Lipase-catalyzed kinetic resolution as key step in the synthesis of enantiomerically pure σ ligands with 2-benzopyran structure.

In order to obtain enantiomerically pure σ1 receptor ligands with a 2-benzopyran scaffold an Oxa-Pictet-Spengler reaction with the enantiomerically pure 2-phenylethanol derivatives (R)-4 and (S)-4 was envisaged. The kinetic resolution of racemic alcohol (±)-4 using Amano Lipase PS-C II and isopropenyl acetate in tert-butyl methyl ether led to the (R)-configured alcohol (R)-4 in 42% yield with an enantiomeric excess of 99.6%. The (S)-configured alcohol (S)-4 was obtained by Amano Lipase PS-C II catalyzed hydrolysis of enantiomerically enriched acetate (S)-5 (76.9% ee) and provided (S)-4 in 26% yield and 99.7% ee. The absolute configuration of alcohol (R)-4 was determined by exciton coupled CD spectroscopy of the bis(bromobenzoate) (R)-7. The next important step for the synthesis of 2-benzopyrans 2 and 3 was the Oxa-Pictet-Spengler reaction of the enantiomerically pure alcohols (R)-4 and (S)-4 with piperidone ketal 8 and chloropropionaldehyde acetal 12. The conformationally restricted spirocyclic 2-benzopyrans 2 revealed higher σ1 affinity than the more flexible aminoethyl derivatives 3. The (R)- and (R,R)-configured enantiomers (R)-2 and (R,R)-3 represent the eutomers of this class of compounds with eudismic ratios of 4.8 (2b) and 4.5 (2c). High σ1/σ2 selectivity (>49) was found for the most potent σ1 ligands (R)-2b, (R)-2c, (R)-2d, and (S)-2d (Ki(σ1) 9-15nM).

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

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.