Synthesis of Labeled Caffeine - Caffeine: Chemistry, Analysis, Function and Effects - page 83

water mixture in the presence of sodium hydroxide (Cherrah et al 1987;
Falconnet et al 1986), or in DMF in the presence of potassium carbonate
(Regal et al 2005). They were purified by preparative TLC (Falconnet et al
1986), silica column chromatography (Cherrah et al 1987) and preparative
HPLC (Regal et al 2005).
Syntheses of [1,3,7-(CD 3 ) 3 ]caffeine
[1,3,7-(CD 3 ) 3 ]caffeine (46) was prepared from xanthine (13) by the action of
CD 3 I in an acetone-water mixture in the presence of sodium hydroxide
(Cherrah et al 1987; Falconnet et al 1986; Horning et al 1979). The purification
of [1,3,7-(CD 3 ) 3 ]caffeine was achieved by preparative TLC (Falconnet et al
1986),

d n 0 t 2 n g | 2

silica column chromatography (Cherrah et al 1987) and repeated

recrystallization from ethanol (Horning et al 1979).

5.3.6 Syntheses of [ 3 H]Caffeine
Tritium labeled caffeine was prepared by catalyzed hydrogen-tritium
exchange. This reaction occurs regioselectively at the H-8 position. Tritium
sources were tritiated water, a tritiated water-tritiated acetic acid mixture or
tritium gas. Catalysts such as alkali or palladium and platinum have been used
(Scheme 5.12).

Scheme 5.12 Synthesis of [8-T]caffeine by hydrogen-tritium exchange. H-8 tritiated

caffeine was prepared by catalyzed hydrogen-tritium exchange with
tritiated water, a tritiated water-tritiated acetic acid mixture or tritium gas.

[8-T]caffeine (47) was prepared by heating caffeine (36) in tritiated water and
alkaline pH was found to improve the tritium incorporation rate (Jelinska and
Sobkowski 1977).
This hydrogen-tritium exchange reaction was performed in tritiated water-
tritiated acetic acid mixture in the presence of platinum black catalyst (Neiman
1981).
The H-8 substitution as in [8-T]caffeine was also performed by the action of
tritium gas in the presence of palladium on barium sulfate (47) (Bloxsidge et al
1981).

5.4 Characterization of Labeled Caffeine
Labeled caffeine is usually characterized by mass spectrometry (MS) and
nuclear magnetic resonance (NMR).