Synthesis of 9-substituted tetrahydrodiazepinopurines - Asmarine A analogues

Article abstract of DOI:10.1016/S0040-4020(03)01058-5

Asmarines are marine alkaloids, with a unique tetrahydro[1,4]diazepino-[1,2,3-g,h]purine (THDAP) structure and interesting biological properties. Three synthetic approaches were employed for the preparation of the THDAP system. Several N-9, of the purine, protecting groups were investigated. ¹⁵N-Chemical shifts measured from ¹⁵NH HMBC experiments for several compounds, that demonstrate the influence of various structural features on the ¹⁵N-resonances are reported.

Full text of DOI:10.1016/S0040-4020(03)01058-5

Tetrahedron 59 (2003) 6493–6501
Synthesis of 9-substituted tetrahydrodiazepinopurines—
asmarine A analogues
*
Doron Pappo and Yoel Kashman
School of Chemistry, Tel-Aviv University, Ramat Aviv 69978, Israel
Received 16 April 2003; revised 9 June 2003; accepted 3 July 2003
Abstract—Asmarines are marine alkaloids, with a unique tetrahydro[1,4]diazepino-[1,2,3-g,h]purine (THDAP) structure and interesting
biological properties. Three synthetic approaches were employed for the preparation of the THDAP system. Several N-9, of the purine,
protecting groups were investigated. 15N-Chemical shifts measured from 15NH HMBC experiments for several compounds, that demonstrate
the influence of various structural features on the 15N-resonances are reported.
q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
The selective cytotoxicity to tumor cells of the marine
sponge metabolites asmarines A and B1 triggered a
synthetic project aimed at the preparation of asmarine
analogues. The asmarines (e.g. asmarine A, 1) are composed
of a diterpene half, chelodane,2 and an adenine half,
constructing the N(10)hydroxy-9-substituted tetra-
hydro[1,4]diazepino-[1,2,3-g,h]purine (THDAP).
This report describes the synthesis of the basic, non-
protected THDAP system of asmarines A and B (2), two 9-
alkylated analogues (3–4) and the 9-oxo-derivative (5)
(Fig. 1).†
Figure 1. Asmarine A (1) and synthetic analogues.
15N NMR is a powerful tool for structure determination.3–5
The suitability of 15N NMR spectroscopy is attributed to the
wide range of chemical shifts (ca. 900 ppm, and over
detection makes it possible to acquire one bond and long-
1
range H–15N correlations, circumventing the low sensi-
tivity, and therefrom obtain the 15N chemical shifts.
300 ppm for natural compounds) and its great sensitivity to
structural and environmental changes. A major disadvan-
Herewith we report the N-atom chemical shifts of the
tage of this spectroscopy is the extremely low sensitivity of
synthesized compounds and show the relationship between
15N at the natural abundance level. However, despite the
the structures and the N-resonances.
inherently low sensitivity, 15N still has useful potential as a
structural probe even at natural abundance, since inverse-
2. Results and discussion
Keywords: asmarine; diazepinopurine; purine; 15NH HMBC; 15N
resonances; Mitsunobu reaction.
2.1. Synthesis of the THDAP system
Abbreviations: MPM, (4-methoxyphenyl)methyl; DMPM, (3,4-
Recently, we reported the synthesis of the diprotected
N(4),N(10)O-dibenzyl-THDAP ring system (8)6 in two
steps starting from N6-benzyloxy-9-benzyladenine (6)7
(Scheme 1). Synthesis of the third, diazepino, ring to afford
compound 8, was achieved by an internal SN2 reaction
between the BnON¼ nitrogen atom and the primary halide
obtained by treatment of the CH2OH group of 7 with SOCl2.
dimethoxyphenyl)methyl; TMPM, (3,4,5-trimethoxyphenyl)methyl;
DPM, diphenylmethyl; MPPM, (4-methoxyphenyl)phenylmethyl; THP,
tetrahydo-2H-pyran-2-yl; SEM, [2-(trimethylsilyl)ethoxy]methyl.
*
Corresponding author. Tel.: þ972-3-6408419; fax: þ972-3-6409293;
e-mail: kashman@post.tau.ac.il
The present atoms numbers for compounds 2–5 are according to the
†
IUPAC nomenclature and differ from our previous numbering which was
based on the purine numbers.
0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4020(03)01058-5

6494
D. Pappo, Y. Kashman / Tetrahedron 59 (2003) 6493–6501
Scheme 1. Synthesis of N(4),N(10)O-dibenzyl-THDAP (8).
Removing both the N-benzyl and O-benzyl protecting
groups of 8 was, however, not straightforward. Pd-catalyzed
hydrogenolysis of 8 occasionally removed the O-benzyl
group leaving the free amine, rather than the hydroxylamine
group, and did not affect the N-benzyl group. Therefore
other deprotection conditions, as well as more suitable
protecting groups, were sought.
difficult to be removed selectively. The DPM group of
compound 13 (prepared from 6-chloropurine in two steps)
was not cleaved during reaction with 3-bromopropanol to
afford compound 14a, in 95% yield, avoiding a second
alkylation. Yet the DPM group could easily be selectively
removed under stronger acidic conditions, TFA in CH2Cl2,
to give compound 15a in 45% yield. Closure of the
diazepine ring was achieved by a Mitsunobu reaction
(DIAD, Ph3P)10 to afford compound 16a. Deprotection of
the O-benzyl group was accomplished by warming
compound 16a in a solution of 30% HBr in AcOH at
1008C for 3.5 h affording compound 2, the heterocyclic
fragment of asmarine A, which was recently also prepared
by a different route.11
The suitability of a variety of nitrogen protecting groups, for
the protection of the N-9 atom of the purine system, was
investigated. Among the groups checked were: (a) benzyl,
(b) MPM, (c) DMPM, (d) TMPM, (e) DPM, (f) MPPM, (g)
THP and (h) SEM.8 Three of the latter groups, f to h,
separated readily upon alkylation of N-7 as a result of the
generation of the imidazolium ion (i), namely, each of the
MPPM, THP and SEM groups donates a pair of electrons to
the imidazolium ion (i) (Scheme 2), detaches from the
heterocycle and, by itself, gives a stable cation which is the
driving force for the cleavage.9 The latter described
cleavage of the N-9 protecting group leaves the purine
system free (intermediate k) for a second alkylation at N-3
or N-9 to afford compounds 10 and 11, respectively, as the
main products (Scheme 2). Hence, protecting groups f–h
are not suitable.
