Total synthesis of (±)-lysergic acid, lysergol, and isolysergol by palladium-catalyzed domino cyclization of amino allenes bearing a bromoindolyl group

Article abstract of DOI:10.1021/ol8022648

(Chemical Equation Presented) Ergot alkaloids and their synthetic analogs have been reported to exhibit broad biological activity. We investigated direct construction of the C/D ring system of ergot alkaloids based on palladium-catalyzed domino cyclization of amino allenes. With this biscyclization as the key step, total synthesis of (±)-lysergic acid, (±)-lysergol, and (±)-isolysergol was achieved.

Full text of DOI:10.1021/ol8022648

ORGANIC
LETTERS
2008
Vol. 10, No. 22
5239-5242
Total Synthesis of (()-Lysergic Acid,
Lysergol, and Isolysergol by
Palladium-Catalyzed Domino Cyclization
of Amino Allenes Bearing a
Bromoindolyl Group
Shinsuke Inuki, Shinya Oishi, Nobutaka Fujii,* and Hiroaki Ohno*
Graduate School of Pharmaceutical Sciences, Kyoto UniVersity, Sakyo-ku,
Kyoto 606-8501, Japan
hohno@pharm.kyoto-u.ac.jp; nfujii@pharm.kyoto-u.ac.jp
Received September 29, 2008
ABSTRACT
Ergot alkaloids and their synthetic analogs have been reported to exhibit broad biological activity. We investigated direct construction of the
C/D ring system of ergot alkaloids based on palladium-catalyzed domino cyclization of amino allenes. With this biscyclization as the key step,
total synthesis of (()-lysergic acid, (()-lysergol, and (()-isolysergol was achieved.
Ergot alkaloids are pharmacologically important indole
alkaloids produced by the fungus ClaViceps purpurea,
which grows parasitically on rye and other grains (Figure
1).1 These alkaloids have been reported to exhibit broad
biological activity, and several synthetic derivatives such
as pergolide or bromocriptine are also used as antiprolactin
and anti-Parkinson’s disease drugs.2 Owing to their
biological importance, ergot alkaloids, particularly lysergic
Figure 1. Indole alkaloids of the ergot family.
acid (1), have been the target of many synthetic studies,
but most of the previous syntheses relied on a stepwise
linear approach for construction of the C/D ring system.3
One exception is Oppolzer’s strategy, which is based on
simultaneous construction of C/D rings by an intramo-
lecular imino-Diels-Alder reaction.3d
We expected domino cyclization of allenes of the type 5
(Scheme 1) to provide direct access to the core structure of
ergot alkaloids 4, including lysergic acid (1), lysergol (2),
and isolysergol (3). In recent years, transition-metal-catalyzed
cyclization of allenes bearing a nucleophilic functionality has
been widely used for construction of various types of
heterocycles, and several efficient domino cyclizations of
(1) For isolation of lysergic acid, see: (a) Jacobs, W.; Craig, L. J. Biol.
Chem. 1934, 104, 547–551. (b) Stoll, A.; Hofmann, A.; Troxler, F. HelV.
Chim. Acta 1949, 32, 506–521. For isolation of lysergol, see: (c) Agurell,
S. Acta Pharm. Suec. 1965, 2, 357–374. For isolation of isolysergol, see.
(d) Agurell, S. Acta Pharm. Suec. 1966, 3, 7–10.
(2) (a) Ninomiya, I.; Kiguchi, T. In The Alkaloids; Brossi, A., Ed.;
Academic Press: San Diego, CA, 1990; Vol. 38, pp 1-156. (b) The Merck
Index, 12th ed.; Merck and Co., Inc.: Whitehouse Station, NY, 1996; pp
231. (c) Somei, M.; Yokoyama, Y.; Murakami, Y.; Ninomiya, I.; Kiguchi,
T.; Naito, T. In The Alkaloids; Cordell, G. A., Ed.; Academic Press: San
Diego, CA, 2000; Vol. 54, pp 191-257.
