Oxa-Pictet–Spengler reaction as key step in the synthesis of novel σ receptor ligands with 2-benzopyran structure

Abstract

The Oxa-Pictet–Spengler reaction of methyl 3-hydroxy-4-phenylbutanoate (8) was explored to obtain novel σ receptor ligands. 1-Acyl protected piperidone ketals 10 and 11 reacted with phenylethanol 8 to yield spirocyclic compounds. Aliphatic aldehyde acetals 19 provided 1,3-disubstituted 2-benzopyrans 20 with high cis-diastereoselectivity. The intramolecular Oxa-Pictet–Spengler reaction of 24 led to the tricyclic compound 25. The spirocyclic compounds 18 show high σ1 affinity (Ki 20–26nM) and σ1/σ2 selectivity (>9-fold), when a large substituent (n-octyl, benzyl, phenylpropyl) is attached to the piperidine N-atom. Opening of the piperidine ring to yield aminoethyl (22, 23) or aminomethyl derivatives (21) resulted in reduced σ1 affinity and σ1/σ2 selectivity.