Tetramethylsuccinimide as a directing/protecting group in purine glycosylates

Article abstract of DOI:10.1021/ol9025028

Chemical Equation Presented Tetramethylsuccinic anhydride can be used to protect the exocyclic amine of 6-aminopurine derivatives by forming the corresponding tetramethysuccinimide. X-ray crystallography confirms that the imide carbonyl and the methyl groups are positioned to sterically block the N7 nitrogen so that glycosylations occur with very high regiochemical control at N9. This approach is particularly effective for 3-substituted purines where the substituent tends to block access to N9 and inhibit glycosylation at that site.

Full text of DOI:10.1021/ol9025028

ORGANIC
LETTERS
2010
Vol. 12, No. 1
120-122
Tetramethylsuccinimide as a Directing/
Protecting Group in Purine Glycosylations
Joseph W. Arico, Amy K. Calhoun, Kerry J. Salandria, and Larry W. McLaughlin*
Department of Chemistry, Merkert Chemistry Center, Boston College,
Chestnut Hill, Massachusetts 02467
mclaughl@bc.edu
Received October 29, 2009
ABSTRACT
Tetramethylsuccinic anhydride can be used to protect the exocyclic amine of 6-aminopurine derivatives by forming the corresponding
tetramethysuccinimide. X-ray crystallography confirms that the imide carbonyl and the methyl groups are positioned to sterically block the N7
nitrogen so that glycosylations occur with very high regiochemical control at N9. This approach is particularly effective for 3-substituted
purines where the substituent tends to block access to N9 and inhibit glycosylation at that site.
Syntheses of purine 2′-deoxynucleosides are challenging,
particularly because the glycosylation step must be both
regioselective and diasterioselective. Reaction yields of the
naturally occurring ꢀ-N9 products are often low to moderate.
Most preparations of purine 2′-deoxynucleosides employ the
sodium salt method,1 which typically uses a 6-chloropurine
heterocycle and 2-deoxy-3,5-di-O-(p-toluoyl)-R-D-erythro-
pentofuranosyl chloride.2,3 For example 6-chloropurine (1a,
Table 1) reacts to form 57% ꢀ-N9 nucleoside and 13% ꢀ-N7
with no R-isomers detected.1 Treatment with NH3/MeOH
deprotects the sugar and displaces the 6-chlorine to form 2′-
deoxyadenosine. Small changes in the sterics or electronics
of the heterocycle can impact the ratio of coupling products.
The favorable coupling ratio for 1a (N9:N7 ) 4.7:1) largely
reverses itself for the 6-chloropurine derivative 2a containing
a 3-deaza-3-methyl substituent (N9:N7 ) 1.0:2.2).4 The
products of 1a and 2a can be separated by flash chroma-
tography, but this is not always the case. Many substrates
produce significant amounts of R-nucleoside products, further
complicating purification and lowering yields.
Robins has described the use of purines bearing a
2-alkylimidazole5,6 or triazole6 in the 6-position to achieve
highly regio- and diastereoselective glycosylations using the
sodium salt method. However, the purine coupling partners
must be prepared from the corresponding oxopurines or the
often expensive chloropurines. In addition, removal of the
2-alkylimidazole was reported to be difficult with most
purines.6 Related studies include the regioselective N9
arylation of purines.7
We attempted to use the Robins method in our work with
2a and other 3-deaza-3-substituted purines, but the 6-(2-
alkylimidazolyl) derivatives of 2a and 3a could not be
prepared, largely because displacement of the 6-chlorine of
3-deazapurines requires harsh conditions (i.e., refluxing
(1) Kazimierczuk, Z.; Cottam, H. B.; Revankar, G. R.; Robins, R. K.
J. Am. Chem. Soc. 1984, 106, 6379–6382.
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3362.
10.1021/ol9025028  2010 American Chemical Society
Published on Web 12/04/2009

