STE 7774
No. of Pages 4, Model 5G
20 April 2015 Steroids xxx (2015) xxx–xxx 1
Contents lists available at ScienceDirect
Steroids journal homepage: www.elsevier.com/locate/steroids 4 5 3
Erroneous epimerization at C-22 in sapogenins
6
Izabella Jastrze˛bska, Jacek W. Morzycki ⇑
7
Institute of Chemistry, University of Białystok, Ciołkowskiego 1K, 15-245 Białystok, Poland
8 9 1 2 1 2 12 13 14 15 16 17 18 19 20 21
a r t i c l e
i n f o
Article history: Received 10 January 2015 Received in revised form 31 March 2015 Accepted 8 April 2015 Available online xxxx
a b s t r a c t A recent report [1] about the epimerization of steroidal sapogenins at C-22 by treatment with BF3OEt2 is incorrect. We proved that the epimerization of sapogenins with BF3OEt2 occurs at C-25 as in the case of other acid-catalyzed reactions previously studied. Ó 2015 Published by Elsevier Inc.
23 24 25 26 27
Keywords: Sapogenins 22-epi-Sapogenins Sapogenin isomerization 28
29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57
Steroidal sapogenins undergo different transformations during acid treatment since they contain the acid-sensitive spiroacetal moiety. These reactions have been intensively studied for many years and investigations are still continued [1]. Sapogenin isomerization was first reported by Marker in 1939 [2]. Sarsasapogenin (1a) underwent isomerization on prolonged heating in ethanol with hydrochloric acid at reflux. The isomerization product (smilagenin 3a) was erroneously identified as 22-epi-sarsasapogenin (2a). Unfortunately, this incorrect structural assignment was later supported by other chemists [3,4]. The proposed mechanism for the Brønsted acid-catalyzed isomerization at C-22 is shown in Scheme 1. It should be noted that the mechanism also allows for the isomerization at C-20 that was reported by Wall et al. [5]. An analogous mechanism for the Lewis acid-catalyzed reactions has been now suggested by Sandoval-Ramírez [1]. However, extensive investigations have established that the isomeric sapogenins differ only in their configuration at C-25, and not at C-22 or C-20, as initially believed. Scheer, Kostic and Mosettig [6] converted sarsasapogenin (1a) and smilagenin (3a) to identical derivatives via catalytic hydrogenolysis of the C(22)– O(26) bond, selective tosylation of the resulting primary alcohol followed by LiAlH4 reduction. The ring F degradation of the isomeric sapogenins [6,7] afforded the enantiomeric a-methylglutaric acids. Finally, isomerization experiments catalyzed with deuterium chloride carried out by Callow and Massy-Beresford [8] resulted in hydrogen-deuterium exchange at positions C-25 and C-20. They explained the acid-catalyzed interconversion of the C-25 isomers in terms of a bimolecular displacement by a ⇑ Corresponding author. Tel.: +48 85 7388260; fax: +48 85 7388099.
proton with inversion [8,9], but such a reaction is without parallel in organic chemistry. A plausible oxidation-reduction mechanism for isomerization at C-25 (Scheme 2) was presented by Woodward, Sondheimer and Mazur in 1958 [10]. The key step of the process is a reversible 1,5-hydride transfer from C-26 to C-22. The isomerization involves a series of equilibria between the oxonium ions produced from sapogenins by acid treatment. This mechanism was supported by the observation that the intermediate C-26 aldehyde was converted to the 25R sapogenin when subjected to the conditions ordinarily used for saponin isomerization (HClaq in boiling ethanol) or to BF3OEt2 in benzene at room temperature. A direct proof for the reverse 1,5-hydride shift came from the study of the acid-catalyzed reaction of aldehyde labeled with deuterium at C-22 [11]. As a matter of fact, all equilibria shown in Schemes 1 and 2 occur during the treatment of steroidal sapogenins with strong Brønsted acids under aqueous (HClaq/ethanol, HClaq/dioxane) or anhydrous (HClgas/Ac2O/dioxane, p-TsOH/CH2Cl2) conditions, as well as with Lewis acids (trimethylsilyl triflate, boron trifluoride). During the reaction, all three stereogenic centers in the spirostane side chain could be affected. However, the reactions catalyzed by strong acids (thermodynamic control) lead to the formation of the most stable product among the eight possible stereoisomers. The calculated steric energies (Table 1) for sarsasapogenin (1a), 22-epi-sarsasapogenin (2a), smilagenin (3a) and the remaining five stereoisomers we reported several years ago [12]. The driving force pushing the reaction forward is a steric strain release accompanying the isomerization of 25S-spirostanes to their 25R-isomers. The reverse transformation is also possible but the content of the 25S isomer in the equilibrium mixture is very low.
