Dedicated to Andrew T. B. Stuart, P.Eng., in recognition of his contributions to clean
hydrogen and deuterium technologies.
Caffeine (1), or 1,3,7-trimethylxanthine, is a naturally occurring alkaloid found in the leaves,
fruit, or nuts of numerous plant species’ including coffee, tea, cacao, and so on.[1] It is a psychoactive member of the methylxanthine family of compounds, commonly
encountered in food or drinks, where it serves as a mild stimulant, increasing alertness
and concentration.[2] In addition to these common uses, it is also considered by the WHO to be an essential
medicine, used for the treatment of bronchopulmonary dysplasia and apnea in premature
babies.[3] Furthermore, recent studies have suggested caffeine to be effective in treating
orthostatic hypotension and hypoxic ischemic encephalopathy,[4] in delaying Alzheimer’s progression,[5] and even in inhibiting MCF-7 human breast cancer cells.[6] However, when consumed in excess, acute poisoning can result, with symptoms including
nausea, vomiting, insomnia, heart palpitations, and, in rare cases, death.[1]
[7] Given the ever-increasing consumer consumption of caffeine, especially at higher
concentrations, the development of a caffeine alternative that is both long-lasting
and equally effective at lower concentrations would have a broad impact.
Figure 1 Caffeine (1), caffeine-d
9 (1-d
9), their parent purine base xanthine (2), and their primary desmethyl metabolites paraxanthine (3), theobromine (4), and theophylline (5)
Caffeine is metabolized in vivo through demethylation by the CYP1A2 enzyme, a member of the cytochrome P450 family
of enzymes, into its three primary desmethyl metabolites paraxanthine (3), theobromine (4), and theophylline (5) (Figure [1]).[8] The pharmacokinetic profiles of caffeine and other compounds metabolized by these
enzymes can be modified through deuteration, as substituting hydrogen for deuterium
can significantly influence drug-protein binding, receptor affinity, enzyme efficiency,
and drug distribution.[1]
[9] Deuteration can also significantly influence metabolic rates through the kinetic
isotope effect, owing to deuterium’s increased mass over hydrogen, and the stronger
bonds it forms with carbon, which makes such bonds harder to break.[10] Given caffeine’s metabolic degradation pathway, it is unsurprising that biological
studies on the impact of deuterating its methyl groups (e.g., 1-d
9) have been conducted over the past few decades.[1]
[8a]
[9]
[11] As caffeine-d
9 remains an important research target with significant potential for societal and
economic impact, devising a cost-efficient, gram-scale synthesis of caffeine-d
9 is warranted.
Scheme 1 Select syntheses of caffeine and caffeine-d
9 (a–c) and synthesis of caffeine-d
9 from xanthine and CD3I (d)
Caffeine is readily obtained as a byproduct of the decaffeinating process, and as
there is ample supply of the natural material, little effort has been dedicated to
its chemical synthesis. It was first synthesized from uric acid in 1895 by Emil Fischer,[12] but in the intervening century, it received scarce attention from the synthetic
community.[13] Syntheses by Traube in 1900,[14] and by Narayan[15] in 2003 followed similar synthetic strategies from 4- or 5-amino-N,N-dimethyluracil, respectively. In the Narayan synthesis, 6 was subjected to formylation, nitration, and reductive heterocyclization steps to
give theophylline (5), which was then N-methylated to give caffeine (Scheme [1a]).[16] In other examples, Ando and co-workers reported a one-step, 61% yield synthesis
of caffeine from xanthine using dimethyl sulfate and KF-coated alumina,[16c] and Bier and co-workers published a 20% yielding synthesis of caffeine-d
9 from xanthine using CD3I and K2CO3 (Scheme [1b,c]).[17] Unfortunately, much of this precedent is not immediately applicable to a concise,
scalable synthesis of caffeine-d
9. For example, existing synthetic routes from natural precursors 3, 4, or 5 would not be viable as they already contain non-deuterated methyl groups. A synthesis
following the Traube/Narayan strategy would result in significant loss of deuterated
material through poor-yielding reactions and purification steps. Ando’s use of dimethyl
sulfate would not be ideal as dimethyl sulfate-d6 would be costly to produce and at least half of its deuterium content would be lost
as waste, and Bier’s synthesis was operationally challenging and very low yielding.
