Elsevier

Steroids

Volume 76, Issues 10–11, September–October 2011, Pages 1043-1050
Steroids

Azido analogs of neuroactive steroids

https://doi.org/10.1016/j.steroids.2011.04.008Get rights and content

Abstract

Analogs of pregnanolone (3α-hydroxy-5β-pregnan-20-one), modified in position 17 were prepared. Compounds with 20-keto pregnane side chain replaced completely by azide (17α- and 17β-azido-5β-androstan-3α-ol), compounds with its part replaced (20-azido-21-nor-5β-pregnan-3α-ol), and compounds with keto group only replaced ((20R)- and (20S)-20-azido-5β-pregnan-3α-ol) were synthesized using tosylate displacements with sodium azide or Mitsunobu reaction with azoimide. All five azido steroids were converted into corresponding sulfates. Subsequent tests for inhibition of glutamate induced response on NMDA receptors revealed that modification of pregnanolone sulfate side chain with azide did not disturb the activity and some of sulfates tested were more active than parent compound.

Highlights

► Azides in hindered position 17 of androstane skeleton were synthesized. ► Substitution of tosylate with azide and Mitsunobu reaction were compared. ► Five azido analogs of pregnanolone sulfate altered in 17 side chain were prepared. ► Azido analogs of pregnanolone sulfate inhibited NMDA receptor activity.

Introduction

Steroidal azides are very often described in connection with preparation of corresponding amines [1], [2], [3], [4], [5]. Optical [6], thermal and photochemical properties [7], [8], [9] of steroidal azides were also studied. Recently, they were used in preparation of nitrogen heterocycles [10] or as intermediates for click chemistry [11].

In connection with broader project on neuroactive steroids we started with preparation of analogs of pregnanolone, modified in position 17. We designed analogs with the 20-keto pregnane side chain replaced with azido group only or with one or two carbon side chain bearing azido group. The rest of neurosteroid structure has to be conserved, so derivatives of 3α-hydroxy-5β-androstane, 21-nor-5β-pregnane or 5β-pregnane are to be synthesized.

In position 17 of androstane skeleton, 17α-azides are accessible by sulfonate substitution by sodium azide or Mitsunobu reaction with azoimide. The first approach was applied to 5α-androstane derivatives with 3-keto group [12], 3β-acetoxy group [13], or 3β-acetoxy-5-ene moiety [14]. On 3-keto-4-ene derivatives, effects of various solvents [15] or sulfonate leaving groups [16] were studied. So far mentioned examples covered substitution of pseudo-equatorial 17β-sulfonate with sodium azide to give 17α-azide. Also substitution in 14β-androstane series was described [17], where leaving 17α-sulfonate gave 17β-azide. The second approach, Mitsunobu reaction, was applied on 3-keto-4-ene derivatives and on 3-keto derivative with 5α-androstane skeleton and 17β-hydroxy group [18]. Nucleophilic substitution by sodium azide has been used also for preparation of 20-azidopregnane derivatives [19], [20], [21]. In this case, concurrent elimination reaction was noticed and the mechanism and solvent influence were studied [21].

Section snippets

General

Melting points were determined on a Boetius micro melting point apparatus (Germany) and are uncorrected. Optical rotations were measured at 25 °C on a AUTOPOL IV polarimeter (Rudolph Research Analytical, USA), and [α]D values are given in 10−1 deg cm2 g−1. Infrared spectra (wavenumbers in cm−1) were recorded on a Bruker IFS 88 spectrometer. 1H and 13C NMR spectra were taken on Bruker AVANCE-400 instrument at 23 °C in deuterochloroform and referenced to tetramethylsilane as the internal standard. 13C

17-Azido 5β-androstanes

For the preparation of steroidal azides in sterically hindered position 17, corresponding tosylates were treated with sodium azide in hexamethylphosphoramide (HMPA). From both isomers, 17α-azide is more easily available (Scheme 1). Starting from 17β-hydroxy-5β-androstan-3-one (1a), available from testosterone [22], tosylate 2a was prepared. The nucleophilic substitution with sodium azide took 60 h at 90 °C for completion. Resulting 17α-azido-5β-androstan-3-one (3a) was obtained in 74% yield. In

Conclusion

Syntheses of pregnanolone analogs with side chain in position 17 modified or replaced with azido group were developed. All five pregnanolone analogs were prepared by nucleophilic substitution of corresponding tosylates with sodium azide in hexamethylphosphoramide. In some cases, concurrent elimination reactions were observed so yields of this key reaction varied between 27% and 89%. For 17-azides 3a and 3b also alternative Mitsunobu reaction with azoimide was checked with comparable yields.

Acknowledgments

We are indebted to Dr. M. Buděšínský for help with interpreting of NMR spectra and especially for full assignment of 13C NMR spectra of pregnane azides 15a and 15b. We thank Mrs. L. Kloučková and Mrs. D. Hybšová for skillful technical assistance. Dr. S. Vašíčková was taking the IR spectra and Dr. P. Fiedler was interpreting them. Mass spectra were measured by the staff of the Laboratory of Mass Spectrometry (Dr. J. Cvačka, Head). Elemental analyses were carried out in the Analytical Laboratory

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