Compounds 3 and 4 were prepared in the same way as
compound 2, namely, reacting compound 13 with suitable
bromo secondary alcohols (Scheme 3; bromo alcohol 24c
was prepared by the reaction of heptylmagnesium bromide
with 3-bromopropanal).
Determination of the structures of compounds 2 to 4 was
achieved by suitable 1H NMR, 13C NMR and MS
spectroscopy and confirmed by 13CH and 15NH HMBC
2
3
experiments. The JCH and JCH correlations proved,
unequivocally, both the THDAP ring system and the
location of the attached alkyl substituent at C-9. The
various CH– correlations for compound 3, for
Among the other tested N-9 protecting groups (a–e), the
DPM group (e) was found to give the best results
(Scheme 3); groups b–d, on the other hand, were too
Scheme 2. Suggested mechanism for removing protecting groups f–h.

D. Pappo, Y. Kashman / Tetrahedron 59 (2003) 6493–6501
6495
Scheme 3. Synthesis of the 10-hydroxy-9-substituted THDAP (2 to 4).
O-benzyl group in the case of compounds 16a–c,
compound 18, under same hydrogenolysis conditions
(10% Pd–C, H2, 1 atm), afforded the required free
hydroxylamine of the purine hydroxamate 5 in 67% yield.
Compound 18 serves as a starting material for the desired
C-9 substituted THDAP’s.
As described above for compounds 2 to 4, the structure of 5
was also established by HMBC experiments. The CH
correlation between H2-7 and the purine carbons (C-5 and
C-6a), confirmed the structure of the THDAP system.
Further support for the structure came from the 15NH
HMBC correlations, which also enabled the measurement of
the 15N chemical shifts (Table 1).
Figure 2. Long-range 1H–15N heteronuclear shift correlations measured
for compound 2, and selective long-tange 1H–13C heteronuclear shift
correlations measured for compound 3.
example Figure 2, demonstrate clearly the confirmation of
the suggested structure. Further support comes from the
15N- chemical shifts and long-range NH correlations (Fig. 2)
vide infra.
In addition to the synthesis of compounds 2 to 4, we also
prepared the THDAP system 21, with the unsubstituted
NH-10 group (Scheme 5). This synthesis started from
compound 19, possessing a N-3 protecting group known to
direct the second alkylation to N-7,13 thus affording
compound 20 which, after the acidic deprotection of the
TMPM group, afforded compound 21. The latter tricyclic
A second approach to the THDAP ring system based on an
amidation and a Michael reaction12 could also be applied to
compound 17 (Scheme 4), affording the expected 9-oxo
derivative 18. In contrast to the failure to deprotect the
Scheme 4. Synthesis of the 9-oxo derivative (5).

6496
D. Pappo, Y. Kashman / Tetrahedron 59 (2003) 6493–6501
Table 1. 15N chemical shifts deduced from long-range 1H–15N heteronuclear shift correlations (40 MHz, d6-DMSO, 258C)
Compound number
Structure
N-1
N-3
N-7
N-9
N6
1a
247.0
233.3
153.9
245.9
166.5
2a
245.7
250.8
238.0
235.3
237.9
235.7
156.0
155.6
155.4
245.4
245.4
245.0
149.3
168.7
92.6
16a
21
20
13
234.0
136.9
161.3
207.9
167.3
247.4
231.9
173.9
119.2
285.3
14a
141.8
204.0
172.2
180.0
288.2
15b
18
5
134.4
n.o.b
n.o.b
226.7
263.5
262.6
164.5
155.9
155.7
250.0
244.5
244.2
280.0
203.9
188.3

D. Pappo, Y. Kashman / Tetrahedron 59 (2003) 6493–6501
6497
N6
Table 1 (continued)
Compound number
Structure
N-1
N-3
N-7
N-9
22
248.0
260.5
154.7
245.4
150.7c
Chemical shifts in ppm upfield from liquid NH3. The N-atoms numbers for all the compounds in this Table are according to the purine numbers.
a
The experiment was performed at 508C in order to separate the line of H-2 from that of H-8.
Not observable.
b
c
The correlation was determined by a 15N-HSQC experiment.
Scheme 5. Synthesis of the 10-hydro-THDAP (21).
system is the heterocyclic system of the recently isolated
asmarine H.14 The structure of 21 was confirmed by the
various NMR techniques discussed, including 15N chemical
shifts.
N-atom resonates at ca. 250 (sp2) and the other (sp3) around
160 ppm.3 In addition, the N-1 atom of 1 is downfield
shifted, compared with adenine, by 19 ppm due to the N6
substituents and, of course, N6 itself is 88 ppm downfield
shifted, in 1, due to the hydroxylamine group and the a to N6
substituents. In the case of compound 21, lacking the N–OH
group, N6 resonates at 92.6 ppm. Further changes of N6 are
seen with the alteration of its own and the neighbor
substituents, e.g. compounds 2 and 16a. In addition,
comparison of N-7 and -9, of compound 14a, with the
values for 13 and 15b suggest that the positive charge in 14a
is evenly spread over both N-7 and -9 as both shift ca.
6–8 ppm downfield from the uncharged sp3 N-atoms. In
addition, removing the DPM group from 14a results in a
70 ppm downfield shift of N-9 (to d 250.0 for 15b) and a
7.7 ppm upfield shift of N-7 due to cancellation of the
positive charge. Hence, change of hybridization has a much
stronger effect than the positive charge.
2.2. 15N NMR as a tool for structure determination
The 15N NMR spectra, as shown above for representative
compounds‡ (Table 1, Fig. 2) demonstrate well the
feasibility of acquiring structural information from these
15N-resonance measurements. It is used, for example, to
establish whether N6 is a free NH, as in compound 21 and
22, or a NOR group as in compounds 1, 2, 5, 16a, 18, to find
the major tautomeric form of the N(1)–C(6)–N6 moiety
(e.g. in compounds 13, 14a, 15b vs 21 and 22) or determine
if a particular N-atom is in the sp2 or sp3 hybridization.
