Stereospecific alkylation of substituted adenines by the Mitsunobu coupling reaction under microwave-assisted conditions

Article abstract of DOI:10.1039/c4ra01968g

A novel and efficient synthetic method has been developed for the preparation of alkylated aminopurines (N,N-dimethyl-, 2-chloro-N-methyl-, and N-methyladenines) with achiral and chiral 3,4-dihydro-2H-1,5-benzoxathiepin-3-ol by the Mitsunobu reaction under microwave-assisted conditions. This reaction reveals a complete inversion of the stereogenic centre of the secondary alcohol giving an alkylated purine linked to a homochiral six-membered ring. Fifty novel purine derivatives have been prepared. Alkylation sites have been determined by 2D NMR techniques and for three compounds have been confirmed by X-ray crystallography. The N-9/N-3 regioselectivity can be justified by the electronic effects of the substituents at positions 2 and 6 of the purine. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra01968g

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Stereospecific alkylation of substituted adenines by
the Mitsunobu coupling reaction under
microwave-assisted conditions†
Cite this: RSC Adv., 2014, 4, 22425
M. E. Garcıa-Rubino,a M. C. Nunez-Carretero,a D. Choquesillo-Lazarte,b J. M. Garcıa-
´
˜
´˜
´
b
c
a
*
Ruiz, Yolanda Madrid and J. M. Campos
A novel and efficient synthetic method has been developed for the preparation of alkylated aminopurines
(N,N-dimethyl-, 2-chloro-N-methyl-, and N-methyladenines) with achiral and chiral 3,4-dihydro-2H-1,5-
benzoxathiepin-3-ol by the Mitsunobu reaction under microwave-assisted conditions. This reaction reveals
a complete inversion of the stereogenic centre of the secondary alcohol giving an alkylated purine linked to
a homochiral six-membered ring. Fifty novel purine derivatives have been prepared. Alkylation sites have
been determined by 2D NMR techniques and for three compounds have been confirmed by X-ray
crystallography. The N-9/N-3 regioselectivity can be justified by the electronic effects of the substituents
at positions 2 and 6 of the purine.
Received 6th March 2014
Accepted 12th May 2014
DOI: 10.1039/c4ra01968g
www.rsc.org/advances
There are reports dealing with the alkylation reaction of
adenine under the Mitsunobu conditions.8–11 Continuing our
Introduction
studies on multident heterocyclic nucleophiles, we examined
the alkylation of adenine using a variety of alkylating
agents.12,13 The wide variety of alkylation conditions involved
in the literature reports makes it difficult to discern what
inuence the structural features of the alkylating agent has on
the alkylation site. To overcome this problem we have used a
typical and standardized alkylation condition to study the
reaction of several substituted adenines with racemic 3,4-
dihydro-2H-1,5-benzoxathiepin-3-ol, and its two enantiomers
through the Mitsunobu reaction under microwave conditions.
We have described a series of eleven 2- and 6-substituted (RS)-
9-(2,3-dihydro-1,4-benzoxathiin-3-ylmethyl)-9H-purine deriva-
tives (1–11) (Fig. 1), by applying a standard Mitsunobu
protocol that led to a six-membered ring contraction from
(RS)-3,4-dihydro-2H-1,5-benzoxathiepin-3-ol via an epis-
ulfonium intermediate.14
In the search for new compounds of therapeutic interest, purine
derivatives are very important for their possible interaction with
many biological targets. We have reported the fast microwave-
assisted synthesis of a series of (RS)-6-substituted-7 or 9-(2,3-
dihydro-5H-1,4-benzodioxepin-3-yl)-7H or -9H-purines, with the
advantage of the selective preparation of the N-7 or N-9
regioisomers. Our interest in the topic lies in the fact that
pyrimidines and purine alkylated products seem to be attractive
structures endowed with anti-proliferative properties.1–6
The essential biological functions of N-substituted adenine
compounds have naturally led to extensive interest in the
synthesis of a wide variety of alkylated purine systems as
potential analogues or antagonists of naturally occurring
adenine derivatives.7 Direct alkylation of adenine, of its simple
derivatives, and of its metal salts is the dominant synthetic
route to N-substituted adenines, largely because of the conve-
nience and simplicity of such reactions.
We wish to report herein the reactivity of the racemic and
enantiomers of 3,4-dihydro-2H-1,5-benzoxathiepin-3-ol (12) and
a
´
´
´
Departamento de Quımica Farmaceutica y Organica, Facultad de Farmacia, c/
Campus de Cartuja s/n, 18071 Granada, Spain. E-mail: jmcampos@ugr.es; Fax: +34
958 243845; Tel: +34 958 243850
b
´
Laboratorio de Estudios Cristalogracos, IACT, CSIC-Universidad de Granada, Avda.
de las Palmeras 4, 18100 Armilla, Granada, Spain
c
´
´
Centro de Instrumentacion Cientıca, Universidad de Granada, Edicio Mecenas,
Campus Universitario de Fuente Nueva, 18071 Granada, Spain
† Electronic supplementary information (ESI) available: Chromatograms and
RMN studies of enantiomers of 12 and 13, and racemates and enantiomers of
17 and 22–24 are provided. CCDC 864711, 864709 and 864710. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c4ra01968g
Fig. 1 2- and 6-substituted (RS)-9-(2,3-dihydro-1,4-benzoxathiin-3-yl-
methyl)-9H-purine derivatives obtained by the Mitsunobu reaction.
