Facile synthesis of O6-alkyl-, O6-aryl-, and diaminopurine nucleosides from 2′-deoxyguanosine

Article abstract of DOI:10.1021/ol005564m

(equation presented) The 3′,5′-bis-O-TBDMS derivative of 2′-deoxyguanosine can be converted to its O⁶-alkyl and O⁶-aryl ethers as well as to N⁶-substituted diaminopurine nucleosides in two simple steps. Also described is a

Full text of DOI:10.1021/ol005564m

ORGANIC
LETTERS
6
6
2000
Vol. 2, No. 7
927-930
Facile Synthesis of O -Alkyl-, O -Aryl-,
and Diaminopurine Nucleosides from
2′-Deoxyguanosine
Mahesh K. Lakshman,* Felix N. Ngassa, John C. Keeler, Yen Q. V. Dinh,
John H. Hilmer, and Larry M. Russon†
Department of Chemistry, UniVersity of North Dakota, Grand Forks, North Dakota
58202-9024, and Mass Spectrometry SerVices, Department of Chemistry and
Biochemistry, UniVersity of Oklahoma, Norman, Oklahoma 73019-3050
mlakshman@mail.chem.und.nodak.edu
Received January 18, 2000
ABSTRACT
6
6
6
The 3′,5′-bis-O-TBDMS derivative of 2′-deoxyguanosine can be converted to its O -alkyl and O -aryl ethers as well as to N -substituted
6
diaminopurine nucleosides in two simple steps. Also described is a novel, nonaqueous, one-step O -desulfonylation method that leads to
deprotection of the carbonyl moiety of 2′-deoxyguanosine.
O6-Alkylated derivatives of 2′-deoxyguanosine (dG) are
extremely important for a variety of reasons. From a
biological standpoint, alkylation at O6 has been proposed to
reduce the number of H-bonding sites in the guanine moiety
which could lead to a mutagenic event.1 In support of this
hypothesis O6-methyl and ethyl dG have been shown to
produce G f A transitions.2,3 Thus, O-alkyl guanines are of
interest for mutagenesis and carcinogenesis studies. From a
synthetic standpoint, several O6-protected deoxyguanosine
derivatives are precursors to electrophilic inosine nucleosides
bearing leaving groups at the C-2 position.4 On the basis of
the severalfold importance of O6-modified dG derivatives,
we began to evaluate methods for their facile synthesis. In
addition, we wanted to identify a strategy wherein a common
intermediate could be converted not only to the O6-alkyl
derivatives but also to O6-aryl as well as the biologically
important 2,6-diaminopurine nucleosides.
Introduction of a sulfonate at the O6 position of N2-3′,5′-
triacyl dG is well documented.5,6 Such a derivative, when
exposed to Me3N, undergoes displacement of the sulfonate,
producing a quaternary ammonium salt that has been
converted to O6-alkyl ethers.5 However, we were interested
only in protection of the carbohydrate hydroxyls and wanted
to avoid acylation of the N2. More recently, O6-allyl 3′,5′-
bis-O-TBDMS dG has been synthesized from an O6-sulfonate
precursor devoid of an amine protection, but the reaction
requires the use of allyl alcohol as solvent.7 This feature poses
obvious problems when high-boiling, solid and/or expensive
alcohols are utilized. Also, in our view, another major
drawback of these procedures is the use of the unpleasant,
highly volatile Me3N (bp 3 °C).5,7 Recognizing this, Reese
† University of Oklahoma.
(1) Basu, A. K.; Essigmann, J. M. Chem Res. Toxicol. 1988, 1, 1-18.
(2) Mehta, J. R.; Ludlum, D. B. Biochim. Biophys. Acta 1978, 521, 770-
778.
(3) Abbott, P. J.; Saffhill, R. Biochim. Biophys. Acta 1979, 562, 51-61.
(4) For examples, see: (a) Zajc, B.; Lakshman, M. K.; Sayer, J. M.;
Jerina, D. M. Tetrahedron Lett. 1992, 33, 3409-3412. (b) Harris, C. M.;
Zhou, L.; Strand, E. A.; Harris, T. M. J. Am. Chem. Soc. 1991, 113, 4328-
4329. (c) Harwood, E. A.; Sigurdsson, S. T.; Edfeldt, N. B. F.; Reid, B. R.;
Hopkins, P. B. J. Am. Chem. Soc. 1999, 121, 5081-5082. (d) Steinbrecher,
T.; Wameling, C.; Oesch, F.; Seidel, A. Angew. Chem., Ind. Ed. Engl. 1993,
32, 404-406.
(5) Gaffney, B. L.; Jones, R. A. Tetrahedron Lett. 1982, 23, 2253-2256.
(6) Tanimura, H.; Sekine, M.; Hata, T. Tetrahedron Lett. 1986, 27, 4047-
4050.
(7) Hayakawa, Y.; Hirose, M.; Noyori, R. J. Org. Chem. 1993, 58, 5551-
5555.
10.1021/ol005564m CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/07/2000

