Squaric acid: an impressive organocatalyst for the synthesis of biologically relevant 2,3-dihydro-1H-perimidines in wate
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Ó Indian Academy of Sciences Sadhana (0123456789().,-volV)FT3](0123456 789().,-volV)
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Squaric acid: an impressive organocatalyst for the synthesis of biologically relevant 2,3-dihydro-1H-perimidines in water SUSHIL KHOPKAR and GANAPATI SHANKARLING* Dyestuff Technology Department, Institute of Chemical Technology, N. P. Marg, Matunga, Mumbai 400 019, Maharashtra, India E-mail: [email protected] MS received 8 May 2019; revised 23 September 2019; accepted 13 October 2019
Abstract. Squaric acid, a green, metal-free and eco-friendly organocatalyst, has been exploited for the synthesis of biologically interesting 2,3-dihydro-1H-perimidines. The reaction was performed using water as a green reaction medium and the organocatalyst can be easily recovered and reused up to four consecutive cycles without much decrease in catalytic activity. Several advantages of the present methodology are low catalyst loading, excellent catalytic performance, easy catalyst handling, mild reaction conditions, operational simplicity, ease of product isolation, avoidance of column chromatographic separation, a higher yield of the desired product in short reaction time, high atom economy and synthesis of benzophenone based perimidine. Keywords. Squaric acid; organocatalyst; 2,3-dihydro-1H-perimidines; green chemistry.
1. Introduction Perimidine derivatives are one of the most important class of nitrogen-containing heterocycles which have diverse biological and pharmacological properties comprising anti-bacterial, anti-microbial, anti-cancer, antiinflammatory activities.1–5 The interest in perimidines originates from the presence of this heterocyclic system in various biologically and pharmacologically active compounds. Due to strong electron-donating character of 2,3-Dihydro-1H-perimidine derivatives, they have been used as intermediates in the synthesis of symmetrical squarylium dyes to shift the absorption at NIR region.6,7 In addition, these compounds have also been used as antioxidant stabilizers,8 photochromic compounds,9 catalysts,10 ligand scaffolds11 and stoppers for supramolecule.12 The structures of a few perimidines with different applications are depicted in Figure 1. The reaction of 1,8-diamino naphthalene and acetophenones have been accelerated by various protonic acids and metal catalysts such as BiCl3,13 RuCl3,14 Yb (OTf)3,15 HBOB,16 Amberlyst-15,17 CMK-5-SO3H18 and MNPs-TBSA.19 However, the time-consuming procedures for catalyst preparation, moisture sensitivity
of catalyst, various side products formation resulting in low yield of desired product, longer reaction time, laborious work up methods, non-recoverability of catalyst, need of chromatographic separation and high environmental factor limits the applicability of these reactions. As every method has merits and demerits, there is a need for improvement of existing methods. Recently, organocatalysts have emerged as powerful catalysts in synthetic chemistry owing to its distinctive properties such as non-toxicity, selectivity, reusability, moisture a
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