E-mail address: [email protected] (J.W. Morzycki). http://dx.doi.org/10.1016/j.steroids.2015.04.001 0039-128X/Ó 2015 Published by Elsevier Inc.
Please cite this article in press as: Jastrze˛bska I, Morzycki JW. Erroneous epimerization at C-22 in sapogenins. Steroids (2015), http://dx.doi.org/10.1016/ j.steroids.2015.04.001
58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87
STE 7774
No. of Pages 4, Model 5G
20 April 2015 2
I. Jastrze˛bska, J.W. Morzycki / Steroids xxx (2015) xxx–xxx 27
21 2 0S
18 12 19 1 2 3
RO
A 4
11
H
13
C
9 10 5
17
2 6 2 5S 23 F 22R 24
S
O
O
7
RO
or Lewis acids
6
H+
H
2a; R = H (22-epi-sarsasapogenin) 2b; R = Ac HO
H O+
-H
O
HClaq, EtOH, Δ
1a; R = H (sarsasapogenin) 1b; R = Ac
1
S
S
16
1 4 15
8
B
D
E
O
- H+
+ O
O
HO
+
H
H+
O
- H+
+
pseudosapogenin HO O H+
H+
+ O
O - H+
O
O
H+
- H+
2 (20R or 20S)
Scheme 1. The erroneous isomerization of sarsasapogenin (1) at C-22 – a suggested mechanism.
S
S
O
O
S
R
R
R
O
O HClaq, EtOH, Δ
RO
RO
or Lewis acids
H 1a; R = H (sarsasapogenin) 1b; R = Ac
1
H+
O
+ O
HO H
HO+ H
- H+
HO+ H O
HO.. H
H O+
O O
H 3a; R = H (smilagenin) 3b; R = Ac
- H+ H
HO.. H + O
.
H+
O
O.
- H+
+
H O+
O - H+
O
O
H+
3
Scheme 2. The oxidation-reduction mechanism of sapogenin isomerization at C-25 [10].
Table 1 Calculated steric energies (MM + force field) for sarsasapogenin (1a) and its diastereoisomers [12].
88 89 90 91
3b-hydroxy-5b-spirostane
Steric energy (kcal/mol)
20S,22R,25S (sarsasapogenin 1a) 20S,22S,25S (22-epi-sarsasapogenin 2a) 20S,22R,25R (smilagenin 3a) 20S,22S,25R 20R,22R,25R 20R,22R,25S 20R,22S,25R 20R,22S,25S
58.64 60.46 57.54 61.71 62.18 63.43 62.58 61.48
The cleavage of the spirostane ring F with acids (usually with acetic anhydride/AlCl3) affords enol ethers commonly called pseudosapogenins (Scheme 1). Their cyclization reactions with strong acids also give the corresponding thermodynamically stable
Table 2 C and 1H NMR assignments for the isomerization product of 1b and smilagenin (3a).
13
Position Isomerization Smilagenin product of 1b (13C) (3a) (13C)
Isomerization Smilagenin product of 1b (1H) (3a) (1H)
20
41.6
41.61
21
14.6
14.51
22 23
109.1 31.4
109.26 31.39
24 25 26
28.8 30.3 66.8
28.80 30.31 66.86
27
17.2
17.14
1.86 (qn, J = 6.8) 0.96 (d, J = 6.9) 0.97 (d, J = 6.9) 1.58, 1.67 (d, J = 4.57) 1.46, 1.64 1.64 3.36 (t, J = 10.3) 3.38 (t, J = 11.0) 3.46 (ddd, J = 10.3, 3.48 (ddd, 4.4, 1.8) J = 11.0, 4.0, 1.9) 0.79 (d, J = 7.2) 0.79 (d, J = 6.4)
Please cite this article in press as: Jastrze˛bska I, Morzycki JW. Erroneous epimerization at C-22 in sapogenins. Steroids (2015), http://dx.doi.org/10.1016/ j.steroids.2015.04.001
STE 7774
No. of Pages 4, Model 5G
20 April 2015 3
I. Jastrze˛bska, J.W. Morzycki / Steroids xxx (2015) xxx–xxx
O
O O
O
O BF 3·OEt 2
RO
O or
RO 4a; R = H (diosgenin) 4b; R = Ac
RO 5a; R = H (22-epi-diosgenin) 5b; R = Ac
6a; R = H (yamogenin) 6b; R = Ac
Scheme 3. BF3OEt2 catalyzed isomerization of diosgenin.
Table 3 C NMR assignments for the isomerization product of 4b [1], yamogenin acetate (6b) [17] and the authentic 22-epi-diosgenin acetate (5b) [14].