The 15N resonances of the herewith described compounds
range from ca. 90–290 ppm, agreeing well with known 15N
resonances3 and, furthermore, adding resonance values for
previously unreported functionalities such as BnO–N¼ and
combinations of functional groups, for example values for
the THDAP-system, purine hydroxamate and purine
hydroxyl amine systems. Comparison of the 15N resonances
of the purine part of the natural compound asmarine A (1)
with those of adenine,§,5 show remarkable changes for N-7
and N-9 (see Table 1). Whereas the most upfield
N-resonance for the adenine ring system is 156.9 for N-9,
it is 153.9 for N-7 of 1; dN-7 for adenine is 239.2 and dN-9 for
1 is 245.9 ppm. The latter N-7/N-9 chemical shift exchange
agrees with their sp2/sp3 exchange. The hybridization effect
can clearly be seen for substituted imidazoales where one
Another example for the sp2/sp3 hybridization change is
seen for the N-1 and N6 atoms in compounds 13 and 21.
Namely, N-1 in compound 13 is in the sp3 hybridization and
6
N6 in the sp2 hybridization (dN-1 136.9 and dN 285.3 ppm)
while, in 21, N-1 is sp2 and N6 is sp3 hybridized (dN-1 238.0
and dN 92.6 ppm).
6
Additional interesting 15N values are those measured for the
amido- and hydroxamate-purine systems, compounds 5, 18
and 22.15 While the amide and hydroxamate functionalities
have a minimal effect on N-7 and 9, in comparison with
compound 2, the N-3 atom is shifted ca. 25 ppm downfield.
Of course, N6 changes notably with its substituent.
Noteworthy is the 13C-resonance of the carbonyl group of
the latter compounds (168.1, 168.2 and 172.2 ppm,
respectively) showing a 4 ppm difference between the
hydroxamates and the amide resonances.
‡
In the following discussion, the atoms numbers of all compounds are
according to the purine numbers.
§
The 15N chemical shifts of adenine reported originally in respect to
CH3NO2 as the reference standard,5 are in this case converted to the liquid
NH3 scale: i.e. 15N (d6-DMSO) d 233.8 (N-1), 228.0 (N-3), 239.2 (N-7),
156.9 (N-9), 78.3 (N6).
The above-described synthetic routes with suitable substrates

6498
D. Pappo, Y. Kashman / Tetrahedron 59 (2003) 6493–6501
(e.g. compounds 4 and 5) as well as further alterations of
compounds, such as 18, by substitution of the carbonyl
group leads to asmarine analogues which are ready for SAR
studies.
d6-DMSO) 11.29 (1H, s, NH), 7.57 (1H, s, H-8), 7.53
(1H, s, H-2), 7.36–7.14 (15H, m, three Ph rings), 6.92 (1H,
s, PhCHPh), 5.00 (2H, s, PhCH2); dC (125 MHz, d6-DMSO)
144.8, 141.8, 141.6, 139.2, 137.7, 129.2, 128.4, 128.0,
127.8, 118.8, 75.0, 61.3; MS (CI) m/z (relative intensity):
408 (8, MHþ), 302 (100, MHþ2C7H7O), 107 (90,
C7H7Oþ); HRMS (CI): MHþ, found 408.1824, requires
C25H22N5O 408.1849.
3. Experimental
3.1. General methods
3.1.3. N6-Benzyloxy-7-(3-hydroxypropyl)-9-diphenyl-
methyladeninium bromide (14a). A mixture of compound
13 (600 mg, 1.47 mmol) and 3-bromopropanol (1.00 g,
7.2 mmol) in DMF (10 mL) was kept at 708C, under an
argon atmosphere, for 3 days. The solvent was then
evaporated and the residue chromatographed under vacuum.
Elution with ethyl acetate–methanol, 10:1, afforded com-
pound 14a (770 mg, 95%) as a colorless oil; nmax (KBr)
3018, 1675, 1618, 1558 cm21; dH (400 MHz, d6-DMSO)
12.20 (1H, br s, NH), 9.08 (1H, s, H-8), 7.79 (1H, s, H-2),
7.25–7.42 (15H, m, three Ph rings), 7.17 (1H, s, PhCHPh),
5.07 (2H, s, PhCH2), 4.62 (1H, br t, OH), 4.38 (2H, m,
H-11), 3.41 (2H, m, H-13), 1.88 (2H, m, H-12); dC
(100 MHz, d6-DMSO) 149.4, 141.3, 137.5, 137.2, 136.0,
135.4, 129.4, 129.3, 128.4, 128.2, 128.0, 127.8, 111.3, 76.4,
64.4, 57.7, 48.4, 32.0; MS (FAB)þm/z (relative intensity):
466 (44, Mþ), 167 (100, C13H11); HRMS (FAB) m/z
(relative intensity): Mþ, found 466.2236, requires
C28H28N5O2 466.2243.
Starting materials and reagents were purchased from
commercial suppliers and were used without further
purification. Petroleum ether refers to the fractions with
bp 64–688C. THF was freshly distilled from sodium and
benzophenone before use. DMSO was distilled from CaH2
under vacuum. Vacuum liquid chromatography (VLC) was
performed using silica gel 60 H (Merck), prewashed with
methanol. Melting points were determined on a Electro-
thermal IA9000 melting points apparatus and are
uncorrected. IR spectra were obtained with a Bruker FTIR
Vector 22 spectrometer and are reported in cm21. MS
spectra were recorded on a Fisons, Autospec Q instrument.