and Skone have used 1-methylpyrrolidine in place of Me3N.8
However, 1-methylpyrrolidine is relatively volatile (bp 80-
81 °C), and only one example involving its use is reported.8
Jones and co-workers have developed a protocol that does
not utilize Me3N.9 This method, though very useful, utilizes
sodium alkoxides which would be incompatible with silyl
protecting groups. The Mitsunobu reaction offers entry to
O6-alkyl dG derivatives devoid of N2 protection,10 and the
conditions are compatible with silyl protecting groups.4,11
However, in many cases separation of products from residual
reactants and byproducts is cumbersome. There are fewer
routes to O6-aryl dG derivatives. The methods of Jones9 and
Reese12 for the preparation of some of these compounds
provide the N2-acylated products. We therefore became
interested in developing a procedure that (a) was operation-
ally simple, (b) provided 3′,5′-O-disilyl nucleoside products
devoid of N2 protection, (c) obviated the use of Me3N, (d)
did not require the reactant to be the reaction medium, and,
importantly, (e) could be utilized to prepare a variety of
compounds from a single precursor.
Scheme 1
Recently, displacement of chloride from the C-6 position
of purines by 1,4-diazabicyclo[2.2.2]octane (DABCO) has
been demonstrated.13,14 Although O6-alkyl purines have been
prepared from the ammonium salts with alkoxides, no
applications of this method for the preparation of O6-alkyl
nucleosides have been reported.14 For our studies the C-6
chloro derivative was not optimal due to the low-yielding
(15%) chlorination of dG.2 On the other hand, the easily
prepared 3′,5′-bis-O-TBDMS O6-mesitylenesulfonate of dG7
(2) appeared to be a much better starting point. However, at
the outset of our experimentation we could not be certain
that DABCO would efficiently displace the mesitylene-
sulfonate (Mes) from the purine.
To test this critical step, we exposed 2 to DABCO in 1,2-
dimethoxyethane (1,2-DME) at rt (Scheme 1). Much to our
delight, immediate progress of the reaction was observed by
TLC with the formation of a very polar, fluorescent spot.
Within 45 min a precipitate, presumably salt 3, began
forming. Complete conversion of 2 to 3 required 13 h, but
as described herein, it is not necessary to isolate 3 prior to
the displacement step.
Preparation of the O6-Alkyl and -Aryl Ethers of dG.15
Synthesis of the O6-alkyl or -aryl ethers (2 f 4) can be
conveniently achieved as a two-step, one-pot reaction. To a
solution of 2 in 1,2-DME (0.1 M) were added 4 Å molecular
sieves (∼320 mg/mmol), the 1° alcohol16 or phenol (5 molar
equiv),17 DABCO (2 molar equiv), and DBU (1.5 molar
equiv). Reactions are normally complete within 24 h at rt.
Upon completion, the reaction mixtures were diluted with
EtOAc (reactions with alcohols) or Et2O (reactions with
phenols). Reaction mixtures with alcohols were extracted
with H2O, while those with phenols were initially extracted
