New Strategy for Synthesis of the Disaccharide Moiety of the Highly Potent Anticancer Natural Product OSW-1

Abstract The facile synthesis of a partially protected OSW-1 disaccharide moiety, having a 2-O-p-methoxybenzoyl-β-D-xylopyranosyl-(1 → 3)-2-O-acetyl-L-arabinopyranoside structure, was elaborated by glycosylation in a β-stereoselective fashion. The xylopyranose donors were synthesized by a short synthetic approach via convenient selective 1,2-diacetal protection of 3,4-trans-diequatorial hydroxyl group. Regioselective ring opening of 1,2-diacetal-protected substrates efficiently led to the arabinopyranose acceptor with a free 3-hydroxyl group. Glycosylation of the xylopyranose donor with the arabinopyranose acceptor provided the β-disaccharide. GRAPHICAL ABSTRACT


INTRODUCTION
OSW-1 (1) (Fig. 1), a highly potent anticancer natural product, is a member of the cholestane glycoside. It exhibited exceptionally potent cytostatic activities against various human malignant tumor cells. The anticancer activities of this compound are found to be between 10 to 100 times more potent than some well-known anticancer agents in clinical use, such as mitomycin C, adriamycin, cisplatin, camptothecin, and even taxol. In addition, it has demonstrated significantly lower toxicity (IC 50 1500 nM) to normal human pulmonary cells. [1,2] The structural novelty of OSW-1 is characterized by the attachment of a disaccharide to the C-16 position of the steroid aglycone. [3] Because of the extraordinary antitumor activities, OSW-1 is an attractive synthetic target. The synthesis of the aglycone part was reported in 1998 by Guo and Fuchs. [4] In 1999, the disaccharide moiety of OSW-1 was synthesized by Deng et al. [5] as part of the first total synthesis of OSW-1. Later in 2001, Jin and Yu [6] reported the synthesis of this disaccharide moiety by using a slightly different approach. Because the disaccharide part of the OSW-1 molecule is important for biological activity, modification of this carbohydrate backbone was biologically evaluated by Suhr and Thiem in 2004. [7] As part of our research on steroidal synthesis and the regioselectivity of ketal ring opening in carbohydrate molecules, our interest focused on alternative pathways and glycosylation directed toward the disaccharide moiety.
The use of 1,2-diacetals as protecting groups for trans-1,2-diols has been shown to be a particularly useful method for the efficient construction of complex, biologically significant oligosaccharides. [8] The high selectivity for trans-1,2-diols, in the presence of other polyols, rapidly leads to protected monosaccharides amenable for further synthetic manipulation. In this article we describe the high-yielding, selective protection of phenylthioxyloside 4 with butane-2,3-dione, affording the corresponding butane-2,3-diacetal (BDA) intermediate 5, which could be further manipulated in the expedient synthesis of the disaccharide moiety 18 of OSW-1 (1).

FACILE SYNTHESIS OF A DISACCHARIDE UNIT OF OSW-1
diequatorial alcohols, giving BDA-protected 5 in good yield. The p-methoxy benzoyl group was then introduced by treatment of 5 with p-methoxybenzoic acid, N,N 0 -dicyclohexylcarbodiimide (DCC), and 4-dimethylaminopyridine (DMAP) in dichloromethane (DCM) affording 6 in 93% yield. The phenylthiol in 6 was removed by treatment with N-bromosuccinimide (NBS) to furnish lactol 7, which was subsequently converted to the corresponding trichloroacetimidate 8 in 96% yield. [9] Meanwhile, the arabinoside acceptor 15 was prepared by selective ring opening of the corresponding 3,4-benzylidene ketal 14, which was readily prepared from tetraacetyl L-arabinose 9 [10] in five steps in 41% yield (Scheme 2). The p-methoxybenzyl a-L-arabinopyranoside 12 was prepared from 9 according to the standard method. [10] The 3,4-diol of 12 was selectively protected as benzylidene acetal to give 13 in 74% yield, which were subsequently acetylated to yield exo-14 and endo-14, respectively. The endo-configuration of the benzylidene ring of 13endo was confirmed on the basis of 1 H and 13 C NMR spectroscopic data; the benzylidene proton resonated at d 5.94 ppm and the signal of the acetalic carbon atom appeared at 104.5 ppm, whereas the benzylidene proton of the exo-isomer 13 was found at d 6.19 ppm and the signal of the acetalic carbon atom appeared at 103.3 ppm. These values are in good agreement with the corresponding data reported in the literature. [11] By using the method discovered by others [12,13] and recently by our group, [11] exo-benzylidene acetal of 14exo was selectively cleaved using TiCl 4 =Et 3 SiH in CH 2 Cl 2 at À78 C to give 4-benzyl ether 15 in 43% yield and 3-benzyl ether 16 in 11% yield, whereas the endo-isomer14 provided ether 15 in 73% and isomeric ether 16 in 14% yields (Scheme 3).
The synthesis of disaccharide 17 was finally achieved by glycosylation of xylosyl trichloroacetimidate 8 and the arabinosyl acceptor 15 in the presence of TMSOTf, furnishing the disaccharide moiety 17 in moderate yield (Scheme 4).
Selective removal of the anomeric p-methoxybenzyl group from the disaccharide 17 with DDQ led to the corresponding hydroxyl compound 18 in 65% yield as a mixture of a-and b-anomer (ratio ¼ 2:3) (Scheme 5), ready for coupling with OSW-1 aglycone.

FACILE SYNTHESIS OF A DISACCHARIDE UNIT OF OSW-1
to give the BDA-protected xylopyranoside whereas the arabinopyranoside acceptor with a free 3-hydroxyl group was prepared by regioselective reductive ring opening of benzylidene acetal using TiCl 4 and Et 3 SiH. This procedure offers a new approach for the construction of complex, biologically significant oligosaccharides by using the 1,2-diacetal as a highly effective protecting groups for trans-vicinal diols and selective monobenzylation of the cis-vicinal diols by reductive ring opening of benzylidene acetal.

EXPERIMENTAL
Proton nuclear magnetic resonance ( 1 H NMR) spectra and carbon nuclear magnetic resonance ( 13 C NMR) spectra were recorded on a Varian Gemini 300 spectrophotometer. Chemical shifts were recorded as d values in parts per million (ppm). Spectra were acquired in CDCl 3 unless otherwise stated. The peak due to residual CHCl 3 (7.26 ppm for 1 H and 77.2 ppm for 13 C) was used as the internal reference. Coupling constants (J) are given in Hz, and multiplicity is defined as follows: , and activated 4 Å MS (1.65 g) in dry dichloromethane (24.8 mL) was stirred for 1 h at room temperature. The reaction mixture was cooled to À78 C for 30 min followed by the dropwise addition of a 0.1 M solution of TMSOTf in dry CH 2 Cl 2 (0.07 mL, 0.007 mmol). Stirring was continued and the reaction mixture was allowed to warm to À20 C for 2 h. Then the reaction mixture was quenched with triethylamine and filtered through celite, and the solvents were evaporated. Purification by flash column chromatography (EtOAc=hexane, 3:7) gave the title compound.