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    2020-08-10

    
    of these studies have been extremely optimistic. This indubitably in-dicates the feasibility along with the ease of inclusion of quinolines as a flexible component in diverse antitumor pharmacophores. Moreover, quinoline-based HSP90 inhibitors have also demonstrated efficacy in some recently conducted studies [31–33]. Based on these revelations, the design of amide bond tethered quinoline-resorcinol hybrids was conceived as a logical strategy in the present study.
    Recent medicinal chemistry campaigns of our research group have focused primarily on antitumor constructs bearing bicyclic planar or non-planar/partially hydrogenated (6,6/6,5-fused) heterocycles. In alignment with our ongoing drug discovery program that involves continuous transposition between single target and dual target antic-ancer chemical architectures, quinoline-resorcinol hybrid constructs as potential Hsp90 inhibitors were designed in the present study (Fig. 2). Employing a fragment linking approach, the designed hybrids were synthesized and their in-vitro antiproliferative effects were evaluated. The impact of regio-variations of the fusion, a resorcinol ring on the quinoline core through an amide linkage, along with the influence of alkyl substitution at the amide NH on the cellular activity as well as HSP90 inhibition were also investigated.
    2. Results
    The synthetic routes to the target compounds are shown in Schemes 1–3. The quinoline-resorcinol fused constructs (9, 13, 15, 17 and 20)
    Fig. 2. Designed hybrids.
    were synthesized by a synthetic route depicted in Scheme 1. The qui-nolines (23–27) bearing the amine functional group at different posi-tions were condensed with 2,4-bis(benzyloxy)-5-isopropylbenzoic 3XFLAG employing amide coupling mediated by EDC/HOBt. The intermediates formed (28–32) were debenzylated to yield the final compounds (9, 13, 15, 17 and 20). A generalized debenzylation procedure using 10% Pd/C in ethanol in a hydrogenation vessel at 40–42 psi could not be applied to all the intermediates (28–32) owing to the tendency of intermediates 29 and 30 to form tetrahydroquinolines. Thus an alternative strategy was followed using Pd/C and formic acid to afford the debenzylated adducts (13, 15).
    To further evaluate the effect of N-alkylation of the amide NH on the cellular activity as well as HSP90 inhibition, the methods depicted in Schemes 2 and 3 were employed. A similar trend of intermediates re-sponding to different reagents and reaction conditions, generating the target compounds as in Scheme 1 was observed while attempts were made to alkylate the amide NH group. Scheme 2 utilizes cesium car-bonate, sodium hydride, potassium t-butoxide assisted proton
    abstraction of the compounds 28, 31 and 32 to generate the alkylated intermediates (33–37). The alkylated intermediates were then sub-jected to debenzylation with a combination of various reagents and solvents, furnishing the target compounds 10, 11, 12, 19, and 22.
    While attempting to accomplish the synthesis of the fused constructs 14, 16, 18 and 21 through Scheme 2 and employing intermediates (29, 30, 31, 32), it was observed that the synthetic methodology could only produce the target compounds in poor yields. The exact reason for the attenuated reactivity with some alkyl iodides and simultaneous for-mation of degradative products in these cases remains unclear, and a different strategy, shown in Scheme 3 was employed to form the N-alkylated target compounds (14, 16, 18, 21). The starting materials (13, 15, 17, 20) were treated with tert-butyldimethylsilyl chloride (TBDMSCl) which led to selective protection of the OH group at the 4′-position. The selective protection of this OH group could be attributed to the hydrogen bonding of the OH at the 2′-position with the carbonyl group of the amide bond. The TBDMS protected intermediates (38–41) were then subjected to alkylation using t-BuOK (42–45) followed by
    subsequent deprotection of the tert-butyldimethylsilyl ethers to furnish the target compounds (14, 16, 18, 21).
    2.2. Biological evaluation
    2.2.1. In-vitro cytotoxicity studies
    We systemically investigated the synthetic compounds (9–22) for their antiproliferative activity on three human tumor cell lines, HCT116 colorectal cancer cell lines, Hep3B liver cancer cell lines, and PC-3 prostate cancer cell lines. BIIB021, 6-chloro-9-((4-methoxy-3,5-di-methyl-pyridin-2-yl)- methyl)-9H-purin-2-amine (2) and 17-AAG were
    employed as standards. The results presented in Table 1 revealed ex-citing insights regarding the antiproliferative effects of the synthetic compounds. The compound 11 (N-ethyl-2,4-dihydroxy-5-isopropyl-N-(quinolin-3-yl)benzamide) bearing the isopropyl resorcinol function-ality tethered via an amide bond at position 3 of the quinoline scaffold displayed substantial cytotoxic effects against HCT116, Hep3B and PC-3 cancer cell lines with GI50 values of 0.17, 0.33 and 0.14 µM. The adduct 11 is endowed with two-fold higher cell growth inhibitory ef-fects against PC-3 cell lines in comparison to BIIB0201 (2) and is equipotent with 17-AAG. Conjugate 11 displays a similar inhibitory profile to that of BIIB021 (2) towards the HCT116 cell lines and was