Abstract
Sharpless asymmetric dihydroxylation of styrene derivative 6 afforded chiral triols (R)-7 and (S)-7, which were cyclized with tosyl chloride in the presence of Bu2SnO to provide 2-benzopyrans (R)-4 and (S)-4 with high regioselectivity. The additional hydroxy moiety in the 4-position was exploited for the introduction of various substituents. Williamson ether synthesis and replacement of the Boc protective group with a benzyl moiety led to potent σ1 ligands with high σ1/σ2-selectivity. With exception of the ethoxy derivative 16, the (R)-configured enantiomers represent eutomers with eudismic ratios of up to 29 for the ester (R)-18. The methyl ether (R)-15 represents the most potent σ1 ligand of this series of compounds, with a Ki value of 1.2 nM and an eudismic ratio of 7. Tosylate (R)-21 was used as precursor for the radiosynthesis of [18F]-(R)-20, which was available by nucleophilic substitution with K[18F]F K222 carbonate complex. The radiochemical yield of [18F]-(R)-20 was 18%–20%, the radiochemical purity greater than 97% and the specific radioactivity 175–300 GBq/µmol. Although radiometabolites were detected in plasma, urine and liver samples, radiometabolites were not found in brain samples. After 30 min, the uptake of the radiotracer in the brain was 3.4% of injected dose per gram of tissue and could be reduced by coadministration of the σ1 antagonist haloperidol. [18F]-(R)-20 was able to label those regions of the brain, which were reported to have high density of σ1 receptors.
Highlights
The σ receptor was firstly described in 1976 by Martin et al It was named after the ligandSKF-10047 and initially regarded as opioid receptor subtype [1]
The Boc-protected piperidone was chosen because the handling of the resulting carbamate was much easier than the handling of alternative benzyl substituted tertiary amines (e.g., 12)
These results indicate that the 2-benzopyran based radiotracer [18F]-(R)-20 with a 2-fluoroethoxy side chain underwent faster biotransformation than the analogous 2-benzofuran based tracers [18F]2b and [18F]2c with a fluoroethyl or fluoropropyl side chain
Summary
The σ receptor was firstly described in 1976 by Martin et al It was named after the ligand. The 2-benzopyran 3 (Ki = 1.3 nM) represents a very potent σ1 receptor antagonist [35], enantiomers of 2-benzopyran based σ1 ligands were not yet investigated Due to their structural similarity to the spirocyclic 3-substituted 2-benzopyrans 3 [36] and 2-benzofurans 1 and 2, 4-substituted 2-benzopyrans 4 were considered as new type of σ1 receptor ligands. 2-benzopyran scaffold was not exploited for the development of a fluorinated PET tracer so far. In this communication we report the first enantioselective synthesis of 4-substituted spirocyclic 2-benzopyrans of type 4, their affinity towards σ receptors and the generation and biological evaluation of a [18F]-labeled PET tracer based on this scaffold
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