Prolinamide/PPTS-catalyzed Hajos-Parrish annulation: Efficient approach to the tricyclic core of cylindricine-type alkaloids

Article abstract of DOI:10.1055/s-0028-1083542

A series of Wieland-Miescher ketone analogues bearing highly functionalized substituents are efficiently constructed in high enantioselectivities and good yields using catalytic amounts of prolinamide and PPTS. We have successfully utilized this reaction as a key step to synthesize the tricyclic core of cylindricine type alkaloids. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0028-1083542

LETTER
2831
Prolinamide/PPTS-Catalyzed Hajos–Parrish Annulation: Efficient Approach
to the Tricyclic Core of Cylindricine-Type Alkaloids
Approachtothe
Tricyclic
a
Core of
C
o
ylindricine-T
-
ype
A
lk
M
aloids ing Zhang, Min Wang, Yong-Qiang Tu,* Chun-An Fan, Yi-Jun Jiang, Shu-Yu Zhang, Fu-Min Zhang*
State Key Laboratory of Applied Organic Chemistry & Department of Chemistry, Lanzhou University, Lanzhou 730000, P. R. of China
Fax +86(931)8912582; E-mail: tuyq@lzu.edu.cn
Received 20 June 2008
and 5–6-fused bicyclic species in the early 1970’s.2 From
Abstract: A series of Wieland–Miescher ketone analogues bearing
then on, various studies have been performed toward
screening the structurally diverse substrates and develop-
ing new catalysts to improve the enantioselectivity and re-
highly functionalized substituents are efficiently constructed in
high enantioselectivities and good yields using catalytic amounts of
prolinamide and PPTS. We have successfully utilized this reaction
as a key step to synthesize the tricyclic core of cylindricine type al- activity, but most of them only focused on establishing the
kaloids.
quaternary fused bicyclic system with simple angular me-
thyl group.3 In connection with our recent synthetic inter-
Key words: prolinamide, Hajos–Parrish annulation, Wieland–Mie-
ests in the polycycle-fused natural products,4 we realize
scher ketone analogues, cylindricine type alkaloids, intramolecular
Schmidt reaction
that the efficient asymmetric construction of the fused bi-
cyclic systems with functionalized chains rather than the
simple methyl or ethyl group will give more impact to ex-
The quaternary 6–6- and 5–6-fused bicyclic systems are peditious synthesis of some natural products with the key
core structures incorporated in a variety of biologically tricycle-fused skeletons (Scheme 1),5 as well as some im-
significant pharmaceutical molecules and natural prod- portant pharmaceutical intermedicates.6 To our knowl-
ucts, especially in alkaloids, steroids, and terpenoids, and edge, however, only few examples about the
thus frequently demonstrated as key building blocks in the enantioselective access to this kind of synthetic structures
total synthesis of these interesting molecules.1 The enan- were revealed.7 Recently, our studies on this subject have
tioselective construction of these bicyclic systems has resulted in more efficient establishment of this kind of
been attracting high attentions since Hajos and Parrish de- fused bicyclic ring system with higher enantioselectivity
veloped L-proline-catalyzed asymmetric Robinson annu- (up to 88% ee) by using simple and easily available
lation strategy to give access to chiral C8a-methyl 6–6- prolinamide and pyridinium p-toluenesulfonate (PPTS) as
O
O
Michael addition
O
Coupling
reaction
Schmidt
reaction
O
O
N
O
O
CO2Me
Aldol reaction
O
O
O
O
Scheme 1 Proposed structural diversity of tricyclic systems
SYNLETT 2008, No. 18, pp 2831–2835
1
4
.1
1
.2
0
0
8
Advanced online publication: 15.10.2008
DOI: 10.1055/s-0028-1083542; Art ID: W09908ST
© Georg Thieme Verlag Stuttgart · New York