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several adenines, such as N,N-dimethyladenine, 2-chloro-N-
When 2-mercaptophenol is reacted with (R)-epichlorohydrin,
methyladenine, and N-methyladenine. The results will be (S)-14 is obtained (90%) with an ee ¼ 99.9% with complete
interpreted in terms of multident nucleophile reactivity and SN2 inversion of the stereocentre of its oxirane moiety.16 This implies
transition state structures for the alkylation reactions. These a complete regioselective process as a consequence of the
concepts should allow more condent prediction of the effects nucleophilic attack of the sulfanyl anion at the less hindered
that variations in structural features and conditions will have on face of the oxirane moiety of (R)-epichlorohydrin, thus resulting
the alkylation pattern of adenine derivatives during the varied in (S)-14 with an excellent ee. Aer removing DMF in vacuo and
syntheses of alkylated purines.
purication by ash column chromatography of the reaction
crude, heating (S)-14 with NaOH in H2O at 100 ꢀC gives the
cyclization products, (S)-12 (62% yield, ee ¼ 95.3%) and the six-
membered primary alcohol (R)-13 (24% yield, ee ¼ 98.5%)
(Scheme 1).
Results and discussion
Synthesis of (RS)-, (S)- and (R)-3,4-dihydro-2H-1,5-
benzoxathiepin-3-ol (12), and (RS)-, (S)- and (R)-2,3-dihydro-
1,4-benzoxathiin-2-methanol (13)
Scheme 2 shows the different stereochemical behaviour
when the experimental workup was modied using (S)-
epichlorohydrin. Although pathway (a) is exactly the same as
that shown in Scheme 1, compound (R)-14 is not isolated and
NaOH and more DMF are added (instead of water) and the
Three-carbon (C-3) epoxides bearing halide substituents are
highly versatile synthetic building blocks because each carbon
is functionalized and a potential site of nucleophilic attack.
Epichlorohydrin, in particular, is a readily available C-3 unit
that is widely employed in organic and polymer synthesis.15
Compound (RS)-, (S)- and (R)-14 were synthesized under
microwave irradiation, rꢀacemic or enantiomeric epichlorohy-
drin as appropriate, 140 C, 10 min. Both (RS)- and (S)-14 were
obtained with a 90% yield, (Scheme 1). Compound (R)-14 was
not isolated to obtain (R)-12 and (S)-13 (Scheme 2).
ꢀ
resulting solution is heated at 100 C for 24 h. Aer removing
DMF and subsequent ash chromatography, (R)-12 (62%, ee ¼
74.7%) and (S)-13 (24%, ee ¼ 77.3%) are obtained.
The interpretation for the formation of (R)-12 [and for (S)-13
would be very similar] with a marked erosion of its optical
purity in relation to the method outlined in Scheme 2 could be
rationalized in Scheme 3. The formation of (R)-14 follows the
same pattern for (S)-14. As (S)-14 was obtained with a high
yield (90%), we decided neither to purify nor isolate (R)-14;
although the reaction workup was reduced, the ee worsened
due to the presence of HCl in the reaction medium.17
Accordingly, (R)-14 follows two different pathways, i.e., a
preferential SN2 process [presumably with an excellent ee, see
Scheme 1 with (R)-epichlorohydrin], and a SN1 process with
concomitant racemization. Finally, cyclization of pure (R)-14
[route (a) of Scheme 3] and (S)-14 with NaOH at 100 ꢀC leads to
(R)-12 and (S)-12, respectively, the enantiomer (R)-12 being
produced mainly through pathway (b) and to a lesser extent via
pathway (a), whilst (S)-12 is formed only through pathway (a)
(Scheme 3).
Due to fact that the syntheses of (R)-14 and (S)-14 are carried
out under the same conditions, the racemization process is not
produced at this stage [conrmed by isolation of (S)-14] and
hence, racemization takes place in the following cyclization
step, which differs in both enantiomers (Scheme 3).
Scheme 1 Reagents and conditions: (a) (R)-epichlorohydrin, pyridine,
MW, 140 ꢀC, 10 min, DMF; (b) NaOH, H2O, 100 ꢀC, 24 h.
Reactivity between racemic and enantiomers of 4-dihydro-2H-
1,5-benzoxathiepin-3-ol and 6-(N,N-dimethyl)adenine
Scheme 4 shows the reaction pattern between (RS)-12 and N,N-
dimethyladenine under microwave irradiation, the N-3 alky-
lated purine (RS)-17 (32%) being the major compound, and the
minor one the N-9 alkylated purine (RS)-18 (5%).
When (S)-12 (ee ¼ 95.3%) is the starting reactant, the prod-
ucts obtained are (R)-17 (38%) and (R)-18 (5%), both of them
with excellent optical purities (ee ¼ 96.9% and ee ¼ 94.2%,
respectively) (Scheme 5).
When (R)-12 (ee ¼ 74.7%) is the starting reactant, (S)-17
(34%, ee ¼ 76%) and (S)-18 (5%, ee ¼ 73.7%) are obtained
(Scheme 6).
Scheme 2 Reagents and conditions: (a) (S)-epichlorohydrin, pyridine,
MW, 140 ꢀC, 10 min, DMF; (b) without isolating (R)-14, NaOH, DMF,
100 ꢀC, 24 h.
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Scheme 3 Starting from (S)-epichlorohydrin tentative explanation for the formation of both (R)-12 major and (S)-12 minor compounds, without
isolating (S)-15. Formation of the minor (S)-13 compound is not included for simplicity, although the mechanism is similar.
Scheme 6 Reactivity between (R)-12 and N,N-dimethyladenine.
Scheme 4 Preparation of the (RS)-(3-alkylated-N,N-dimethyl)adenine
[(RS)-17] and (RS)-(9-alkylated-N,N-dimethyl)adenine [(RS)-18].
suggested by non-equivalent N-Me2 signals in the 1H NMR
spectrum of 17 and is substantiated by its X-ray analysis. This is
consistent with the electron-releasing nature of the dimethyla-
mino group: sp ¼ ꢁ0.83,18 or ꢁ0.82.19 Although the electronic
inuence of the NMe2 group on N-1 and N-3 of the purine
nucleus is basically the same (its mesomeric electron-releasing
effect is much more important than its inductive electron-
withdrawing one), the lack of alkylation of N,N-dimethylade-
nine at its N-1 site could be attributed to the steric hindrance
Scheme 5 Reactivity between (S)-12 and N,N-dimethyladenine.
