohols: access to hydroxyalkylation of quinoxalin-2(1H)-ones

An efficient protocol for the synthesis of hydroxyl-containing quinoxalin-2(1H)-ones has been developed via the copper-catalyzed cross-coupling reaction of quinoxalin-2(1H)-ones with alcohols with moderate to good yields.

In-depth structure–selectivity investigations on asymmetric, copper-catalyzed oxidative biaryl coupling in the presence of 5-cis-substituted prolinamines

Copper complexes of 5-cis-substituted prolinamines provided up to 87% ee in the enantioselective oxidative biaryl coupling of 3-hydroxy-2-naphthoates.