Biomimetic monoacylation of diols in water. Lanthanide-promoted reactions of methyl benzoyl phosphate

Article abstract of DOI:10.1021/ja049538l

The direct monoacylation of diols by acyl phosphate monoesters in water is a biomimetic analogy to the enzymic aminoacylation of tRNA by aminoacyl adenylates. Without catalysis, acyl phosphate monoesters react rapidly with amines but very slowly with water and alcohols. Lanthanide ions dramatically and selectively facilitate the base-catalyzed monoacylation of diols in water by methyl benzoyl phosphate (MBP), a typical acyl phosphate monoester, in neutral solutions where reactive amines are protonated and unreactive. The reaction patterns and reactivity of various diols with MBP in the presence of lanthanides are consistent with a mechanism that involves internal addition from the conjugate base of the bis-bidentate complex of the lanthanide with the diol and MBP. The method is also applicable to reactions of nucleosides as evidenced by the selective monoacylation of the 2′- or 3′-hydroxyl group of adenosine, without reaction of the 5′-hydroxyl group or the 6-amino group. Analogues of adenosine without the diol are unreactive. This suggests that the method will selectively monoacylate the hydroxyl groups at the unique diol in tRNA that forms the 3′-terminus.

Full text of DOI:10.1021/ja049538l

Published on Web 08/05/2004
Biomimetic Monoacylation of Diols in Water.
Lanthanide-Promoted Reactions of Methyl Benzoyl Phosphate
Lisa L. Cameron, Sheila C. Wang, and Ronald Kluger*
Contribution from the DaVenport Chemistry Research Building, Department of Chemistry,
UniVersity of Toronto, Toronto, Ontario, Canada M5S 3H6
Received January 26, 2004; E-mail: rkluger@chem.utoronto.ca
Abstract: The direct monoacylation of diols by acyl phosphate monoesters in water is a biomimetic analogy
to the enzymic aminoacylation of tRNA by aminoacyl adenylates. Without catalysis, acyl phosphate
monoesters react rapidly with amines but very slowly with water and alcohols. Lanthanide ions dramatically
and selectively facilitate the base-catalyzed monoacylation of diols in water by methyl benzoyl phosphate
(MBP), a typical acyl phosphate monoester, in neutral solutions where reactive amines are protonated and
unreactive. The reaction patterns and reactivity of various diols with MBP in the presence of lanthanides
are consistent with a mechanism that involves internal addition from the conjugate base of the bis-bidentate
complex of the lanthanide with the diol and MBP. The method is also applicable to reactions of nucleosides
as evidenced by the selective monoacylation of the 2′- or 3′-hydroxyl group of adenosine, without reaction
of the 5′-hydroxyl group or the 6-amino group. Analogues of adenosine without the diol are unreactive.
This suggests that the method will selectively monoacylate the hydroxyl groups at the unique diol in tRNA
that forms the 3′-terminus.
Amino acids are activated for protein synthesis as esters of
a 3′-terminal hydroxyl group of transfer RNA (tRNA), catalyzed
by aminoacyl tRNA synthetases.1 The pathway involves initial
formation of an acyl phosphate monoester (aminoacyl adenyl-
ate),2 which then aminoacylates the terminal hydroxyl group
(Scheme 1).
The preparation of tRNAs acylated with unnatural amino
acids is an important step in the Hecht-Schultz scheme that
uses the ribosome to produce protein-specific residues with
novel side chains.3-5 A biomimetic system would utilize
synthetic aminoacyl phosphate monoesters6,7 to acylate hy-
droxyls at the 3′-terminal of tRNAs.