13
Position Isomerization product of 4b 20 21 22 23 24 25 26 27
a
42.1 14.3 109.7 25.9 25.7 27.0 65.1 16.1
Yamogenin acetate (6b)
22-epi-diosgenin acetate (5b)
42.18 14.36 109.75 26.02 25.83 27.12 65.14 16.08
41.5 16.4 111.2 25.5 23.4 27.5 68.6 16.9
a
This chemical shift is given in the text of Ref. [1]. In Experimental the value 40.2 is given.
Table 4 1 H NMR assignments of methyl groups in the isomerization product of 4b [1], yamogenin acetate (6b) [18], and 22-epi-diosgenin acetate (5b) [14].
a
92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107
Position Isomerization product of 4b
Yamogenin acetate (6b)
22-epi-diosgenin acetate (5b)
18 19 21 27
0.79 1.04 1.01 1.08
0.92 1.05 1.01a 1.06a
0.79 1.03 1.00 1.07
These signals may be interchanged.
sapogenins as major products. However, the reactions of pseudosapogenins with weak acids proceed in a different way, producing the kinetically favored sapogenin products [13,14]. According to Sandoval-Ramírez [1], the reaction of sarsasapogenin acetate (1b) with boron trifluoride etherate in chloroform at 50 °C afforded 22-epi-sarsasapogenin acetate (2b) in a 70% yield. However, the physical data for the product obtained (mp 144– 146 °C; [a]D 52.2) were very similar to those of smilagenin acetate (3b): mp 147–149 °C; [a]D 60.4 [15]. Also 13C and 1H NMR spectra (side chain region) of the obtained sapogenin were very close to the analogous smilagenin (3a) spectra [16] (see Table 2). Starting from diosgenin acetate (4b) upon treatment with BF3OEt2, Mexican chemists [1] obtained sapogenin identified as 22-epi-diosgenin acetate (5b) in 17% yield (Scheme 3). However, on the basis of the similarity of the NMR spectra (Tables 3 and 4), we assume that the real product of the described reaction could
be yamogenin acetate (6b), though the physical data provided by the authors (oil, [a]D 18.4) are different from those reported in the literature (mp 180–182 °C, [a]D 130.8) [17]. We have recently described the synthesis of 22-epi-sapogenins, including 22-epi-diosgenin acetate (5b), via the photoinduced isomerization of 23-oxosapogenins at C-22 followed by a deoxygenation procedure [14]. Compound 5b synthesized by us (mp 159–162 °C) showed different 1H and 13C NMR spectra than those described by Sandoval-Ramírez [1]. The down-field shift of 18-methyl protons in 1H NMR of compound 5b seems to be diagnostic for 22-epi-sapogenins. In addition to that, compound 5b obtained by us was proven acid sensitive and could not be obtained under conditions described in Ref. [1]. Yamogenin (6a) was also subjected by Sandoval-Ramírez [1] to epimerization with BF3OEt2 (Scheme 4). According to the paper an inseparable mixture of 22-epi-yamogenin (7a) and the unreacted starting material was obtained in the ratio 1.4:1, as judged by the integration of axial H-26 proton signals in their 1H NMR spectra. The diagnostic signal of the isomerization product appeared at d 3.37 as a triplet with a large coupling constant (the exact J value was not given). However, the H-26 proton signals in 1H NMR of diosgenin (4a) resonate at the same region [19] and this sapogenin is the likely isomerization product. The 1H NMR spectrum shown by Sandoval-Ramírez [1] as a proof of the isomerization of yamogenin (6a) to 22-epi-yamogenin (7a) in fact represents the mixture of yamogenin (6a) and diosgenin (4a). In Fig. 1a fragment of the spectrum (a) of the isomerization mixture shown in Ref. [1] is compared with the analogous fragment of 1H NMR spectrum (b) of pure diosgenin recorded by us (both spectra were measured at 400 MHz). It can be noted that all signals coming from diosgenin are present in the spectrum of the isomerization mixture. Finally, we repeated the BF3OEt2 catalyzed reactions of sarsasapogenin acetate (1b) and diosgenin acetate (4b). The reaction product of 1b proved identical (TLC Rf, IR and NMR spectra) with smilagenin acetate (3b). An analogous reaction of purified diosgenin acetate (4b) gave the isomerization product only in a very low yield (3%; in Ref. [1] a yield of 17% is given). The alleged product structure according to Ref. [1] was 22-epi-diosgenin acetate (5b), but the product obtained by us proved identical with an authentic sample of yamogenin acetate (6b). It should be stressed that yamogenin is a common contamination of commercial diosgenin, sometimes present in a concentration of up to 15% [17]. So, compound 6b could be already present in the starting material.
O O
O BF3·OEt2
HO
O or
HO 6a (yamogenin)
O
O
HO 7a (22-epi-yamogenin)
4a (diosgenin)
Scheme 4. BF3OEt2 catalyzed isomerization of yamogenin (6a).