10.1021/ol8022648 CCC: $40.75
Published on Web 10/29/2008
2008 American Chemical Society

constructed by Claisen rearrangement of enol ether 6, which
could be readily obtained by conjugate addition of propargyl
alcohol 7 to methyl propiolate followed by reduction/
protection.
Scheme 1. Retrosynthetic Analysis of 4
Scheme 2. Synthesis of Allenic Amides 15 and 16
amino allene derivatives have also been reported.4,5 However,
the reaction of allenes having an aryl halide and amino group
at both ends of internal allenes is unprecedented. We describe
herein a new entry to the ergot alkaloids skeleton using a
palladium-catalyzed domino cyclization of amino allenes 5
bearing a protected 4-bromoindol-3-yl group. Total synthesis
of lysergic acid, lysergol, and isolysergol using this strategy
is also presented.
Retrosynthetic analysis of the amino allenes 5 is shown
in Scheme 1. We envisioned that the allene unit of 5 can be
(3) For synthesis of lysergic acid, see: (a) Kornfeld, E. C.; Fornefeld,
E. J.; Kline, G. B.; Mann, M. J.; Morrison, D. E.; Jones, R. G.; Woodward,
R. B. J. Am. Chem. Soc. 1956, 78, 3087–3114. (b) Julia, M.; LeGoffic, F.;
Igolen, J.; Baillarge, M. Tetrahedron Lett. 1969, 10, 1569–1571. (c)
Armstrong, V. W.; Coulton, S.; Ramage, R. Tetrahedron Lett. 1976, 17,
4311–4314. (d) Oppolzer, W.; Francotte, E.; Ba¨ttig, K. HelV. Chim. Acta
1981, 64, 478–481. (e) Rebek, J., Jr.; Tai, D. F. Tetrahedron Lett. 1983,
24, 859–860. (f) Kiguchi, T.; Hashimoto, C.; Naito, T.; Ninomiya, I.
Heterocycles 1982, 19, 2279–2282. (g) Kurihara, T.; Terada, T.; Yoneda,
R. Chem. Pharm. Bull. 1986, 34, 442–443. (h) Cacchi, S.; Ciattini, P. G.;
Morera, E.; Ortar, G. Tetrahedron Lett. 1988, 29, 3117–3120. (i) Hen-
drickson, J. B.; Wang, J. Org. Lett. 2004, 6, 3–5. (j) Moldvai, I.; Temesva´ri-
´
Major, E.; Incze, M.; Szentirmay, E.; Ga´cs-Baitz, E.; Sza´ntay, C. J. Org.
Preparation of the requisite allenic amides of the type 5
for the palladium-catalyzed domino cyclization is outlined
in Scheme 2. 3-(Bromomethyl)indole 9 is easily accessible
from commercially available 4-bromoindole 8.6 Lithiation
and addition of 1,3-dithiane 107 to the bromide 9 gave
thioacetal 11 in 96% yield. Subsequent functional-group
modifications, including hydrolysis of the thioacetal,8 reduc-
tion, desilylation, and conjugate addition to methyl propiolate,
provided the enoate 12.9 The propargyl vinyl ether 13 was
obtained by DIBAL reduction and silylation of 12. Claisen
rearrangement under thermal conditions (m-xylene, 170 °C)
gave the desired allenic alcohol 14 (a:b ) ca. 33:67) in only
38% yield (Table 1, entry 1). Microwave irradiation10 in
CHCl3 dramatically improved the yield to 82% (entry 2).11
Furthermore, use of 5 mol % of gold-oxo complex
[(Ph3PAu)3O]BF4 resulted in 78% yield of 14, in favor of
the opposite diastereomer (a:b ) ca. 80:20, entry 3).12
Mitsunobu reaction of 14 with NsNH2 or TsNHFmoc13
(followed by piperidine treatment) gave N-nosyl and N-
tosylamide derivatives 15 and 16 (a:b ) 80:20), respectively.