Acyl (and aminoacyl) phosphate monoesters are water-soluble
and can be prepared by convenient synthetic routes.8 They are
generally unreactive toward attack by hydroxyl groups, as
exemplified by their very slow hydrolysis in neutral solution.9,10
It is of interest that enzymic aminoacylation of hydroxyl groups
in tRNA is promoted primarily by the enzymes binding the
reactants in reactive proximity.11 We considered that metal
cations, which coordinate to the anionic phosphate oxygens of
acyl phosphate monoesters,12,13 would activate the adjacent
carbonyl group through bidentate coordination and charge
neutralization.14 However, divalent cations only weakly enhance
the hydrolysis of acyl phosphate monoesters through such
complexation.6 We reasoned that trivalent lanthanide ions would
be more likely to promote the desired reactions based on their
size, lability, and charge. Indeed, we found that lanthanide ions
dramatically accelerate base-promoted hydrolysis and metha-
nolysis of methyl benzoyl phosphate (MBP) in water.14 This
established the general activation pattern and suggests a
mechanism in which the coordinated conjugate base of the
hydroxyl group is the target of acyl transfer from the coordinated
acyl phosphate monoester. We assumed that lanthanide ions
coordinate both the phosphate and the carbonyl oxygens of
MBP.14 Because the desired acylation sites in tRNA are the
vicinal hydroxyl groups in the unique diol functionality at the
3′-terminus of tRNA, we examined the reaction patterns of diols
with MBP and lanthanide ions and have found that monoacyl-
ation of appropriate diols occurs readily and selectively.
Experimental Section
UV kinetic measurements were performed in triplicate in the
temperature-controlled cell compartment maintained within 0.1 °C.
HPLC analysis was performed with a C18 reverse phase analytical
column and eluted with HPLC-grade acetonitrile. The methanolysis of
MBP was following at 240 nm. Cells containing 2.85 mL of dry
methanol were equilibrated at 25 °C for 30 min. MBP was dissolved
in methanol, and 0.15 mL of this solution was immediately added to
(1) DeMoss, J. A.; Genuth, S. M.; Novelli, G. D. Proc. Nat. Acad. Sci. USA
1956, 42, 325-332.
(2) Berg, P. J. Biol. Chem. 1958, 233, 601-607.
(3) Pezzuto, J. M.; Hecht, S. M. J. Biol. Chem. 1980, 255, 865-869.
(4) Robertson, S. A.; Ellman, J. A.; Schultz, P. G. J. Am. Chem. Soc. 1991,
113, 2722-2729.
(5) Noren, C. J.; Anthony-Cahill, S. J.; Griffith, M. C.; Schultz, P. G. Science
1989, 244, 182-188.
(6) Kluger, R.; Loo, R. W.; Mazza, V. J. Am. Chem. Soc. 1997, 119, 12089-
12094.
(7) Kluger, R.; Li, X.; Loo, R. W. Can. J. Chem. 1996, 74, 2395-2400.
(8) Kluger, R. Synlett 2000, 1708-1720.
(12) Sigel, H.; Da Costa, C. P.; Song, B.; Carloni, P.; Gregan, F. J. Am. Chem.
Soc. 1999, 121, 6248-6257.
(13) Sigel, H.; Da Costa, C. P. J. Inorg. Biochem. 2000, 79, 247-251.
(14) Kluger, R.; Cameron, L. L. J. Am. Chem. Soc. 2002, 124, 3303-3308.
(9) Di Sabato, G.; Jencks, W. P. J. Am. Chem. Soc. 1961, 83, 4393-4400.
(10) Di Sabato, G.; Jencks, W. P. J. Am. Chem. Soc. 1961, 83, 4400-4405.
(11) Fersht, A. R. Biochemistry 1987, 26, 8031-8037.
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J. AM. CHEM. SOC. 2004, 126, 10721-10726
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A R T I C L E S
Cameron et al.
Scheme 1
the cuvette with a concentration of MBP of 5 × 10-5 M. The
spontaneous methanolysis of MBP follows an approximately straight-
line change in concentration versus time between 0 and 100 min, which
amounts to less than 1% conversion. Using this slope, the observed
first-order rate constant was determined by dividing the initial rate by
the initial concentration of MBP. The lanthanide concentration depen-
dence of the methanol reaction was studied by adding different amounts
of lanthanum trifluoromethanesulfonate (0.01 M in methanol) to MBP
in methanol. Lanthanide catalysis was studied over a small concentration
range (2.5 × 10-5 to 1 × 10-4 M) because at higher concentrations
the reactions are very fast. Scans of the product mixtures show a shift
of λmax from that of MBP at 234 nm to 227 nm, which is the λmax of
methyl benzoate in methanol. Reactions were followed to at least five
half-lives.