Please cite this article in press as: Jastrze˛bska I, Morzycki JW. Erroneous epimerization at C-22 in sapogenins. Steroids (2015), http://dx.doi.org/10.1016/ j.steroids.2015.04.001
108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151
STE 7774
No. of Pages 4, Model 5G
20 April 2015 4
I. Jastrze˛bska, J.W. Morzycki / Steroids xxx (2015) xxx–xxx
(b)
O
H-26ax
O H-26eq
HO
diosgenin
(a)
Fig. 1. Comparison of spectra of the isomerization mixture (a) and pure diosgenin (b).
152
Acknowledgment
153 154
Financial support from the University of Białystok within the project BST-124 is gratefully acknowledged.
155
References
156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174
[1] Vinas-Bravo O, Merino-Montiel P, Romero-Lopez A, Montiel-Smith S, MezaReyes S, Melendez FJ, Sandoval-Ramírez J. Epimerization of C-22 in (25R)- and (25S)-sapogenins. Steroids 2015;93:60–7. [2] Marker RE, Rohrmann E. Sterols. LIII. The structure of the side chain of sarsasapogenin. J Am Chem Soc 1939;61:846–51. [3] Djerassi C, Martinez H, Rosenkranz G. Steroidal sapogenins. VII. Experiments in the hecogenin series. J Org Chem 1951;16:303–8. [4] Wall ME, Serota S. Steroidal sapogenins XX. Configuration of spiroketal side chain at carbon 22. J Am Chem Soc 1954;76:2850–1. [5] Wall ME, Eddy CR, Serota S. Steroidal sapogenins XIX. Stereochemistry of sapogenins and cholesterol at carbon 20. J Am Chem Soc 1954;76:2849–50. [6] Scheer I, Kostic RB, Mosettig E. The C-25 isomerism of sarsasapogenin and smilagenin. J Am Chem Soc 1953;75:4871–2. [7] James VHT. The configuration at C-25 in the natural spirostan sapogenins. J Chem Soc 1955:637–9. [8] Callow RK, Massy-Beresford PN. Steroidal sapogenins: the isomerization of normal to iso-sapogenins. J Chem Soc 1958:2645–53. [9] Cornforth JW. Organic chemistry. Steroids. Ann Repts Progr Chem (Chem Soc London) 1953;50:216–32.
[10] Woodward RB, Sondheimer F, Mazur Y. The mechanism of the isomerization of steroidal sapogenins at C-25. J Am Chem Soc 1958;80:6693–4. [11] Seo S, Uomori A, Takeda K. Direct observation of the reverse 1,5-hydride shift: the mechanism of acid-catalyzed isomerization at C-25 of spirostanols. J Org Chem 1986;51:3823–7. [12] Iglesias-Arteaga MA, Jastrze˛bska I, Morzycki JW. Reactions of sapogenins with m-chloroperoxybenzoic acid catalyzed by Lewis acids. Polish J Chem 2006;80:667–71. [13] Tobari A, Teshima M, Koyanagi J, Kawase M, Miyamae H, Yoza K, Takasaki A, Nagamura Y, Saito S. Spirostanols obtained cyclization of pseudosaponin derivatives and comparison of anti-platelet agglutination activities of spirostanol glycosides. Eur J Med Chem 2000;35:511–27. [14] Jastrze˛bska I, Górecki M, Frelek J, Santillan R, Siergiejczyk L, Morzycki JW. Photoinduced isomerization of 23-oxosapogenins: conformational analysis and spectroscopic characterization of 22-isosapogenins. J Org Chem 2012;77:11257–69. [15] Steraloids Inc., Online Catalogue. [16] Agrawal PK, Bunsawansong P, Morris GA. Complete assignment of the 1H and 13 C NMR spectra of steroidal sapogenins: smilagenin and sarsasapogenin. Magn Reson Chem 1997;35:441–6. [17] Uomori A, Seo S, Sato T, Yoshimura Y, Takeda K. Synthesis of (25R)-[262H1]cholesterol and 1H NMR. and HPLC resolution of (25R)- and (25S)-26hydroxycholesterol. J Chem Soc, Perkin Trans 1987;1:1713–8. [18] Tori K, Nishikawa J, Seo S, Uomori A, Yasuda F. 1H NMR Spectra of (25S)steroidal sapogenins. Reassignments of the C-20 and C-25 methyl signals. Steroids 1982;39:73–80. [19] Puri R, Wong TC, Puri RK. Solasodine and diosgenin: 1H and 13C assignments by two-dimensional NMR spectroscopy. Magn Reson Chem 1993;31:278–82.
