Mechanistic insights on the reactivity of furospirostanes with the 16β,22:22,25-diepoxy-23-acetoxymethyl-24-methyl side chain
Graphical abstract
Introduction
The spiroketal assemblies (see Fig. 1) are present in widespread compounds that can be isolated from many marine and terrestrial organisms: plants, fungi and insects among others. The vast number and the growing pharmacological importance of compounds containing spiroketal assemblies have triggered increasing interest in both their synthesis and studies of their chemical reactivity [1], [2], [3], [4], [5]. In addition to the spirostanic sapogenins which bear the 1,6-dioxaspiro[4,5] decane moiety, furospirostanes, an emerging family of steroids bearing the 16β,22:22,25-diepoxy moiety in the chain, that are considered 1,6-dioxaspiro[4.4] nonane derivatives are attracting increasing attention.
The increasing number of naturally occurring bioactive steroids that bear the furospirostane side chain includes compounds that have shown interesting antitumor activity such as ritterazines (1), cephalostatins (2) [6], [7], [8], [9], [10], hippuristanols (3) [11], [12], [13], [14], [15], as well as the antihypertensive glycosides of nuatigenin (4) [16] among others (see Fig. 2).
Unlike spirostanic sapogenins, the side-chain reactivity of which has received considerable attention in the past 70 years selected Ref. [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], the studies of the reactivity of the furospirostane side chain are rather scarce [38], [39], [40], [41], [42]. In particular, Fuchs and coworkers hypothesized that the cytotoxic activity of cephalostatins may be related to the possibility of the generation of oxacarbenium ions around C-22 [43], [44]. This fact, added to the increasing number of cytotoxic steroids bearing the furospirostane side chain that have been identified, prompts the exploration of the reactivity of this moiety.
As a part of our ongoing program directed to the synthesis of potentially bioactive spiroketals derived from rearranged steroid sapogenins, we envisaged the bromination at C-23 of our previously described furospirostane sapogenin 23S-5 [45] as a possible route to the introduction of additional functionality in the F ring (see Scheme 1).
In the course of the bromination experiments employing pyridine hydrobromide perbromide (PyrBr2·HBr) we found that, in addition to the desired bromination at C-23, an unexpected substitution of the C-23′ acetoxy moiety also took place. This led us to explore the particular reactivity of this structural fragment. Herein we describe our findings on the reactivity of the 16β,22:22,25-diepoxy-23-acetoxymethyl-24-methyl side chain.
Section snippets
Experimental
Reactions were monitored by TLC on ALUGRAM® SIL G/UV254 plates from MACHEREY–NAGEL. Chromatographic plates were sprayed with a 1% solution of vanillin in 50% HClO4 and heated until color developed. Purifications and separations were performed in pressurized chromatographic columns packed with MACHEREY–NAGEL silica gel 60 (230–400 mesh ASTM). Melting points were measured on a Melt-Temp II apparatus. Mass spectra were registered in a Thermo-Electron spectrometer model DFS (Double Focus Sector).
Bromination of 23S-5 with
(1.28 g, 3.6 mmol) was added to a solution of 23S-5 (518 mg, 1.0 mmol) [45] in acetic acid (25 mL). The mixture was stirred for 1 h at 50 °C and poured into ice/water. The produced solid was filtered off washed with water and dissolved in CH2Cl2. The organic solution was dried (anh. Na2SO4) and evaporated to produce a mixture of 23R-6, 23S-6, 23R-7 and 23S-7 that were separated in a pressurized chromatographic column packed with silica gel (18.5 g) using hexane/ethyl acetate 15:1 as eluent.
Reaction of 23S-5 with HBr in acetic acid
The diacetate 23S-5 (258.4 mg, 0.5 mmol) was added to a 33 wt.% solution of HBr in acetic acid (5 mL, 27.2 mmol of HBr). The mixture was stirred for 45 min., poured into ice/water and extracted with ethyl acetate (40 mL). The organic layer was washed with 5% aqueous Na2CO3 (4 × 25 mL) and water (2 × 25 mL), dried (anh. Na2SO4) and evaporated to produce a mixture of 23R-8 and 23S-8 that were separated in pressurized chromatographic column packed with silica gel (12 g) using hexane/ethyl acetate 40:1 as
Reaction of 23S-5 with acetic acid and BF3·Et2O
BF3·Et2O (1.5 mL, 12.2 mmol) was added to a solution of 23S-5 (518 mg, 1.0 mmol) in acetic acid and the mixture was stirred for 5 h before slow addition of 10% aqueous NaHCO3 solution (20 mL) and extraction with ethyl acetate (25 mL). The organic layer was washed with 10% aqueous NaHCO3 solution (8 × 20 mL) water (3 × 20 mL), dried (anh. Na2SO4) and evaporated to afford a mixture of the starting material, 23S-5, the epimeric 23R-5, and olefin 9 that were separated in pressurized chromatographic column
Results and discussion
Treatment of the diacetate 23S-5 with PyrBr2·HBr in acetic acid afforded the epimeric 23-brominated furospirostanes 23R-6 and 23S-6 together with the unexpected dibrominated compounds 23R-7 and 23S-7 (see Scheme 2).
Although the MS of compound 23R-6 does not show a molecular ion that shows the presence of a bromine, the occurrence of a fragment in m/z 515 may be interpreted as the product of the loss of Br from the molecular ion (M+−Br or more likely MH+−HBr). The signals of C-16, C-22 and C-26
Conclusions
We have found that the presence of the C-23′ acetoxy moiety confers a special reactivity to the furospirostane side chain. The F-ring opening produces a Δ22-intermediate in which the C-23′-acetoxy is placed in position allowing its substitution or elimination. This reactivity feature opens new and useful possibilities for the transformation of the 16β,22:22,25-diepoxy-23-acetoxymethyl-24-methyl side chain.
Acknowledgments
The authors thank to the Dirección General de Asuntos del Personal Académico (DGAPA-UNAM) for financial support via project IN221911 and CONACyT for scholarship granted to MM-A. Thanks are due to Rosa I. del Villar Morales, Georgina Duarte Lisci (USAI-UNAM) for recording NMR and MS spectra. We want to express our gratitude to Dr. Carlos Cobas from Mestrelab® for assistance with the MestreNova NMR processing program and to Dr. John Boulton for correcting the manuscript.
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