The addition reactions of other alcohols and MBP in the presence
of lanthanide and EPPS buffer were also followed via monitoring of
the decrease of absorbance from MBP at 240 nm. Fresh solutions of
EPPS buffer and lanthanum salts were prepared and adjusted to pH 7
at 25 °C with 0.25 M ethanol, ethylene glycol, glycerol, D-glucose,
ethanolamine, methylamine, or diethylamine. The alcohol concentrations
varied from 0 to 2 M with methanol and 0 to 0.1 M with the other
alcohols. Stock solutions of amines were adjusted to pH 7. First-order
rate coefficients for the lanthanide-catalyzed reaction of MBP in the
presence of added alcohols were calculated from nonlinear regression
fitting of the absorbance versus time data to the equation for a first-
order decay. The observed first-order rate constants were plotted against
alcohol concentration, and the second-order rate constants for alco-
holysis were obtained from the slopes.
We have previously reported that lanthanides promote the formation
of methyl benzoate from methanol, MBP, and lanthanides in water.14
The products resulting from reaction of MBP with other alcohols in
the presence of lanthanide ions in aqueous solution were analyzed by
HPLC and comparison with genuine samples of likely products. The
alcohols utilized were ethanol, n-propanol, 2-propanol, 1,2-propanediol,
1,3-propanediol, ethylene glycol, glycerol, cis-1,2-cyclopentanediol,
trans-1,2-cyclopentanediol, 1,3-cyclopentanediol, and cyclopentanol.
Stock solutions of lanthanum trifluoromethanesulfonate (0.01 M), EPPS
(0.1 M), and MBP (5 × 10-3 M) were prepared as described for kinetic
studies. The final reagent concentrations were 1 M alcohol, 1 × 10-3
M La(OTf)3, 0.01 M EPPS (pH 8), and 5 × 10-4 M MBP. The solution
pH was varied from 7 to 8 and the concentration of lanthanum
trifluoromethanesulfonate was varied from 5 × 10-4 to 0.01 M, to study
the effect of pH and lanthanum concentration on the product ratio. The
pH range that we used was limited because at higher pH the lanthanide
salts precipitate. Europium ion (as europium chloride) was also studied
in some cases, giving results that were essentially identical to those
with lanthanum salts. Europium triflate gave the same results as
europium chloride.
We performed HPLC analysis with a reverse phase column. The
eluent was a mixture of water and acetonitrile containing 0.1%
trifluoroacetic acid (TFA) with a flow rate of 1 mL/min. The eluting
species were detected at 230 nm. The column was equilibrated in a
solution of 5% acetonitrile in water, and a linear gradient of acetonitrile
(2%/min) was initiated after injection of the sample (25 µL). In the
case of the reactions of 1,2-propanediol, two monoester products formed
(the hydroxyl groups are not equivalent). An isocratic eluent, 25%
acetonitrile in water with 0.1% TFA, was used to improve the separation
of the two esters in the HPLC for the 1,2-propanediol reaction. The
identities of the resulting acid and esters were tested by HPLC by co-
injecting genuine samples with the lanthanide-catalyzed reaction
solutions. Stock solutions (5 × 10-3 M) of benzoic acid, methyl
benzoate, ethyl benzoate, 2-hydroxyethyl benzoate, and cis-1,2-hy-
droxycyclopentyl benzoate were prepared.15 The solutions were added
to a lanthanide-containing reaction mixture after absorbance from MBP
had decreased to less than 1% of its original value and the combined
solution was injected into the HPLC. Overlap on elution of the genuine
sample and the product peaks was used to identify the reaction product.
The 1H NMR spectra of genuine samples were also compared to those
obtained in the lanthanide-catalyzed reaction of the diols and MBP.