2832
X.-M. Zhang et al.
LETTER
TBDPS
Besides, several L-proline derivatives 4 and 5a–c were
also examined using DMF as the solvent and PPTS as an
additive to improve the ee values and the yields of the cur-
rent reaction. In the review of some literatures on the re-
actions catalyzed by L-proline and its derivatives, the
catalyst 4 has been successfully used in O-nitroso-aldol–
Michael reaction and Mannich reaction, and 5a–c have
shown high entioselectivity for direct Aldol reactions.8–10
The further screening results are tabulated in entries 2–11
of Table 1. The employment of 4 only gave the low ee val-
ue (61%) and low yield (42%).9 However, using L-prolin-
amide (5a) or its derivatives 5b and 5c (entries 3–5) could
dramatically improve the ee values and afford reasonable
yields.10 So L-prolinamide (5a) was then subjected to the
optimization of reaction media (entries 6–8). From the ob-
tained results, it could be seen that the reaction solvent
had an obvious influence on the enantioselectivity and re-
activity. For example, using DMSO afforded a higher ee
(70%) as well as a better yield (50%), and employing
DCE gave much higher ee (81%) in a similar yield (54%).
Gratifyingly, while using MeCN as solvent, the best ee
(87%) and an improved yield (65%) could be afforded
(entry 8). Finally, the effect of the additive was also exam-
ined. The parallel reaction without any additive only gave
the desired product in low yield (32%), but could maintain
the enantioselectivity (80% ee, entry 9). While using other
additive such as PTS and HClO4, there was no improve-
ment for the ee and the yield (entries 10 and 11).
O
O
O
N
N
H
N
H
H
OH
OH
NH2
4
3
5a
H
N
H
N
S
N
H
N
H
O
O
O
O
5b
5c
Figure 1 Catalysts screened in asymmetric Hajos–Parrish annula-
tion
combined catalyst, and we have successfully utilized this
method to achieve an efficient synthesis of the tricyclic
core of cylindricine-type alkaloids. Herein we present our
experimental results in detail.
Initially, the substrate 1a with a C2-propanate ester chain
was chosen as a model in the asymmetric annulation to
construct the corresponding 6–6-fused bicyclic units by
using the easily available catalyst L-proline (3, 0.3 equiv,
Figure 1) with PPTS (0.3 equiv) in DMF at 50 °C. How-
ever, the expected 6–6-fused bicyclic product 2a was ob-
tained only in 66% ee and 45% yield (Table 1, entry 1).
Table 1 Optimization of Hajos–Parrish Reaction Conditions of 1aa
CO2Me
O
cat. (30 mmol%)
additive (30 mmol%)
Table 2 L-Prolinamide-Catalyzed Hajos–Parrish Reaction of Vari-
O
O
ous 1,3-Dionesa
solvent (0.3 M)
time, 50 °C
O
O
R1
R1
O
O
O
CO2Me
2a
L-prolinamide, PPTS
MeCN (0.3 M)
R2
O
1a
R1
R1
R2
O
2
1
Entry Catalyst
Solvent
Additive Time Yield
ee
(h)
(%)b
(%)c,d
Entry Substrate
Product Yield ee
(%)b,e (%)c–e
1
2
3
DMF
PPTS
PPTS
PPTS
PPTS
PPTS
PPTS
PPTS
PPTS
None
PTS
24
45
42
43
32
46
50
54
65
32
42
35
66
61
79
78
76
70
81
87
87
80
80
4
DMF
144
144
144
72
1
2
1a, R1 = H, R2 = CH2CH2CO2Me 2a
1b, R1 = H, R2 = CH2CH2CO2Et
2b
1c, R1 = H, R2 = CH2CH2CO2n-Bu 2c
1d, R1 = Me, R2 = CH2CH2CO2Me 2d
1e, R1 = H, R2 = allyl
1f, R1 = H, R2 = CH2CBr=CH2
1g, R1 = H, R2 = CH2CH2CN
1h, R1 = H, R2 = Me
65
51
57
41
56
46
59
68
70
56
87
82
82
70
85f
83
88
87
86
86
3
5a
5b
5c
5a
5a
5a
5a
5a
5a
DMF
4
DMF
3
5
DMF
4
6
DMSO
DCE
96
5
2e
2f
2g
2h
2i
7
144
144
144
144
144
6
8
MeCN
MeCN
MeCN
MeCN
7
9
8
10
11
9
1i, R1 = H, R2 = Et
1j, R1 = H, R2 = Bn
HClO4
10
2j
a Reaction conditions: substrate (0.3 mmol), catalyst (30 mmol%), ad-
ditive (30 mmol%), solvent (1 mL).
a See Experimental Section.
b Isolated yield.
b Isolated yield.
c Determined by chiral HPLC (see the Supporting Information for detail).
d The absolute configuration was not determined.
e Reaction conditions: L-prolinamide (30 mmol%), PPTS (30
mmol%), MeCN (0.3 M), 50 °C.
c Determined by chiral HPLC analysis using a Chiralcel OD column.
d The absolute configuration was not determined.
f The absolute configuration R was assigned by the comparison of the
known optical rotation value.
Synlett 2008, No. 18, 2831–2835 © Thieme Stuttgart · New York