We next investigated construction of the ergot alkaloid
skeleton via the palladium-catalyzed domino cyclization
Chem. 2004, 69, 5993–6000. For synthesis of lysergol and isolysergol, see:
(k) Kiguchi, T.; Hashimoto, C.; Ninomiya, I. Heterocycles 1985, 23, 1377–
1380. (l) Ninomiya, I.; Hashimoto, C.; Kiguchi, T.; Naito, T.; Barton,
D. H. R.; Lusinchi, X.; Milliet, P. J. Chem. Soc., Perkin Trans. 1 1990,
707–713
.
(4) For a recent book and review of allene, see: (a) Zimmer, R.; Dinesh,
C. U.; Nandanan, E.; Khan, F. A. Chem. ReV. 2000, 100, 3067–3125. (b)
Bates, R. W.; Satcharoen, V. Chem. Soc. ReV. 2002, 31, 12–21. (c) Mandai,
T. In Modern Allene Chemistry; Krause, N., Hashmi, A. S. K., Eds.; Wiley-
VCH: Weinheim, 2004; Vol. 2, pp 925-972. (d) Ohno, H. Chem. Pharm.
Bull. 2005, 53, 1211–1226. (e) Ma, S. Chem. ReV. 2005, 105, 2829–2871.
(f) Widenhoefer, R. A. Chem. Eur. J. 2008, 14, 5382–5391
.
(5) For related examples of Pd-catalyzed cyclization of allenes, see: (a)
Larock, R. C.; Berrios-Peña, N. G.; Fried, C. A. J. Org. Chem. 1991, 56,
2615–2617. (b) Davies, I. W.; Scopes, D. I. C.; Gallagher, T. Synlett 1993,
85–87. (c) Rutjes, F. P. J. T.; Tjen, K. C. M. F.; Wolf, L. B.; Karstens,
W. F. J.; Schoemaker, H. E.; Hiemstra, H. Org. Lett. 1999, 1, 717–720. (d)
Ohno, H.; Toda, A.; Miwa, Y.; Taga, T.; Osawa, E.; Yamaoka, Y.; Fujii,
N.; Ibuka, T. J. Org. Chem. 1999, 64, 2992–2993. (e) Hiroi, K.; Hiratsuka,
Y.; Watanabe, K.; Abe, I.; Kato, F.; Hiroi, M. Synlett 2001, 263–265. (f)
Ohno, H.; Anzai, M.; Toda, A.; Oishi, S.; Fujii, N.; Tanaka, T.; Takemoto,
Y.; Ibuka, T. J. Org. Chem. 2001, 66, 4904–4914. (g) Grigg, R.; Ko¨ppen,
I.; Rasparini, M.; Sridharan, V. Chem. Commun. 2001, 964–965. (h) Ohno,
H.; Miyamura, K.; Takeoka, Y.; Tanaka, T. Angew. Chem., Int. Ed. 2003,
42, 2647–2650. (i) Ohno, H.; Takeoka, Y.; Kadoh, Y.; Miyamura, K.;
Tanaka, T. J. Org. Chem. 2004, 69, 4541–4544. (j) Ohno, H.; Miyamura,
K.; Mizutani, T.; Kadoh, Y.; Takeoka, Y.; Hamaguchi, H.; Tanaka, T. Chem.
Eur. J. 2005, 11, 3728–3741. (k) Ma, S.; Yu, F.; Li, J.; Gao, W. Chem.
Eur. J. 2007, 13, 247–254. (l) Okano, A.; Mizutani, T.; Oishi, S.; Tanaka,
T.; Ohno, H.; Fujii, N. Chem. Commun. 2008, 3534–3536
.