The structure of 3-substituted-6-N,N-dimethyladenine is
Scheme 7 Resonance hybrid 20 can explain the alkylation at the N-3
conditioned by the importance of the dipolar iminium form (20)
atom of the purine moiety, as suggested by non-equivalent –NMe2
to the possible resonance hybrid [19 4 20 4 21] (Scheme 7), as signals in the 1H NMR spectrum of (RS)-17.
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caused by the dimethylamino group. On the other hand, its and CH-8 (which is the CH group that is not correlated with C-6)
steric inuence also avoids the alkylation at the N-7 position of of the purine ring. This correlation proves unequivocally that
the adenine-derived compound.
the linkage between the six-membered moiety and the purine
The structure of (RS)-17 has been determined by 1H, 13C base takes place through N-9 in (RS)-18 (Fig. 3).
NMR, HMBC (Fig. 2), HSQC and moreover by X-ray crystallog-
raphy. HMBC gives 2-bond and 3-bond information, i.e., which
Reactivity between racemic and enantiomers of 4-dihydro-2H-
1
of the H are 2-bonds or 3-bonds away from a particular 13C.
1,5-benzoxathiepin-3-ol and 2-chloro-N-methyladenine
1
HSQC gives 1-bond information, i.e., which H is attached to
The Mitsunobu coupling of allylic and benzylic alcohol with
which 13C. The combination of 1H NMR and 1H-13C HSQC
adenine and 2-chloroadenine has been reported previously with
good N-9 selectivity.21–23 However, poor to modest yield (20–
spectra is very powerful, and oen allows the complete assign-
ment of all protons in a molecule. In the HMBC experiment to
60%) and limited substrate scope were observed,24–26 which
three bonds, the following two correlations are important: (a)
signicantly limits the application of this method. Currently,
the rst one between the exocyclic methylene group and the
there is no detailed study regarding the Mitsunobu coupling
quaternary carbon C-4 of the purine (common carbon atom that
between 2-chloroadenine and a secondary alcohol through
neighbouring-group participation.
is correlated through three bonds to CH-2 and CH-8) of 17; and
the second one: (b) between the exocyclic methylene group and
CH-2 (which is correlated to C-6) of the purine ring. This
correlation proves unequivocally that the linkage between the
The methylamino moiety is an electron-releasing group with
a sp similar to that of the dimethylamino moiety {sp (MeHN) ¼
ꢁ0.76;21 sp (Me2N) ¼ ꢁ0.83,18 or 0.82 (ref. 19)} and then favours
six-membered moiety and the purine base takes place through
the nucleophilicity of the N-3 atom of purine. However the
N-3 in (RS)-17 (Fig. 2).
presence of an electron-withdrawing atom such as chlorine {sI
When the same reaction is carried out between (RS)-12, (R)-
(Cl) ¼ 0.47 (ref. 27)} neutralizes the effect of the methylamino
12, (S)-12 and N,N-dimethyladenine, the 3-substitution on N,N-
group and consequently the alkylation site moves to the N-9
dimethyladenine is still preferred, and is accompanied by the
atom of 2-chloro-N-methyladenine, i.e., going from 17 and 18 to
formation of the corresponding 9-substituted-N,N-dimethyla-
22, there is a shi of s-electron-density from the six- to the ve-
denine (RS)-18, (S)-18 and (R)-18. Miyaki and Shimizu20 found
membered ring of purine. Moreover, no exocyclic N-alkylation
that, aer chromatographic separation of the reaction products,
takes place due to its relatively weak nucleophilic character.
the benzylation of N,N-dimethyladenine with PhCH2OH in THF
The structure of (RS)-22 has been determined by 1H, 13C
gave the N-3 and N-9-regioisomers.
NMR, HMBC and by X-ray in (R)-22 (Schemes 8–10). The HMBC
The structure of (RS)-18 has been determined by 1H, 13C
experiment to three bonds proves unequivocally that the
NMR, HMBC and HSQC (Fig. 3). In the HMBC experiment to
linkage between the six-membered moiety and the purine base
three bonds the following two correlations are important: (a)
does not take place through N-3 because the exocyclic methy-
the rst one between the exocyclic methylene group and the
lene group is correlated with CH-8 of the purine, and accord-
ingly the linkage could be through N-9 or N-7 (Fig. 4). It was
quaternary carbon C-4 of the purine (common carbon atom that
is correlated through three bonds to CH-2 and CH-8) of (RS)-18
and the second one: (b) between the exocyclic methylene group
Fig. 2 Representation of the HMBC interactions (with double-tipped
arrows) observed between the exocyclic methylene group and the
purine atoms, when the linkage is N-3 in compound (RS)-17.
Scheme
adenine [(RS)-22].
8 Preparation of (RS)-(N-9-alkylated-2-chloro-N-methyl)
Fig. 3 Representation of the HMBC interactions (with double-tipped
arrows) observed between the exocyclic methylene group and the Scheme
9
Preparation of (R)-(N-9-alkylated-2-chloro-N-methyl)
purine atoms, when the linkage is N-9 in compound (RS)-18.
adenine [(R)-22].
22428 | RSC Adv., 2014, 4, 22425–22433
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Scheme 10 Preparation of (S)-(N-9-alkylated-2-chloro-N-methyl)
adenine [(S)-22].
Scheme 12 Preparation of (S)-(N-3-alkylated-N-methyl)adenine [(S)-
23] and (S)-(N-9-alkylated-N-methyl)adenine [(S)-24].
nally determined by (R)-22 X-ray crystal structure because in
the HMBC experiment it was impossible to differentiate
between C-4 and C-5 of the purine.
The stereochemistry of (S)-22 shows the complete inversion
of the stereogenic alcohol center. This result is consistent with a
SN2 mechanism and strongly enhances the application of the
reported method as a general approach for the stereospecic
synthesis of alkylated purines.