LETTER
Approach to the Tricyclic Core of Cylindricine-Type Alkaloids
2833
O
S
O
a
S
ref. 3e
65%
b
O
O
O
O
O
O
O
10
8
9
S
S
e, f
d
g
c
N
O
O
O
11
12
13
14
HO
HO
N3
[H]
[H]
H
favored
H
h
N
N
X
N
O
disfavored
O
15 X = O
16 X = S
i
Scheme 2 Reagents and conditions: (a) D-prolinamide (0.3 equiv), PPTS (0.3 equiv), MeCN, 50 °C, 60%, 83% ee; (b) HS(CH2)3SH (1.1
equiv), TMSCl (0.3 equiv), CHCl3, r.t., 75%; (c) BH3·SMe2 (1 equiv), cyclohexene (2 equiv), THF, 0 °C, then NaBO3·4H2O, H2O, r.t., 95%;
(d) W-2 Raney Ni, EtOH, r.t., 82%; (e) MsCl (1.1 equiv), Et3N (1.1 equiv), CH2Cl2, r.t.; (f) NaN3 (5 equiv), DMF, r.t., 87% over 2 steps; (g)
BF3·OEt2, r.t., 70%; (h) H2, Pd/C, EtOAc, r.t., 83%; (i) Lawesson’s reagent (0.75 equiv), toluene, reflux, 96%.
Hajos–Parrish reaction
Under the optimized conditions, a variety of 1,3-cyclo-
hexanedione substrates were examined. As shown in
O
H
H
Table 2, the substrates bearing various of propanate esters
B
N
A
*
1b–c (entries 1–3) gave the expected Wieland–Miescher
ketone analogues 2b–c in high enantioselectivities and
good yields, while 1d bearing two methyl groups at the
six-membered ring gave a lower ee of 70% and a yield of
40% (entry 4). Similar results were also obtained in the
cases of dione substrates 1e and 1f with allyl or 2-bromo-
allyl group, respectively (entries 5 and 6).11 In addition,
the cyano dione 1g also proved to be well effective in this
reaction, and gave the highest ee value (88%, entry 7).
Furthermore, the well-known substrate, 2-methyl-2-(3-
oxobutyl)-1,3-cyclohexanedione (1h), was examined (en-
try 8), and it proceeded smoothly to afford the expected
product 2h in 87% ee in 68% yield, being better than the
very recent literature report (78% ee, 57% yield).3b Final-
ly, when ethyl- or benzyl-substituted substrates 1i and 1j
were subjected to this annulation, similar ee values could
also be obtained (entries 9 and 10). These results indicated
that the substitution at the six-membered ring of the 1,3-
dione substrate has some influence on the enantioselectiv-
itiy and the yield of this reaction, but the property of func-
tionalized carbon chain at C-2 position has no significant
influence.
R1
N
O
C6H13
N
C
R2
R
intramolecular Schmidt reaction
R = CH2Cl: cylindricine A (6a)
R = CH2OH: cylindricine C (6b)
R = CH2OMe: cylindricine D (6c)
R = CH2OAc: cylindricine E (6d)
R = CH2SCN: cylindricine F (6e)
R
R
R
1 = C6H13, R2 = CH2OH (7a)
1 = Bu, R2 = CH2OH (7b)
1 = C6H13, R2 = H (7c)
Figure 2 Some cylindricines and lepadiformines
6a–e, along with the lepadiformines 7a–c isolated by
Biard’s group in 1994,13 possess a novel tricyclic core and
some other important biological activities.13 Due to their
unique structures, lots of synthetic efforts appeared after
their isolation and most of them focused on strategies to
form the key tricyclic ring system.14
As shown in Figure 2, our strategy was featured by the
current prolinamide/PPTS-catalyzed Hajos–Parrish annu-
lation and an intramolecular Schmidt reaction,15 establish-
ing the fused A,B,C-ring system.
Our synthesis commenced with substrate 8 (Scheme 2),
which could be easily accessed from 1,3-cyclopentadione
in a higher yield than Landais and Vincent’s conditions
(up to 65% over two steps).16
After we had established this kind of fused bicyclic ring
system with higher enantioselectivity, we tried to utilize
this method to construct the core tricyclic structure of cy-
lindricine-type alkaloids. The cylindricines were isolated
by Blackman from the Tasmanian ascidian Clavelina cy-
lindrica in 1993.12 Further investigations showed their
toxicity in a brine shrimp assay.12 Part of these alkaloids
When we treated 8 with a catalytic amount of D-prolin-
amide under the optimized conditions, the desired inter-
mediate 9 could be obtained with 83% ee in 60% yield.
Direct Brown hydroboration of 9 gave a sluggish reaction
Synlett 2008, No. 18, 2831–2835 © Thieme Stuttgart · New York

