Synthesis and characterization of dimeric steroids based on 5-oxo-4,5-seco-yne units linked by a diyne spacer

New dimeric steroids in which two 5-oxo-4,5-seco-3-yne steroids units are linked by a flexible diyne spacer, were prepared by both Eglinton and Pd-catalyzed coupling of the corresponding monomers. X-Ray crystallography shows that one of the obtained dimers displays a novel supramolecular network in which the facial hydrophobicity of the steroidal skeleton plays an important role. The crystal packing is dominated by interactions that accommodate the steroid cores in a highly crowded packed columnar self-assembly


Introduction
Steroids are a family of lipophilic compounds that are widespread in nature.Their intrinsic biological activity has kept this kind of compound in the focus of attention for more than a century.The subfamily of steroid dimers added its first members when a few compounds were isolated from nature 1 or occurred as by-products of various reactions. 2The interesting chemical, biological and physical properties of the rising number of steroid dimers isolated from living organisms or obtained by synthesis [3][4][5][6][7] has prompted intensive activity in this developing field.
Although dimeric steroids are known to form well-defined crystals, reports on the crystal structure of this type of compound are rather scarce, compared to those of monomeric steroids.As part of a program on the development of steroid-based crystalline systems, we have become interested in the preparation of compounds with flexible structural arrangements.In a previous publication we described the synthesis of dimeric terephthalates derived from epimeric 4,5-seco-cholest-3-yn-5-ols that display novel supramolecular networks in the solid state in which the facial hydrophobicity of the steroidal skeletons plays an important role. 8This prompted us to set up procedures to prepare dimeric steroids in which the steroid cores are joined by a diyne moiety that acts as a flexible spacer.
1,3-Diynes are considered important building blocks for organic synthesis and material science.This has prompted the development of a large collection of synthetic methods for the preparation of such compounds. 10,11In particular, the synthesis of symmetric conjugated diynes can be obtained by several methods that include the different modifications of the Glaser-Eglinton-Hay oxidative coupling.
Herein we report the synthesis and NMR characterization of bis-4,4-(5-oxo-4,5-seco-3-yne-steroids) by coupling the corresponding 5-oxo-4,5-seco-3-yne steroid units.An X-ray diffraction analysis of compound 5b confirmed the proposed structure and provided detailed information about the bond lengths, bond angles and the conformation that the molecules adopt in the crystal state.

Results and Discussion
The starting alkynones 4a,b were obtained following our previously described sequences that include, as key steps, the epoxidation of ,-unsaturated ketones and the Eschenmoser-Tanabe fragmentation of a mixture of diastereomeric epoxides (Scheme 1).Scheme 1.Our previously reported synthesis of the 5-oxo-4,5-seco-3-yne steroids 4a and 4b. 8,9,12he first attempts at dimerization included the conditions of the Hay coupling 13 that employs TMEDA as base and a catalytic amount of CuCl in acetone under an oxygen atmosphere.Although this procedure produced the desired dimerization, TLC analysis indicated that after 16 hours the amount of starting material was significant.The more drastic conditions of the Eglinton version 14 in which the alkyne and an excess of cupric acetate are refluxed in methanolic pyridine produced the desired dimers in moderate yields after chromatographic purification (Scheme 2, and Table 1, Method A).
During the course of a parallel investigation 9 we observed that Sonogashira coupling of the alkynone 4a with o-iodobenzyl alcohol employing Pd(PPh3)4 and CuI in triethylamine produced a significant amount of the dimer 5a as side product.Exclusion of the oxygen by consecutive purging cycles with Ar under sonication solved this problem.This prompted us to drive the reaction to the desired dimers by removal of the iodinated derivative and saturation with oxygen.We were happy to find that treatment of the alkynones 4a,b in such conditions afforded good yields of the desired dimers 5a,b in reactions with a clean, fast and easy work-up, followed by chromatographic purification (Scheme 2, and NMR study allowed unambiguous characterization of the obtained compounds.With the exception of the 13 C signals of C-2, C-3 and C-4, the dimerization does not induce significant changes in the NMR signals of the neighboring nuclei compared with those of the starting monomer 4 (See Tables 2 and 3).This suggests that in solution the dimeric compounds adopt a linear conformation in which each steroid core is placed well away from the other, resembling the conformation observed in solid state (vide infra).