Reactivity between racemic and enantiomers of 4-dihydro-2H-
1,5-benzoxathiepin-3-ol and N-methyladenine
Reactions between (RS)-12, (R)-12 or (S)-12 and N-methyl-
adenine produce the same regioisomers (N-9 and N-3) and the
sum of the global yields of the nal products are in the same
range [90% from (RS)-12, 89% from (S)-12 and 90% from (R)-12]
(Schemes 11–13). Reactions between (RS)-12, (R)-12 and (S)-12
and 6-(N-methyl)adenine produce the regioisomers (RS)-23 (N-3:
45%) and (RS)-24 (N-9: 45%), (S)-23 (N-3: 48%) and (S)-24 (N-9:
Scheme 13 Preparation of (R)-(N-3-alkylated-N-methyl)adenine [(R)-
23] and (R)-N-9-alkylated-N-methyl)adenine [(R)-24].
42%), (R)-23 (N-3: 47%) and (R)-24 (N-9: 42%), respectively. This
is consistent with the electron-releasing nature of the methyl-
amino group: sp ¼ ꢁ0.76.19 In the case of the NHMe group
compared to the NMe2 one, the lesser electron-releasing power
of the former in relation to the latter explains the approximately
equal amount of both N-3 (23) and N-9 (24) regioisomers in the
reaction between 12 and N-methyladenine, whilst in the case of
N,N-dimethyladenine the major product is the N-3 regioisomer
(RS)-, (R)-, and (S)-17, being the ratio N-9/N-3 z 7/1.
In the HMBC experiment to three bonds, the two following
correlations are important: (a) the rst one between the exocy-
clic methylene group and the quaternary carbon C-4 of the
purine (common carbon atom which is correlated to CH-2 and
CH-8) of (RS)-23; and the second one: (b) between the exocyclic
methylene group and CH-2 (which is the group that is not
correlated with C-6) of the purine ring. This correlation proves
unequivocally that the linkage between the six-membered
moiety and the purine base takes place through N-3 in (RS)-23
(Fig. 5).
Fig. 4 Representation of the HMBC interactions (with double-tipped
arrows) observed between the exocyclic methylene group and the
purine atoms of (RS)-22 when the linkage is through N-9.
The structure of (RS)-24 has been determined by 1H, 13C
NMR, HMBC and HSQC. In the HMBC experiment to three
bonds the following two correlations are outstanding: (a) the
rst one between the exocyclic methylene group and the
quaternary carbon C-4 of the purine (common carbon atom that
is correlated through three bonds to CH-2 and CH-8) of (RS)-24;
and the second one: (b) between the exocyclic methylene group
and CH-8 (which is the CH group that is not correlated with C-6)
Scheme 11 Preparation of (RS)-(N-3-alkylated-N-methyl)adenine
[(RS)-23] and (RS)-(N-9-alkylated-N-methyl)adenine [(RS)-24].
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600 MHz 1H and 150 MHz 13C NMR, 500 MHz 1H and 125 MHz
13C NMR, 400 MHz H and 100 MHz 13C NMR or 300 MHz H
and 75 MHz 13C NMR spectrometers at ambient temperature.
Chemical shis (d) are quoted in parts per million (ppm) and
are referenced to the residual solvent peak. Signals are desig-
nated as follows: s, singlet; bs, broad singlet, d, doublet; dd,
doublet of doublets; ddd, double doublet of doublets; t, triplet;
m, multiplet. The HMBC spectra were measured using a pulse
sequence optimized for 10 Hz (inter-pulse delay for the evolu-
tion of long-range couplings: 50 ms) and a 5/3/4 gradient
combination. In this way, direct responses (J couplings) were
not completely removed. High-resolution Nano-Assisted Laser
Desorption/Ionization (NALDI-TOF) or Electrospray Ionization
(ESITOF) mass spectra were carried out. Small scale microwave-
assisted synthesis was carried out in a single-mode microwave
instrument producing controlled irradiation at 2.450 GHz and
sealed reaction vessels were used. Reaction time refers to hold
time from 130 C to 160 C, not to total irradiation time. The
temperature was measured with an IR sensor outside the reac-
tion vessel. Anhydrous DMF was used as received. Anhydrous
conditions were performed under argon. X-ray data were
collected on a diffractometer equipped with graphite a mono-
chromated Mo Ka radiation. The structure was solved by direct
methods shelxs97 and rened by full-matrix least-squares
against F2 using shelxl97 soware.
1
1
Fig. 5 Representation of the HMBC interactions (with double-tipped
arrows) observed between the exocyclic methylene group and the
purine atoms, when the linkage is through N-3 in compound (RS)-23.
Fig. 6 Representation of the HMBC interactions (with double-tipped
arrows) observed between the exocyclic methylene group and the
purine atoms, when the linkage is through N-9 in compound (RS)-24.
ꢀ
ꢀ
of the purine ring. This correlation proves unequivocally that
the linkage between the six-membered moiety and the purine
base takes place through N-9 in (RS)-24 (Fig. 6).
Neither 2,6-diaminopurine nor 6,8-dichloro-2-methyl-
thiopurine, as possible purine bases in the Mitsunobu coupling,
produced the desired coupling products with the secondary
racemic and homochiral alcohols 12. These results may be due
to the poor solubility of the bases.
