Bromoallenes as allyl dication equivalents in the absence of palladium(0): Synthesis of bicyclic sulfamides by tandem cyclization of bromoallenes

Article abstract of DOI:10.1002/anie.200462557

Who needs a metal? A highly regioselective synthesis of cyclic sulfamides containing a bicyclo[3.3.0]octane skeleton has been developed (see scheme). The first intramolecular nucleophilic attack of the internal sulfamide nitrogen atom takes place at the c

Full text of DOI:10.1002/anie.200462557

Angewandte
Chemie
Cyclization Reactions
Bromoallenes as Allyl Dication Equivalents in the
Absence of Palladium(0): Synthesis of Bicyclic
Sulfamides by Tandem Cyclization of
Bromoallenes**
Hisao Hamaguchi, Shohei Kosaka, Hiroaki Ohno,* and
Tetsuaki Tanaka*
Reactions of bromoallenes have attracted much interest in
recent years because of the interesting chemical properties
associated with their cumulated double bonds and bromine
atom.[1] However, except for our recent study on ring-forming
reactions,[2,3] all the reactions of bromoallenes reported to
date are intermolecular reactions, such as organocopper-
mediated substitutions,[4] palladium-catalyzed cross-coupling
reactions,[5] and formation of allenyl–metal reagents.[6]
Recently, we found that bromoallenes can act as allyl dication
equivalents in the presence of palladium(0), which is an
extremely useful process for the synthesis of medium-sized
heterocycles (Scheme 1).[3] Thus, reaction of bromoallene 1
Scheme 1. Palladium(0)-catalyzed medium-ring formation from bro-
moallenes.
with sodium alkoxide in the presence of a palladium(0)
catalyst and alcohol affords the (h3-allyl)palladium(ii) inter-
mediate 2 by intramolecular nucleophilic attack at the central
carbon atom of the allenic moiety. A second nucleophilic
reaction with alkoxide provides 3 in good to high yields. In
light of this chemistry, we expected that bicyclic compounds 6
could be formed by tandem cyclization of the bromoallene 4
via (h3-allyl)palladium(ii) intermediate 5 (Scheme 2).
In the field of medicinal chemistry, a sulfamide is one of
the most important structural motifs that exist in many
pharmaceutically useful compounds. For example, some
sulfamides, including cyclic ones, are known to be effective
HIV[7] and serine protease inhibitors,[8] and as both agonists
[*] H. Hamaguchi, S. Kosaka, Dr. H. Ohno, Prof. Dr. T. Tanaka
Graduate School of Pharmaceutical Sciences
Osaka University
1-6 Yamadaoka, Suita, Osaka 565-0871 (Japan)
Fax: (+81)6-6879-8214
E-mail: t-tanaka@phs.osaka-u.ac.jp
[**] This work was supported by a Grant-in-Aid for Encouragement of
Young Scientists from the Ministry of Education, Culture, Sports,
Science, and Technology of Japan, and the Mitsubishi Chemical
Corporation Fund. H. H. is grateful to Research Fellowships from
the Japan Society for the Promotion of Science (JSPS) for Young
Scientists.
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DOI: 10.1002/anie.200462557
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Communications
Scheme 2. Synthesis of bicyclic heterocycles.
and antagonists of serotonin[9] and histamine receptors.[10]
Furthermore, cyclic sulfamides have also been used as
effective chiral auxiliaries in asymmetric aldol reactions.[11]
However, most of the reported methods for the synthesis of
sulfamides rely on the reaction of diamines with sulfuryl
chloride or the sulfamide H2NSO2NH2 under drastic reaction
conditions.[12] Accordingly, we planned a novel synthesis of
bicyclic sulfamides by tandem cyclization of bromoallenes.
We describe here a highly regioselective construction of cyclic
sulfamides with a bicyclo[3.3.0]octane skeleton from bro-
moallenes containing a sulfamide moiety as nucleophile, and
show that some bromoallenes can act as allyl dication
equivalents even in the absence of palladium(0).