NMR Spectra were recorded on a Bruker ARX-500 and
Bruker Avance-400 spectrometers using standard Bruker
pulse sequences. The H–15N HMBC experiments were
1
optimized for delays of 55, 70 and 90 ms. the 15N chemical
shifts are reported with respect to liquid NH3 as the
reference standard. 3-Bromopropanol (24a) is commercially
available. 4-Bromo-2-butanol (24b) was prepared according
to a literature procedure.16
3.1.4. N6-Benzyloxy-7-(3-hydroxybutyl)-9-diphenyl-
methyladeninium bromide (14b). Compound 14b was
prepared from 13 under the same procedure as described for
the preparation of 14a. A mixture of compound 13 (1.18 g,
2.9 mmol) and 4-bromobutan-2-ol (1.0 g, 6.5 mmol), in
DMF (10 mL), was kept at 708C for 3 days under an argon
atmosphere, afforded after VLC eluting with ethyl acetate–
methanol, 6:1, compound 14b (1.34 g, 83%) as a colorless
oil; nmax (CHCl3) 3019, 1671, 1594, 1208 cm21; dH
(400 MHz, d6-DMSO) 12.19 (1H, br s, NH), 9.20 (1H, s,
H-8), 7.80 (1H, s, H-2), 7.40–7.18 (15H, m, three Ph rings),
7.08 (1H, s, PhCHPh), 5.08 (2H, s, PhCH2), 4.64 (1H, br s,
OH), 4.36 (2H, m, H-11), 3.63 (1H, m, H-13), 1.76 (2H, m,
H-12), 1.01 (3H, d, J¼6.0 Hz, H-14); dC (100 MHz,
d6-DMSO) 149.1, 141.4, 138.1, 137.5, 137.1, 136.8,
129.4, 129.0, 128.7, 128.6, 128.2, 111.5, 76.0, 64.3, 63.3,
49.0, 48.4, 24.1; MS (FAB) m/z (relative intensity): 480
(100, Mþ), 374 (10, Mþ2C6H6O), 314 (21, Mþ2C13H11);
HRMS (FAB) m/z (relative intensity): Mþ, found 480.2411,
requires C29H30N5O2 480.2400.
3.1.1. 9-Diphenylmethyl-6-chloropurine (12). To a stirred
mixture of triphenyl-phosphine (5 g, 20 mmol), benzhydrol
(3 g, 16 mmol) and 6-chloropurine (2 g, 13 mmol) in dry
THF (70 mL) at 08C under an argon atmosphere, was added
diisopropyl azodicarboxylate (3.7 g, 18 mmol). The mixture
was stirred for 5 min while a yellow precipitate formed. The
mixture was allowed to warm up to room temperature and
was then stirred overnight. The solvent was evaporated
under vacuum, and the residue vacuum chromatographed.
Elution with petroleum ether–ethyl acetate, 5:1, afforded
the protected purine contaminated with diisopropyl
hydrazodicarboxylate. The mixture was triturated with
ethanol and filtered, affording compound 12 (1.5 g, 40%)
as a white solid. Recrystallization with ethanol gave
colorless crystals, mp 161–1628C; nmax (KBr) 3114, 1584,
1557, 1203 cm21; dH (400 MHz, CDCl3) 8.73 (1H, s, H-2),
7.95 (1H, s, H-8), 7.13–7.40 (11H, m, two Ph rings and
PhCHPh); dC (125 MHz, CDCl3) 146.9, 146.6, 146.0, 139.4,
132.2, 123.9, 126.5, 123.6, 122.8, 57.0; MS (CI) m/z
(relative intensity): 321 (77, MHþ), 167 (100, C13H1þ1);
HRMS (CI): MHþ, found 321.0907, requires C18H14N4Cl
321.0908.
3.1.5. N6-Benzyloxy-7-(3-hydroxydecanyl)-9-diphenyl-
methyladeninium bromide (14c). Compound 14c was
prepared from 13 under the same procedure described for
the preparation of 14a. A mixture of compound 13 (710 mg,
1.74 mmol) and 24c (1.3 g, 5.48 mmol), in DMF (10 mL),
was kept at 708C for 5 days under an argon atmosphere,
afforded after VLC eluting with ethyl acetate–methanol,
20:1, compound 14c (980 mg, 87%) as a colorless oil; nmax
3.1.2. N6-Benzyloxy-9-diphenylmethyladenine (13). A
mixture of 12 (2.27 g, 7 mmol), O-benzyl hydroxylamine
(2 g, 16 mmol), and potassium carbonate (0.8 g, 6 mmol) in
ethanol (30 mL) was heated under reflux for 18 h. The
cooled mixture was then filtered and the precipitate washed
twice with cold water (2 mL) providing compound 13
(2.73 g, 94%) as a white solid. Recrystallization from
methanol gave colorless crystals, mp 2128C; nmax (KBr)
3028, 2856, 1663, 1594, 1535 cm21; dH (400 MHz,
(CHCl3) 3382, 3019, 1671, 1594, 1208 cm21
; dH
(400 MHz, d6-DMSO) 12.17 (1H, br s, NH), 9.09 (1H, s,
H-8), 7.79 (1H, s, H-2), 7.25–7.42 (15H, m, three Ph rings),
7.15 (1H, s, PhCHPh), 5.07 (2H, s, PhCH2), 4.52 (1H, d,
J¼5.2 Hz, OH), 4.44 (2H, m, H-11), 3.55 (1H, m, H-13),

D. Pappo, Y. Kashman / Tetrahedron 59 (2003) 6493–6501
6499
1.88 (2H, m, H-12), 1.26–1.42 (12H, m, –CH2–), 0.88 (3H,
t, J¼6.0 Hz, CH2CH3); dC (100 MHz, d6-DMSO) 148.5,
141.2, 137.1, 136.5, 136.1, 135.2, 135.1, 129.6, 129.5,
128.6, 128.3, 128.2, 111.8, 76.0, 68.9, 64.4, 49.1, 37.7, 36.5,
31.8, 29.5, 29.2, 25.5, 22.6, 14.1; MS (FAB) m/z (relative
intensity): 564 (70, Mþ), 398 (7, Mþ2C13H11), 166 (100,
C13Hþ11); HRMS (FAB) m/z (relative intensity): Mþ, found
564.3341, requires C35H42N5O2 564.3339.
29.6, 29.2, 22.7, 22.6, 14.0; MS (CI) m/z (relative intensity):
398 (38, MHþ), 292 (10, MHþ2C7H7O), 107 (100,
C7H7Oþ), 91 (75, C7Hþ7 ); HRMS (CI): MHþ, found
398.2545, requires C22H32N5O2 398.2556.
