Selective construction of aliphatic polycycles utilizing platinum-catalyzed cyclization of 2-(enynyl)cycloalkenals

Article abstract of DOI:10.1055/s-0033-1338805

An efficient method for the selective construction of aliphatic polycycles through platinum-catalyzed cyclization of 2-(enynyl)cycloalkenals has been developed by considering electronic (ligand) and structural (olefinic tether) effect. The presence or abs

Full text of DOI:10.1055/s-0033-1338805

LETTER
▌1433
lSeteterlective Construction of Aliphatic Polycycles Utilizing Platinum-Catalyzed
Cyclization of 2-(Enynyl)cycloalkenals
Selective Construction of Aliphatic Polycycles
Jin Wook Han, Ji Ho Lee, Chang Ho Oh*
Department of Chemistry and Research Institute for Natural Science, College of Natural Sciences, Hanyang University, Seoul 133 791, Korea
Fax +82(2)22990762; E-mail: changho@hanyang.ac.kr
Received: 15.03.2013; Accepted after revision: 22.04.2013
3) PtCl2 in wet toluene yielded the type 5 products; 4)
Abstract: An efficient method for the selective construction of ali-
PtCl2 in toluene yielded the type 6 products.11
phatic polycycles through platinum-catalyzed cyclization of 2-(eny-
nyl)cycloalkenals has been developed by considering electronic
(ligand) and structural (olefinic tether) effect. The presence or ab-
CHO
sence of ligand and the length of olefinic tether determine the course
of the reaction after the formation of platinum–carbene complex, so
that four types of aliphatic polycycles can be formed selectively.
n
1
O
O
Key words: cyclization, homogeneous catalysis, platinum,
carbenes, carbocycles
5
"M(+)"
H
6
"Pt"
H
6
O
2
H
"Rh"
H2O
Z = CR2 or N-Ts
H
Conjugated enynals serve versatile substrates for the syn-
thesis of [m,7,n]-tricyclic skeletons via metal-catalyzed
reactions1 since those readily form metal–pyrylium inter-
mediates which undergo cycloaddition with an alkene
inter- and/or intramolecularly.2 Such [m,7,n]-tricyclic
structural motifs are found in a variety of natural products,
many of which have very challenging structures and a
broad spectrum of biological activities.3 We have been in-
terested in constructing [m,7,n]-tricyclic skeletons bear-
ing a central seven-membered ring. Among various
strategies for the construction of seven-membered carbo-
cycles,4 transition metals are utilized as catalysts because
of their inherent potential for causing a rapid increase in
skeletal complexity.5 During the course of our scientific
endeavors leading to a general and modular entry to poly-
cyles, we have reported a highly unique behavior of met-
al–carbene complexes A, formed via a Huisgen-type
[3+2] cycloaddition between metal–pyrylium species and
a double bond (Scheme 1).6 In the substrates involving an
aromatic ring such as 2-enynylbenzaldehydes, we ob-
served a variety of polycycles depending on the type of
substrates, catalysts, and reaction conditions. First, 2-(6-
en-1-ynyl)benzaldehydes (1, n = 5) would form the metal–
carbene intermediates A in the presence of transition-met-
al cations: 1) [Rh(COD)Cl]2 (5 mol%) and dppp (10
mol%) in refluxing wet xylene yielded the type 2 prod-
ucts;7 2) AuBr3 in 1,2-dichloroethane (DCE) yielded the
type 3 products;8 3) Cu(OTf)2 yielded the type 4 prod-
ucts;9 4) PtCl2 in wet toluene yielded the type 5 products.10
Second, 2-(7-en-1-ynyl)benzaldehydes (1, n = 6) would
form the metal–carbene intermediates A in the presence of
transition-metal cations: 1) AuBr3 in DCE yielded the
type 3 products; 2) Cu(OTf)2 yielded the type 4 products;
O
"Pt"
H2O
n
"Au"
O
HO
M
A
"Cu"
5, 6
5, 6
O
5
3
5, 6
O
4
Scheme 1 Metal-catalyzed cyclization of 2-enynylbenzaldehydes 1
These results could provide an interesting approach, not
only for simple fused polycycles but also for the synthesis
of complex natural products, which have a core skeleton
of seven-membered carbocycles.12 However, in the ma-
jority of cases, it has been necessary to use aromatic sub-
strates for the efficient formation of the pyrylium
species.13
In the line of our extension of this protocol, we turned our
attention to the aliphatic version using 2-(enynyl)cycloal-
kenals 7 (Scheme 2). Previously, we have reported a pre-
liminary result that both 2-(6-en-1-ynyl)cycloalkenals and
2-(7-en-1-ynyl)cycloalkenals underwent cyclization into
the corresponding tricycles 8 having a diene group when
PtCl2(PPh3)2 was used as a catalyst.14 Note that the inter-
mediate A could be rearranged into the [m,6,6]-tricycle 9.
In continuing study, we found that 2-(7-en-1-ynyl)-
cycloalkenals catalyzed into the corresponding 10 when
PtCl2 was used as a catalyst.15 Here we wish to report the
PtCl2-catalyzed cyclization of 2-(6-en-1-ynyl)cycloalke-
nals into the corresponding [m,7,5]-tricycles 11 in a high-
ly efficient manner. First of all, modes of cyclization are
very sensitive to the catalytic conditions: PtCl2(PPh3)2 in
dioxane and PtCl2 in toluene resulted in the formation of
SYNLETT 2013, 24, 1433–1437
Advanced online publication: 15.05.2013
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1338805; Art ID: ST-2013-U0230-L
© Georg Thieme Verlag Stuttgart · New York

