br Author contributions br Conflicts

Author contributions
Conflicts of interest
Introduction Current the 5-year survival rates for lung and pancreatic cancers are 18% and 8%, respectively. These low survival rates are partially because of the late diagnosis of one-half of cases, in which 5-year survival decrease to 4% and 3%, respectively. Currently, FDA has approved more than ten anticancer drugs for the treatment of pancreatic cancer but poor survival rates indicates the urgency of the development of novel anticancer drug candidates. In this work, development of novel klavuzon derivatives, as new anticancer drug candidates for the treatment of pancreatic cancer, was studied. In recent years, there is an increase in the number of studies for the discovery of drug candidates, which act by covalent binding to the nucleophilic sites of the macromolecules inside the cells. Almost, 30% of drugs in use today act as covalent inhibitor. One of the most common motif in the structure of natural products is α,β-unsaturated-δ-lactone that can act as an irreversible inhibitor through a BMS 193885 formation. It is believed that, nucleophilic site of the enzymes can bind to the β-carbon of the lactone pharmacophore through by Michael addition. (R)-Goniothalamin (1) is one of these natural products and was isolated first in 1967 by Hlubucek from Cryptocarya caloneura. Its selective cytotoxic activity against cancer cell lines5, 6 draw the attention of scientist and many research groups have put effort in synthesizing novel derivatives of goniothalamin to understand the structure activity relationship.7, 8, 9, 10, 11, 12, 13, 14, 15, 16 On the other hand, a few number of studies were dedicated to explore the mechanism of action of goniothalamin derivatives such as inhibition of CRM1 nucleocytoplasmic transport receptor protein and inhibition of topoisomerase I. In terms of cell death, it was shown that (R)-enantiomer of goniothalamin induces apoptosis while (S)-enantiomer induces autophagy. Efficiency of racemic and enantiomerically pure forms of goniothalamin was also demonstrated in Ehrlich solid tumor in mice. Previously, synthesis and antiproliferative activity of conformationally constrained (R)-goniothalamin (()-2) was reported by our group. Although it was found that antiproliferative activity of compound ()-2 was higher than that of (R)-goniothalamin (1), naphthalen-1-yl substitution at position-6 of 5,6-dihydro-2H-pyran-2-one pharmacophore further increases the activity. The inventors named this compound as “klavuzon”. In the same study, (R)-4′-methylklavuzon (()-3) and (R)-2′-methylklavuzon (()-4) were found to be the most cytotoxic compound. In a recent study, racemic 4′-methoxyklavuzon (5) and (R)-2′-methoxyklavuzon (6) derivatives were also prepared and they were found to be less cytotoxic compared to their methyl counterparts (()-3 and ()-4). In the same study, topoisomerase I inhibitory property of klavuzon derivatives was also represented (Fig. 1). Multi-target drug discovery gained significant attention in the last decade to overcome the difficulty of curing diseases having complex etiology and drug-resistance issues. In this respect, goniothalamin derivatives can be considered as a dual acting inhibitor because of its capability to inhibit both CRM1 and topoisomerase I. Due to their structural similarity to goniothalamin, klavuzons may also have similar dual acting inhibitor property. Previously, topo I inhibitory property of klavuzons was shown. In this study, synthesis and antiproliferative properties of novel 4′-alkyl substituted klavuzon derivatives in pancreatic cancer cells (MIA PaCa-2) and Human pancreatic ductal epithelial cells (HPDEC) will be presented. Their time and concentration dependent CRM1 inhibitory properties will also be discussed.
Results and discussion Racemic syntheses of 4′-alkylklavuzon derivatives (16a–h) were completed in eight steps. Strategically important ethyl 4-alkyl-1-naphthoates (10a–h) were prepared according to the procedures reported in literature starting from 4-methyl-1-naphthoic acid (7) in three steps. As can be seen in Scheme 1, benzylic bromination of ethyl 4-methyl-1-naphthoate (8) was performed by a NBS-benzoyl peroxide mixture to produce compound 9 in 92% yield. In the next step, the copper (I)-catalyzed addition of various freshly prepared Grignard reagents to 4-bromomethyl-1-naphthoate (9) was successfully performed. Interestingly, varying amounts of dimerization product (11) of 4-bromomethyl-1-naphthoate (9) were also observed.