Abstract Overweight and Obesity Are Increasing Health Risk Worldwide
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Abstract Overweight and obesity are increasing health risk worldwide, causing serious diseases such as diabetes, cardiovascular diseases and dyslipidemia. Apart from the changes of dietary and physical activity, the use of appetite suppressant is also a practical way for body weight control. Hoodia gordonii (Asclepiadaceae) has been used as an appetite suppressant in Africa for thousands of years. Although its extract has been proposed to be an agent for body weight control, the mechanism of its activities and in vivo targets remain unclear. Our recent study showed that gordonoside F, a pregnane glycoside isolated from H. gordonii, was able to specifically active GPR119 and consequently induce glucose-stimulated insulin secretion both in vitro and in vivo, reducing the food intake in mice. Surprisingly we found P57, the previously known active component, was inactive to GPR119, which implied the existence of different mechanisms of P57. Complicated composition and limited content make it difficult to isolate enough P57 and gordonoside F from H. gordonii. In this case, further biological and pharmacological studies have been suspended. Chemical synthesis become a better choice to provide these compounds in large scale. Herein, we developed an efficient approach to the synthesis of Hoodigogenin A (13 steps, 14% overall yield) in gram-scale from cheap commercial available dehydroepiandrosterone. A glycal method under the promotion of TPHB was applied successfully to the glycosylation of Hoodigogenin A to synthesize P57 precursor in 70% yield with an excellent -selectivity ( = 6: 1). Gordonoside F was also synthesized expeditiously in gram-scale, featuring assembly of the deoxytetrasaccharide with glycosyl o-alkynylbenzoates as donors (92% yield, = 9:1, for the glycosylation of the aglycone). These synthetic Hoodia saponins are used in biological and pharmacological studies, and the results will be reported in due course. Principal Investigator: Biao Yu Grant Title: Decagram-scale synthesis of appetite-suppressant glycosides P57 & gordonoside F PROGRESS REPORT for Mizutani Foundation Research Grant Reference Number: No. 180129 Principal Investigator: Biao Yu Organization: Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences Period: April 1, 2018 to March 31, 2019 Grant Title: Decagram-scale synthesis of appetite-suppressant glycosides P57 & gordonoside F Progress Report: Introduction and objective: Overweight and obesity are increasing health risk worldwide, causing serious diseases such as diabetes, cardiovascular diseases and dyslipidemia. Besides the change of dietary and physical activity, the use of appetite suppressant has been a practical alternative to body weight control. Hoodia gordonii (Asclepiadaceae) has been used as an appetite suppressant in Africa for thousands of years. The active components and their mechanism of action remain unclear. We found that gordonoside F, a pregnane glycoside isolated from H. gordonii, was able to specifically active GPR119 and consequently induce glucose-stimulated insulin secretion both in vitro and in vivo, to reduce the food intake in mice. Surprisingly, P57, the previously proposed active component of H. gordonii, was found inactive to GPR119. Complicated composition and limited content make it difficult to isolate enough pure P57 or gordonoside F from H. gordonii. Therefore, we plan to develop an efficient approach to the synthesis of these two Hoodia saponins, which can then be used in further biological and pharmacological studies. Principal Investigator: Biao Yu Grant Title: Decagram-scale synthesis of appetite-suppressant glycosides P57 & gordonoside F Methods used: Scheme 1 The structures of P57 (1) and gordonoside F (2) and their synthetic strategies In 2012, we reported the chemical synthesis of pregnane glycoside P57 in linear 20 steps with an overall yield of 2.4% from cardiac glycoside digoxin ($ 145.00/g from TCI), wherein the last coupling was achieved via gold(I)-catalyzed glycosylation with disaccharide o-alkynylbenzoate donor in 70% yield in milligram-scale and a poor -selectivity ( = 1:1). In our later synthesis of gordonoside F reported in 2014, we conducted the last coupling between tetrasaccharide donor and hoodigogenin A via gold(I)-catalyzed glycosylation, leading to an excellent 98% yield in milligram-scale. Thus, the chemical access to this family of pregnane glycosides is hampered by the preparation of the aglycone Hoodiagenin A and the stereoselectivie glycosylation. Dehydroepiandrosterone (DHEA), an endogenous steroid hormone, become our cheap starting material for decagram-scale synthesis of the aglycone Hoodigogenin A. Thus, introduction of the oxy groups at the C12 and C14 