2834
X.-M. Zhang et al.
LETTER
Acknowledgment
We are grateful for the financial support of the NSFC (Nos.
20672048, 20621091 and 20732002) and the Chang Jiang Scholars
Program. We thank Mr. Yongliang Shao for X-ray analysis of com-
pound 16.
References and Notes
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Figure 3 X-ray crystal structure of 16
mixture.17 Therefore we first protected the carbonyl group
of a,b-unsaturated enone of 9 to afford 10,18 then under
Kabalka’s conditions,17 the Brown hydroboration elabo-
rated 11 in good chemo- and regioselectivity as well as
high yield (up to 95%). Reduction of the thioether 11 with
W-2 Raney Ni in EtOH delivered 12. Conversion of the
hydroxyl group of 12 to its methylsulfonate ester, fol-
lowed by the substitution of azide provided 13 in 87%
yield over two steps. At this stage, we attempted the cru-
cial intramolecular Schmidt reaction to construct the third
ring.15 A variety of experiments were conducted, with a
particular emphasis on the Lewis acid and the solvent.
Both TiCl4 in CH2Cl2 and BF3·OEt2 in Et2O or CH2Cl2
could promote the reaction, but the yield was low (around
40–50%). Finally, we found that when the reaction was
carried out with BF3·OEt2 without any solvent, the lactam
product 14 could be obtained in up to 70% yield. Catalytic
hydrogenation of the C–C double bond of 14 afforded the
tricyclic core 15.
As we expected, the catalytic hydrogenation proceeded
from a less hindered side of the double bond. The relative
configuration between A and B rings in 15 was deter-
mined to be same as the natural occurring cylindricine-
type alkaloids, and it was further confirmed by X-ray
analysis of 16 (Figure 3) which was derived from 15 un-
der Lawesson’s conditions.19,20
In summary, we have demonstrated that prolinamide/
PPTS is an effective catalyst for the Hajos–Parrish reac-
tion. This desymmetric annulation can be applied to the
efficient and enantioselective synthesis of 6–6- or 5–6-
fused Wieland–Miescher ketone analogues, especially
bearing the functionalized quaternary angular chain. Fi-
nally, we have successfully applied this method to an ex-
peditious asymmetric construction of the tricyclic core of
cylindricines.
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Synlett 2008, No. 18, 2831–2835 © Thieme Stuttgart · New York

LETTER
Approach to the Tricyclic Core of Cylindricine-Type Alkaloids
2835
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