We also used HPLC analysis to determine the products from the
reaction of adenosine and its analogues as the diol component with
MBP. On the basis of the selectivity we observed in the reactions of
MBP with diols and lanthanides (and the suppressed reactions of
amines), we extended our studies to include reactions of the nucleoside
at the 3′-terminal of tRNA, adenosine, and some analogues: uridine,
2′-deoxyadenosine, 2′3′-isopropylideneadenosine, and 5′-chloro-5′-
deoxyadenosine.16 The reaction mixture contained 2 mM nucleoside
or nucleoside derivative, 0.5 mM MBP, 1 mM La(OTf)3, 10 mM pH
8 EPPS. Products were monitored by HPLC, using an isocratic eluent
(25% acetonitrile in water with 0.1% trifluoroacetic acid), at a flow
rate of 1 mL/min with a 25 µL injection volume. The eluting species
were detected at 230 nm.
Results
Diols undergo efficient lanthanide-enhanced monoacylation
in water, while esterification of ethanol and larger monofunc-
tional alcohols in water does not compete effectively with
(15) Iwasaki, F.; Maki, T.; Onomura, O.; Nakashima, W.; Matsumura, Y. J.
Org. Chem. 2000, 65, 996-1002.
(16) Robins, M. J.; Hansske, F.; Wnuk, S. F.; Kanai, T. Can. J. Chem. 1991,
69, 1468-1474.
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Biomimetic Acylation of Diols
A R T I C L E S
Table 1. Lanthanum Ion-Catalyzed Formation of Monoesters and
Competing Hydrolysis of MBP from Alcoholsa
product ratio
acid:ester
alcohol
ester
methanol
ethanol
ethylene glycol
1,2-propanediol
1:0.36
1:0.03
1:3.1
methyl benzoate
ethyl benzoate
2-hydroxyethyl benzoate
1:1.5:0.2 2-hydroxy-1-propyl benzoate,
1-hydroxy-2-propyl benzoate
cis-1,2-cyclopentanediol
1:0.84
(D,L)-cis-2-hydroxycyclopentyl
benzoate
trans-1,2-cyclopentanediol
1,3-propanediol
no ester formed
3-hydroxy-1-propyl benzoate
trans-3-hydroxycyclopentyl benzoate
1:0.31
1:0.9
1,3-cyclopentanediolb
a See Experimental Section for conditions. b Mixture of cis and trans
isomers.
hydrolysis. Due to the large excess of water as compared to
diol, hydrolysis of MBP does accompany monobenzoylation of
the diols. The monoesters themselves are stable both to
hydrolysis and to further benzoylation. This pattern indicates
that the use of excess acylating agent will overcome competition
from hydrolysis when the hydroxyl substrate is limiting. The
reaction pattern is predictive of appropriate biomimetic reactivity
with tRNA and agrees with patterns reported by Clarke for
reactions between diols and acetic anhydride in organic
solvents.17-19
We also find that lanthanide ions do not promote competing
amide-forming reactions of amines and MBP. The uncatalyzed
reactions of amines with MBP depend on the concentration of
free amine and would normally be done at pH 9 where basic
amines would not be heavily protonated.20 The weakly basic
amino group of adenosine (pKa of the conjugate acid is 3.6)
would be unlikely to compete as the rate of amide formation
from acyl phosphate monoesters is directly proportional to the
basicity of the amino group with ânuc ≈ 1. For example, the
rate constant for aminolysis MBP with free n-propylamine is
1.1 M-1 s-1 (pKa of conjugate acid ) 10.5), while for 2,2-
difluoroethylamine, k ) 1.7 × 10-3 M-1 s-1 (pKa of conjugate
acid ) 7.5).
Figure 1. Formation of methyl benzoate from MBP as a function of
methanol concentration in pH 7, 0.01 M EPPS, with 2.5 × 10-3 M EuCl3,
25 °C. Similar results were obtained with lanthanum triflate in place of
europium chloride.