5240
Org. Lett., Vol. 10, No. 22, 2008

To obtain some mechanistic insight of the domino cy-
clization, diastereomerically pure 15a and 15b(obtained by
careful HPLC separation of 14 followed by Mitsunobu
reaction) were subjected to the reaction conditions shown
in entry 4 (Table 2). Domino cyclization of the major isomer
15a gave an 83:17 diastereomixture, in 78% yield, in which
the major cyclized product 17a predominated (Scheme 3).
In contrast, reaction of the minor isomer 15b favored the
diastereomer 17b (a:b ) 21:79) in 67% yield.
Table 1. Claisen Rearrangement of Propargyl Ether 13
entry
conditionsa
yield (%)b
dr (a:b)c
1
2
m-xylene, 170 °C, 50 min
38
82
ca. 33:67
ca. 33:67
CHCl3, MW, 120 °C, 12 min
then 150 °C, 12 min
3
[(Ph3PAu)3O]BF4 (5 mol %)
78
ca. 80:20
CH2Cl2, 40 °C, 10 h
a MW ) microwave irradiation. b Isolated yields after reduction with
NaBH4. Determined by HPLC and H NMR analysis.
c
1
Scheme 3. Palladium-Catalyzed Domino Cyclization of
Diastereomerically Pure Substrates
Table 2. Palladium-Catalyzed Domino Cyclizationa
entry
Pd/ligand
solvent base yield (%)b dr (a:b)c
1
Pd(PPh3)4
DMF
DMF
DMF
DMF
toluene K2CO3
dioxane K2CO3
DMSO K2CO3
DMF
DMF
DMF
Na2CO3
Cs2CO3
K2CO3
K2CO3
31
41
84:16
75:25
73:27
74:26
NDf
80:20
74:26
88:12
92:8
2
Pd(PPh3)4
3
Pd(PPh3)4
83
4d
5
Pd(PPh3)4
78
Scheme 4
.
Proposed Mechanism for Domino Cyclization
Pd(PPh3)4
trace
68
6
Pd(PPh3)4
7d
8
Pd(PPh3)4
68
Pd(OAc)2/PPh3
PdCl2(dppf)
Pd(OAc)2/P(o-tol)3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
61
9
10
41
20
>95:5
72:28
87:13
11e Pd(OAc)2/rac-BINAP DMF
31
12g Pd(PPh3)4
DMF
65
a Reactions were carried out using a diastereomixture of 15 or 16 (a:b
) 80:20) at 0.06 M for 2.5-5 h. b Isolated yields. c Determined by 1H NMR
analysis. d Reaction was performed at 120 °C. e Reactions were carried out
using Pd (5 mol %) and ligand (5 mol %) f Not determined. g Reaction was
carried out using a substrate 16 at 120 °C.
(Table 2). The reaction was conducted using a 80:20
diastereomixture of 15 and 16 because separating the
diastereomeric mixtures resulting from Claisen rearrangement
was difficult. Reaction of 15 with 5 mol % of Pd(PPh3)4
and Na2CO3 in DMF at 100 °C afforded desired product 17
in 31% yield (a:b ) 84:16, entry 1). Among the several bases
investigated, K2CO3 has proven to be the most effective to
give 83% of 17 as a 73:27 diastereomixture (entry 3).14
Although the reaction at 120 °C slightly decreased the yield
of the desired product, unidentified side products were easily
removed from the desired product 17 (entry 4). Changing
the solvent from DMF to toluene, dioxane or DMSO did
not enhance the yield of desired product (entries 5-7).