175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203
Please cite this article in press as: Jastrze˛bska I, Morzycki JW. Erroneous epimerization at C-22 in sapogenins. Steroids (2015), http://dx.doi.org/10.1016/ j.steroids.2015.04.001
No. of Pages 4, Model 5G
20 April 2015 Steroids xxx (2015) xxx–xxx 1
Contents lists available at ScienceDirect
Steroids journal homepage: www.elsevier.com/locate/steroids 4 5 3
Erroneous epimerization at C-22 in sapogenins
6
Izabella Jastrze˛bska, Jacek W. Morzycki ⇑
7
Institute of Chemistry, University of Białystok, Ciołkowskiego 1K, 15-245 Białystok, Poland
8 9 1 2 1 2 12 13 14 15 16 17 18 19 20 21
a r t i c l e
i n f o
Article history: Received 10 January 2015 Received in revised form 31 March 2015 Accepted 8 April 2015 Available online xxxx
a b s t r a c t A recent report [1] about the epimerization of steroidal sapogenins at C-22 by treatment with BF3OEt2 is incorrect. We proved that the epimerization of sapogenins with BF3OEt2 occurs at C-25 as in the case of other acid-catalyzed reactions previously studied. Ó 2015 Published by Elsevier Inc.
23 24 25 26 27
Keywords: Sapogenins 22-epi-Sapogenins Sapogenin isomerization 28
29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57
Steroidal sapogenins undergo different transformations during acid treatment since they contain the acid-sensitive spiroacetal moiety. These reactions have been intensively studied for many years and investigations are still continued [1]. Sapogenin isomerization was first reported by Marker in 1939 [2]. Sarsasapogenin (1a) underwent isomerization on prolonged heating in ethanol with hydrochloric acid at reflux. The isomerization product (smilagenin 3a) was erroneously identified as 22-epi-sarsasapogenin (2a). Unfortunately, this incorrect structural assignment was later supported by other chemists [3,4]. The proposed mechanism for the Brønsted acid-catalyzed isomerization at C-22 is shown in Scheme 1. It should be noted that the mechanism also allows for the isomerization at C-20 that was reported by Wall et al. [5]. An analogous mechanism for the Lewis acid-catalyzed reactions has been now suggested by Sandoval-Ramírez [1]. However, extensive investigations have established that the isomeric sapogenins differ only in their configuration at C-25, and not at C-22 or C-20, as initially believed. Scheer, Kostic and Mosettig [6] converted sarsasapogenin (1a) and smilagenin (3a) to identical derivatives via catalytic hydrogenolysis of the C(22)– O(26) bond, selective tosylation of the resulting primary alcohol followed by LiAlH4 reduction. The ring F degradation of the isomeric sapogenins [6,7] afforded the enantiomeric a-methylglutaric acids. Finally, isomerization experiments catalyzed with deuterium chloride carried out by Callow and Massy-Beresford [8] resulted in hydrogen-deuterium exchange at positions C-25 and C-20. They explained the acid-catalyzed interconversion of the C-25 isomers in terms of a bimolecular displacement by a ⇑ Corresponding author. Tel.: +48 85 7388260; fax: +48 85 7388099.
proton with inversion [8,9], but such a reaction is without parallel in organic chemistry. A plausible oxidation-reduction mechanism for isomerization at C-25 (Scheme 2) was presented by Woodward, Sondheimer and Mazur in 1958 [10]. The key step of the process is a reversible 1,5-hydride transfer from C-26 to C-22. The isomerization involves a series of equilibria between the oxonium ions produced from sapogenins by acid treatment. This mechanism was supported by the observation that the intermediate C-26 aldehyde was converted to the 25R sapogenin when subjected to the conditions ordinarily used for saponin isomerization (HClaq in boiling ethanol) or to BF3OEt2 in benzene at room temperature. A direct proof for the reverse 1,5-hydride shift came from the study of the acid-catalyzed reaction of aldehyde labeled with deuterium at C-22 [11]. As a matter of fact, all equilibria shown in Schemes 1 and 2 occur during the treatment of steroidal sapogenins with strong Brønsted acids under aqueous (HClaq/ethanol, HClaq/dioxane) or anhydrous (HClgas/Ac2O/dioxane, p-TsOH/CH2Cl2) conditions, as well as with Lewis acids (trimethylsilyl triflate, boron trifluoride). During the reaction, all three stereogenic centers in the spirostane side chain could be affected. However, the reactions catalyzed by strong acids (thermodynamic control) lead to the formation of the most stable product among the eight possible stereoisomers. The calculated steric energies (Table 1) for sarsasapogenin (1a), 22-epi-sarsasapogenin (2a), smilagenin (3a) and the remaining five stereoisomers we reported several years ago [12]. The driving force pushing the reaction forward is a steric strain release accompanying the isomerization of 25S-spirostanes to their 25R-isomers. The reverse transformation is also possible but the content of the 25S isomer in the equilibrium mixture is very low.