3.1.9. 7,8,9,10-Tetrahydro-10-benzyloxy-[1,4]diaze-
pino[1,2,3-g,h]purine (16a). Diisopropyl azodicarboxylate
(245 mg, 1.2 mmol) was added to a stirred solution of
compound 15a (180 mg, 0.6 mmol) and triphenylphosphine
(331 mg, 1.26 mmol) in dry THF (10 mL) under an argon
atmosphere. After 24 h the solvent was evaporated and the
residue chromatographed under vacuum. Elution with ethyl
acetate–methanol, 5:1, afforded compound 16a (160 mg,
95%) as a colorless oil; nmax (CHCl3) 3018, 1672, 1597,
1208 cm21; dH (400 MHz, d6-DMSO) 8.52 (1H, s, H-2),
8.42 (1H, s, H-5), 7.55 (2H, m, PhH), 7.40 (3H, m, PhH),
5.10 (2H, s, PhCH2), 4.22 (2H, t, H-7), 3.78 (2H, t, H-9),
2.18 (2H, m, H-8); dC (100 MHz, d6-DMSO) 159.6, 151.8,
151.7, 145.9, 135.9, 129.4, 128.3, 109.1, 75.8, 54.0, 46.9,
26.2; MS (CI) m/z (relative intensity): 282 (30, MHþ), 176
(85, MHþ2C7H7O), 107 (100, C7H7Oþ); HRMS (CI):
MHþ, found 282.1358, requires C15H16N5O 282.1355.
3.1.6. N6-Benzyloxy-7-(3-hydroxypropyl)adenine (15a).
TFA (10 mL) was added slowly to a solution of 14a
(726 mg, 0.82 mmol) in CH2Cl2 (10 mL) at 08C. The
solution was stirred for 24 h and then evaporated under
vacuum. The residue was taken into ethyl acetate and
washed with 0.5 M aq. NaHCO3, brine and dried over
sodium sulfate. The solvent was then evaporated and the
residue chromatographed under vacuum. Elution with ethyl
acetate–methanol, 20:1, afforded compound 15a (180 mg,
45%) as a colorless oil; nmax (CHCl3) 3398, 3018, 1655,
1597, 1208 cm21; dH (400 MHz, d6-DMSO) 11.20 (1H, br,
NH), 8.26 (1H, s, H-2), 7.68 (1H, s, H-8), 7.31–7.45 (5H, m,
PhH), 5.01 (2H, s, PhCH2), 4.26 (2H, m, H-11), 3.35 (2H, t,
H-13), 1.84 (2H, m, H-12); dC (100 MHz, d6-DMSO) 147.5,
145.2, 140.4, 139.1, 138.4, 128.8, 128.5, 128.1, 110.0, 75.7,
57.7, 45.5, 33.6; MS (CI) m/z (relative intensity): 300
(10, MHþ), 194 (27, MHþ2C7H7O), 107 (75, C7H7Oþ);
HRMS (CI): MHþ, found 300.1462, requires C15H18N5O2
300.1460.
3.1.10. 7,8,9,10-Tetrahydro-10-benzyloxy-9-methyl-
[1,4]diazepino[1,2,3-g,h]purine (16b). Compound 16b
was prepared from 15b under the same procedure described
for the preparation of 16a. A mixture of diisopropyl
azodicarboxylate (355 mg, 1.7 mmol), compound 15b
(110 mg, 0.35 mmol) and triphenylphosphine (475 mg,
1.7 mmol) in dry THF (10 mL) under an argon atmosphere
was stirred for 1 h at 408C and then kept at room
temperature over night. VLC eluting with ethyl acetate–
methanol, 6:1, afforded compound 16b (70 mg, 68%) as a
3.1.7. N6-Benzyloxy-7-(3-hydroxybutyl)adenine (15b).
Compound 15b was prepared from 14b under the same
procedure described for the preparation of 15a; TFA (3 mL)
was added to a solution of compound 14b (535 mg,
0.95 mmol) in CH2Cl2 (7 mL). The solution was stirred
for 24 h at ambient temperature, affording after VLC eluting
with ethyl acetate–methanol, 20:1, compound 15b (120 mg,
40%) as a colorless oil; nmax (CHCl3) 3014, 1655, 1598,
1208 cm21; dH (400 MHz, d6-DMSO) 11.23 (1H, br s, NH),
7.84 (1H, s, H-2), 7.54–7.24 (6H, m, 1H-8 and 5H-Ph), 5.00
(2H, s, PhCH2), 4.70 (1H, d, J¼4 Hz, OH), 4.19 (2H, m,
H-11), 3.56 (1H, m, H-13), 1.71 (2H, m, H-12), 0.98 (3H, d,
J¼6.0 Hz, H-14); dC (100 MHz, d6-DMSO) 150.7, 143.4,
141.4, 140.1, 138.7, 128.5, 127.9, 109.6, 75.3, 63.4, 44.5,
40.6, 23.9; MS (CI) m/z (relative intensity): 314 (63, MHþ),
208 (51, MHþ2C7H7O), 107 (88, C7H7Oþ), 91 (100,
C7Hþ7 ); HRMS (CI): MHþ, found 314.1609, requires
C16H20N5O2 314.1617.
colorless oil; nmax (CHCl3) 3018, 1672, 1597, 1209 cm21
;
dH (400 MHz, d6-DMSO) 8.50 (1H, s, H-2), 8.41 (1H, s,
H-5), 7.53 (2H, m, PhH), 7.40 (3H, m, PhH), 5.11 (1H, d,
J¼10.2 Hz, PhCH2), 5.10 (1H, d, J¼10.2 Hz, PhCH2), 4.38
(1H, m, H-7), 4.21 (1H, m, H-7), 4.08 (1H, m, H-9), 2.25
(2H, m, H-8), 1.23 (3H, d, 6.8 Hz, H-11); dC (100 MHz,
d6-DMSO) 159.3, 151.9, 150.8, 145.8, 136.0, 129.4, 128.1,
109.3, 76.2, 57.7, 41.7, 31.0, 13.7; HRMS (CI): MHþ, found
296.1511, requires C16H18N5O 296.1511.