Scheme 4. Synthesis of bicyclic sulfamide 11 in the presence or
absence of palladium(0): a) [Pd(PPh3)4] (10 mol%), NaH (2.5 equiv),
MeOH, 608C, 11.5 h; b) NaH (2.5 equiv), MeOH, 608C, 16 h.
the medium-ring formation,[3] the palladium catalyst is
essential for successful conversion.[16] Thus, the reaction of
bromoallenes as an allyl dication in the absence of a
palladium catalyst can be applied to highly reactive bromoal-
lenes which easily form cyclized products, such as five-
membered rings.
To investigate the synthesis of bicyclic sulfamides by
tandem cyclization of bromoallenes, as depicted in Scheme 2,
the bromoallene 9 bearing a sulfamide moiety as nucleophile
was prepared from propargyl alcohol 7[13] as shown in
Scheme 3. Thus, treatment of 7 with MsCl and Et3N gave
Next, we investigated the effect of the solvent on the
palladium-free reaction using bromoallene 13. The results are
summarized in Table 1. Treatment of bromoallene 13 with
Table 1: Cyclization of bromoallene 13 in the absence of palladium(0).[a]
Entry
Solvent
T
t [h]
Product (yield[b] [%])
Scheme 3. Synthesis of bromoallene 9 bearing a sulfamide group:
a) MsCl, Et3N; b) CuBr·SMe2, LiBr; c) 1% HCl/EtOH; d) BocNH-
SO2NHBn, PPh3, diethylazodicarboxylate (DEAD); e) 3n HCl, EtOAc.
1
2
3
4
5
6
DMF[c]
DMF[d]
THF[d]
room temp
room temp
room temp
508C
room temp
608C
0.5
0.5
1.5
2.5
20
14 (76) and 15 (13)
14 (77) and 16 (7)
no reaction
THF[d]
14 (48) and 15 (16)
MeOH[c]
MeOH[c]
15 (8)[e]
the corresponding mesylate, which was then converted into
the bromoallenol 8 by treatment with CuBr·SMe2/LiBr[14]
followed by desilylation. Condensation of 8 with BocNH-
SO2NHBn[15] under the Mitsunobu conditions gave the
corresponding N-Boc bromoallene, the Boc group of which
was removed by treatment with 3n HCl to afford the desired
bromoallene 9 bearing two nitrogen nucleophiles.
7
16 (94)
[a] Reactions were carried out with NaH (1.5 equiv) or NaOMe
(1.5 equiv) as base in the absence of [Pd(PPh3)4]. [b] Yields of isolated
products. [c] NaH was used as base. [d] NaOMe was used as base.
[e] 71% of 13 was recovered.
We next investigated the tandem cyclization of the
bromoallene 9 in the presence of palladium(0). To realize
the desired cyclization, selective addition of the internal
sulfamide nitrogen to the central allenic carbon, followed by
preferential reaction of the terminal sulfamide nitrogen over
that of alkoxide, is essential. Fortunately, cyclization of the
bromoallene 9 gave the bicyclic sulfamide 11 (72% yield) as
the only isolable product (Scheme 4), presumably via the
intermediate 10. Surprisingly, the same reaction also pro-
ceeded in the absence of palladium(0) to afford 11 in better
yield (81%) along with a small amount of the six-membered-
ring product 12 (9%), although the required reaction time
was longer. These results demonstrate that bromoallene 9 can
act as an allyl dication equivalent in the absence of
palladium(0). However, it should be clearly noted that, in
NaH in DMF gave the azetidine 14 as the major product[17]
and a small amount of dihydropyrrole 15 containing a
bromomethyl group (Table 1, entry 1). Similar results were
obtained when using NaOMe as the base in DMF or THF
(Table 1, entries 2 and 4). Although the reaction of 13 with
NaH in MeOH at room temperature gave 15 in low yield
(8%; Table 1, entry 5), the reaction at 608C gave dihydro-
pyrrole 16, which contains a methoxymethyl group, in high
yield (94%; Table 1, entry 6). From these observations, it can
clearly be seen that bromoallenes can effectively act as allyl
dication equivalents in an alcoholic solvent in the absence of
palladium(0).
We next investigated the tandem cyclization reaction with
other bromoallenes, which were prepared from the corre-
sponding bromoallenols by a similar procedure to that shown
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in Scheme 3. The results of these cyclizations are summarized
in Table 2. As we expected, treatment of the bromoallene 17,
which contains a protected diol, gave the bicyclic sulfamide 22
in 91% yield (Table 2, entry 1). Similarly, the bromoallene 18
Table 2: Synthesis of bicyclic sulfamide in the absence of palladium(0).[a]
Entry Substrate
Time [h] Product
Yield
[%][b]
1
2
3
4.5
6
91
89
Scheme 5. Possible reaction courses.
