Synthesis and biological evaluation of conformationally restricted σ1receptor ligands with 7,9-diazabicyclo[4.2.2]decane scaffold

Abstract

The key step in the synthesis of the 7,9-diazabicyclo[4.2.2]decane system was a modified Dieckmann condensation of piperazinebutyrate 11, which makes use of trapping the first cyclized intermediate with TMS-Cl. Reduction of the bicyclic ketone 14 with LiBH₄ at −90 °C provided diastereoselectively (>99 : 1) the syn-configured alcohol 15a, which was converted into the final alcohol and ethers 16a–g. The configuration at the 2-position was established by X-ray structure analysis of methyl and ethyl ethers 15b and 15c. In contrast to bicyclic systems with a three-carbon bridge, inversion of the configuration at the 2-position of the alcohol 15a failed to give the inverted alcohol 19a. However, an unselective reduction of the ketone 24 with L-Selectride led to the diastereomeric alcohols 16a and 25a in the ratio 36 : 64. LiAlH₄ reduction of the tosylate 20 and the alkene 18 yielded the diazabicyclo-decane 26 and -decene 27 without further substituents at the four-carbon bridge. The σ₁ and σ₂ receptor affinities were investigated in receptor binding studies with radioligands. All test compounds showed a lower σ₁ affinity than the corresponding bicyclic derivatives with a three-membered bridge. The reduced σ₁ receptor affinity is attributed to the larger four-membered bridge. This hypothesis is supported by the alkene 27, which represents the most potent σ₁ ligand of this series (K_i = 7.5 nM). In the alkene 27 the size and flexibility of the bridge is considerably reduced by the double bond. The methyl ether 25b and the unsubstituted derivatives 26 and 27 revealed moderate inhibition of the growth of the human tumor cell lines A-427, 5637 and MCF-7. Again, these compounds are less potent than the analogues with a three-membered bridge. The IC₅₀-value of the most potent σ₁ ligand 27 against the small cell lung cancer cell line A-427 (IC₅₀ = 10 μM) should be emphasized, since this cell line is particularly sensitive to homologues with a three-carbon bridge.