We then considered aliphatic anhydrides that might exhibit
better hydrolytic stability, beginning with 1,4-cis-cyclohex-
anedicarboxylic anhydride.13 Unfortunately this compound
polymerizes easily and attempts to prepare simple imides
from ammonia or methyl amine have failed. Attempts to
prepare the cyclic imide from adenine in this work also failed.
The third compound we considered was tetramethylsuc-
cinic anhydride (M4SA). Upon formation of the imide with
adenine (Figure 1) the methyl groups should partially shield
Table 1. Regioselective Purine Glycosylations
a See ref 1. b Unpublished results. c A procedure with KOH and
cryptand catalyst TDA-14 was used instead of the sodium salt method.
d See ref 4.
Figure 1.
Structure of adenyl-N6-tetramethylsuccinimide from X-ray
diffraction methods: (a) face view and (b) edge view.
hydrazine followed by Raney nickel reduction4,8). Further-
more, the glycosylation products of 3a could not be separated
easily, leading us to seek an alternative procedure.
the N7 nitrogen from glycosylation. The cyclic anhydride
M4SA is readily obtained in two steps14 from AIBN.
M4SA reacted smoothly with adenine and other purines
in refluxing pyridine in the presence of DBU (Scheme 1).
Here we describe 2,2,3,3-tetramethylsuccinimide (M4SI)
as a new directing/protecting group for the synthesis of ꢀ-N9
nucleosides with high regio- and diastereoselectivity.
We reasoned that 6-aminopurines protected with a cyclic
imide might perform well as glycosylation partners. The
cyclic imide would (i) protect the amino group from side
reactions, (ii) permit direct glycosylation to N9, and (iii) be
removed by mild base to expose the amino group, with or
without deprotection of the sugar, as needed. We first
examined phthaloyl protection of adenine using phthalic
anhydride. Phthalimide has been reported as a protecting
group in DNA synthesis9,10 and 15N-phthalimide has been
used for the introduction of isotopic nitrogen into nucleo-
sides.11,12 The N6-phthalimide derivative of adenine could
be prepared (1b), although the yield was poor (∼40%) under
all conditions examined. It participated in glycosylation
reactions to give almost exclusively the N9 regioisomer but
in only 36% isolated yield. Deprotection with NH3/MeOH
removed the phthalimide in less than 15 min and the sugar
toluoyl esters after several hours at room temperature. The
amine could be reprotected as needed for DNA synthesis,
but that approach was less than ideal.
Scheme 1. Synthesis of M4SI-Protected Purines
Surprisingly, no reaction took place without base, and other
bases such as triethylamine and proton sponge failed to
promote the reaction. A small amount of the amide was
always present due to incomplete cyclization, but this could
be converted to the cyclic imide with refluxing SOCl2,
resulting in 78-81% yields of the purine-N6-tetramethyl-
succinimide. It is important to convert the residual amide
(8) Itoh, T.; Yamaguchi, T.; Mizuo, Y. J. Carbohydr., Nuleosides,
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(9) Kume, A.; Sekine, M.; Hata, T. Tetrahedron Lett. 1982, 23, 4365–
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(11) Kamaike, K.; Kinoshita, K.; Niwa, K.; Hirose, K.; Suzuki, K.;
Ishido, Y. Nucleosides, Nucleotides Nucleic Acids 2001, 20, 59–75.
(12) Kamaike, K.; Takahashi, M.; Utsugi, K.; Tomizuka, K.; Ishido, Y.
(13) Hall, H. K., Jr. J. Am. Chem. Soc. 1958, 80, 6412–6420.
(14) Whitesides, G. M.; Hackett, M.; Brainard, R. L.; Lavalleye,
J. P. P. M.; Sowinski, A. F.; Izumi, A. N.; Moore, S. S.; Brown, D. W.;
Staudt, E. M. Organometallics 1985, 4, 1819–1830.
Tetrahedron Lett. 1995, 36, 91–94
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Org. Lett., Vol. 12, No. 1, 2010
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into the cyclic imide since the amide (Scheme 1) does not
function well as a blocking group. We obtained the structure
of the M4SI of adenine by X-ray crystallography and two
views are shown in Figure 1. In the “face view” (Figure 1a)
the four methyl groups can be observed positioned “umbrella-
like” above the N7 edge of the adenine ring system. The
distance between the imide carbonyl oxygen and the N7