Table 2. Second-Order Rate Constants (M-1 s-1) for the
Lanthanum-Catalyzed Reaction of MBP with Alcohols, 25 °Ca
alcohol
k
methanol
ethanol
ethylene glycol
glycerol
(2.6 ( 0.04) × 10-3
no reaction observed
(1.2 ( 0.04) × 10-2
(3.0 ( 0.02) × 10-2
(6.2 ( 0.5) × 10-3
D-glucose
a See Experimental Section for conditions.
ester). This is consistent with a steric preference. While the 1,3-
substituted diols also react readily with MBP in the presence
of lanthanum ion, the results in Table 1 indicate that the
hydroxyl groups of the cyclic diol are esterified in preference
to hydrolysis by ratios of 49 and 17 as compared to the similar
groups in the linear 1,3-diol. Because 1,3-cyclopentanediol is a
mixture of cis and trans isomers, we did not determine individual
reactivities.
In our initial screening of reaction patterns, we found that
the combination of lanthanide ion, MBP, and hydroxyl group
is most effective for reactions of (cis)-1,2-diol substrates. These
are converted to monobenzoyl esters in water (Table 1) and
were identified as described in the Experimental Section. While
the reactions of MBP with water and with methanol are
accelerated, we observed no ester formation for higher mono-
functional alcohols (propanol, butanol, cyclopentanol) and only
a slow reaction with ethanol that does not compete effectively
with hydrolysis. The reactions of diols are sufficiently acceler-
ated to permit them to be converted to monoesters in water by
the combination of acyl phosphate monoester and lanthanide.
Ethylene glycol, 1,2-propanediol, and cis-1,2-cyclopentanediol
react rapidly with MBP. Symmetrical diols react to give only
one product, the monobenzoyl ester. 1,2-Propanediol, which has
heterotopic hydroxyl groups, gives two products that form at
different rates, with the primary hydroxyl group being more
reactive (initial product ratio is 92:8 in favor of the primary
The products from the reactions of glycerol and D-glucose
elute in the HPLC with retention times similar to those of MBP
and benzoic acid. Broad peaks are consistent with a mixture of
isomeric products. It is likely that the products are monoesters
because we do not observe products eluting with the longer
retention times expected for higher weight materials.
Kinetic measurements of the disappearance of MBP in water
containing reactive alcohols were performed with methanol,
ethylene glycol, glycerol, and glucose. The reactions are all first
order in alcohol for kobs ) khyd + kalc[ROH]. The linear
dependence on methanol concentration between 0 and 2 M is
shown in Figure 1. Second-order rate constants for the reactions
were obtained from the slope of the linear dependences of kobs
on alcohol concentrations (Table 2). The reaction with glucose
is complicated by ring and anomeric equilibria. We have
undertaken a new set of studies with specific cyclic glycosides
to get a clearer picture of the specificity of the reaction.
Amines inhibit the lanthanide-catalyzed hydrolysis of MBP,
suggesting that they coordinate to the metal ion in competition
with the alcohols. In no case did we observe formation of
amides, which occurs in solutions where the amines are present
to a significant extent in an unprotonated state (pH > 8).21 For
example, methylamine and ethanolamine reduce the observed
(17) Clarke, P. A. Tetrahedron Lett. 2002, 43, 4761-4763.
(18) Clarke, P. A.; Kayaleh, N. E.; Smith, M. A.; Baker, J. R.; Bird, S. J.; Chan,
C. J. Org. Chem. 2002, 67, 5226-5231.
(19) Clarke, P. A.; Arnold, P. L.; Smith, M. A.; Natrajan, L. S.; Wilson, C.;
Chan, C. J. Chem. Soc., Chem. Commun. 2003, 2588-2589.
(20) Kluger, R.; Grant, A. S.; Bearne, S. L.; Trachsel, M. R. J. Org. Chem.
1990, 55, 2864-2868.
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Cameron et al.
Figure 2. Dependence of kobs for lanthanide-catalyzed hydrolysis of MBP
on methylamine (b), and ethanolamine (O) concentration. 0.01 M EPPS,
pH 7, containing 2.5 × 10-3 M lanthanum triflate, repeated with europium
trichloride, 25 °C.