Preparation of (S)-3-[(2-hydroxyphenyl)thio]-1,2-epoxypropane
[(S)-14]
NMR data for adenine-derived compounds
A mixture of 2-mercaptophenol (200 mg, 1.59 mmol) and 60%
pyridine (130 mg, 1.59 mmol) in anhydrous DMF (2.5 mL) under
argon, was stirred at r.t. for 5 min, (R)-epichlorohydrin (170 mg,
1.83 mmol) was added and the resulting mixture was irradiated
under microwaves conditions at 140 ꢀC for an additional 10
min. The mixture was cooled, and the pH was xed to 2–3 with 1
N HCl. The DMF was removed in vacuo, and the residue was
extracted (H2O–CH2Cl2). Organic layers were combined and
dried (Na2SO4). Aer ltration the solvent was removed in vacuo
and the crude was puried by ash column chromatography
using an 8/2 hexane–EtOAc mixture. (S)-14 (260 mg, 90%) was
obtained as a brownish syrup. ee ¼ 99.9%, [a]2D5 ꢁ46.6 (c 1.0 in
CH2Cl2); HPLC analysis: hexane–2-propanol ¼ 90/10, ow rate
1.0 mL minꢁ1, l ¼ 250 nm, tR (S)-14 ¼ 11.346 min.
The 1H NMR (CDCl3) CH-2 signal of adenines in N-9 isomer (RS)-
18 is shied downeld in relation to the corresponding signal of
the N-3 isomer (RS)-17, whereas the CH-8 signal of (RS)-18 is
shied upeld compared with the corresponding signal of N-3
[(RS)-17]. In summary, the signals of both proton atoms are
interchanged in both regioisomers. Compounds (RS)-23 and -24
cannot be compared because the 1H NMR spectra of both (RS)-
23 and -24 were registered in different deuterated solvents
(CDCl3 and DMSO-d6, respectively).
Chemical shis (CDCl3) of the carbon atom for the CH-2
group follows the same tendency in N-3 [(RS)-17 and -23] and N-
9 [(RS)-18 and -24] isomers. These signals are shied downeld
in N-3 isomers [(RS)-17 and (RS)-23, d 141.9–145.1 ppm]
compared with the corresponding signals in N-9 isomers [(RS)-
18 and -24, d 152.8–152.4 ppm].
Preparation of (S)-3,4-dihydro-2H-1,5-benzoxathiepin-3-ol [(S)-
12] and (R)-2,3-dihydro-1,4-benzoxathiin-2-methanol [(R)-13]
Chemical shi (CDCl3) of the carbon atom for CH-8 is shied
up-eld in N-9 isomers [(RS)-18 and (RS)-24, d 152.7–148.2 ppm]
compared with the corresponding signals in N-3 isomers [(RS)-
17 and -23, d 139.5–140.7 ppm].
Compounds (S)-12 (62 mg, 62%) and (R)-13 (24 mg, 24%) were
synthesized according to our previously reported procedure for
the racemic compounds,5,14 but using (R)-epichlorohydrin
instead. (S)-12: ee ¼ 95.3%, [a]2D5 ꢁ36.7 (c 1.0 in CH2Cl2); HPLC
analysis: hexane–CH2Cl2 ¼ 65/35, ow rate 1.0 mL minꢁ1, l ¼
250 nm, tR (R)-12 ¼ 10.896 min, tR (S)-12 ¼ 12.079 min
Experimental
General methods
Melting points were taken in open capillaries and are uncor- [Fig. (S3)†].
rected. Flash chromatography was performed on silica gel 60
(R)-13: ee ¼ 98.5%, [a]2D5 + 38.1 (c 1.0 in CH2Cl2); HPLC
with a particle size of 0.040–0.063 mm (230–400 mesh ASTM). analysis: hexane–CH2Cl2 ¼ 65/35, ow rate 1.0 mL minꢁ1, l ¼
Nuclear magnetic resonance spectra have been recorded on a 250 nm, tR (S)-13 ¼ 14.682 min, tR (R)-13 ¼ 15.890 min.
22430 | RSC Adv., 2014, 4, 22425–22433
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1H-exocyclic), 6.89 (2H, m, CH-aromatics), 7.02 (2H, m, CH-
Preparation of (R)-12 and (S)-13
aromatics), 7.71 (1H, s, H8-purine), 8.33 (1H, s, H2-purine). 13
C
The procedure was exactly the same as described to obtain (S)-
12 and (R)-13, but compound (R)-14 was not isolated, NaOH (63
mg, 1.59 mmol) and more DMF (10 mL) were added (instead of
water) and the resulting solution was heated at 100 ꢀC for 24 h.
Aer removing DMF and subsequent ash chromatography, (R)-
12 (179 mg, 62%) and (S)-13 (69 mg, 24%) were obtained.
(R)-12: ee ¼ 74.7%, [a]2D5 + 28.3 (c 1.0 in CH2Cl2); HPLC
analysis: hexane–CH2Cl2 ¼ 65/35, ow rate 1.0 mL minꢁ1, l ¼
250 nm, tR (R)-12 ¼ 9.854 min, tR (S)-12 ¼ 11.114 min.
NMR: dC (125 MHz, CDCl3) 29.93 (CH3), 37.72 (CH-3), 45.76
(CH2-exocyclic), 65.91 (CH2-2), 116.25 (C-4a), 118.82 (CH-
aromatics), 120.54 (C-5-purine), 122.63, 126.17, 127.96, 139.15
(CH-8-purine), 150.65 (C-4-purine), 151.45 (C-8a), 152.84 (CH-2-
purine), 155.25 (C-6-purine). HR LSIMS m/z calcd for
C16H17N5OS [M + H]+ 328.1232, found 328.1233.
(R)-N,N-Dimethyl-9-(2,3-dihydro-1,4-benzoxathiin-3-ylmethyl)-
9H-adenine [(R)-18] (9 mg, 5%): ee ¼ 94.2%, [a]2D5 + 67.1 (c 0.34 in
MeOH); HPLC analysis: hexane–EtOH ¼ 80/20, ow rate 1.0 mL
minꢁ1, l ¼ 250 nm, tR (R)-18 ¼ 20.053 min, tR (S)-18 ¼ 25.755 min.
(S)-N,N-Dimethyl-9-(2,3-dihydro-1,4-benzoxathiin-3-ylmethyl)-
9H-adenine [(S)-18] (9 mg, 5%): ee ¼ 73.7%, [a]2D5 ꢁ53.3 (c 0.34 in
MeOH); HPLC analysis: hexane–EtOH ¼ 80/20, ow rate 1.0 mL
minꢁ1, l ¼ 250 nm, tR (R)-18 ¼ 20.050 min, tR (S)-18 ¼ 25.764 min.