3.1.11. 7,8,9,10-Tetrahydro-10-benzyloxy-9-heptyl-
[1,4]diazepino[1,2,3-g,h]purine (16c). Compound 16c
was prepared from 15c under the same procedure described
for the preparation of 16a. A mixture of diisopropyl
azodicarboxylate (255 mg, 1.27 mmol), compound 15c
(170 mg, 0.42 mmol) and triphenylphosphine (350 mg,
1.33 mmol) in dry THF (5 mL) under an argon atmosphere
was stirred for 4 h at 458C and then kept at room
temperature over night. VLC eluting with ethyl acetate–
methanol, 10:1, afforded compound 16c (130 mg, 80%) as a
3.1.8. N6-Benzyloxy-7-(3-hydroxydecanyl)adenine (15c).
Compound 15c was prepared from 14c under the same
procedure described for the preparation of 15a; TFA (6 mL)
was added to a solution of compound 14c (532 mg,
0.82 mmol) in CH2Cl2 (14 mL). The solution was stirred
for 24 h affording after VLC eluting with ethyl acetate–
methanol, 20:1, compound 15c (130 mg, 40%) as a colorless
oil; nmax (CHCl3) 3398, 3018, 1655, 1597, 1208 cm21; dH
(400 MHz, CDCl3) 10.96 (1H, br s, NH), 8.11 (1H, s, H-2),
7.43 (2H, m, PhH), 7.34 (3H, m, PhH), 7.04 (1H, s, H-8),
5.06 (2H, s, PhCH2), 4.45 (1H, m, H-11), 4.05 (1H, m,
H-11), 3.37 (1H, m, H-13), 1.69–1.82 (2H, m, H-12), 1.48–
1.24 (12H, m, –CH2–), 0.86 (3H, t, J¼6.0 Hz, CH2CH3);
dC (125 MHz, CDCl3) 150.6, 143.3, 141.2, 140.7, 137.1,
129.1, 128.5, 128.3, 110.0, 76.3, 66.7, 43.6, 39.7, 37.1, 31.7,
colorless oil; nmax (CHCl3) 3018, 1672, 1597, 1208 cm21
;
dH (400 MHz, d6-DMSO) 8.46 (1H, s, H-2), 8.37 (1H, s,
H-5), 7.51 (2H m, PhH), 7.40 (3H, m, PhH), 5.08 (1H, d,
J¼10.5 Hz, PhCH2), 5.00 (1H, d, J¼10.5 Hz, PhCH2), 4.38
(1H, m, H-7), 4.12 (1H, m, H-7), 3.93 (1H, m, H-9), 2.36
(1H, m, H-8), 2.19 (1H, m, H-8), 1.71 (1H, m, H-11), 1.52
(1H, m, H-11), 1.19–1.34 (10H, m, –CH2–), 0.82 (3H, t,
J¼6.5 Hz, CH2CH3); dC (100 MHz, d6-DMSO) 159.0,
151.9, 150.3, 145.7, 136.0, 129.3, 128.4, 128.3, 109.1,
75.9, 61.5, 41.4, 39.8, 31.2, 28.7, 28.5, 27.4, 25.7, 22.1,

6500
D. Pappo, Y. Kashman / Tetrahedron 59 (2003) 6493–6501
14.0; HRMS (CI): MHþ, found 380.2450, requires
C22H30N5O 380.2450.
2-propanol (3 mL) affording compound 18 (200 mg, 32%)
as a white solid dec. 2608C; nmax (KBr) 3092, 3039, 1695
(CO), 1618, 1554 cm21; dH (400 MHz, d6-DMSO) 8.87
(1H, s, H-2), 8.65 (1H, s, H-5), 7.60–7.38 (5H, m, PhH),
5.18 (2H, s, PhCH2), 4.50 (2H, br t, H-7), 3.23 (2H, br t,
H-8); dC (100 MHz, d6-DMSO) 168.2, 161.8, 152.6, 147.7,
145.1, 135.6, 130.2, 129.5, 129.1, 113.5, 78.0, 42.3, 37.8;
MS (EI) m/z (relative intensity): 295 (10, Mþ), 278 (45,
Mþ2OH), 189 (Mþ2C7H7O), 91 (100, C7Hþ7 ); HRMS
(CI): MHþ, found 296.1149, requires C15H14N5O2 296.1148.
3.1.12. 7,8,9,10-Tetrahydro-10-hydroxy-[1,4]diaze-
pino[1,2,3-g,h]purine (2). Compound 16a (49 mg,
0.13 mmol) was dissolve in cold 30% HBr in glacial acetic
acid (3 mL) and warmed to 1008C for 3.5 h. The cooled
reaction mixture was then evaporated, the residue triturated
twice with ether and the hydrobromide salt dissolved in
methanol and basified with K2CO3. The remained gum after
evaporation was chromatographed under vacuum. Elution
with ethyl acetate–methanol, 7:3, afforded compound 2
(35 mg, 93%) as an amorphous solid; nmax (KBr) 3412
(OH), 1607, 1559, 1345 cm21; dH (500 MHz, d6-DMSO)
10.00 (1H, br s, OH), 8.31 (1H, s, H-2), 8.30 (1H, s, H-5),
4.28 (2H, t, H-7), 3.82 (2H, t, H-8), 2.24 (2H, m, H-9); dC
(125 MHz, d6-DMSO) 159.2, 152.3, 151.9, 145.4, 109.1,
56.0, 47.1, 26.7; MS (EI) m/z (relative intensity): 191 (18,
Mþ), 175 (100, Mþ2100), 120 (78, Mþ2C3H5NO); HRMS
(EI): Mþ, found 191.0803, requires C8H9N5O 191.0807.
3.1.16. 7,8,10-Trihydro-10-hydroxy-[1,4]diaze-
pino[1,2,3-g,h]purin-9-one (5). A solution of compound
18 (46 mg, 0.16 mmol), in methanol (2 mL) was hydrogen-
ated over 10% Pd–C (42 mg) at atmospheric pressure at
room temperature for 1.5 h. Removing the catalyst by
filtration and evaporating the solvent left a crude gum. The
residue was triturated with ether (5 mL) and ethyl acetate
(5 mL) affording compound 5 (22 mg, 67%) as a white
solid. The material was unstable while attempting crystal-
lization; nmax (KBr) 3379 (OH), 1669 (CO), 1619,
1559 cm21; dH (400 MHz, d6-DMSO) d 8.78 (1H, s, H-2),
8.60 (1H, s, H-5), 4.53 (2H, br t, H-7), 3.17 (2H, br t, H-8);
dC (100 MHz, d6-DMSO) 168.1, 161.1, 152.0, 147.0, 145.6,
113.1, 42.1, 37.2; MS (EI) m/z (relative intensity): 189 (65,
MHþ216), 135 (62, C5H5N5þ); HRMS (EI): Mþ2OH,
found 188.0572, requires C8H6N5O 188.0572.
