INTRODUCTION:

Dimerization is an association of two identical molecules in such a way that they behave as single unit. This may take place by a number of ways i.e. simple association of two units, by self condensation or by condensation of two molecules or more involving other simple bifunctional moieties. Dimeric steroids were observed as synthetic by products^1-2,followed by a number of discoveries of such compounds from natural as well as synthetic sources^3-4.The dimeric steroids have been shown to exhibit such unique characteristics as medicinal⁵, catalytic⁶and liquid crystal properties⁷ that will lead to important applications. This resulted in the increase of interests of chemists to synthesize such dimeric steroids. Dimeric steroids have also long been investigated because of their useful biological activities⁸ and they are obtained by the use of many reagents. This sustained interest in them, prompted us to develop new synthetic approach to obtain steroidal dimers¹¹. 1,3-Dibromopropane has proved to be a good reagent for the synthesis of steroidal dimers in excellent yield. The availability and safety of reagent and mild reaction conditions make it a suitable method for such process.

MATERIALS AND METHODS:

Melting point are uncorrected, IR (KBr) was carried out on Perkin-Elmer 237 spectrophotometer. ¹HNMR and C¹³NMR spectra were run in CDCl₃. FAB mass spectra were recorded on JEOL S x 102 mass spectrometer using Argon/Xenon (6KV, 10mA) as the FAB gas.

TLC plates were coated with silica gel. Light petroleum refers to a fraction of b.p. 60^o-80^oC.

To a solution of steroidal substrate (1) (4g, 1.8 mmoles) in dry pyridine (10ml, 123.65 mmoles), p-toluenesulfonylchloride (4g, 20.98 mmoles) was added and then the mixture was shaken for 5 – 10 minute at room temperature, a white precipitate was obtained. The reaction mixture was allowed to stand overnight. The organic matter was dissolved in ether and ethereal solution was washed successively with dilute H₂SO₄, NaHCO₃ (5%) and water and dried over anhydrous Na₂SO₄.Evaporation of ether left an oily residue,which was crystallized from petroleum ether to give corresponding tosylate (3) yield = 3.7 g, 6.85 mmoles and m.p. = 130^o – 132^oC (reported m.p.= 131^o – 133^oC) ¹⁰. Similarly the steroidal substrate (2) gave the corresponding tosylate (4) yield = 3.5 g, 6.17 mmoles and m.p. = 133^oC.

To a solution of tosylated product (3) (1g, 1.85 mmoles) in dichloromethane (30 ml, 468 mmoles), cupric chloride (0.5g, 2.93 mmoles), 1,3-dibromopropane (0.10 ml, 0.98 mmoles) and acetic acid (0.10 ml, 1.74 mmoles) was added and the reaction mixture was refluxed with constant stirring for 72 hrs. The progress of the reaction was checked by TLC. When the reaction was complete the reaction mixture was taken in ether and ethereal solution was washed successively with water, NaHCO₃ (5%) solution, and again with water and then dried over anhydrous Na₂SO₄. Evaporation of ether left a solid mixture which on column chromatography over silica gel afforded two products. Elution with light petroleum afforded a solid which was crystallized from methanol having melting point 93^oC (reported m.p. = 92.5^oC) was characterized as cholesta-4,6-diene (7) yield = 20%, molecular formula = C₂₇H₄₄, molecular weight = 368, C% = 88.00 (88.04), H% = 11.92 (11.95), IR (KBr) cm^-1 = 1635(-C=C-C=C-), ¹HNMR (CDCl₃) d (ppm) = 2.49 (q, C3-H₂), 5.37 (t, C4-H and C7-H), 3.76 (t, C8-H), 4.76 (d, C6-H), 0.85, 0.86 and 0.87 for other methyl, C¹³NMR (CDCl₃) d (ppm) = C1 (36.44), C2 (31.85), C3 (33.46), C4 (122.56), C5 (140.86), C6 (122.92), C7 (138.51), C8 (39.60), C9 (50.14), C10 (35.87), C11 (21.04). C12 (42.38), C13 (43.49), C14 (60.38), C15 (23.93), C16 (28.10), C17 (56.22), C18 (11.94), C19 (18.80), C20 (36.27), C21(19.34), C22 (39.20), C23(24.35), C24 (39.78), C25 (28.31), C26 (22.66), C27 (22.92). Further elution with petroleum ether : ether (98:2) afforded a solid which was crystallized from methanol having melting point 82^oC was characterized as 1,3 (bicholest-5-enyl-3,3'-dioxy) propane (5) yield = 80%,molecular formula = C₅₇H₉₆O₂, molecular weight = 812 (794 by Rast’s method), C% = 84.00(84.23), H% = 11.80(11.82),IR (KBr) cm^-1 = 1095 (ether linkage), 1184 (C-O stretching), 1652 (C=C), ¹HNMR (CDCl₃) d (ppm) = 3.26 (t, C6-H and C6'-H) (vinylic proton), 4.76 (brs, -2OCH₂), 3.35 (d, C4-H₂ and C4'-H₂), 5.36 (m, C3-aH and C3'-aH), 1.02 (m, -CH₂), 0.97, 0.98 and 0.99 for other methyl, C¹³NMR (CDCl₃) d (ppm) = C1 (80.42), C2 (28.32), C3 (80.41), C4 (39.59), C5 (140.93), C6 (121.66), C7 (31.96), C8 (32.02), C9 (50.27), C10 (36.96), C11 (22.66). C12 (40.09), C13 (42.39), C14 (56.85), C15 (23.94), C16 (28.09), C17 (56.23), C18 (11.94), C19 (18.79), C20 (36.27), C21 (19.45), C22 (37.01), C23 (23.92), C24 (36.69), C25 (28.32), C26 (22.91), C27 (22.92), Ca (76.68), Cb (35.88), m/z = 812 (M⁺, 5.0), 658 (7.5), 657 (10.0), 399 (75.0), 386 (10.0), 369 (87.5), 368 (27.5), 367 (55.0), 255 (12.5), 213 (17.5), 161 (32.5), 145 (62.5), 105 (70.0), 95 (92.5), 93 (75.0), 81 (32.5).