1434
J. W. Han et al.
LETTER
different products. The platinum cation is alkynophilic path to deprotonation of H4 was rather achieved simply by
and is prone to form platinum-bound pyrylium complexes using PtCl2 as a catalyst.
via the 6-endo-dig mode which would react with the
Although not known deeply, platinum–carbene com-
pendant double bond to form the intermediate A.16
plexes A, generated by PtCl2 with 2-(6-en-1-ynyl)cycloal-
kenals, could undergo deprotonation of H4 to furnish the
H
[m,7,5]-tricyclic enones 11 in excellent yields under mild
m
m
O
conditions. We initiated our studies by focusing on the se-
lective reactivity depending on structural aspects derived
from core skeletons. In order to make a systematic con-
struction of a core substructure such as 11, we first de-
signed the substrate 7a as a simplified model compound
and reexamined its cyclization reaction under a variety of
platinum-catalysis conditions (Table 1). First of all, the
reaction of 7a with PtCl2 as the catalyst proceeded well
at 80 °C in toluene to furnish products 8a and 11a in 15%
and 68% yields, respectively (Table 1, entry 1). However,
increased temperature to 120 °C under the same condi-
tions resulted in product 11a in 98% yield without form-
ing product 8a (Table 1, entry 2). It is noted that more
elevated temperature facilitates elimination of H4 from in-
termediate A to form the carbocyclic products 11. This re-
action was also working in other solvents such as DCE
and p-dioxane to afford product 11a in 48% and 42%
yields, respectively (Table 1, entries 3 and 4).Whereas the
use of tetrahydrofuran (THF) gave product 8a as a sole
product (Table 1, entry 5). In the cases of n-hexane and di-
ethyl ether, selectivity of the reaction has diminished un-
der similar reaction conditions (Table 1, entries 6 and 7).
PtCl4 exhibited similar catalytic activity toward 7a to 11a
(Table 1, entry 8). Interestingly, when PtCl2(PPh3)2 was
used as the catalyst in p-dioxane, diene product 8a was
5, 6
6
8
O
9
– H2
– H1
H4
O
H1
H2
H3
CHO
H
– H4
Pt
m
m
O
m
n
5, 6
n
11
7
Pt
A
– H3
m
6,7
O
10
Scheme 2 Four possible reaction pathways for the platinum–carbene
complex A
Depending on the natures of C–H bonds, intermediate A
might undergo subsequent reactions: deprotonation (of
H1, H2, and H3) and subsequent fragmentations leading to
8, 9, and 10, respectively. Our investigation scrutinizing a
Table 1 Platinum-Catalyzed Cyclization of Enynal 7aa
O
CHO
Pt(II)
(5 mol%)
H
H
O
+
E
E
E
E
E
E
7a
8a
11a
Entry
Catalyst
Solvent
Temp (°C)
Time (h)
Yield of products (%)b
1
2
PtCl2
toluene
toluene
DCE
80
120
100
100
100
100
100
120
100
120
6
1
8a 15, 11a 68
11a 98
PtCl2
3
PtCl2
3
11a 48
4
PtCl2
1,4-dioxane
THF
3
11a 42
5
PtCl2
12
12
8
8a 54
6
PtCl2
n-hexane
Et2O
8a 16, 11a 32
8a 15, 11a 28
11a 74
7
PtCl2
8
PtCl4
toluene
1,4-dioxane
toluene
1
9
PtCl2(PPh3)2
PtCl2(PPh3)2
2
8a 78
10
18
8a 62, 11a 28
a Catalyst loading: 5 mol%, E = CO2Et.
b Isolated yield.
Synlett 2013, 24, 1433–1437
© Georg Thieme Verlag Stuttgart · New York