(RS)-2-Chloro-N-methyl-9-(2,3-dihydro-1,4-benzoxathiin-3-yl-
(S)-13: ee ¼ 77.3%, [a]2D5 ꢁ33.2 (c 1.0 in CH2Cl2); HPLC
analysis: hexane–CH2Cl2 ¼ 65/35, ow rate 1.0 mL minꢁ1, l ¼
250 nm, tR (S)-13 ¼ 14.899, tR (R)-13 ¼ 16.249 min.
General procedure for the microwave-assisted synthesis of
racemic and homochiral compounds 17, 18, 22, 23 and 24
DIPAD (2 equiv.) was slowly added to a suspension of the benzo- methyl)-9H-adenine [(RS)-22]: white solid (86 mg, 45%), mp 117–
1
ꢀ
fused six-membered ring 12 (1 equiv.), the purine base (1 equiv.) 119 C. H NMR: dH (500 MHz, CDCl3) 3.11 (3H, m, CH3), 3.82
and triphenylphosphine (1.1 equiv.) in dry THF (5 mL mmolꢁ1
in a microwave vial, cooled in an ice-bath. The vial was 1.9 Hz, CH-3), 4.25 (2H, m, CH2-2), 4.37 (1H, dd, Jgem ¼ 14.2 Hz,
)
(1H, m, CH-3), 4.23 (1H, ddd, Jgem ¼ 12.0 Hz, J1,2 ¼ 3.0 Hz J1,3 ¼
sealed and microwave irradiated at a temperature of 140 ꢀC for J1,2 ¼ 8.0 Hz, CH2 exocyclic), 4.47 (1H, dd, Jgem ¼ 14.2 Hz, J1,2
¼
10 min. Compounds 17, 18, 22, 23 and 24 were puried by 7.5 Hz, CH2 exocyclic), 6.88 (2H, m, CH-aromatics), 7.02 (2H, m,
ash column chromatography on silica gel using a mixture CH-aromatics), 7.73 (1H, s, H8-purine). 13C NMR: dC (126 MHz,
of hexane–EtOAc (5/1
(10/0.1 / 7/3). The solvent was evaporated in vacuo and the (CH2-2), 115.72 (C-4a), 118.21 (C-5-purine), 118.74 (CH-
products were obtained pure.
aromatics), 122.64, 126.20, 127.84, 140.45 (CH-8-purine), 149.63
/ 0/1) and then CH2Cl2–MeOH CDCl3) 27.75 (CH3), 37.53 (CH-3), 45.79 (CH2-exocyclic), 65.70
(RS)-N,N-Dimethyl-3-(2,3-dihydro-1,4-benzoxathiin-3-ylmethyl)- (C-4-purine), 151.19 (C-8a), 155.86 (C-6-purine). HR LSIMS m/z
3H-adenine [(RS)-17]: white solid (57 mg, 32%), mp 152–154 ꢀC. calcd for C16H14N5ClOS [M + H]+ 348.0686, found 348.0687.
1H NMR: dH (500 MHz, CDCl3) 3.35 (3H, br s, CH3), 3.92 (3H, br
(R)-2-Chloro-N-methyl-9-(2,3-dihydro-1,4-benzoxathiin-3-yl-
s, CH3), 4.18 (1H, m, CH-3), 4.27 (1H, d, Jgem ¼ 12.0 Hz, J1,2 ¼ 1.7 methyl)-9H-adenine [(R)-22] (85 mg, 42%): ee ¼ 99.9%, [a]D25
+
Hz, CH-2), 4.37 (1H, dd, Jgem ¼ 12.0 Hz, J1,2 ¼ 2.9 Hz, CH-2), 4.42 33.59 (c 0.65 in DMSO); HPLC analysis: hexane–EtOH ¼
(1H, dd, Jgem ¼ 13.7 Hz, J1,2 ¼ 8.4 Hz, 1H-exocyclic), 4.65 (1H, dd, 80/20, ow rate 1.0 mL minꢁ1, l ¼ 250 nm, tR (R)-22 ¼
Jgem ¼ 13.7 Hz, J1,2 ¼ 6.9 Hz, 1H-exocyclic), 6.89 (2H, m, CH- 13.353 min.
aromatics), 7.03 (2H, m, CH-aromatics), 7.87 (1H, s, H2-purine),
(S)-2-Chloro-N-methyl-9-(2,3-dihydro-1,4-benzoxathiin-3-yl-
7.96 (1H, s, H8-purine). 13C NMR: dC (125 MHz, CDCl3) 36.99 methyl)-9H-adenine [(S)-22] (100 mg, 49%): ee ¼ 73.7%, [a]2D5
(CH-3), 38.23 (CH3), 39.95 (CH3), 51.77 (CH2-exocyclic), 66.14 ꢁ22.8 (c 0.65 in DMSO); HPLC analysis: hexane–EtOH ¼ 80/20,
(CH2-2), 115.92 (C-4a), 122.02 (C-5-purine), 118.91 (CH- ow rate 1.0 mL minꢁ1, l ¼ 250 nm, tR (R)-22 ¼ 13.352 min, tR
aromatics), 122.74, 126.38, 128.15, 141.92 (CH-2-purine), 150.11 (S)-22 ¼ 14.560 min.
(C-4-purine), 151.27 (C-8a), 152.74 (CH-8-purine), 153.85 (C-6-
(RS)-N-Methyl-3-(2,3-dihydro-1,4-benzoxathiin-3-ylmethyl)-3H-
purine). HR LSIMS m/z calcd for C16H17N5OS [M + H]+ 328.1232, adenine [(RS)-23]: white solid (77 mg, 45%), mp 199–201 ꢀC. 1H
found 328.1238.
