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J. Chem. Sci. (2020)132:147 https://doi.org/10.1007/s12039-020-01850-w

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Sulfated polyborate as an eco-compatible solid acid catalyst for efficient and facile solvent-free synthesis of polyhydroquinolines DILIP AUTEa,b, AKSHAY KSHIRSAGARa, BHAGWAT UPHADEa and ANIL GADHAVEa,* a Department

of Chemistry and Research Centre, Padmashri Vikhe Patil College (Affiliated to Savitribai Phule Pune University, Pune), Pravaranagar District, Ahmednagar, Maharashtra 413 713, India b Department of Chemistry, Arts Commerce & Science College, Satral, Maharashtra, India E-mail: [email protected] MS received 16 April 2020; revised 29 August 2020; accepted 2 September 2020

Abstract. An environmentally viable, green and efficient protocol for the synthesis of polyhydroquinolines using sulfated polyborate catalyst have been developed by Hantzsch four-component condensation between aromatic aldehydes, dimedone, ammonium acetate and ethylacetoacetate or ethylcyanoacetate at 100 °C under solvent-free condition. The sulfated polyborate catalyst is characterized by FT-IR, XRD, SEM and EDAX techniques. The noticeable features of this methodology are mild Brønsted acidic nature of the catalyst, good to excellent product yields (85–94%), short reaction time (18–30 min), solvent-free condition, operational simplicity and applicability to a wide range of substrates (29 examples). Keywords. Sulfated polyborate; polyhydroquinolines; Hantzsch condensation; solvent-free; solid acid catalyst.

1. Introduction The solid acid catalyst plays an important role to develop economically and environmentally sustainable processes.1 Boric acid is a readily available inexpensive mild catalyst used in a variety of organic transformations.2 It is known that above 100 °C it undergoes polymerization and liberates water molecule which may hamper the rate of reactions.3 This led to the development of sulfated polyborate with enhanced catalytic activity.4 Over the past years, the sulfated polyborate has emerged as a potential solid acid catalyst used in several MCRs with excellent yields, short reaction time and operational simplicity.5–7 The interesting feature of this catalyst is, it possesses Lewis acidic property due to polymeric borate backbone and Brønsted acidic nature due to support of -SO3H group which makes it more applicable. Polyhydroquinolines (PHQs) are privileged scaffolds due to the presence of 1,4-dihydropyridine (DHP) moiety which is a core part of calcium channel functioning as well as hypertension preventive drugs

viz., nimodipine, lacidipine, felodipine, amlodipine, nicardipine, nifedipine, etc.8 The polyhydroquinolines show diverse biological functions such as antimalarial, bronchodilatory, vasodilator, antidiabetic and antiasthematic activities.9 In 1881, synthesis of substituted 1,4-dihydropyridines was first reported by Arthur Hantzsch and further Safak’s group synthesized 1,4DHP-annulated novel heterocycle known as polyhydroquinoline (Figure 1).10 The straightforward