Received: 14 July 1997 / Uploaded: 21 July 1997/updated: 10 September 1997

Introduction

Digitalis cardiac glycosides are well known drugs clinically used for treatment of congestive heart failure.¹ Their action is mainly due to inhibition of Na⁺,K⁺-ATPase, an enzyme located in the cell membrane and promoting the outward transport of Na⁺ and the inward transport of K⁺.² The most potent inhibitors of Na⁺,K⁺-ATPase are cardenolides such as digoxin, digitoxin, digitoxigenin and gomphoside (Figure 1). The first three compounds have some common features, typical of digitalis: 17b-unsaturated lactone; 14b-hydroxy; A/B and C/D cis ring junctions; 3b-hydroxy or 3b-glycosyl linkage with digitoxose. A quite different molecule is gomphoside, an A/B trans cardiac glycoside from Asclepias fruticosa RBr,³ in which the aglycone (gomphogenin) is linked to a 4,6-dideoxyhexosulose trhough its 2a- and 3b-hydroxy groups.

Templeton⁴ and, more recently, Repke⁵ explored the possibility to obtain novel and highly potent digitalis derivatives by functionalization of the 2a-hydroxy group of gomphogenin.
With the aim of having analogues of gomphoside in the A/B cis digitalis skeleton, and evaluating the importance of different configuration at positions 2 and 3, we planned the synthesis of the 2-hydroxy derivatives of digitoxigenin and 3-epidigitoxigenin (Figure 2).

Three of the four isomers of 2,3-dihydroxy derivatives of the A/B cis series: 2b,3a 4; 2b,3b 5d and 2a,3a 8 were synthesized.

Chemistry

Treatment of the known 4b-bromo-3-oxo-14b-hydroxy-5b-card-20(22)-enolide 1⁴ (Scheme 1) with anhydrous potassium acetate in refluxing acetic acid,⁶ gave the key compound 2b-acetoxy-3-oxo derivative 2 (60% yield). The reduction of 2 with lithium tri-tertbutoxyaluminum hydride in THF gave the 2b-acetoxy-3a-hydroxy compound 3 (52% yield). Hydrolysis of the acetoxy group with 10% aq. HCl in methanol gave the desired 2b,3a,14b-trihydroxy-5b-card-20(22)-enolide 4 (70% yield).
The 2b,3b-dihydroxy derivative 5d was obtained by exploiting the high selectivity of L-Selectride⁷ in reducing the 3-keto group of compound 2 to axial 3b-hydroxy group. The reaction gave, together with the 2b,3b,14b-trihydroxy-5b-card-20(22)-enolide 5d a mixture of acetates: 2b,3b-diacetoxy 5a, 2b-acetoxy-3b-hydroxy 5b, and 3b-acetoxy-2b-hydroxy 5c in roughly equal amount. The mixture was hydrolized with 10% aq. HCl in methanol to give compound 5d in an overall yield of 70% from 2.

The 2a,3a-dihydroxy compound 8, was prepared as described in Scheme 2. Digitoxigenin 6 was reacted with trifluoromethansulfonic anhydride in pyridine to give a mixture of D² and D³ derivatives 7 in a 6/4 ratio that become 8/2 after crystallization from acetone. This mixture was reacted as such with a catalytic amount of OsO₄ in the presence of 4-methylmorpholine-4-oxide, to give a mixture of three 2,3- and 3,4-dihydroxy compounds. Purification by silica gel chromatography gave the desired 2a,3a-dihydroxy compound 8 (20% yield), together with 5d (25% yield) and the 3b,4b-dihydroxy derivative 9 (25% yield).

Biological Data

The preliminary biological data of some of the synthesized compounds, in comparison with digitoxigenin and 3-epidigitoxigenin, in the displacement of the specific [³H]-ouabain binding⁸ on Na⁺,K⁺-ATPase are reported in Table 1.

^aAverage of three values (-log IC₅₀). The affinity for the receptor site of Na⁺,K⁺-ATPase was evaluated by the displacement of the specific [³H]-ouabain binding from Na⁺,K⁺-ATPase receptor^8a isolated from dog kidney and purified according to Jørghensen.^8b
bNot tested.

References

1. Hofman, B. F.; Bigger, J. T. In The Pharmacological Basis of Therapeutics; Goodman Gilman, A.; Nies, A. S.; Rall, T. W.; Taylor, P., Eds.; Pergamon Press, New York, 1990, Section VII, Chapter 34.
2. Repke, K. R. H.; Schönfeld, W. Trends Pharmacol. Sci., 1984, 5, 393.
3. Watson, T. R.; Wright S. E. Aust. J. Chem., 1957, 10, 79.
4. Templeton, J. F.; Cheung, H. T. A.; Sham, C. R.; Watson, T. R.; Kong, J. J. Chem. Soc. Perkin Trans. I, 1983, 251.
5. Weiland, J.; Ritzau, M.; Megges, R.; Schön, R.; Watson, T. R.; Repke, K. R. H. Eur. J. Med. Chem., 1995, 30, 763.
6. a) Satoh, Y.; Mukoh, M.; Ogaki, Y.; Takahashi, T.; Kimura, T.; Aoki, H.; Hagitani, A. Bull. Chem. Soc. Jpn. 1966, 39, 855. b) Iida, T.; Komatsubara, I.; Chang, F. C.; Goto, J.; Nambara, T. Steroids, 1991, 114.
7. a) Brown, H. C.; Krishnamurthy, S. J. Am. Chem. Soc., 1972, 94, 7159. b) Krishnamurthy, S.; Brown, H. C. J. Am. Chem. Soc., 1976, 98, 3383.
8. a) Brown, L.; Erdmann, E. Arzneim. Forsh., 1984, 34, 1314. b) Jørghensen, P. L. Biochim. Biophys. Acta, 1974, 356, 36.
9. Thomas, R.; Gray, P.; Andrews, J. Adv. Drug Res., 1990, 19, 312.

All comments on this poster should be sent by e-mail to ecsoc@listserv.arizona.edu with A0012 as the message subject of your e-mail.