NMR: dH (500 MHz, CDCl3) 3.24 (3H, m, CH3), 4.08 (1H, m, H-3),
(R)-N,N-Dimethyl-3-(2,3-dihydro-1,4-benzoxathiin-3-ylmethyl)- 4.31 (1H, dd, Jgem ¼ 12.1 Hz, J1,2 ¼ 1.5 Hz, CH2-2), 4.40 (1H, dd,
3H-adenine [(R)-17] (68 mg, 38%): ee ¼ 96.9%, [a]2D5 ꢁ135.49 (c Jgem ¼ 12.11 Hz, J1,2 ¼ 2.8 Hz, CH2-2), 4.49 (1H, dd, Jgem ¼ 13.7
1.0 in CH2Cl2); HPLC analysis: hexane–EtOH ¼ 80/20, ow rate Hz, J1,2 ¼ 8.5 Hz, CH2-exocyclic), 4.72 (1H, dd, Jgem ¼ 13.8 Hz,
1.0 mL minꢁ1, l ¼ 250 nm, tR (R)-17 ¼ 17.84 min, tR (S)-17 ¼ J1,2 ¼ 6.7 Hz, CH2-exocyclic), 6.90 (2H, m), 7.04 (2H, m), 8.00
23.430 min.
(1H, s, H8-purine), 8.07 (1H, s, H2-purine). 13C NMR: dC (126
(S)-N,N-Dimethyl-3-(2,3-dihydro-1,4-benzoxathiin-3-ylmethyl)- MHz, CDCl3) 28.32 (CH3), 37.11 (CH-3), 51.81 (CH2-exocyclic),
3H-adenine [(S)-17] (61 mg, 34%): ee ¼ 76%, [a]2D5 + 104.74 (c 1.0 65.90 (CH2-2), 115.12 (C4a), 117.19 (C-5-purine), 118.96 (CH-
in CH2Cl2); HPLC analysis: hexane–EtOH ¼ 80/20, ow rate 1.0 aromatics), 122.91, 126.59, 128.10, 145.14 (CH-2-purine), 147.47
mL minꢁ1, l ¼ 250 nm, tR (R)-17 ¼ 18.242 min, tR (S)-17 ¼ (C-4-purine), 148.24 (CH-8-purine), 151.20 (C-8a), 153.86 (C-6-
23.055 min.
purine). HR (LSIMS) m/z calcd for C15H16N5OS [M + H]+
(RS)-N,N-Dimethyl-9-(2,3-dihydro-1,4-benzoxathiin-3-ylmethyl)- 314.1076, found 314.1076.
9H-adenine [(RS)-18]: white solid (9 mg, 5%), mp 170–172 ꢀC. 1H
(R)-N-Methyl-3-(2,3-dihydro-1,4-benzoxathiin-3-ylmethyl)-3H-
NMR: dH (500 MHz, CDCl3) 3.52 (6H, br s, N(CH3)2), 3.89 (1H, m, adenine [(R)-23] (80 mg, 47%): ee ¼ 96.1%, [a]2D5 ꢁ61.81 (c 1.0 in
CH-3), 4.25 (2H, m, CH2-2), 4.41 (1H, dd, Jgem ¼ 14.2 Hz, J1,2
¼
MeOH); HPLC analysis: hexane–EtOH ¼ 80/20, ow rate 1.0 mL
7.7 Hz, 1H-exocyclic), 4.50 (1H, dd, Jgem ¼ 14.2 Hz, J1,2 ¼ 7.7 Hz, minꢁ1, l ¼ 250 nm, tR (R)-23 ¼ 14.343 min, tR (S)-23 ¼ 17.190 min.
This journal is © The Royal Society of Chemistry 2014
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(S)-N-Methyl-3-(2,3-dihydro-1,4-benzoxathiin-3-ylmethyl)-3H- 00592 and by the ERDF (European Regional Development
adenine [(S)-23] (83 mg, 48%): 78.1 ee, [a]2D5 + 59.43 (c 1.0 in Fund). The M. E. G.-R. FPU grant AP2007-02954 from the Min-
´
MeOH); HPLC analysis: hexane–EtOH ¼ 80/20, ow rate 1.0 mL isterio de Ciencia e Innovacion of Spain is greatly acknowl-
minꢁ1, l ¼ 250 nm, tR (R)-23 ¼ 15.004 min, tR (S)-23 ¼ 17.299 min. edged. The project “Factorıa de Cristalizacion” Consolider
(RS)-N-Methyl-9-(2,3-dihydro-1,4-benzoxathiin-3-ylmethyl)-9H- Ingenio-2010 (CSD2006-00015) provided X-ray structural facili-
adenine [(RS)-24]: white solid (77 mg, 45%), mp 168–170 ꢀC. 1H ties for this work.
´
´
NMR: dH (600 MHz, DMSO-d6) 2.95 (3H, s, CH3), 4.12 (1H, m, H-
3), 4.28 (2H, m, CH2-2), 4.40 (1H, dd, Jgem ¼ 14.1 Hz, J1,2 ¼ 8.2
Hz, CH2-exocyclic), 4.56 (1H, dd, Jgem ¼ 14.1 Hz, J1,2 ¼ 6.7 Hz,
Notes and references
CH2-exocyclic), 6.90 (2H, m), 7.06 (2H, m), 8.13 (1H, s, H8-
purine), 8.23 (1H, s, H2-purine). 13C NMR: dC (150 MHz, DMSO
d6) 26.96 (CH3), 37.82 (CH-3), 44.65 (CH2-exocyclic), 65.95 (CH2-
2), 116.45 (C4a), 118.99 (C-4-purine), 118.26 (CH-aromatics),
122.10, 125.65, 127.29, 140.77 (CH-8-purine), 148.55 (C-4-
purine), 151.07 (C-8a), 152.48 (CH-2-purine), 154.85 (C-6-
purine). HR (LSIMS) m/z calcd for C15H16N5OS [M + H]+
314.1076, found 314.1077.