LETTER
Selective Construction of Aliphatic Polycycles
1435
formed as the major product (78%; Table 1, entry 9). In were isolated in 62% and 28% yields, respectively (Table
the treatment with PtCl2(PPh3)2 in toluene, 8a and 11a 1, entry 10).
O
O
H
O
O
OBn
BnO
OTBS
OTBS
11b (97%)
7j
11j (98%)
7b
O
O
O
O
H
E
E
E
OBn
BnO
E
7c
11c (98%)
7k
11k (32%)
O
O
O
O
OTBS
TBSO
11d (98%)
OBn
BnO
11l (26%)
7d
7l
O
O
O
O
OBn
OBn
E
OBn
OBn
E
E
E
E
7e
11e (72%)
7m
11m (46%)
O
O
O
O
E
E
E
11f (80%)
7n
11n (68%)
7f
O
O
O
O
N
Ts
N
Ts
OTBS
TBSO
11o (64%)
7g
11g (54%)
7o
O
O
O
O
OBn
OBn
N
Ts
N
Ts
OBn
OBn
11h (34%)
7p
11p (46%)
7h
O
O
O
O
H
E
E
E
E
BnO
BnO
11q (96%)
11i (93%)
7q
7i
Figure 1 Products 11 of Pt-catalyzed cyclization of substrates 7
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1433–1437