65[c]
contains a tosylamide group, in MeOH at room temperature
(Table 1, entry 5), yielded the dihydropyrrole 15 (8%), which
contains a bromomethyl group, along with a considerable
amount of the starting bromoallene 13 (71%). In contrast, the
same reaction at higher temperature (Table 1, entry 6) gave
the methoxymethyl derivative 16, which is produced by the
nucleophilic substitution of 15 with methoxide. Formation of
the six-membered-ring product 12 can be rationalized by an
SN2’-type intramolecular attack by the terminal nitrogen.
Finally, we investigated the synthesis of cyclic sulfamides
from bromoallene 33, which has a three-atom tether between
the sulfamide and bromoallene (Scheme 6). Interestingly, the
12
4
5
10
12
57[d]
50[e]
[a] Reactions were carried out with NaH (2.5 equiv) in MeOH at 608C.
[b] Yields of isolated products. [c] Six-membered-ring product 27 was
obtained in 10% yield as a minor product. [d] Six-membered-ring
products 28a and 28b were obtained in 9% and 5% yield, respectively.
[e] Six-membered-ring product 29 was obtained in 24% yield.
was converted into the tricyclic sulfamide 23 in 89% yield
(Table 2, entry 2). Bicyclic sulfamides 24 and 25 were
obtained as the major products from the reaction of
bromoallenes 19, which has a methyl group on the terminal
nitrogen, or racemic 20, which has a single methyl substituent
on the carbon between the allenyl and sulfamide groups
(Table 2, entries 3 and 4). When the bromoallene 21, which
contains an unsubstituted carbon tether, was used, bicyclic
sulfamide 26 was isolated (50%) along with a considerable
amount of the six-membered-ring product 29 (24%; Table 2,
entry 5).
Possible reaction courses are shown in Scheme 5. In the
tandem cyclization, the first intramolecular nucleophilic
attack takes place at the central carbon atom of bromoallene
30 and is followed by protonation by MeOH to produce
intermediate 31 bearing a bromomethyl group. The second
intramolecular nucleophilic addition affords the bicyclic
sulfamide 11. This reaction pathway via the intermediate 31
is supported by the experimental results shown in Table 1:
NaH-mediated cyclization of the bromoallene 13, which
Scheme 6. Reaction of bromoallene 33, which has a three-atom tether
between the sulfamide and bromoallene.
first cyclization occurs regioselectively at the allenic carbon
close to the sulfamide group to give 34 (66%) and 35 (18%).
Pyrrolidine 37 is considered to be a plausible intermediate,
formed by SN2’-type intramolecular amination of 36.[18] This is
followed by exo- or endo-cyclization to afford 34 or 35,
respectively. This result reveals that the base-induced cycliza-
tion of bromoallenes in MeOH in the absence of palladium(0)
favors five-membered-ring formation, which is in striking
contrast to the palladium(0)-catalyzed reaction.[3]
In conclusion, we have developed a novel synthesis of
cyclic sulfamides containing a bicyclo[3.3.0]octane skeleton
from bromoallenes having a sulfamide moiety as nucleo-
philes. Although the palladium-free cyclization reaction is
limited to the reaction of relatively reactive substrates, we
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Communications
2000, 41, 811 – 814; f) A. M. Caporusso, S. Filippi, F. Barontini, P.
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have demonstrated for the first time that bromoallenes can
act as allyl dication equivalents even in the absence of
palladium(0), and that the intramolecular reaction with two
nucleophiles enables the direct formation of bicycles by
tandem cyclization. These cyclizations are extremely useful
for the synthesis of bicyclic sulfamides and could extend the
potential application of sulfamides as new pharmaceutically
useful agents in medicinal chemistry.
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Experimental Section
General procedure for the cyclization of bromoallenes in the absence
of palladium(0). Reaction of 9 (Scheme 4): NaH (60% suspension in
mineral oil; 12 mg, 0.3 mmol) was added to MeOH (0.5 mL) at room
temperature under nitrogen, and the mixture was stirred for 10 min at
this temperature.