To support the ongoing efforts towards evaluating caffeine-d
9 as an alternative to caffeine, we wished to develop an efficient synthesis of caffeine
from xanthine. We report here that a new, one-step synthesis of caffeine from xanthine
has been developed, and also that substituting conventional CH3I with CD3I enabled a caffeine-d
9 synthesis without any adverse effect on the reaction outcome (Scheme [1d]).
We began our investigation by following the example of González-Calderón’s synthesis
from theobromine (4), and heating xanthine, iodomethane, and sodium methoxide in methanol at reflux (Scheme
[2], equation 1).[16d] Unfortunately, no caffeine was observed after 4 h, and extending the reaction time
to 24 hours failed to improve on this result. Crude 1H NMR analysis of the reaction mixtures suggested that only mono-methylated xanthine
derivatives had been produced. The lack of reactivity in these reactions was attributed
to the poor solubility of 2 in both water and alcoholic solvents.[18] We also attempted the reaction in acetonitrile with K2CO3, modifying the procedure of Bier;[17] however, our eventual 25% yield was little better than their reported 20% yield
(Scheme 2, equation 2).
Scheme 2 Preliminary attempts at caffeine synthesis using NaOMe in MeOH or K2CO3 in CH3CN
Table 1 Optimization of Caffeine Synthesis Using NaH/DMSO and CH3Ia
|
|
Entry
|
CH3I (equiv)
|
NaH (equiv)
|
DMSO (equiv)
|
Time (h)
|
Yield (%)
|
|
1
|
5
|
4
|
16
|
24
|
45
|
|
2
|
7
|
4
|
16
|
24
|
53
|
|
3
|
5
|
6
|
24
|
24
|
75
|
|
4
|
5
|
8
|
32
|
24
|
78
|
|
5
|
5
|
8
|
0
|
24
|
0
|
|
6
|
5
|
6
|
24
|
18
|
46
|
|
7
|
5
|
6
|
24
|
48
|
64
|
|
8
|
4
|
6
|
24
|
24
|
50
|
|
9b
|
5
|
6
|
24
|
24
|
77
|
a Reactions conducted on a 1 mmol scale in THF (7 × volume of DMSO) at rt. 1 was recrystallized from EtOH.
b Reaction conducted on a 1 g scale.
We next consulted the Narayan synthesis from theophylline (5),[15] which used 5 equiv of CH3I to achieve a single methylation, and also used the very hazardous base dimsyl sodium.[19] But given the improved solubility of 2 in DMSO, the ability to moderate the risks of NaH/DMSO by diluting with THF,[20]
[21] and given the significant room for improving the CH3I stoichiometry, we next studied these reaction conditions. We were surprised to find
that reacting xanthine with CH3I (5 equiv), NaH (4 equiv), and DMSO (16 equiv) in THF at room temperature gave 45%
yield of caffeine after 24 hours (Table [1], entry 1). While increasing the loading of CH3I to 7 equiv resulted in a 53% yield of caffeine (entry 2), this modification was
not adopted as we believed it to constitute a poor outcome given the 40% increased
loading of methylating agent. Conversely, increasing the loading of NaH in DMSO to
6 equiv gave caffeine in 75% yield, and further increasing it to 8 equiv gave caffeine
in a similar 78% yield (entries 3 and 4). Conversely, using NaH in THF in the absence
of DMSO caused the reaction to fail completely (entry 5). Varying the reaction time
also failed to improve the yield, as stopping the reaction after 18 hours gave 1 in 46% yield and stopping it after 48 hours gave 1 in 64% yield (entries 6 and 7). At this stage we also attempted to decrease the loading
of CH3I to 4 equiv, but as this only gave caffeine in 50% yield (entry 8), it appeared that
the 5 equiv were required to achieve a good yield. Finally, we conducted the reaction
on a 1 g (6.6 mmol) scale, to potentially overcome any material losses occurring during
the small-scale recrystallization. To our delight, this reaction proceeded without
incident, and 1 was recovered in 77% yield (entry 9), in excellent accord with the earlier results.