Figure 3. (a) Adenosine and derivatives that react with MBP and
lanthanides to give two acylation products. (b) Adenosine derivatives that
do not react with MBP and lanthanides.
Table 3. Effect of Lanthanum Concentration on the Products from
MBP and 1 M Ethylene Glycol in Water Containing EPPS Buffer
pH
103 [La3+], M
acid:ester
benzoic acid and two benzoyl esters of adenosine. The reaction
of MBP with water is slower than its reaction with the
nucleoside.
7.0
0.50
1.0
2.0
1.0
2.0
1:1.3
1:3.6
1:6.2
1:5.3
1:5.3
Four compounds were studied for comparison with adenosine
to establish the site of reaction because our attempts at NMR-
based identification did not provide a definitive assignment. Two
analogues contain the 2′- and 3′-hydroxyl groups but lack other
potential nucleophiles (group “a”): 5′-chloro-5′-deoxyadenosine,
which lacks the primary hydroxyl group, and uridine, which
lacks an amino group. We also examined the reactions of
compounds that do not contain either or both the 2′- and 3′-
hydroxyl groups but which do contain the other potential
nucleophiles found in adenosine (group “b”): 2′,3′-isopropyl-
idine-adenosine has a 5′-hydroxyl group and a 6-amino group;
2′-deoxyadenosine has 5′- and 3′-hydroxyl groups (nonadjacent)
and the 6-amino group.
As in the case of adenosine, the lanthanide-catalyzed reactions
of MBP with uridine and 5′-chloro-5′deoxyadenosine give two
monoacylation products in addition to benzoic acid (from
competing hydrolysis), a pattern similar to that of adenosine.
No esters are observed in the lanthanide-catalyzed reaction of
MBP with 2′-deoxyadenosine and 2′,3′-isopropylideneadenosine,
both of which contain the other two potentially nucleophilic
sites but no vicinal diols.
8.0
first-order rate coefficient for hydrolysis of MBP (Figure 2).
Ethanolamine did not give any ester or amide product. Isopro-
pylamine, diisopropylamine, diethylamine, and triethylamine
also depress MBP hydrolysis. In a separate study, we examined
the effects of ionic strength upon the rate and observed a
depressing salt effect that is more evident at higher concentra-
tions, following the Debye-Hu¨ckel expression. Thus, a portion
of the depression is due to a nonspecific salt effect.
Changing the pH of the reaction solution from 7 to 8 increases
the rates of reaction of MBP but does not affect the relative
amounts of alcoholysis and hydrolysis. Increasing the concentra-
tion of lanthanum ion results in a slight increase in the rate of
the MBP reaction, suggesting that the concentrations are nearly
saturating. In all cases but one, increasing the lanthanide
concentration has no effect on the product ratio. The exception
is that in the reaction of ethylene glycol, an increase in
lanthanum concentration results in a modest increase in the rate
of alcoholysis at pH 7 but not at pH 8 (Table 3).
On the basis of a referee’s suggestion, we also tested the
effects of amino acids on the reaction with adenosine. We used
the same conditions for the lanthanum-promoted benzoylation
of adenosine with alanine and serine in concentrations equal to
that of adenosine. In each case, the amino acid had no effect
on the yield of 2′- and 3′-benzoyl adenosines (R. Ren,
unpublished).
Diol-Selective Monobenzoylation of Adenosine. The se-
lectivity we have seen in the reactions of lanthanides and MBP
with diols in the presence of amines indicates that a reaction
with adenosine would be selective for monoacylation of its 2′-
or 3′-hydroxyl group, avoiding reaction of the 6-amino group
exocyclic to the purine and the 5′-hydroxyl group. To test this
prediction, we examined the reaction of adenosine and analogues
of adenosine with lanthanides and MBP (Figure 3). Lanthanide
ion catalyzes the reaction of MBP with adenosine at pH 7 and
pH 8 (in the absence of lanthanide ions, MBP does not react
Discussion
The enhanced reactivity of diols with MBP in the presence
of lanthanides is consistent with coordination of the diol being
advantageous for lanthanide catalysis in water, as was observed
by Clarke for reactions of diols with acetic anhydride in organic
solvents.19 Lanthanides should preferentially coordinate ap-
propriate diols due to the entropic advantage of chelation.
with adenosine). The reaction solution contained 5 × 10-4
M
MBP, 1 × 10-3 M La(OTf)3, 0.01 M EPPS, pH 8. MBP gives
(21) Kluger, R. Can. J. Chem. 1994, 72, 2193-2197.