E-mail address: [email protected] (J.W. Morzycki). http://dx.doi.org/10.1016/j.steroids.2015.04.001 0039-128X/Ó 2015 Published by Elsevier Inc.
Please cite this article in press as: Jastrze˛bska I, Morzycki JW. Erroneous epimerization at C-22 in sapogenins. Steroids (2015), http://dx.doi.org/10.1016/ j.steroids.2015.04.001
58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87
STE 7774
No. of Pages 4, Model 5G
20 April 2015 2
I. Jastrze˛bska, J.W. Morzycki / Steroids xxx (2015) xxx–xxx 27
21 2 0S
18 12 19 1 2 3
RO
A 4
11
H
13
C
9 10 5
17
2 6 2 5S 23 F 22R 24
S
O
O
7
RO
or Lewis acids
6
H+
H
2a; R = H (22-epi-sarsasapogenin) 2b; R = Ac HO
H O+
-H
O
HClaq, EtOH, Δ
1a; R = H (sarsasapogenin) 1b; R = Ac
1
S
S
16
1 4 15
8
B
D
E
O
- H+
+ O
O
HO
+
H
H+
O
- H+
+
pseudosapogenin HO O H+
H+
+ O
O - H+
O
O
H+
- H+
2 (20R or 20S)
Scheme 1. The erroneous isomerization of sarsasapogenin (1) at C-22 – a suggested mechanism.
S
S
O
O
S
R
R
R
O
O HClaq, EtOH, Δ
RO
RO
or Lewis acids
H 1a; R = H (sarsasapogenin) 1b; R = Ac
1
H+
O
+ O
HO H
HO+ H
- H+
HO+ H O
HO.. H
H O+
O O
H 3a; R = H (smilagenin) 3b; R = Ac
- H+ H
HO.. H + O
.
H+
O
O.
- H+
+
H O+
O - H+
O
O
H+
3
Scheme 2. The oxidation-reduction mechanism of sapogenin isomerization at C-25 [10].
Table 1 Calculated steric energies (MM + force field) for sarsasapogenin (1a) and its diastereoisomers [12].
88 89 90 91
3b-hydroxy-5b-spirostane
Steric energy (kcal/mol)
20S,22R,25S (sarsasapogenin 1a) 20S,22S,25S (22-epi-sarsasapogenin 2a) 20S,22R,25R (smilagenin 3a) 20S,22S,25R 20R,22R,25R 20R,22R,25S 20R,22S,25R 20R,22S,25S
58.64 60.46 57.54 61.71 62.18 63.43 62.58 61.48
The cleavage of the spirostane ring F with acids (usually with acetic anhydride/AlCl3) affords enol ethers commonly called pseudosapogenins (Scheme 1). Their cyclization reactions with strong acids also give the corresponding thermodynamically stable
Table 2 C and 1H NMR assignments for the isomerization product of 1b and smilagenin (3a).
13
Position Isomerization Smilagenin product of 1b (13C) (3a) (13C)
Isomerization Smilagenin product of 1b (1H) (3a) (1H)
20
41.6
41.61
21
14.6
14.51
22 23
109.1 31.4
109.26 31.39
24 25 26
28.8 30.3 66.8
28.80 30.31 66.86
27
17.2
17.14
1.86 (qn, J = 6.8) 0.96 (d, J = 6.9) 0.97 (d, J = 6.9) 1.58, 1.67 (d, J = 4.57) 1.46, 1.64 1.64 3.36 (t, J = 10.3) 3.38 (t, J = 11.0) 3.46 (ddd, J = 10.3, 3.48 (ddd, 4.4, 1.8) J = 11.0, 4.0, 1.9) 0.79 (d, J = 7.2) 0.79 (d, J = 6.4)
Please cite this article in press as: Jastrze˛bska I, Morzycki JW. Erroneous epimerization at C-22 in sapogenins. Steroids (2015), http://dx.doi.org/10.1016/ j.steroids.2015.04.001
STE 7774
No. of Pages 4, Model 5G
20 April 2015 3
I. Jastrze˛bska, J.W. Morzycki / Steroids xxx (2015) xxx–xxx
O
O O
O
O BF 3·OEt 2
RO
O or
RO 4a; R = H (diosgenin) 4b; R = Ac
RO 5a; R = H (22-epi-diosgenin) 5b; R = Ac
6a; R = H (yamogenin) 6b; R = Ac
Scheme 3. BF3OEt2 catalyzed isomerization of diosgenin.
Table 3 C NMR assignments for the isomerization product of 4b [1], yamogenin acetate (6b) [17] and the authentic 22-epi-diosgenin acetate (5b) [14].