Scheme 3 Synthesis of caffeine-d
9
Having developed an effective, one-step synthesis of caffeine from xanthine, we then
attempted the synthesis using CD3I as the methylating agent. When conducted on a 1 g (6.6 mmol) scale, the reaction
proceeded without incident to produce caffeine-d
9 in 86% isolated yield after recrystallization (Scheme [3]).[21] Comparison of the 1H NMR spectra for caffeine and caffeine-d
9 showed the expected disappearance of the three methyl peaks (δ = 3.94, 3.51, and
3.34), consistent with their deuteration (Figure [2]). Furthermore, while the peak for the C8 proton was still present, it was shifted
0.05 ppm upfield (δ = 7.42 vs 7.47) for deuterated caffeine, consistent with long-range
shielding by the N7–CD3 group. Comparison of the 13C NMR spectra for caffeine and caffeine-d
9 also showed the expected changes consistent with deuteration (Figure [3]). While the five carbon signals from the purine core were relatively unchanged,
the signals associated with the methyl groups were markedly different. These were
observed as septets (2nI + 1, where I
D = 1 and n = 3) with 21.6 Hz coupling constants. Furthermore, their heights were significantly
depressed due to the lack of decoupling-based NOE enhancement, and they were shifted
0.8 ppm upfield, again due to the shielding effect of deuterium.[22]
Figure 2 Overlaid 1H NMR spectra of caffeine (bottom) and caffeine-d
9 (top)
Figure 3 Overlaid 13C NMR spectra of caffeine (bottom) and caffeine-d
9 (top)
In conclusion, we have developed an efficient, gram-scale synthesis of caffeine and
caffeine-d
9 from xanthine. Many of the various conditions reported in the literature for methylating
xanthine and other desmethyl caffeine precursors were tested, and optimal reaction
conditions were found to be a 1.6-fold excess of the methylating agent and using dimsyl
sodium as the base. Additionally, the use of THF as the solvent was critical to mitigating
the hazards associated with thermal decomposition of the NaH/DMSO mixture. This synthetic
route offers a significant improvement over existing strategies for preparing caffeine-d
9, and we anticipate that this route will be benefit researchers seeking gram-scale
access to deuterated caffeine and its related derivatives.
Caffeine-d
9 (1-d
9)
To a flame-dried round bottom flask was added THF (78.5 mL) followed by anhyd DMSO
(11.2 mL, 158 mmol, 24 equiv). The solution was cooled to 0 °C in an ice bath and
NaH (1.58 g, 60% in mineral oil, 39.5 mmol, 6 equiv) was slowly added (300 mg/min),
after which the resulting grey suspension was stirred at 0 °C for 30 min. The reaction
mixture warmed to rt, xanthine (1 g, 6.6 mmol, 1 equiv) was added and the resulting
was mixture stirred for 30 min. CD3I (2.09 mL, 32.9 mmol, 5 equiv) was then added dropwise over ~5 min and the reaction
was then stirred for 24 h at rt. Water (100 mL) was added to the reaction and the
resulting biphasic mixture was extracted with DCM (3 × 70 mL). The combined organic
phases were concentrated under vacuum to give a crude yellow solid, and this was purified
by recrystallization (EtOH) to yield fine, white, fiber-like crystals. The crystals
were isolated by vacuum filtration, and the resulting filtrate could be concentrated
and subject to further recrystallization. The combined crystals were dried under high
vacuum to yield caffeine-d
9 (1.154 g, 86%). NMR and melting point data matched that found in the literature;[17] mp 235–237 °C (uncorrected; Lit.[17] 236.1 °C).
1H NMR (CDCl3, 300 MHz): δ = 7.42 (singlet).
13C NMR (CDCl3, 75 MHz): δ = 155.1, 151.5, 148.5, 141.3 (CH), 107.3, 32.8 (septet, J = 21.6 Hz, CD3), 28.9 (septet, J = 21.6 Hz, CD3), 27.1 (septet, J = 21.6 Hz, CD3).