Further screening using Pd(OAc)2/PPh3 (entry 8), PdCl2(dppf)
(entry 9), Pd(OAc)2/P(o-tol)3 (entry 10) and Pd(OAc)2/rac-
BINAP (entry 11) was done. As diastereoselectivity im-
proved, yield of desired product decreased (entries 3, 8-10),
except for using Pd(OAc)2/rac-BINAP (entry 11). When the
N-tosyl derivative 16 was employed, desired product 18 was
isolated in 65% yield with good diastereoselectivity (87:13,
entry 12).15
A rationale for stereoselectivities of the domino cyclization
of internal amino allenes is depicted in Scheme 4. This domino
cyclization could proceed through two pathways: (1) carbopal-
ladation4,5 and (2) amidopalladation.5b,k Because of a steric
reason, carbopalladation of indolylpalladium(II) bromide,
formed in situ by oxidative addition of the bromoindole
moiety to Pd(0), would proceed through 6-exo type cycliza-
tion as depicted in A to generate η3-allylpalladium complex
B. The second cyclization by the nosylamide group in an
anti manner then gives the minor isomer 17b. On the other
hand, coordination of the indolylpalladium(II) to the allenic
moiety would promote anti attack of the nosylamide group
as shown in C (amidopalladation pathway) to give a
palladacycle D, which gives the isomer 17a by reductive
elimination. Predominant formation of 17a can be rational-
ized by considering the strained bicyclic structure A in the
carbopalladation step.
Org. Lett., Vol. 10, No. 22, 2008
5241

hydrolysis of 20 with NaOH, accompanying isomerization
to the favorable isomer.3i Physical data were in agreement
with those of natural and synthetic lysergic acid, lysergol
and isolysergol reported in the literature.3i,j,l
Scheme 5. Total Synthesis of Isolysergol (3), Lysergol (2), and
Lysergic Acid (1)
In conclusion, we developed a novel entry to direct
construction of an ergot alkaloid skeleton based on pal-
ladium-catalyzed domino cyclization of amino allenes. With
this biscyclization as the key step, total synthesis of lysergic
acid, lysergol and isolysergol was achieved.
Acknowledgment. This work was supported by a Grant-
in-Aid for Encouragement of Young Scientists (A) (H.O.)
from the Ministry of Education, Culture, Sports, Science and
Technology of Japan, and Targeted Proteins Research
Program. S.I. is grateful to Research Fellowships of the Japan
Society for the Promotion of Science (JSPS) for Young
Scientists. Appreciation is expressed to Fundamental Studies
in Health Sciences of the National Institute of Biomedical
Innovation (NIBIO).
Supporting Information Available: Experimental pro-
cedures and characterization data for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL8022648
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With the ergot alkaloid derivatives 17 and 18 bearing the
requisite functionalities in hand, the final stage was set for
the completion of the total synthesis of lysergic acid, lysergol,
and isolysergol (Scheme 5). Deprotection of the Ns group
of 17 and N-methylation gave a separable mixture of
diastereomers, each of which was readily converted into
isolysergol (3) and lysergol (2) by removal of TIPS and Ts
groups. We chose tosylamide 18 as the precursor of lysergic
acid.16 Cleavage of the TIPS group of 18, oxidation of the
resulting primary alcohol by standard protocol, and esteri-
fication with TMSCHN2 gave the corresponding methyl ester
19a (62%, 4 steps).17 Removal of tosyl group with sodium
naphthalenide and subsequent N-methylation led to a dias-
tereomixture of methyl isolysergate 20a and lysergate 20b
(35:65). Total synthesis of lysergic acid was completed by
(10) Trost, B. M.; Dong, G.; Vance, J. A. J. Am. Chem. Soc. 2007, 129,
4540–4541.
(11) The relative configuration of 14a was determined by NOE analyses
of the corresponding pyran, obtained by Au-catalyzed stereospecific
cyclization18 (see Supporting Information).
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15979.
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(14) The relative configurations of 17a and 17b were confirmed by their
conversion to isolysergol and lysergol, respectively.
(15) The relative configurations of 18a and 18b were confirmed by
derivatization of 17a and 17b to the same compounds, respectively (see
Supporting Information).
(16) Cleavage of nosyl group in the ester derived from 17 was less
effective (20-30% yield) under standard conditions.
(17) The relative configuration of 19a was confirmed by conversion to
18a (see Supporting Information).
(18) Gockel, B.; Krause, N. Org. Lett. 2006, 8, 4485–4488.
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