13
Position Isomerization product of 4b 20 21 22 23 24 25 26 27
a
42.1 14.3 109.7 25.9 25.7 27.0 65.1 16.1
Yamogenin acetate (6b)
22-epi-diosgenin acetate (5b)
42.18 14.36 109.75 26.02 25.83 27.12 65.14 16.08
41.5 16.4 111.2 25.5 23.4 27.5 68.6 16.9
a
This chemical shift is given in the text of Ref. [1]. In Experimental the value 40.2 is given.
Table 4 1 H NMR assignments of methyl groups in the isomerization product of 4b [1], yamogenin acetate (6b) [18], and 22-epi-diosgenin acetate (5b) [14].
a
92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107
Position Isomerization product of 4b
Yamogenin acetate (6b)
22-epi-diosgenin acetate (5b)
18 19 21 27
0.79 1.04 1.01 1.08
0.92 1.05 1.01a 1.06a
0.79 1.03 1.00 1.07
These signals may be interchanged.
sapogenins as major products. However, the reactions of pseudosapogenins with weak acids proceed in a different way, producing the kinetically favored sapogenin products [13,14]. According to Sandoval-Ramírez [1], the reaction of sarsasapogenin acetate (1b) with boron trifluoride etherate in chloroform at 50 °C afforded 22-epi-sarsasapogenin acetate (2b) in a 70% yield. However, the physical data for the product obtained (mp 144– 146 °C; [a]D 52.2) were very similar to those of smilagenin acetate (3b): mp 147–149 °C; [a]D 60.4 [15]. Also 13C and 1H NMR spectra (side chain region) of the obtained sapogenin were very close to the analogous smilagenin (3a) spectra [16] (see Table 2). Starting from diosgenin acetate (4b) upon treatment with BF3OEt2, Mexican chemists [1] obtained sapogenin identified as 22-epi-diosgenin acetate (5b) in 17% yield (Scheme 3). However, on the basis of the similarity of the NMR spectra (Tables 3 and 4), we assume that the real product of the described reaction could
be yamogenin acetate (6b), though the physical data provided by the authors (oil, [a]D 18.4) are different from those reported in the literature (mp 180–182 °C, [a]D 130.8) [17]. We have recently described the synthesis of 22-epi-sapogenins, including 22-epi-diosgenin acetate (5b), via the photoinduced isomerization of 23-oxosapogenins at C-22 followed by a deoxygenation procedure [14]. Compound 5b synthesized by us (mp 159–162 °C) showed different 1H and 13C NMR spectra than those described by Sandoval-Ramírez [1]. The down-field shift of 18-methyl protons in 1H NMR of compound 5b seems to be diagnostic for 22-epi-sapogenins. In addition to that, compound 5b obtained by us was proven acid sensitive and could not be obtained under conditions described in Ref. [1]. Yamogenin (6a) was also subjected by Sandoval-Ramírez [1] to epimerization with BF3OEt2 (Scheme 4). According to the paper an inseparable mixture of 22-epi-yamogenin (7a) and the unreacted starting material was obtained in the ratio 1.4:1, as judged by the integration of axial H-26 proton signals in their 1H NMR spectra. The diagnostic signal of the isomerization product appeared at d 3.37 as a triplet with a large coupling constant (the exact J value was not given). However, the H-26 proton signals in 1H NMR of diosgenin (4a) resonate at the same region [19] and this sapogenin is the likely isomerization product. The 1H NMR spectrum shown by Sandoval-Ramírez [1] as a proof of the isomerization of yamogenin (6a) to 22-epi-yamogenin (7a) in fact represents the mixture of yamogenin (6a) and diosgenin (4a). In Fig. 1a fragment of the spectrum (a) of the isomerization mixture shown in Ref. [1] is compared with the analogous fragment of 1H NMR spectrum (b) of pure diosgenin recorded by us (both spectra were measured at 400 MHz). It can be noted that all signals coming from diosgenin are present in the spectrum of the isomerization mixture. Finally, we repeated the BF3OEt2 catalyzed reactions of sarsasapogenin acetate (1b) and diosgenin acetate (4b). The reaction product of 1b proved identical (TLC Rf, IR and NMR spectra) with smilagenin acetate (3b). An analogous reaction of purified diosgenin acetate (4b) gave the isomerization product only in a very low yield (3%; in Ref. [1] a yield of 17% is given). The alleged product structure according to Ref. [1] was 22-epi-diosgenin acetate (5b), but the product obtained by us proved identical with an authentic sample of yamogenin acetate (6b). It should be stressed that yamogenin is a common contamination of commercial diosgenin, sometimes present in a concentration of up to 15% [17]. So, compound 6b could be already present in the starting material.
O O
O BF3·OEt2
HO
O or
HO 6a (yamogenin)
O
O
HO 7a (22-epi-yamogenin)
4a (diosgenin)
Scheme 4. BF3OEt2 catalyzed isomerization of yamogenin (6a).