3.1.13. 7,8,9,10-Tetrahydro-10-hydroxy-9-methyl-
[1,4]diazepino[1,2,3-g,h]purine (3). Compound 3 was
prepared from 16b under the same procedure as described
for the preparation of compound 2. Compound 16b (18 mg,
0.06 mmol) in 30% HBr in glacial acetic acid (3 mL) was
warmed to 1008C for 3.5 h. VLC eluting with ethyl acetate–
methanol, 7:3, afforded compound 3 (8 mg, 66%) as an
amorphous solid; nmax (KBr) 3412 (OH), 1607, 1559,
1345 cm21; dH (400 MHz, d6-DMSO) 9.73 (1H, br s, OH),
8.33 (2H, br s, H-2 and H-5), 4.46 (1H, m, H-7), 4.13 (1H,
m, H-7), 3.91 (1H, m, H-9), 2.22 (2H, m, H-8), 1.17 (3H, d,
J¼6.4 Hz, H-11); dC (100 MHz, d6-DMSO) 159.4, 159.1,
151.9, 144.9, 109.7, 59.0, 41.8, 31.4, 14.2; MS (CI) m/z
(relative intensity): 205 (7, Mþ), 189 (Mþ216), 120 (16,
Mþ2C4H7NO), 84 (56, C4H6NOþ); HRMS (CI): MHþ,
found 206.1037, requires C9H12N5O 206.1041.
3.1.17. 3-[(3,4,5-Trimethoxyphenyl)methyl]-adenine
(19). A mixture of adenine (4 g, 29 mmol) and 3,4,5-
trimethoxybenzylbromide (9.25 g, 35 mmol) in DMF
(50 mL) was heated to 708C for 36 h. The cooled solution
was then evaporated and the residue triturated with ethanol.
The filtered hydrobromide salt was triturated with 10% aq.
NaHCO3, filtrated and washed with cold water. Recrystal-
lization from ethanol gave 19 (3.9 g, 43%) as a white solid
dec. 2288C; nmax (KBr) 3005, 1672, 1618, 1127 cm21; dH
(400 MHz, d6-DMSO) 8.55 (1H, s, H-2), 8.22 (1H, br, NH),
8.10 (1H, br, NH), 7.80 (1H, s, H-8), 6.91 (2H, s, ArH), 5.41
(2H, s, ArCH2), 3.72 (6H, s, ArOCH3), 3.61 (3H, s,
ArOCH3); dC (100 MHz, d6-DMSO) 157.1, 155.1, 154.7,
151.9, 145.5, 139.5, 133.7, 122.6, 108.4, 62.1, 58.1, 54.6;
MS (CI) m/z (relative intensity): 316 (100, MHþ), 183 (58,
C10H15Oþ3 ), 136 (77, C5N5H6); HRMS (CI): MHþ, found
316.1410, requires C15H18N5O3 316.1410.
3.1.14. 7,8,9,10-Tetrahydro-10-hydroxy-9-heptyl-
[1,4]diazepino[1,2,3-g,h]purine (4). Compound 4 was
prepared from 16c under the same procedure as described
for the preparation of compound 2. Compound 16c (14 mg,
0.04 mmol) in 30% HBr in glacial acetic acid (3 mL) was
warmed to 1008C for 3.5 h. VLC eluting with ethyl acetate–
methanol, 7:3, afforded compound 4 (8 mg, 67%) as an
amorphous solid; nmax (KBr) 3412 (OH), 1607, 1559,
1345 cm21; dH (400 MHz, d6-DMSO) 9.79 (1H, br s, OH),
8.28 (2H, br s, H-2 and H-5), 4.33 (1H, m, H-7), 4.13 (1H,
m, H-7), 3.91 (1H, m, H-9), 2.38 (1H, m, H-8), 2.26 (1H, m,
H-8), 1.77 (1H, m, H-11), 1.52 (1H, m, H-11), 1.34–1.13
(10H, m, –CH2–), 0.79 (3H, t, J¼6.5 Hz, CH2CH3); dC
(100 MHz, d6-DMSO) 158.3, 151.7, 150.4, 144.8, 108.9,
63.0, 41.5, 31.2, 28.9, 28.7, 28.5, 27.7, 25.6, 22.0, 13.9; MS
(CI) m/z (relative intensity): 290 (40, MHþ), 274 (100,
MHþ216); HRMS (CI): MHþ, found 290.1984, requires
C15H24N5O 290.1980.
3.1.18. 3,7,8,9-Tetrahydro-3-[(3,4,5-trimethoxyphenyl)-
methyl]-[1,4]diazepino-[1,2,3-g,h]purine (20). A mixture
of compound 19 (1 g, 3.2 mmol), 1,3-dibromopropane
(1.5 g, 7.4 mmol) and triethylamine (2 g, 19 mmol) in
DMA (10 mL) was heated to 808C for 36 h. The solution
was then cooled and the triethylamine hydrobromide filtered
off. The solvent was evaporated and the residue triturated
with ether, leaving an oil which was dissolved in methanol
and basified with K2CO3. The residue after evaporation was
chromatographed under vacuum. Elution with ethyl acet-
ate–methanol, 7:3, afforded compound 20 (0.9 g, 79%) as a
white solid subl. 2458C; nmax (KBr) 3125, 1655, 1591,
1397 cm21; dH (400 MHz, d6-DMSO) 9.14 (1H, s, H-2),
8.76 (1H, s, H-5), 6.93 (2H, s, ArH), 5.49 (2H, s, ArCH2),
4.46 (2H, br t, H-7), 3.70 (6H, s, ArOCH3), 3.64 (2H, m,
H-9), 3.56 (3H, s, ArOCH3), 2.23 (2H, br t, H-8); dC
(100 MHz, d6-DMSO) d 155.1, 155.0, 152.2, 149.6, 149.5,
3.1.15. 7,8,10-Trihydro-10-(benzyloxy)-[1,4]diaze-
pino[1,2,3-g,h]purin-9-one (18). A mixture of N-benzyl-
oxyaminopurine (17)17 (0.5 g, 2.1 mmol) and vinyl acrylate
(0.4 g, 4.14 mmol) in dry DMSO (3 mL) under an argon
atmosphere was heated to 808C for 24 h. The solution was
then cooled and the obtained solid filtered and washed with

D. Pappo, Y. Kashman / Tetrahedron 59 (2003) 6493–6501
6501
148.5, 139.7, 132.6, 113.6, 108.6, 62.2, 58.2, 54.3, 51.1,
46.3, 28.8; MS (CI) m/z (relative intensity): 356 (8, MHþ),
176 (100, MHþ2C10H13O3); HRMS (CI): MHþ, found
356.1723, requires C18H22N5O3 356.1724.