Similarly the tosylate (4) also gave two products elution with light petroleum (100%) afforded a solid which was crystallized from methanol having melting point 96^oC was characterized as stigmasta-4,6,22-triene (8) yield = 20%, molecular formula = C₂₉H₄₆, molecular weight = 394, C% = 88.20 (88.32), H% = 11.62 (11.67), IR (KBr) cm^-1 = 1628 (-C=C-C=C-), ¹HNMR (CDCl₃) d (ppm) = 2.56 (q, C3-H₂), 5.36 (t, C4-H and C7-H), 3.73 (t, C8-H), 4.75 (d, C6-H), 0.89, 0.90 and 0.91 for other methyl, C¹³NMR (CDCl₃) d (ppm) = C1 (36.42), C2 (31.87), C3 (33.47), C4 (122.58), C5 (140.88), C6 (122.94), C7 (138.51), C8 (39.63), C9 (50.61), C10 (35.88), C11 (21.07). C12 (42.39), C13 (43.49), C14 (60.39), C15 (23.95), C16 (28.11), C17 (56.24), C18 (11.95), C19 (18.82), C20 (36.28), C21 (19.35), C22 (124.37), C23 (139.20), C24 (39.79), C25 (28.32), C26 (22.68), C27 (22.94), C28 (37.23) C29 (37.93). Further elution with petroleum ether : ether (98:2) afforded a solid which was crystallized from methanol having melting point 88^oC was characterized as 1,3 (bistigmasta-5,22-dienyl-3,3'-dioxy) propane (6) yield = 50%, molecular formula = C₆₁H₁₀₀O₂, molecular weight = 864 (827 by Rast’s method), C% = 84.62 (84.72), H% = 11.56 (11.57), IR (KBr) cm^-1 = 1107 (ether linkage), 1184 (C–O stretching), 1652 (C=C), ¹HNMR (CDCl₃) d (ppm) = 3.35 (t, C6-H and C6'-H), 4.79 (brs,-2OCH₂), 3.39 (d, C4-H₂ and C4'-H₂), 5.38 (m, C3-aH and C3'-aH), 1.00 (m,-CH₂), 0.99, 1.00 and 1.02 for other methyl, C¹³NMR (CDCl₃) d (ppm) = C1 (80.45), C2 (28.36), C3 (80.43), C4 (39.59), C5 (140.94), C6 (121.69), C7 (31.98), C8 (32.04), C9 (50.28), C10 (36.96), C11 (22.68), C12 (40.12), C13 (42.40), C14 (56.88), C15 (23.96), C16 (28.10), C17 (56.24), C18 (11.96), C19 (18.80), C20 (36.29), C21(19.46), C22 (123.94), C23(137.01), C24 (38.70), C25 (28.34), C26 (22.94), C27 (22.90), C28 (37.23), C29 (37.92), Ca (76.88), Cb (35.88), m/z = 864 (M⁺, 7.5), 684 (7.5), 683 (15.0), 425 (57.5), 395 (35.0), 394 (17.5), 393 (27.5), 255 (30.0), 213 (17.5), 161 (22.5), 145 (57.5), 105 (80.0), 95 (77.5), 93 (67.5), 91 (87.5), 82 (92.5), 81 (52.5).

RESULTS AND DISCUSSION:

Here in we are presenting a novel and convenient method for the synthesis of such compounds^5,6. The reaction was carried out on 3b-tosyloxycholest-5-ene (3) and 3b-tosyloxystigmasta-5, 22-diene (4) in dichloromethane, 1,3-dibromopropane, acetic acid in presence of CuCl₂ as shown in Scheme 1.

The formations of the dimer with ether linkages was confirmed by Rast’s method and can be explained by the formation of oxygen free radical from tosylate⁹ and then the substitution of bromine in the 1,3-dibromopropane at the two ends by alkoxy free radicals as shown in Scheme 2.

The formation of cholesta-4,6-diene (7) and stigmasta-4,6,22-triene (8) also obtained during the reaction can be explained by the mechanism given in Scheme 3.