Please cite this article in press as: Jastrze˛bska I, Morzycki JW. Erroneous epimerization at C-22 in sapogenins. Steroids (2015), http://dx.doi.org/10.1016/ j.steroids.2015.04.001
108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151
STE 7774
No. of Pages 4, Model 5G
20 April 2015 4
I. Jastrze˛bska, J.W. Morzycki / Steroids xxx (2015) xxx–xxx
(b)
O
H-26ax
O H-26eq
HO
diosgenin
(a)
Fig. 1. Comparison of spectra of the isomerization mixture (a) and pure diosgenin (b).
152
Acknowledgment
153 154
Financial support from the University of Białystok within the project BST-124 is gratefully acknowledged.
155
References
156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174
[1] Vinas-Bravo O, Merino-Montiel P, Romero-Lopez A, Montiel-Smith S, MezaReyes S, Melendez FJ, Sandoval-Ramírez J. Epimerization of C-22 in (25R)- and (25S)-sapogenins. Steroids 2015;93:60–7. [2] Marker RE, Rohrmann E. Sterols. LIII. The structure of the side chain of sarsasapogenin. J Am Chem Soc 1939;61:846–51. [3] Djerassi C, Martinez H, Rosenkranz G. Steroidal sapogenins. VII. Experiments in the hecogenin series. J Org Chem 1951;16:303–8. [4] Wall ME, Serota S. Steroidal sapogenins XX. Configuration of spiroketal side chain at carbon 22. J Am Chem Soc 1954;76:2850–1. [5] Wall ME, Eddy CR, Serota S. Steroidal sapogenins XIX. Stereochemistry of sapogenins and cholesterol at carbon 20. J Am Chem Soc 1954;76:2849–50. [6] Scheer I, Kostic RB, Mosettig E. The C-25 isomerism of sarsasapogenin and smilagenin. J Am Chem Soc 1953;75:4871–2. [7] James VHT. The configuration at C-25 in the natural spirostan sapogenins. J Chem Soc 1955:637–9. [8] Callow RK, Massy-Beresford PN. Steroidal sapogenins: the isomerization of normal to iso-sapogenins. J Chem Soc 1958:2645–53. [9] Cornforth JW. Organic chemistry. Steroids. Ann Repts Progr Chem (Chem Soc London) 1953;50:216–32.
[10] Woodward RB, Sondheimer F, Mazur Y. The mechanism of the isomerization of steroidal sapogenins at C-25. J Am Chem Soc 1958;80:6693–4. [11] Seo S, Uomori A, Takeda K. Direct observation of the reverse 1,5-hydride shift: the mechanism of acid-catalyzed isomerization at C-25 of spirostanols. J Org Chem 1986;51:3823–7. [12] Iglesias-Arteaga MA, Jastrze˛bska I, Morzycki JW. Reactions of sapogenins with m-chloroperoxybenzoic acid catalyzed by Lewis acids. Polish J Chem 2006;80:667–71. [13] Tobari A, Teshima M, Koyanagi J, Kawase M, Miyamae H, Yoza K, Takasaki A, Nagamura Y, Saito S. Spirostanols obtained cyclization of pseudosaponin derivatives and comparison of anti-platelet agglutination activities of spirostanol glycosides. Eur J Med Chem 2000;35:511–27. [14] Jastrze˛bska I, Górecki M, Frelek J, Santillan R, Siergiejczyk L, Morzycki JW. Photoinduced isomerization of 23-oxosapogenins: conformational analysis and spectroscopic characterization of 22-isosapogenins. J Org Chem 2012;77:11257–69. [15] Steraloids Inc., Online Catalogue. [16] Agrawal PK, Bunsawansong P, Morris GA. Complete assignment of the 1H and 13 C NMR spectra of steroidal sapogenins: smilagenin and sarsasapogenin. Magn Reson Chem 1997;35:441–6. [17] Uomori A, Seo S, Sato T, Yoshimura Y, Takeda K. Synthesis of (25R)-[262H1]cholesterol and 1H NMR. and HPLC resolution of (25R)- and (25S)-26hydroxycholesterol. J Chem Soc, Perkin Trans 1987;1:1713–8. [18] Tori K, Nishikawa J, Seo S, Uomori A, Yasuda F. 1H NMR Spectra of (25S)steroidal sapogenins. Reassignments of the C-20 and C-25 methyl signals. Steroids 1982;39:73–80. [19] Puri R, Wong TC, Puri RK. Solasodine and diosgenin: 1H and 13C assignments by two-dimensional NMR spectroscopy. Magn Reson Chem 1993;31:278–82.
175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203
Please cite this article in press as: Jastrze˛bska I, Morzycki JW. Erroneous epimerization at C-22 in sapogenins. Steroids (2015), http://dx.doi.org/10.1016/ j.steroids.2015.04.001