Crystal structure discussion
While all attempts at production of monocrystals of compounds 5a suitable for X-ray diffraction were unsuccessful, crystallization of compound 5b from ethyl acetate-acetone afforded monocrystals suitable for such studies.
The molecular structure of 4,4'-bis(4,5-secocholest-3-yn-5,5'-dione) (5b) was solved in the monoclinic space group C2 and Z = 2.The asymmetric unit contains only half of the dimeric molecule and the other half is generated by a 2-fold rotation axis perpendicular to the alkyne moiety which deviates slightly from linearity (175.6°), in accordance with similar steroidal dimeric structures 15,16 as shown in Figure 1.Additionally, the planes of both steroidal cores adopt an orthogonal position with respect to the dialkyne axis.
The dimeric molecules of compound 5b are packed in parallel layers propagated by C(2 64Å] interactions between neighboring molecules along the crystallographic bc plane.This particular arrangement favors the interaction between the hydrophobic faces of adjacent steroidal dimers generating a multidimensional highly crowded packed columnar self-assembly (see Figure 2).
The absolute configuration of compound 5b can be assumed without risk, as that known for the starting material considering that the synthetic transformations carried out do not affect the absolute configuration of © ARKAT USA, Inc the chiral centres in the naturally occurring steroid framework.Crystal data and experimental details of the structure determination of compound 5b are listed in Table 4. 17

Conclusions
We have developed straightforward procedures that allow the synthesis of dimers constituted of two units of a 5-oxo-4,5-seco-3-yne steroid derived from the naturally occurring compounds testosterone (1a), and cholesterol (1b).Although both procedures afforded the desired compounds, the palladium-catalyzed homocoupling emerged as more convenient than the Eglinton procedure due to its more environmental friendly conditions, higher yields and easier work-up of the reaction mixture.Crystallographic studies showed that the flexible diyne spacer of dimer 5b favors a crystal packing dominated by interactions that are capable of accommodationg the steroid cores in a highly crowded packed columnar self-assembly.Further studies to explore the solid-state properties and synthetic applications of dimers derived from the obtained compounds are in development.

Experimental Section
General.Reactions were monitored by TLC on ALUGRAM SIL G/UV254 plates from MACHEREY-NAGEL.TLC plates were sprayed with a 1% solution of vanillin in 50% HClO4 and heated until color developed.Melting points were measured on a Melt-Temp II apparatus.NMR spectra were recorded in CDCl3 solutions in a Varian INOVA 400 spectrometer using the solvent signal as reference.NMR signals assignments were carried out with the aid of combined 1D and 2D NMR techniques that included 1 H, 13 C, COSY, Nuclear Overhauser Effect Spectroscopy (NOESY), Heteronuclear Single Quantum Correlation (HSQC) and Heteronuclear Multiple Bond Correlation (HMBC).High resolution mass spectra were registered in a Jeol, AccuTOF JMS-T100LC spectrometer.The starting alkynones 4a and 4b were obtained following our previously described protocols (vide supra Scheme 1). 8,9,12Previously obtained physical and NMR data for 4a and 4b are provided for comparison purposes.

Figure 1 .
Figure 1.ORTEP perspective of compound 5b with thermal ellipsoids drawn at 50% probability level for all atoms other than hydrogen, with their corresponding unit cell viewed along the crystallographic c axis.

Table 1 .
Yields of the coupling reactions

Table 3 .
Selected 13 C NMR signals of the starting alkynones 4a,b and the obtained dimers 5a,b* (δ, ppm) = mean value, * Since the dimers 5a,b are C2 symmetrical, the chemical shifts of carbon atoms are identical for both halves

Table 4 .
Crystal data and structure refinement for the dimeric steroid 5b