position through C-H functionalization and Norrish type I/prins reaction were desired. Considering the difficulty in constructing the -glycosidic linkage to the aglycone, we developed a glycal approach to the synthesis of P57 via glycosylation of the aglycone with a trisaccharide glycal donor. Principal Investigator: Biao Yu Grant Title: Decagram-scale synthesis of appetite-suppressant glycosides P57 & gordonoside F Results and Discussion: The aglycone, Hoodigogenin A, was prepared from the cheap DHEA ($ 1.70/g from Energy Chemical) as depicted in Scheme 2. Protection of the 3-OH with Bn group, formation of the 2-picolylimine at C17 position, then C-H functionalization with Schönecker method resulted in hydroxylation at the unactivated C12 to produce 3. Treatment of 3 with Wittig reaction and borohydride oxidation gave compound 4. With the suitable protection group and oxidation state at C12 and 17-OH, Norrish type I/Prins reaction afforded 12,14-diol 5. Finally, acylation and debenzylation provided Hoodigogenin A in gram-scale (13 steps, 14 % overall yield). Scheme 2 Synthesis of Hoodigogenin A from DHEA The coupling of glycals 6-8 (1.0 eq) with DHEA (9, 1.5 eq) under the conditions of TPHB (0.15 eq) in CHCl3 at rt (Table 1, entries 1-3) were used as a model reaction, leading to the corresponding glycosylated products 10-12 with poor to modest yields and -selectivities. Glycal 6 with the electron-withdrawing 4-O-Bz group was found to be a relatively inert donor and the corresponding reaction underwent sluggishly with good selectivity but poor yield (14%). In contrast, glycal 7 bearing the electron-donating 4-O-Bn group was much more reactive upon treatment of TPHB. However, the desired 11 was obtained in a poor 21% yield due to the Ferrier I side-reaction, and the ratio was decreased to 1.1:1.0. Delightfully, glycal 8 with the bulky 4-O-TBDPS group turned out to be a satisfactory donor, giving the desired 12 with good -selectivity in an acceptable 47% yield. Next, we examined the solvent effect of CHCl3, CH3CN, THF, PhCl, and toluene (Table 1, entries 3-7) with glycal 8 as the donor. PhCl (entry 6) turned out to be optimal solvent, resulting in 12 in a modest 50% yield with satisfactory -selectivity (entry 6, 50%, = 7.0:1.0) when 0.1 eq of TPHB was used at rt. Principal Investigator: Biao Yu Grant Title: Decagram-scale synthesis of appetite-suppressant glycosides P57 & gordonoside F Table 1 Model Screening with glycals 6-8 and dehydroepiandrosterone (9) O H Me H O dehydroepiandrosterone (9) O H RO TPHB, 4Å MS, solvent, temp. RO O OMe OMe R=Bz,6 R=Bz,10 R=Bn,7 R=Bn,11 R = TBDPS, 8 R = TBDPS, 12 Entry Glycal Solvent Temp. TPHB (eq)Product Yield ( b) 1 6 CHCl3 rt 0.15 10 14% (3.9:1.0) a 2 7 CHCl3 rt 0.15 11 21% (1.1:1.0) a 3 8 CHCl3 rt 0.15 12 47% (4.1 :1.0) 4a 8 rt 0.5012 17% (3.0:1.0) CH3CN 5a 8 THF rt 0.20 12 Trace 6 8 PhCl rt 0.10 12 50% (7.0:1.0) 7a 8 toluene rt 0.20 12 68% (5.7:1.0) 8 8 toluene 46 ºC 0.10 12 55% (7.6:1.0) 9 8 toluene 86 ºC 0.10 12 <10% (3.4:1.0) a b 1 TPHB was added in portion. Determined by HNMR. With the optimized TPHB-catalyzed glycosylation conditions at hand, we conducted the glycosylation of hoodigogenin A (1.5 eq) with trisaccharide glycal (13, 1.0 eq). As depicted in Table 2, the most satisfactory result was achieved by the addition of 1.5 eq of TPHB in portion at rt, leading to 70% yield and a ratio to 6:1. Trisaccharide 14 was subjected to the final deprotection of the benzoyl groups in the presence of the 12-O-tiglic ester, indeed exposure of 14 to basic conditions furnished saponin P57 (2.4 g, 64% yield). Principal Investigator: Biao Yu Grant Title: Decagram-scale synthesis of appetite-suppressant glycosides P57 & gordonoside F Table 2 Glycosylation of hoodigogenin A with trisaccharide glycal 13 In the previous synthesis, glycosylation of hoodigogenin A (1.4 eq) with o-alkylbenzoate donor (15, 1.0 eq) led to tetrasaccharide 16 in 92% yield ( = 1.0: 1.0) (Scheme 3). Now, we have improved the selectivity to 9:1 via optimizing the glycosylation conditions (unpublished results). Final saponification of the β-anomer 16 furnished Gordonoside F (1.1 g, 93% yield). AcO O Hoodigogenin A, O O O MeO O O PPh3AuOTf, 5Å MS O O 92%, = 1.0: 1.0 OMe OMe OMe O O 15 n-Bu O O O AcO O O OH MeO O O O O O OMe OMe OMe 16 16 KOH, PhMe, MeOH, rt, 3.5 h, 93% Gordonoside F (2) Scheme 3 Glycosylation of hoodigogenin A with tetrasaccharide o-alkylbenzoate donor 15 In summary, we have accomplished the gram-scale synthesis of P57 (15 steps, 5.4% overall yield) and Gordonoside F (15 steps, 6.1% overall yield) from cheap DHEA. The present synthesis provides Principal Investigator: Biao Yu Grant Title: Decagram-scale synthesis of appetite-suppressant glycosides P57 & gordonoside F an efficient approach to obtain in gram-scale P57, Gordonoside F and other Hoodia saponins.