with 1 N NaOH to accomplish removal of excess (potentially
recoverable) phenol, followed by H2O extraction. When
water-immiscible alcohols were used, excess alcohol was
(15) To a solution of 2 (0.074-0.15 mmol) in 1,2-DME were added
DABCO, 4 Å molecular sieves, and the alcohol or phenol. After allowing
the mixture to stir at room temperature for 30 min DBU was added. The
mixture was allowed to stir at room temperature for 24 h and then diluted
with either EtOAc (for reactions with alcohols) or Et2O (for reactions with
phenols). These mixtures were subjected either to aqueous extraction
(reactions involving alcohols) or to sequential extractions with 1 N NaOH
and H2O (reactions involving phenols). The organic layer in each case was
dried over Na2SO4 and evaporated. The crude products were chromato-
graphed on silica gel using appropriate solvent systems.
(8) Reese, C. B.; Skone, P. A. J. Chem. Soc., Perkin Trans. 1 1984,
1263-1271.
(9) Fathi, R.; Goswami, B.; Kung, P.-P.; Gaffney, B. L.; Jones, R. A.
Tetrahedron Lett. 1990, 31, 319-322.
(10) (a) Trichtinger, T.; Charubala, R.; Pfleiderer, W. Tetrahedron Lett.
1983, 24, 711-714. (b) Schulz, B. S.; Pfleiderer, W. Tetrahedron Lett.
1985, 26, 5421-5424.
(16) This reaction provides good yields with 1° alcohols. With a 2°
alcohol (2-propanol) although the desired O6-isopropyl ether was obtained,
the yield was ∼15%. O6-Isopropyl-3′,5′-bis-O-tert-butyldimethylsilyl-2′-
deoxyguanosine: clear oil, Rf(silica/CH2Cl2) ) 0.75. 1H NMR (deacidified
CDCl3): 7.80 (s, 1H8), 6.23 (t, 1H1′, J ) 6.4), 5.46 (sept, 1H, OCH, J )
6.2), 4.71 (s, 2H, NH2), 4.51 (m, 1H3′), 3.87 (app q, 1H4′, Japp ) 3.3), 3.74
(dd, H5′, J ) 4.3, 11.1), 3.65 (dd, 1H5′, J ) 3.2, 11.1), 2.46 (app quint,
1H2′, Japp ) 6.1), 2.24 (ddd, 1H2′, J ) 3.6, 6.0, 13.1), 1.32 (d, 6H, CH3, J
) 6.2), 0.82, 0.81 (2s, 18H, t-Bu), 0.04-0.01 (3s, 12H, SiCH3). HRMS
calcd for C25H48N5O4Si2 (M+ + 1) 538.3245, found 538.3242.
(11) (a) Edwards, C.; Boche, G.; Steinbrecher, T.; Scheer, S. J. Chem.
Soc., Perkin Trans. 1 1997, 1887-1893. (b) Steinbrecher, T.; Wameling,
C.; Oesch, F.; Seidel. A. In Polycyclic Aromatic Compounds: Synthesis,
Properties, Analytical Measurements, Occurrence and Biological Effects;
Garrigues, P., Lamotte, M., Eds.; Gordon and Breach Science Publishers:
Langhorne, PA, 1993; pp 223-230.
(12) Jones, S. S.; Reese, C. B.; Sibanda, S.; Ubasawa, A. Tetrahedron
Lett. 1981, 22, 4755-4758.
(13) Linn, J. A.; McLean, E. W.; Kelley, J. L. J. Chem. Soc., Chem.
Commun. 1994, 913-914.
(17) In the reaction involving EtOH, 10 molar equiv of the alcohol was
used. The lower yield of 4b compared to the other O6-alkyl ethers may be
due to competing elimination by deprotonation at the methyl group in 4b.
(14) Lembicz, N. K.; Grant, S.; Clegg, W.; Griffin, R. J.; Heath, S. L.;
Golding, B. T. J. Chem. Soc., Perkin Trans. 1 1997, 185-186.
928
Org. Lett., Vol. 2, No. 7, 2000