1436
J. W. Han et al.
LETTER
Various substrates were subjected to optimized reactions atom onto the alkyne in a 6-endo mode. Platinum-bound
for the formation of 11a to explore the scope and limita- pyrylium species pre-A were successfully cyclized via
tion of this process (Figure 1). The substrates 7b and 7c, Huisgen-type [3+2] cycloaddition to the tetracyclic plati-
with a benzyloxy group (OCH2Ph) attached to the propar- num–carbene complexes A. These complexes subse-
gylic carbon atom, were designed and prepared to consid- quently undergo selective proton abstraction followed by
er the insertion of the highly electron-deficient carbene in elimination of H4 to produce aliphatic carbocycles 11.
complex A into a benzylic bond. In the case of these com- Here, the platinum catalyst in the presence of a ligand
pounds, elimination of H4 occurred easier than benzylic such as PtCl2(PPh3)2 may possibly serve as an elimination
C–H insertion and afforded 11b and 11c in 97% and 98% of H1 from platinum–carbene intermediate A. That is,
yield, respectively. It is noteworthy that rigid ring sizes in electronic (ligand) effect17 could result in selective H1 ab-
these polycyclic skeletons had a great impact an access to straction eventually to form the oxygen-bridged dienes 8.
a core reaction site such as platinum–carbene. The treat- Increasing olefinic tether (n = 6 or larger) in enynals 7 af-
ment of 7d and 7e, which contains two bulky benzyl ether fect the potential flexibility of platinum–carbene com-
(CH2OCH2Ph) and tert-butyldimethylsilyl ether (OTBS) plexes A which undergo insertion into a C–H bond of the
groups, under our optimized conditions provided also the β-position to afford the fused cyclopropane intermediates
corresponding carbocycles 11d and 11e in high yield. B. Their tandem rearrangement afforded diverse types of
Substrate 7f, having three- and four-atom tethers, was also spiranes 10 depending on the tethered alkenes of the
converted into the five-membered ring (3th cycle) product enynals.
11f without forming the six-membered cycle. Note that
the formation of 5-membered cycle was more easily than
In conclusion, we have developed an efficient method for
the selective construction of aliphatic polycycles through
six-membered cycle in cycloaddition process. Additional-
platinum-catalyzed cyclization of the 2-(enynyl)cycloal-
ly, nitrogen-containing substrates 7g and 7h were investi-
kenals 7 by considering electronic (ligand) and structural
gated for this transformation and smoothly underwent the
(olefinic tether) effect. The presence or absence of ligand
present reaction to afford the corresponding products 11g
and the length of olefinic tether determine the course of
the reaction after the formation of platinum–carbene com-
plex A, so that four types of aliphatic polycycles can be
formed selectively.
and 11h, respectively. In order to survey impact on reac-
tivity according to ring size (1st cycle) increased from five
to eight, substrates 7i–m were treated with the platinum-
catalyst system. The cyclization of these substrates pro-
ceeded well under suitable conditions to furnish products
11i–m in good yields, respectively. Although these reac-
tions involve several reaction pathways, all products 11
To a 5 mL test tube were placed PtCl2 (2.2 mg, 0.0084 mmol, 5.0
mol%) and dried toluene (1.0 mL) under argon atmosphere. To this
solution was cannulated a substrate 7a (56.1 mg, 0.168 mmol) solu-
tion in toluene (1.0 mL) over 1 min at 0 °C. The reaction mixture
were formed with high selectivity and high efficiency. We
next examined substrates 7n–q with remote olefinic teth-
ers (n = 6). At high temperatures, all substrates were con-
verted into the products 11n–q in high yields. However,
reduced temperature to about 80 °C under the same con-
ditions resulted in products 11n–q with forming 10n–q as
minor products.
was sealed and stirred at 120 °C for 1 h. After the reaction was com-
pleted, the reaction mixture was cooled down to r.t., quenched with
a drop of Et3N, and concentrated under reduced pressure. The con-
centrated residue was then rapidly purified by flash silica gel chro-
matography by using a 1:10 mixture of EtOAc and hexane to give
the product 11a (54.7 mg, 98% yield) as a colorless oil. All other
substrates 11b–q were subjected to similar conditions with different
reaction temperatures and times and specified in the Supporting
Information.
Based on the above experimental results, a possible mech-
anism was proposed as shown in Scheme 3. The electro-
philic activation of the alkyne moiety in enynals 7 by
platinum catalyst produces the platinum-bound pyrylium
species pre-A through the attack of the carbonyl oxygen
Compound 11a: IR (NaCl): 2978, 1731, 1672, 1647, 1566, 1242
1
cm–1. H NMR (400 MHz, CDCl3): δ = 5.95 (d, J = 2.4 Hz, 1 H),
4.25–4.13 (m, 4 H), 3.13 (dd, J = 2.4, 17.2 Hz, 1 H), 3.02–2.81 (m,
2 H), 2.65–2.57 (m, 1 H), 2.54–2.39 (m, 2 H), 2.36–2.25 (m, 3 H),
O
O
O+
O+
m
H
Pt
m
n
m
n
–
m
n
5, 6
Pt–
pre-A1
7
Pt
pre-A2
11
n = 5, 6, 7
– H4
H4
O
H1
H
H3
– H3
– H1
O
m
m
rearrangement
O
m
m
n
6, 7
5, 6
n
O
10
Pt-H
8
A
B
Pt
Scheme 3 A possible mechanism for the formation of 8, 10, and 11
Synlett 2013, 24, 1433–1437
© Georg Thieme Verlag Stuttgart · New York

LETTER
Selective Construction of Aliphatic Polycycles
1437
2.08–1.86 (m, 3 H), 1.69–1.64 (m, 1 H), 1.27–1.21 (m, 6 H). 13C
NMR (100 MHz, CDCl3): δ = 195.21, 170.75, 170.71, 155.17,
143.68, 124.60, 122.70, 61.86, 61.82, 53.96, 44.76, 34.98, 33.50,
32.07, 31.87, 30.78, 24.97, 14.07. HRMS: m/z calcd for C19H25O5
[M + H]: 333.1702; found: 333.1708.
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Financial support was provided by a grant from Hanyang Univer-
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