Synthesis of Hoodigogenin A, the Aglycone of Hoodigosides Extracted from Hoodia gordonii

Hoodia gordonii is a succulent plant (asclepiadaceae family) which grows in the Kalahari desert in South Africa. On a historical point of view, it was claimed that the San people, a Bushmen tribe leaving in the Kalahari desert, were able to make long hunting trips without feeling thursty and hungry after chewing fresh sap from H. gordonii. Therefore, it was claimed that H. gordonii could represent a new help for fighting obesity, which is one of the major health problems in the 21st century. Indeed, in 2014, more than 1.9 billion adults, 18 years and older, were overweight. Of these over 600 million were obese [1].


Introduction
Hoodia gordonii is a succulent plant (asclepiadaceae family) which grows in the Kalahari desert in South Africa. On a historical point of view, it was claimed that the San people, a Bushmen tribe leaving in the Kalahari desert, were able to make long hunting trips without feeling thursty and hungry after chewing fresh sap from H. gordonii. Therefore, it was claimed that H. gordonii could represent a new help for fighting obesity, which is one of the major health problems in the 21st century. Indeed, in 2014, more than 1.9 billion adults, 18 years and older, were overweight. Of these over 600 million were obese [1].
In 2007, Van Herdeen et al. reported for the first time the appetite suppressant properties of H. gordonii [2,3]. According to these authors, the compound responsible for that, is the preganane glycoside P57AS3. Later on, Shukla et al. reported that more than 40 different pregnane glycosides (Hoodigosides) were isolated from H. gordonii [4,5]. The common aglycone of all these compounds is Hoodigogenin A (or Gordonoside A) ( Figure 1).
The yield of extraction of the Hoodigosides from Hoodia gordonii lies between 0.003% and 0.02%. On the other hand, the isolation of analytically pure compounds proved to be highly time consuming and tedious. Therefore, in the frame of a collaborative study concerning the synthesis, the extraction and the biological evaluation of Hoodigogenin A, we developed an original synthesis of Hoodigogenin A starting from commercially available reagents. Indeed, Hoodigogenin A could provide different Hoodigosides after running a glycosylation reaction with the required sugar moiety. We report herein the synthesis of Hoodigogenin A, the key step being a Norrish type I-Prins reaction.

Synthesis of Hoodigogenin A
The synthesis started from the commercially available compound 1. After a regioselective deprotection of the 3α-acetoxy group and oxidation of the resulting hydroxy group, a regioselective α-bromination of the diketo derivative 2 yielded stereospecifically the β-bromoketo derivative 3. A dehydrobromination reaction led to the α,β-unsaturated keto derivative 4 [6]. The latter was treated with acetic anhydride and acetyl chloride to give readily the dienol acetate 5 [7]. After protection of the carbonyl group with ethyleneglycol, the dienol acetate was reduced in the presence of NaBH4 [8,9] followed by a KOH promoted deprotection of the 12 α-acetate group, allowing the introduction of the double bond in the B ring and yielding the unstable diol 6. A regioselective protection of the 3 β-hydroxyl group gave compound 7, which was subjected to a Dess Martin periodane oxidation [10] to afford the keto derivative 8 ( Figure 2).
Compound 8 was then suitable to undergo a Norrish type I reaction [11][12][13][14][15]. It has to be noted that the photochemical ring opening of pregnenolone derivatives was never reported in the literature. We were pleased to see that the photolysis of compound 8, which was carried out in a quartz apparatus with a 125 W high pressure mercury lamp, led readily to the formation of aldehyde 9. The latter proved to be instable. Therefore the subsequent Prins reaction was directly carried out on the crude photolysis reaction mixture. Under these reactions conditions, three compounds were isolated: compound 10 that resulted from a deprotection of the dioxolane 8, the spiro derivative 11 and the desired compound 12, which was isolated in 25% yield ( Figure 3) [16].
Finally, the protection of the 12β-hydroxy group with tigloyl chloride in the presence of pyridine and DMAP afforded compound 13. Deprotection of the 3β-acetate group gave Hoodigogenin A, whose structure was confirmed by 1 H and 13 C NMR analysis and by X-ray analysis (Figures 4 and 5) [17].

Conclusion
To overcome the limited availability of Hoodigogenin A, we have disclosed an original synthetic route that affords the latter in 3% overall yield. This synthesis represents a very interesting alternative to the extraction methods for which the yields are 100 times lower. Moreover, analogs of Hoodigogenin A are now accessible by our method [18]. Concerning the biological activities, Smith et al. reported recently a summary of the current knowledge concerning the efficiency of H. gordonii as an appetite suppressant drug. Many contradictory results were obtained and it clearly came out that more studies are absolutely necessary to elucidate the mode of action of hoodigosides [19].

Experimental section
Melting points were measured on a Stuart Scientific melting point apparatus (SMP 3) and are uncorrected. Reactions were carried out under argon with magnetic stirring and degassed solvents. Et 2 O and THF were distilled from Na/benzophenone. Thin layer chromatography (TLC) was carried out on silica gel plates (Merck 60F254) and the spots were visualized under UV lamp (254 or 365 nm) and sprayed with phosphomolybdic acid solution (25 g phosphomolybdic acid, 10 g cerium sulfate, 60 mL H 2 SO 4 , 940 mL H 2 O) followed by heating on a hot plate. For column chromatography, silica gel (Merck Si 60 40-60 µm) was used. IR spectra were recorded on Bruker Alpha (ATR) spectrophotomer. 1 H NMR spectra were recorded at 300 MHz (Bruker AC-300) and 13 C NMR spectra at 75 MHz (Bruker AC-300) using the signal of the residual nondeuterated solvent as internal reference. Significant 1 H NMR data are tabulated in the following order: chemical shift (δ) expressed in ppm, 6 multiplicity (s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet), coupling constants in hertz, number

3β-acetoxy,12-keto, cyclic 20-(ethylene acetal) pregn-5-ene (8)
To a solution of compound 7 (143 mg, 0.34 mmol) in CH 2 Cl 2 (3 mL) at 0°C, was added the Dess Martin reagent (174 mg, 0.41 mmol). The reaction mixture was stirred for 1 h at r.t., then was treated with sat. Na 2 S 2 O 3 (8 mL) and the reaction mixture was stirred for 5 min. To the medium was added sat. NaHCO 3 (8 mL) and the reaction mixture was stirred for 5 min. The layers were separated and the aqueous layer was extracted with CH 2 Cl 2 (3 × 10 mL). The combined organic layers were dried over Na 2 SO 4 , filtered and concentrated under reduced pressure. The crude material was purified by column chromatography ( 13

Synthesis of compounds 10, 11 and 12
A solution of compound 8 (895 mg, 2.15 mmol) in CH 2 Cl 2 (280 mL) was degassed with argon for 15 min and then was irradiated with a high pressure mercury lamp Philips HPK 125 for 15 min in a quartz vessel. The solvent was then evaporated under reduced pressure, to afford the crude seco aldehyde 9 (931 mg, 100%).