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Biomimetic Acylation of Diols
A R T I C L E S
Scheme 2
Scheme 3
Scheme 4
Although the diol is competing with MBP for binding, bis-
chelation of the lanthanide ion by the electrophile and nucleo-
phile would be likely to facilitate the reaction between the
coordinated components. A mechanism for the reaction of cis-
1,2-cyclopentanediol to form a coordinated intermediate is
shown in Scheme 2 and is based on Clarke’s proposal.19
This intermediate will give a mixture of enantiomeric esters
and a lanthanide-stabilized methyl phosphate as shown in
Scheme 3.
Of course, the mechanism of conversion of the complex to
the products remains to be determined. The properties that make
lanthanides versatile catalysts, such as the lability of the
complexes, highly variable coordination geometry, and coor-
dination number, provide many potential routes.22,23 Scheme 4
shows a possible detailed route to the products with ionization
of the coordinated hydroxyl undergoing internal addition.
The geometry of the cyclic diols affects the reactivity of the
hydroxyl-derived nucleophile. trans-1,2-Cyclopentanediol is the
only diol we studied that did not react. While Clarke and co-
workers report the acylation of the trans-diol by acetic anhydride
with YbCl3 in THF,24 the yield is significantly lower than that
with other diols. As well, this is the only substrate they report
that gave significant amounts of the bis-acylation product.
Because their reaction conditions do not involve competing
(22) Neverov, A. A.; Gibson, G.; Brown, R. S. Inorg. Chem. 2003, 42, 228-
234.
(23) Neverov, A. A.; Brown, R. S. Inorg. Chem. 2001, 40, 3588-3595.
(24) Clarke, P. A.; Holton, R. A.; Kayaleh, N. E. Tetrahedron Lett. 2000, 41,
2687-2690.
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A R T I C L E S
Cameron et al.
hydrolysis, it is likely that a slower monodentate route is
responsible. In our work, the addition of trans-1,2-cyclopen-
tanediol slows the rate at which MBP is hydrolyzed (roughly
25% hydrolysis at 1 h, as compared to roughly 100% hydrolysis
in the absence of the trans-diol), as is observed for the cis-diol.
This suggests the lanthanide is coordinating the trans-diol but
that the interaction does not lead to acylation.
In the absence of lanthanides, unprotonated amino groups
are rapidly converted to amides by acyl phosphate monoesters,
while reactions with oxygen-centered nucleophiles are very
slow.8 The lanthanide-promoted reactions of diols occur in
neutral solution where free amine concentrations are very low.
While lanthanides form weak complexes with amines, this would
not promote their nucleophilic reactions in water because the
coordinated amine will not form a highly basic amide ion.25
The lanthanide-promoted reaction of adenosine with MBP
gives preferential monobenzoylation of the secondary hydroxyl
groups (2′ and 3′) while avoiding reaction of the 5′-OH and the
normally more reactive amino group based on reaction patterns
with related compounds. This reaction selectivity of MBP and
lanthanides with adenosine and its analogues is a clear indication
that the combination of lanthanide, acyl phosphate monoester,
and RNA will lead to the desired biomimetic acylation. We are
currently extending our studies to the reactions of nucleotides,
dinucleotides, and more complex entities.
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada (NSERC) for support
through a Discovery Grant. S.C.W. received an NSERC summer
undergraduate research fellowship.
Supporting Information Available: Illustrative chromato-
grams and tabular summary for product analysis. This material
is available free of charge via the Internet at http://pubs.acs.org.
(25) Evans, C. H. Biochemistry of the Lanthanides; Plenum Press: New York,
1990; pp 9-46.
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