1 M. C. Nunez, M. Dıaz-Gavilan, A. Conejo-Garcıa, O. Cruz-
˜
´
´
´
´
´
Lopez, M. A. Gallo, A. Espinosa and J. M. Campos, Curr.
Med. Chem., 2008, 15, 2614.
2 J. A. Marchal, M. C. Nunez, A. Aranega, M. A. Gallo,
A. Espinosa and J. M. Campos, Curr. Med. Chem., 2009, 16,
1166.
˜
´
´
´
´
3 C. Lopez-Cara, A. Conejo-Garcıa, J. A. Marchal,
´ ´
G. Macchione, O. Cruz-Lopez, H. Boulaiz, M. A. Garcıa,
(R)-N-Methyl-9-(2,3-dihydro-1,4-benzoxathiin-3-ylmethyl)-9H-
adenine [(R)-24] (72 mg, 42%): ee ¼ 99.0%, [a]2D5 ꢁ65.83 (c 1.0 in
MeOH); HPLC analysis: hexane–EtOH ¼ 80/20, ow rate 1.0 mL
minꢁ1, l ¼ 250 nm, tR (R)-24 ¼ 26.679 min, tR (S)-24 ¼ 29.495 min.
(S)-N-Methyl-9-(2,3-dihydro-1,4-benzoxathiin-3-ylmethyl)-9H-
adenine [(S)-24] (72 mg, 42%): ee ¼ 59.1%, [a]2D5 + 38.3 (c 1.0 in
MeOH); HPLC analysis: hexane–EtOH ¼ 80/20, ow rate 1.0 mL
minꢁ1, l ¼ 250 nm, tR (R)-24 ¼ 26.576 min, tR (S)-24 ¼ 28.695 min.
´
´
F. Rodrıguez-Serrano, A. Ramırez, C. Cativiela,
´
´
A. I. Jimenez, J. M. Garcıa-Ruiz, D. Choquesillo-Lazarte,
´
A. Aranega and J. M. Campos, Eur. J. Med. Chem., 2011, 46,
249.
´
´
´
4 O. Caba, M. Dıaz-Gavilan, F. Rodrıguez-Serrano, H. Boulaiz,
´
A. Aranega, M. A. Gallo, J. A. Marchal and J. M. Campos, Eur.
J. Med. Chem., 2011, 46, 3802.
´
´
˜
5 A. Conejo-Garcıa, M. E. Garcıa-Rubino, J. A. Marchal,
˜
´
´
´
M. C. Nunez, A. Ramırez, S. Cimino, M. A. Garcıa,
Conclusions
´
A. Aranega, M. A. Gallo and J. M. Campos, Eur. J. Med.
We have described herein a novel and efficient synthetic
method between alkylated aminopurines (N,N-dimethyl-, 2-
chloro-N-methyl-, and N-methyl-adenines) and the achiral and
chiral secondary alcohol 3,4-dihydro-2H-1,5-benzoxathiepin-3-
ol 12 by the Mitsunobu reaction under microwave-assisted
conditions to obtain 17, 18, 22–24. We have conrmed that the
sulfur atom of the seven-membered heterocycle exerts a
remarkable neighboring group effect on its reaction with several
amino-purines, producing a complete SN2 inversion at the
stereogenic centre aer the corresponding ring-contraction.
Chem., 2011, 46, 3795.
´
´
´
6 O. Caba, F. Rodrıguez-Serrano, M. Dıaz-Gavilan, A. Conejo-
´
´
´
Garcıa, P. Alvarez, R. Ortiz, A. Martınez-Amat,
´
J. M. Campos, M. A. Gallo, J. A. Marchal and A. Aranega,
Eur. J. Med. Chem., 2012, 50, 376.
7 A. M. Michelson, in The Chemistry of Nucleosides and
Nucleotides, Academic Press, Inc., London, New York, 1963.
8 J. Reynolds and M. Kassiou, Curr. Org. Chem., 2009, 13, 1610.
9 W. Li, X. Yin and S. W. Schneller, Bioorg. Med. Chem. Lett.,
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Alkylation of purines usually results in the formation of N-9 and 10 H. Kumamoto, N. Takahashi, T. Shimamura, H. Tanaka,
N-7 alkylpurines, the N-9 regioisomer normally being the major
product.1–3 Nevertheless, N-3 regioisomers of purines are
K. T. Nakamura, T. Hamasaki, M. Baba, H. Abe, M. Yano
and N. Kato, Tetrahedron, 2008, 64, 1494.
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the purine ring, and these substructures could lead to specic
Nucleic Acids, 2008, 27, 1238.
inhibitors of known biological targets and pave the way to new 12 J. A. Gomez, J. M. Campos, J. A. Marchal, M. A. Trujillo,
´
´
inhibitors or antagonists of targets that have yet to be described.
Several biological studies on the chiral and achiral molecules
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˜
´
´
´
´
described here are underway and the corresponding results will 13 M. C. Nunez, M. G. Pavani, M. Dıaz-Gavilan, F. Rodrıguez-
´
´
be communicated in due course.
Serrano, J. A. Gomez-Vidal, J. A. Marchal, A. Aranega,
M. A. Gallo, A. Espinosa and J. M. Campos, Tetrahedron,
2006, 62, 11724.
Conflict of interest
´
´
´
´
14 M. Dıaz-Gavilan, A. Conejo-Garcıa, O. Cruz-Lopez,
˜ ´ ´
´
M. C. Nunez, D. Choquesillo-Lazarte, J. M. Gonzalez-Perez,
The authors declare no competing nancial interest.
´
´
F. Rodrıguez-Serrano, J. A. Marchal, A. Aranega,
M. A. Gallo, A. Espinosa and J. M. Campos,
ChemMedChem, 2008, 3, 127.
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RSC Adv., 2014, 4, 22425–22433 | 22433