nitrogen is 3.1 Å, and that to C5 is 3.0 Å. These distances
likely prevent a coplanar conformation between the imide
and heterocycle, and in combination with the four methyl
groups seem sufficient to limit access to N7 by the gylco-
sylation reagent. The “edge view” (Figure 1b) illustrates that
the plane of the imide functionality is in fact rotated 52°
relative to the plane of the heterocycle, and this rotation is
necessary to prevent the steric clash between the imide
carbonyls and the N7 nitrogen and C5 carbon.
Although some coupling reactions generated small amounts
of the unwanted regioisomer or diastereomers, with the M4SI
group in place the desired ꢀ-N9 products could be resolved
from the ꢀ-N7 and R-nucleosides by flash chromatography.
The R-nucleosides result from anomerization of the chloro-
sugar (structure illustrated in Scheme 2), which is accelerated
in polar solvents.16 While the M4SI group is quite effective
in blocking access to N7, the 3-substituted purines such as
2c and 4a are more hindered nucleophiles; the reaction is
then slower to reach completion and larger amounts of
R-nucleosides are formed.
The M4SI group displayed the desired stability that
phthalimide lacked. Treatment of the coupling products of
1c and 2c with NaOMe/MeOH or 7 M NH3/MeOH at room
temperature removed the two toluoyl esters on the sugar but
not the M4SI (64% and 82% yield, respectively), and in 86%
yield for the coupling product of 4a (see the Supporting
Information).
We next examined a number of adenine-based heterocycles
for their ability to undergo glycosylation with M4SI as a
directing/protecting group at the 6-position (Scheme 2) and
Purine nucleosides with substituents at the 3-position are
valuable to probe hydration effects in the DNA minor groove.
Even a simple methyl substituent at C3 will be directed into
the minor groove of duplex DNA where it may disrupt or
effect a reorganization of water structure.
Scheme 2. Purine Glycosylations with the Sodium Salt Method
As a further example of this protecting group’s utility we
elaborated M4SI-protected 2′-deoxy-3-deaza-3-methyladeno-
sine, the coupling product of 2c, to the DMT-protected
phosphoramidite for use in DNA solid-phase synthesis (see
the Supporting Information). The M4SI can be cleanly
removed with NH3/MeOH or concd NH4OH at 55 °C
overnight, both compatible with DNA synthesis protocols.
The use of M4SI-protected phosphoramidites in solid-phase
DNA synthesis will be reported elsewhere.
M4SI is a valuable new directing/protecting group that
allows rapid access to ꢀ-N9 6-aminopurine-2′-deoxynucleo-
sides and functions as an effective base-labile protecting
group. It is especially valuable for 3-substituted purines. It
should also serve in other applications where bidentate amine
protection is desired.
compared these results with those obtained for the corre-
sponding 6-chloropurine, when available (Table 1). Phthal-
imide-protected heterocycles 1b, 2b, and 3b showed a better
directing efffect for N9 than did the 6-Cl derivatives, but
only gave low to moderate yields; varying and poorly
reproducible amounts of R-nucleosides were always isolated
as well. The N7 and N9 regioisomers could be separated
but the R and ꢀ diasteriosomers could not.
In contrast, glycosylation of M4SI-protected adenine 1c
yielded only the N9 isomer. The best yield (71%) was
obtained with phase transfer conditions using KOH/TDA-
115 in comparison with NaH (54%). M4SI performed well
with 3-deaza-3-substituted purines. 2c gave a 6.4:1 ratio of
N9 to N7 ꢀ-nucleosides, and this ratio increased to 7.59:1
for a 3-deaza analogue bearing a silyl-protected linker (4a).
Acknowledgment. We thank Dr. Bo Li for the crystal-
lographic analysis of 1c and Dr. Nick Greco for critical
discussions. This work was supported by an award from the
NSF to L.W.M. (MCB 0451488).
Supporting Information Available: Experimental pro-
cedures and characterization of all compounds along with
NMR spectra. This material is available free of charge via
the Internet at http://pubs.acs.org.
OL9025028
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(16) Kotick, M. P.; Szantay, C.; Bardos, T. J. J. Org. Chem. 1969, 34,
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