A solution of the bromoallene 9 (43.1 mg,
0.12 mmol) in MeOH (0.7 mL) was added at room temperature to
the stirred mixture, which was stirred for 16 h at 608C. Concentration
under reduced pressure gave an oily residue, which was purified by
column chromatography over silica gel with n-hexane/EtOAc (3:1) to
give, in order of elution, 11 (27 mg, 81%) and 12 (3.5 mg, 9%). 11:
À1
= À
colorless solid; m.p. 488C; IR (KBr): n˜ = 1682 (C C N), 1317 cm
(NSO2); 1H NMR (300 MHz, CDCl3): d = 1.21 (s, 6H; 2 ꢀ CMe), 3.32
(s, 2H; CH2), 3.73 (d, J = 1.5 Hz, 2H; CH2), 4.32 (s, 2H; CH2Ph), 4.79
(t, J = 1.5 Hz, 1H; C CH), 7.30–7.37 ppm (m, 5H; Ph); 13C NMR
=
(67.8 MHz, CDCl3): d = 27.5 (2C), 46.1, 48.4, 51.6, 59.2, 111.6, 128.1,
128.4 (2C), 128.7 (2C), 134.7, 136.3 ppm; MS (FAB) m/z (%): 279
(89) [MH+], 91 (100); HRMS (FAB) calcd. for C14H19N2O2S [MH+]:
279.1167; found 279.1169. 12: colorless crystals; m.p. 1188C (n-
ꢀ
hexane/EtOAc); IR (KBr): n˜ = 3284 (NHSO2), 2114 (C C),
1336 cmÀ1 (NSO2); 1H NMR (300 MHz, CDCl3): d = 1.05 (s, 3H;
ꢀ
CMe), 1.08 (s, 3H; CMe), 2.56 (d, J = 2.4 Hz, 1H; C CH), 2.94 (ddd,
J = 14.4, 5.4, 1.8 Hz, 1H; CHaHb), 3.56 (dd, J = 2.4, 1.8 Hz, 1H; CH-
ꢀ
C CH), 3.64 (dd, J = 14.4, 10.8 Hz, 1H; CHaHb), 4.09 (d, J = 14.1 Hz,
1H; PhCHaHb), 4.70–4.76 (m, 1H; NH), 4.78 (d, J = 14.1 Hz, 1H;
PhCHaHb), 7.28–7.39 ppm (m, 5H; Ph); 13C NMR (75 MHz, CDCl3):
d = 23.16, 23.22, 33.7, 49.4, 52.4, 59.5, 77.0, 77.1, 127.9, 128.6 (2C),
128.9 (2C), 135.2 ppm; elemental analysis calcd (%) for C14H18N2O2S:
C 60.40, H 6.52, N 10.06; found: C 60.19, H 6.52, N 10.00.
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Received: November 9, 2004
Published online: January 21, 2005
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Keywords: allenes · cyclization · halogen compounds ·
palladium · sulfamides
.
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[16] Treatment of the bromoallene 38 with NaH in MeOH in the
presence of [Pd(PPh3)4] gave the seven-membered-ring product
39 in 76% yield as a result of initial intramolecular nucleophilic
attack on the central carbon of the allene moiety followed by a
second nucleophilic attack by methoxide.[3] However, treatment
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of the bromoallene 38 with NaH in MeOH in the absence of
[Pd(PPh3)4] gave the six-membered-ring product 40 in 46% yield
by an SN2’-type, intramolecular nucleophilic attack on the
proximal carbon of the allene moiety .
[17] We have already reported the formation of azetidines by NaH-
mediated intramolecular amination of bromoallenes: see,
ref. [2b].
[18] To determine the intermediate of this tandem cyclization
reaction, we investigated the stepwise reaction of the bromoal-
lene 33. Treatment of the bromoallene 33 with NaH in DMF
gave the pyrrolidine 37 in 86% yield. Reaction of 37 under the
same reaction conditions as the one-pot reaction afforded the
exo-cyclized product 34 (67%) and endo-cyclized product 35
(20%) in essentially the same product ratio. Related five-
membered-ring formation by intramolecular amination of a
carbon–carbon triple bond has been reported, see: a) Y. Tamaru,
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