readily removed by careful chromatography. Yields of both
O6-alkyl and -aryl ethers (4a-4k) were quite reasonable (see
Figure 1).
reasoned that a direct SNAr displacement of the sulfonate
by amines would offer the simplest route to N6-substituted
diaminopurine nucleosides.
After initial experimentation we discovered that, unlike
the reactions with alcohols and phenols which proceed at rt,
reactions with amines required elevated temperatures. At 50
°C, these reactions proceeded smoothly to provide the N6-
substituted diaminopurine nucleosides (5a-5d) in good
yields (see Figure 1).
Some features of and products from these reactions are
noteworthy. For example, the O6-benzyl and -allyl derivatives
(4c, 4d), which are intermediates in the synthesis of
electrophilic nucleosides,4,11 can be conveniently prepared
without the need for the Mitsunobu reaction. The relatively
exotic furfuryl derivative 4e can be synthesized in good yield.
Noteworthy among the O6-aryl derivatives, the N-acetyl
moiety of N-acetylaminophenol, remains unaffected during
the course of the reaction (2 f 4i). The amino group in 4i
can be deprotected and utilized for additional functionaliza-
tion. Reaction of salicylaldehyde is also interesting. Despite
the presence of the o-carbonyl group, salicylaldehyde reacts
smoothly to provide 4k in high yield. In this case also, the
aldehyde functionality offers a site for subsequent function-
alization.
At this stage we were interested in evaluating whether the
Mes group in 2 could be used as a transient O6-protection
for dG, which could be displaced in a manner that would
effect restoration of the dG carbonyl. Although 4c and 4d
can be deprotected,4a,b,7 additional steps are involved. We
wanted to exploit the chemistry described herein for devel-
oping a nonaqueous, one-step procedure. It is known that
the 2-cyanoethoxy group can be cleaved under basic condi-
tions.21 The question that remained was whether introduction
and removal of an O6-cyanoethyl group could be ac-
complished in one-step.
When 3 (produced in situ from 2) was subjected to the
reaction conditions with 2-cyanoethanol (Scheme 2), a single
product (1) was obtained in 81% yield. This is the first
example of a one-step desulfonylation at the C-6 position
of purine nucleosides and shows that the Mes group, which
can be readily removed under nonaqueous conditions, can
be used for O6-protection.
Mechanistically, we believe the reaction proceeds through
the ammonium salt 3 and not by attack of the alcohol at the
sulfur. This is because the desulfonylation is not observed
in reactions with other alcohols. Thus, we propose that salt
3 reacts with 2-cyanoethanol as shown in Scheme 2 to
produce the O6-cyanoethyl derivative, which subsequently
undergoes deprotection under the reaction conditions to
furnish the 3′,5′-bis-O-TBDMS derivative of dG (1).
In conclusion, we have developed a very convenient and
concise strategy for the synthesis of O6-alkyl, O6-aryl, and
Figure 1. List of products prepared and yields.
Preparation of the N6-Substituted Diaminopurine Nu-
cleosides.18 More commonly, N6-substituted 2,6-diaminopu-
rine nucleosides are also prepared by displacement of
chloride from 2-amino-6-chloropurine nucleosides.19 Again,
based on the yield-limiting C-6 chlorination of dG, we
(18) Reactions with amines were performed in an identical fashion to
those with alcohols or phenols,15 except that DABCO and DBU were not
added. Additionally, these reactions were conducted at 50 °C. For reactions
involving nonvolatile amines, reaction workup consisted of dilution with
EtOAc, followed by sequential extractions with 10% aqueous citric acid
and saturated aqueous NaHCO3. For water miscible or volatile amines,
dilution of the reaction mixture with EtOAc was followed by aqueous
extraction. In each instance drying of the organic layer over Na2SO4,
evaporation, and chromatography provided the pure products.
(19) For examples, see: (a) Trivedi, B. K.; Bruns, R. F. J. Med. Chem.
1989, 32, 1667-1673. (b) Trivedi, B. K. Nucleosides Nucleotides 1988, 7,
393-402.
(20) For the reaction involving dimethylamine, a 2 M THF solution of
the amine (5 molar equiv) was used. The overall molarity of the reaction
mixture was adjusted to 0.1 M with 1,2-DME.
(21) (a) Gaffney, B. L.; Jones, R. A. Tetrahedron Lett. 1982, 23, 2257-
2260. (b) Beaucage, S. L.; Iyer R. P. Tetrahedron 1992, 48, 2223-2311.
Org. Lett., Vol. 2, No. 7, 2000
929

in a facile manner to a wide variety of compounds, this
chemistry can be easily incorporated either into combinatorial
technology or automated parallel synthesis. In addition, we
have delineated a novel one-step desulfonylation that has
important potential applications. For instance, the O6-
mesitylenesulfonate could serve as a dG protecting group
during oligonucleotide assembly and can be removed using
nonaqueous conditions. More interestingly, this method offers
the possibility for facile isotopic labeling of the carbonyl
groups in dG and other hypoxanthine nucleosides through
the use of H18OCH2CH2CN.
Scheme 2
Acknowledgment. This work has been supported in part
by NSF Grant OSR 9452892.
Supporting Information Available: Compilation of the
proton NMR and HRMS data for 4a-4k and 5a-5d. This
material is available free of charge via the Internet at
http://pubs.acs.org.
N6-substituted diaminopurine nucleosides from 2′-deoxygua-
nosine. The reactions proceed smoothly with a variety of 1°
alcohols, with phenols having different substituents and
substitution patterns, as well as with 1° and 2° amines.
Furthermore, since a single intermediate can be converted
OL005564M
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Org. Lett., Vol. 2, No. 7, 2000