ISSN:
1434-1948
Keywords:
Asymmetric catalysis
;
Allylic alkylation
;
1-Methyl-5-(1′-methylpent-2′-ynyl)barbituric acid
;
Palladium catalysts
;
Chiral phosphane imine ligands
;
Methohexital
;
Anesthetic dose
;
Chemistry
;
General Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Allylation of 1-methyl-5-(1′-methylpent-2′-ynyl)barbituric acid (MBS) with allyl acetate using in situ catalysts of palladium(II) acetylacetonate and chiral phosphane imine ligands resulted in the enantioselective formation of 5-allyl-1-methyl-5-(1′-methylpent-2′-ynyl)barbituric acid (Methohexital), an important anesthetic drug. Both, MBS and Methohexital contain two stereogenic carbon atoms. In MBS, the asymmetric centre in the barbiturate system is labile due to enolization. The asymmetric centre in the hexyne side chain is stable and racemic. The two asymmetric centres of Methohexital are stable and give rise to four stereoisomers, two diastereomeric racemates. An analysis of the isomers of MBS and Methohexital was established on the basis of 1H NMR and, in particular, GC including a base-line separation of the four stereoisomers of Methohexital. The stereoselectivity of the allylation is difficult to control, because the new quaternary asymmetric centre in the barbiturate ring of Methohexital is formed within the nucleophile, attacking the η3-allyl ligand of the catalyst from the side opposite to the palladium atom. Classical optically active ligands, such as diop or norphos, give only 2-6 % ee. Chiral phosphane imine ligands are a successful class of compounds, synthesized by Schiff base condensation of (2-formylphenyl)diphenylphosphane with optically active primary amines. The most efficient ligands have a hydroxymethyl and a bulky alkyl substituent at the asymmetric centre in the imine part, e.g. the L-iso-leucinol and the L-tert-leucinol derivatives 5 and 7. In the Pd-catalyzed allylation of MBS a kinetic resolution and the effect of the enantioselective catalyst interplay, the contributions of which are separated. For MBS the best stereoselectivity factor of the kinetic resolution s = kR/kS was 2.6 and 83 % “ee” were achieved. The corresponding values for Methohexital were s = 3.5 and 80 % ee in the α-dl pair. For 10 mixtures of Methohexital stereoisomers the anesthetic doses for rats were determined. With 9.1 mg/kg body weight of the animal the sample obtained from the catalysis with the D-α-phenylglycinol derivative 8 gave a much lower anesthetic dose than the widely used narcotic Brevimytal®Natrium, the sodium salt of the α-dl racemate of Methohexital, with 13.0 mg/kg body weight.
Additional Material:
2 Ill.
Type of Medium:
Electronic Resource
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