CDCl3) 3.83 (1H, m, H-3), 3.57 (2H, t, H-1), 1.97 (2H, m,
H-2), 1.52–1.29 (12H, m, –CH2–), 0.89 (3H, t, CH2CH3);
dC (100 MHz, d6-DMSO) 69.8, 39.9, 37.2, 31.8, 30.5, 29.5,
29.2, 25.5, 22.6, 14.0; MS (CI) m/z (relative intensity): 157
(100, MHþ280).
3.1.19. 7,8,9,10-Tetrahydro-[1,4]diazepino[1,2,3-
g,h]purine (21). Compound 20 (140 mg, 0.39 mmol) was
dissolved in 30% HBr in glacial acetic acid (5 mL) and
warmed to 1008C for 1 h. The mixture was then cooled to
room temperature and evaporated. The residue was
triturated with ether, dissolved in methanol and basified
with K2CO3. The residue after evaporation was chromato-
graphed under vacuum. Elution with ethyl acetate–
methanol, 8:2, afforded compound 21 (64 mg, 94%) as a
white solid subl. 2158C; nmax (KBr) 3125 (NH), 1635,
1399 cm21; dH (400 MHz, d6-DMSO) 8.28 (1H, s, H-2),
8.16 (1H, s, H-5), 7.78 (1H, br s, NH), 4.23 (2H, t, H-7), 3.43
(2H, t, H-9), 2.13 (2H, m, H-8); dC (100 MHz, d6-DMSO)
160.3, 153.6, 153.2, 145.9, 112.3, 49.7, 44.9, 29.9; MS (EI)
m/z (relative intensity): 175 (100, Mþ), 120 (65,
Mþ2C3H5N); HRMS (CI): MHþ, found 176.0936, requires
C8H10N5 176.0933.
Acknowledgements
We wish to thank Dr Amira Rudi for her help with NMR
measurements.
References
1. Yosief, T.; Rudi, A.; Kashman, Y. J. Nat. Prod. 2000, 63,
299–304.
2. Rudi, A.; Kashman, Y. J. Nat. Prod. 1992, 55, 1408–1414.
3. Martin, E. G.; Hadden, E. C. J. Nat. Prod. 2000, 63, 543–585.
4. Hocek, M.; Votruba, I.; Dvorakova, H. Tetrahedron 2003, 59,
607–611.
3.1.20. 7,8-Dihydro-[1,4]diazepino[1,2,3-g,h]purin-
9(1H)-one (22). Compound 22 was prepared as described
in the literature;15 nmax (KBr) 3417 (NH), 3100, 2885, 1680
(CO), 1631, 1574 cm21; dH (400 MHz, d6-DMSO) d 11.1
(1H, br s, NH), 8.58 (1H, s, H-2), 8.52 (1H, s, H-5), 4.48
(2H, br t, H-7), 3.07 (2H, br t, H-8); dC (100 MHz,
d6-DMSO) 172.2, 162.5, 152.2, 146.5, 145.2, 107.9, 41.7,
38.6; MS (EI) m/z (relative intensity): 189 (100, Mþ), 161
(13, Mþ2CO), 135 (35, C5H5Nþ5 ).
5. Laxer, A.; Major, D. T.; Gottlieb, H. E.; Fischer, B. J. Org.
Chem. 2001, 66, 5463–5481.
6. Pappo, D.; Rudi, A.; Kashman, Y. Tetrahedron Lett. 2001, 42,
5941–5943.
7. Fujii, T.; Itaya, T.; Wu, C. C.; Tanaka, F. Tetrahedron 1971,
27, 2415–2423.
8. Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic
Synthesis; 3rd ed. Wiley: New York, 1999.
9. Fujii, T.; Saito, T.; Iguchi, K. Chem. Pharm. Bull. 1994, 42,
495–499.
3.1.21. 1-Bromodecan-3-ol (24c). A solution of 3-bromo-
propanal (8 g, 60 mmol) in THF (10 mL) was added, over a
period of 30 min. to a solution of heptylmagnesium bromide
prepared from Mg (0.84 g, 35 mmol) and heptyl bromide
(6.5 g, 35 mmol) in THF (20 mL), at 08C under an argon
atmosphere. The temperature was allowed to warm up to
room temperature and the solution was stirred for another
2 h. The reaction mixture was then poured onto 10% aq.
NH4Cl and extracted twice with ether (30 mL). The
combined organic layer was washed with brine and dried
over magnesium sulfate. The residue after evaporation was
chromatographed under vacuum. Elution with CH2Cl2–
petroleum ether, 1:1, afforded compound 24c as a colorless
oil; nmax (CHCl3) 3400 (OH), 1450 cm21; dH (400 MHz,
10. Hughes, D. L. Organic Reactions; Wiley: New York, 1992;
Vol. 42.
11. Masashi, O.; Takahiro, T. Heterocycles 2002, 7, 1235–1238.
12. Fujii, T.; Sato, T.; Itaya, T. Chem. Pharm. Bull. 1971, 19,
1731–1734.
13. Fujii, T.; Itaya, T. Hetrocycles 1998, 48, 1673–1724, and
references therein.
14. Rudi, A.; Shalom, H.; Kashman, Y. J. Nat. Prod. 2003.
15. Brahme, N. M.; Smith, W. T., Jr. J. Heterocycl. Chem. 1985,
22, 109–112.
16. Matikainen, J.; Kaltia, S.; Hase, T.; Kuronen, P. J. Nat. Prod.
1995, 58, 1622–1624.
17. Fujii, T.; Sato, T.; Itaya, T. Chem. Pharm. Bull. 1971, 19,
1731–1734.