Stereodirected synthesis of alkaloid-like quinolizidine systems

Abstract

New stereoselective methods for the chemical modification of cytisine based on T-reactions are reported. A reaction of cytisine with 2-chloro-5-nitrobenzaldehyde and followed condensation with 1,3-dimethylbarbituric acid affords N-(5-nitro-2-{1,3-dimethylperhydropyrimidine-2,4,6-trione-5-methynyl})cytisine, which undergoes a cyclization with the tetrahydropyridine ring closure. The cyclization proceeds via two competing routes yielding 5,5-spirobarbituric acid derivatives with 11,19-diaza-pentacyclo[11.7.1.0^2,11.0^5,10.0^14,19]henicosane and 11,15-diazapentacyclo-[11.7.1.0^2,11.0^5,10.0^15,20]henicosane skeletons. The cyclization reaction in solutions afford either 24.25-trans and 15,16-trans isomers or trans and cis isomer mixtures, depending on the specific solvent. Meanwhile, 24,25-cis and 15,16-cis isomers are formed stereoselectively under heterogeneous conditions in water suspensions. Trans-5,5-spirobarbiturates under similar conditions undergo isomerization into more stable cis-analogs by the configuration inversion at the C7 atom. The synthesized 5,5-spirobarbituric acid derivatives were successfully converted into alkaloid-like quinolizidine systems (1R,2R,3R,13S)-7-nitro-18-oxo-11,19-diazapentacyclo[11.7.1.0^2,11.0^5,10.0^14,19]henicosa-5(10),6,8,14,16-pentaene-3-carboxylic acid and (1R,2S,3S,13S)-nitro-16-oxo-11,15-diazapentacyclo[11,7,1.0^2,11,0^5,10,0^15,20]henicosa-5,7,9,17,19-pentaene-3-carboxylic acid and their derivatives via the spiropyrimidine moiety removal by the stereoselective hydrolysis. The molecular and crystal structures of the target substances were elucidated by X-ray crystallography and NMR.

Graphical Abstract

Keywords:

• Cytisine
• 5,5-spirobarbituric acid derivatives
• T-reactions
• hydrolysis
• quinolizidine alkaloids
• diastereomers
• synchrotron X-ray diffraction

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Russian Foundation for Basic Research (grant No. 16-29-10782-ofi_m) and the RUDN University Program “5-100”. X-ray crystallographic studies using synchrotron radiation were performed at the unique scientific facility Kurchatov Synchrotron Radiation Source supported by the Ministry of Education and Science of the Russian Federation (project code RFMEFI61917X0007).