Microwave-Assisted Soluble Polymer-Supported Synthesis of Benzo­piperazinones

Abstract

Microwave-assisted liquid-phase combinatorial synthesis of quinoxaline-2-ones is reported. Immobilized aryl fluoride was aminated via ipso-fluoro displacement and then reduced the adjacent nitro group to give polymer bound o-phenylenediamine in the microwave oven. Spontaneous condensation and intramolecular cyclization of diamines with chloroacetyl chloride under microwave irradiation resulted in the formation of quinoxaline-2-ones as a combinatorial scaffold. Analytically pure target molecules were isolated after cleavage of the polymer support. All reactions involved were performed completely within a few minutes under microwave exposure.

Key words

microwave-assisted - combinatorial synthesis - quinoxaline-2-ones

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  Jacobsen et al. reported the solution phase synthesis of 1,2,3,4-tetrahydroquinoxalin-2-ones using similar cyclization. They observed primary amino group stands to be more nucleophilic than adjacent substituted NH group. Probably steric hindrance of t-butyl group is the key factor to facilitate the acid chloride to opt primary amine in their work (Scheme [3] ).

13

Discover focused microwave oven from CEM was used for our study.

16

General Experimental Procedure to Prepare 1,2,3,4-Tetrahydroquinoxalin-2-ones: All the microwave assisted polymer-supported reactions described here were performed in a 100 mL round bottom flask (attached to the reflux condenser) with CEM Discover Microwave System at a frequency of 2450 Hz (0-300 W). A mixture of PEG bound diamine 5m, freshly distilled chloroacetyl chloride (4 equiv), and Et₃N (4 equiv) in CH₂CL₂ (10 mL) was irradiated under CEM focused microwave (150 W) for 6 min. Completion of the reaction was confirmed by monitoring on ¹H NMR. Reaction mixture was then precipitated out by addition of EtOH (80 mL) under vigorous stirring. The separated precipitate of PEG bound quinoxalinone 8m was filtered and washed thoroughly with Et₂O and dried in vacuum. Polymer support of 8m was cleaved by exposing it to microwave (150 W) for 10 min in sodium methoxide solution. Progress of cleavage was monitored on TLC [solvent: EtOAc-n-hexane (1:2)]. Separation of 2-arm PEG from the desired products was achieved by precipitation in ethanol to give crude mixtures of compounds 9m and 10m in 90% combined yield and 93% (53% and 40%) HPLC purity (UV detection at λ = 254 nm. Column: Sphereclone 5u Si (250 × 4.6 nm); gradient: 50% EtOAc/n-hexane; flow rate: 1 mL/min).