Design and synthesis of new lupeol derivatives and their α-glucosidase inhibitory and cytotoxic activities

Abstract A series of lupeol derivatives 2, 2a-2f, 2a-2h, 3a-3e, and 4a-4b were designed, synthesised and evaluated for their α-glucosidase inhibitory and cytotoxic activities. Among synthetic derivatives, lupeol analogues 2b and 2e containing a benzylidene chain exhibited the best activity against α-glucosidase and superior to the positive agent with the IC50 values of 29.4 ± 1.33 and 20.1 ± 0.91 μM, respectively. Lupeol analogues 2d and 3a showed weak cytotoxicity against K562 cell line with the IC50 values of 76.6 ± 2.40 and 94.4 ± 1.51 μM, respectively. Graphical Abstract


Introduction
Diabetes mellitus (DM), also known as diabetes, is a condition that impairs the body's ability to process blood glucose, otherwise known as blood sugar. There are three major types of the disease: type 1, type 2, and gestational DM. With all three, the body can't make or use insulin. (Tahlan and Verma 2017). Uncontrolled diabetes is manifested by a very high rise in triglycerides and fatty acid levels (Ferrannini et al. 1991). It can lead to major complications such as heart attack, stroke, eye disease, kidney disease, nerve disease, and infection if left untreated. Antidiabetic drugs comprise a chemically and pharmacologically heterogeneous group of drugs and presently available to reduce hyperglycaemia in DM but have side effects. Therefore, searching for a new class of compounds is essential to overcome these problems. Synthetic transformation of plant metabolites is a significant area of medicinal chemistry that can create new and potential drugs. Lupeol is a pharmacologically active pentacyclic triterpenoid found in a variety of plants (Gallo and Sarachine 2009;Siddique and Saleem 2011;Starks et al. 2011). Reddy and co-workers synthesised few novel lupeol derivatives and screened for their in vivo antihyperglycemic activity. Most derivatives lowered the blood glucose levels, in sucrose challenged streptozotocin induced diabetic rats (STZ-S) model (Reddy et al. 2009).
Recent reports also revealed that lupeol exhibit various cancer types such as prostate, skin, pancreatic, and breast cancer (Saleem 2009). Magalhães and co-workers (Magalhães et al. 2017) designed lupeol derivatives with adequate carboxylic acid and DIC/DMAP reagents. Some synthetic analogues displayed a selective cytostatic effect with low GI 50 values, representing a promising prototype for the development of new anticancer drugs. Due to the several limitations associated with the use of existing synthetic antidiabetic and anticancer drugs, the search for newer antidiabetic and anticancer agents from natural source continues. In continuation of our ongoing search for new antidiabetic and cytotoxic agents from derivatized secondary metabolite (Sichaem et al. 2017;Nguyen et al. 2019), we described herein the synthesis of new lupeol derivatives via three different routes. In addition, the evaluation of a-glucosidase inhibitory and cytotoxic activities of the synthetic derivatives were also assayed.
In an attempt to obtain more derivatives of the starting material (1), the double bond at C-28-C-29 was modified through the oxidation using OxoneV R (2KHSO 5 .KHSO 4 .K 2 SO 4 ) of 2 (Uyanik et al. 2009). As the results, five products 3a-3e were obtained. Synthetic analogue 3c, a natural product was obtained as a major component of the reaction. NMR data of 3c was identical with the (19R)-isomer which was previously reported from P. acidus ). Interesting, this compound was selectively synthesised by da Silva Martins and co-workers using manganese porphyrins (da Silva Martins et al. 2016). In our case, OxoneV R is an effective catalyst to form 3c without protection of 3-OH group. The structures of 3d and 3e were identified using 1D and 2D NMR. HMBC correlations of 30-CHO, 29-CH 3 , H-18 to C-28 and C-19; and of 29-CH 3 to C-30 defined the E-ring of 3d and 3e. Corbett and co-workers (Corbett et al. 1985(Corbett et al. , 1987 indicated that lupeol derivatives containing a chiral C-20 carbon and/or a 20-CHO group have presented as (20R/S)-epimeric mixtures. Both diastereoisomers could be distinguished by NMR data. In our scenario, analogues 3d and 3e was synthetically prepared as pure diastereoisomers.

General
High resolution electrospray ionisation mass spectra (HRESIMS) was recorded with a Bruker microTof spectrometer. 1 H, 13 C and 2D NMR spectra were recorded (CDCl 3 as solvent) at 400 and 100 MHz, respectively, on a Bruker (Avance) 400 NMR spectrometer using tetramethylsilane (TMS) as an internal standard. Chemical shifts are reported in ppm downfield from TMS. Reactions were monitored by thin layer chromatography (TLC) was performed on aluminium sheets precoated with silica gel (Merck Kieselgel 60 PF254). Silica gel (No. 7729,7734,and Merck) was used as stationary phase on quick column chromatography and open column chromatography.
3.3. General procedure to synthesise 2 and 2a-2g by oxidation of 1 Lupeol (1, 200 mg, 0.469 mmol) in acetic acid (35.5 mL) was treated with 50 mL of Jones reagent (0.05 M K 2 Cr 2 O 7 in a 3 M solution of H 2 SO 4 ) and stirred at 100 C for 3 hrs. The mixture was extracted with ethyl acetate-water (1:1) to gain organic layer.
The organic layer was washed with brine and dried over anhydrous Na 2 SO 4 . The residue was subjected to silica gel column chromatography (CC) eluted with n-hexanechloroform (4:1) to afford lupeone (2). This lupeone (2, 70 mg, 0.165 mmol) together with NaOH (35 mg, 0.875 mmol) in ethanol (7 mL) was stirred at 55 C for 15 minutes. Then, the mixture was added appropriate benzaldehyde derivatives (0.33 mmol) and ethanol (7 mL). The reaction was conducted at 55 C for 2 hrs under stirring and was periodically monitored by TLC. The mixture was then extracted with ethyl acetatewater (1:1) in the same procedure with the previous reaction. The residue was absorbed onto column chromatography successively eluted with n-hexane-ethyl acetate (100:1) to yield products 2a-2g. The 1 H and 13 C NMR and ESIMS spectra data of 2a-2g were supplied as supplementary material.
3.4. General procedure to synthesise analogues 3a-3e by oxidation of 1 Lupeol (1, 200 mg, 0.469 mmol) was oxidised with OxoneV R (317 mg, 0.516 mmol) in acetic acid (40 mL) at 100 C for 3 hrs. The mixture was stirred and continuously monitored by TLC. The mixture was extracted with ethyl acetate-water (1:1) in the similar manner with the reaction of 1 and Jones reagent. The residue was then purified by silica gel CC to give products 3a-3e. The 1 H and 13 C NMR and ESIMS spectra data of 3a-3e were supplied as supplementary material.

General procedure to synthesise analogues 4a and 4 b
In the same manner, compound 3c (70 mg, 0.163 mmol) father reacted with 4-chlorobenzaldehyde (49.2 mg, 0.35 mmol). The reaction were applied to silica gel CC using the gradient system of n-hexane-ethyl acetate (10:1) to obtain a new derivative 4b. On the other hand, compound 3c was oxidised with Jones reagent to afford a known product 4a. The 1 H and 13 C NMR and ESIMS spectra data of 4a-4b were supplied as supplementary material.

Disclosure statement
No potential conflict of interest was reported by the authors.