The mass fragmentation of dimers (5) and (6) are shown in Scheme 4.

The IR spectrum of the compound (7) exhibited band at 1635 cm^-1 indicating the presence of –C=C-C=C- bond. On the basis of composition and IR spectrum the structure of the compound can be written as cholesta-3,5-diene but the m.p. = 93^oC observed discards this possibility [reported m.p. = 80^oC]¹ and suggested the conjugated 4,6-diene which can be explained to have formed according to the mechanism given in scheme-4. This finds support from the ¹H NMR spectral study of the compound which exhibited a quartet centered at d 2.49 for two protons can be ascribed to C3-H₂. A triplet centered at d 5.37 for one proton each can be ascribed to C4-H and C7-H. A triplet for one proton at d 3.76 can be ascribed to C8-H. A doublet at d 4.76 can be ascribed to one proton of C6-H. C¹³ NMR shows peak at C4=122.56, C5=140.86, C6=122.92, C7=138.51 indicating the presence of conjugated double bond.

On the basis of above spectral data the compound, m.p. = 93^oC may be tentatively characterized as cholesta-4,6-diene (7).

The IR spectrum of the compound (8) exhibited band at 1628 cm^-1(–C=C-C=C-) this shows that diene is formed which is further supported from its NMR spectral study. The ¹H NMR spectrum exhibited a quartet centered at d 2.56 for two protons can be ascribed to C3-H₂. A triplet centered at d 5.36 for one proton each can be ascribed to C4-H and C7-H. A triplet for one proton at d 3.73 can be ascribed to C8-H. A doublet at d 4.75 can be ascribed to one proton of C6-H. These NMR values are compatible with the NMR spectrum of the earlier compound (7) discussed and hence with the same reasoning the structure (8) is preferred over the other structure showing 3,5-diene. Other methyl group signals were observed at d0.89, 0.90 and 0.91. C¹³ NMR shows peak at C4=122.58, C5=140.88, C6=122.94,

C7=138.51, C22=124.37 and C23=139.20 indicating the presence of double bond at C4, C6 and C22.

On the basis of above spectral data the compound, m.p. = 96^oC may be tentatively characterized as stigmast-4,6,22-triene (8).

The IR spectrum of compound (5) exhibited bands at, 1095 cm^-1 indicating the presence of ether linkage, 1184 cm^-1 for C-O stretching and at 1652 cm^-1 showing C=C bond. The composition and IR spectrum suggested the formation of expected dimer (5) which finds support from its ¹H NMR spectral study and Rast’s method. The ¹H NMR spectrum of the compound exhibited a multiplet centered at d 5.36 which can be ascribed to C3-aH and C3'-aH. A triplet at d 3.26 can be ascribed to C6-H and C6'-H, vinylic protons. A broad singlet at d 4.76 can be ascribed for two –OCH₂. A doublet centered at d 3.35 can be ascribed to the two allylic methylene groups (C4-H₂and C4'-H₂). A multiplet centered at d 1.02 for two protons can be ascribed to the protons of –CH₂ groups of propane. Other methyl group signals were observed at d 0.97, 0.98 and 0.99. C¹³ NMR shows peak at C3=80.42, Ca=76.68 and Cb=35.88. Mass spectrum shows the molecular ion peak at m/z = 812 and other mass fragments which are tentatively explained as shown in Scheme 4.

The IR spectrum of the compound (6) exhibited bands at 1107 cm^-1 indicating the presence of ether linkage, band at 1184 cm^-1 for C-O stretching and 1652 cm^-1 for C=C. The composition and IR spectrum suggested the formation of expected dimer (6) which finds support from its ¹H NMR spectral study and Rast’s method. The ¹H NMR spectrum of the compound exhibited a multiplet centered at d 5.38 which can be ascribed to C3-aH and C3'-aH. A triplet at d 3.35 can be ascribed to C6-H and C6'-H, vinylic protons. A broad singlet at d 4.79 can be ascribed for two –OCH₂. A doublet centered at d 3.39 can be ascribed to the two alylic methylene groups (C4-H₂and C4'-H₂). A multiplet centered at d 1.00 for two protons can be ascribed to the protons of –CH₂ groups of propane. Other methyl group signals were observed at d 0.96, 0.97 and 0.98. C¹³ NMR shows peak at C3=80.43, C5=140.94, C6=121.69, C22=123.94 and C23=137.01 indicating the presence of double bond at C5 and C22. Mass spectrum showed the molecular ion peak at m/z = 864 and other mass fragments which are tentatively explained as shown in Scheme 4.

We conclude that the above results which provide the products (5, 6, 7 and 8) from their respective olefins (3 and 4) make the present method highly useful.

ACKNOWLEDGEMENT:

The authors are thankful to chairman Department of Chemistry, AMU, Aligarh for providing necessary research facilities.

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11. Contributed for oral presentation in 97th Indian science Congress held during 3 to 7 Jan. 2010 at Thiruvanthapuram, India.Book of abstract pp-44.

Received on 28.04.2010 Modified on 11.05.2010

Asian J. Research Chem. 3(3): July- Sept. 2010; Page 758-762