Total synthesis of the natural enantiomer of (-)-lepadiformine and determination of its absolute stereochemistry

Article abstract of DOI:10.1002/1521-3773(20020816)41:16<3017::AID-ANIE3017>3.0.CO;2-1

A short synthesis: The naturally occurring (-)-lepadiformine ((-)-3) was prepared in nine steps in 31.4% overall yield. The key step involved the formation of 2 by the spirocyclization of the N-acyliminium ion generated from 1. Furthermore, HPLC analysis

Full text of DOI:10.1002/1521-3773(20020816)41:16<3017::AID-ANIE3017>3.0.CO;2-1

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has antiarrhythmic properties.[2] On the basis of extensive
NMR spectroscopic experiments, its structure and relative
stereochemistry were reported as the zwitterion 1 (Scheme 1)
with a novel azatricyclic skeleton.[1] Although its specific
rotation value in a chloroform solution was reported to be
zero, it was believed that lepadiformine is not racemic.
However, the absolute configuration has remained hitherto
unknown. The unique structural features and biological
significance of this novel marine alkaloid have made it an
important target for synthesis. Weinreb and co-workers[3]
reported the synthesis of the structure 1 proposed for
lepadiformine and found that the synthetic material exists as
a nonzwitterionic form 2 and that neither the free amine nor
its hydrochloride salt corresponds to the natural product.
Moreover, Pearson et al.[4] synthesized the other three C3/C5
diastereomers of 2 and found that they different from
lepadiformine (Scheme 1). Following these synthetic efforts,
we recently published the total synthesis of compound 3 in the
racemic form, based on an intramolecular acylnitroso Diels
Alder-based approach and found by spectral comparison that
the corresponding hydrochloride salt was identical to the
reported natural product.[5] We thus concluded that the
originally assigned structure of lepadiformine was actually
that of the hydrochloride salt and that its relative stereo-
chemistry should be revised to be that of 3 (Scheme 1).
[6] a) For an excellent review of Lewis acid mediated radical reactions,
see: P. Renaud, M. Gerster, Angew. Chem. 1998, 110, 2704; Angew.
Chem. Int. Ed. 1998, 37, 2562; for recent examples of Lewis acid
promoted atom-transfer radical reactions, see: b) C. L. Mero, N. A.
Porter, J. Am. Chem. Soc. 1999, 121, 5155; c) A. J. Clark, F. D. Campo,
R. J. Deeth, R. P. Filik, S. Gatard, N. A. Hunt, D. Lastecoueres, G. H.
Thomas, J.-B. Verlhac, H. Wongtap, J. Chem. Soc. Perkin Trans. 1 2000,
671; d) O. Kitagawa, H. Fujiwara, T. Taguchi, Tetrahedron Lett. 2001,
42, 2165; e) O. Kitagawa, Y. Yamada, H. Fujiwara, T. Taguchi, J. Org.
Chem. 2002, 67, 922.
[7] For details about side products, see Supporting Information.
[8] For examples on the use of 4-ä molecular sieves in asymmetric
reactions, see: a) R. M. Hanson, K. B. Sharpless, J. Org. Chem. 1986,
51, 1922; b) K. Narasaka, N. Iwasawa, M. Inoue, T. Yamada, M.
Nakashima, J. Sugimoto, J. Am. Chem. Soc. 1989, 111, 5340; c) K.
Mikami, M. Terada, T. Nakai, J. Am. Chem. Soc. 1990, 112, 3949;
d) D. A. Evans, M. M. Faul, M. T. Bilodeau, B. A. Anderson, D. M.
Barnes, J. Am. Chem. Soc. 1993, 115, 5328; e) T. Iida, N. Yamamoto,
H. Sasai, M. Shibasaki, J. Am. Chem. Soc. 1997, 119, 4783.
[9] For recent reviews on lanthanide Lewis acids catalysis, see: a) S.
Kobayashi, Synlett 1994, 689; b) M. Shibasaki, K.-I. Yamada, N.
Yoshikawa in Lewis Acids in Organic Synthesis, Vol. 2 (Ed: H.
Yamamoto), Wiley, New York, 2000, chap. 20, p. 911.
[10] For recent reviews on the use of C2-symmetric chiral bisoxazoline
ligands in asymmetric catalysis, see: a) A. Pfaltz, Acta Chem. Scand.
1996, 50, 189; b) A. K. Ghosh, P. Mathivanan, J. Cappiello, Tetrahe-
dron: Asymmetry 1998, 9, 1; c) J. S. Johnson, D. A. Evans, Acc. Chem.
Res. 2000, 33, 325.
[11] a) For a recent review on the effect of additives on asymmetric
reactions, see: E. M. Vogl, H. Grˆger, M. Shibasaki, Angew. Chem.
1999, 111, 1672; Angew. Chem. Int. Ed. 1999, 38, 1570; for examples of
the additive effect in chiral ytterbium triflate mediated asymmetric
reactions, see: b) S. Kobayashi, H. Ishitani, J. Am. Chem. Soc. 1994,
116, 4083; c) S. Kobayashi, H. Ishitani, I. Hachiya, M. Araki,
Tetrahedron 1994, 50, 11623; d) T. Saito, M. Kawamura, J.-I.
Nishimura, Tetrahedron Lett. 1997, 38, 3231; e) M. Kawamura, S.
Kobayashi, Tetrahedron Lett. 1999, 40, 3213.
7a
11a
N+
3
N
[12] D. A. Evans, Z. K. Sweeney, T. Rovis, J. S. Tedrow, J. Am. Chem. Soc.
2001, 123, 12095.
5
H
HO
O–
2
1
HO
Total Synthesis of the Natural Enantiomer of
(ꢀ)-Lepadiformine and Determination of Its
Absolute Stereochemistry**
N
N
H
H
Hideki Abe, Sakae Aoyagi, and Chihiro Kibayashi*
HO
Lepadiformine was isolated by Biard et al. in 1994from the
marine tunicates Clavelina lepadiformis collected in Tunisia[1]
and from Clavelina moluccensis found along the Djibouti
coast.[2] It has been shown to exhibit moderate cytotoxic
activity against various tumor cell lines in vitro.[1] Moreover, a
recent study indicated that lepadiformine is very active in the
cardiovascular system in vivo and in vitro and suggested that it
3
Scheme 1. The originally proposed structure of lepadiformine was that of
1. The revised structure is that of 3.
After establishment of the relative stereochemistry, two
syntheses of racemic lepadiformine were reported by the
groups of Weinreb[6] and Funk[7] based on a spirocyclization of
an allylsilane N-acyliminium ion and on a amidoacrolein-
derived Diels Alder reaction, respectively. However, because
the natural product is not crystalline and its derivatives could
not be prepared, efforts to obtain an X-ray structure of
natural lepadiformine for the determination of the absolute
configuration have so far been unsuccessful.[1,8] This prompted
us to undertake the enantioselective synthesis of lepadifor-
mine and to determine the absolute configuration of the
natural product.
[*] Prof. Dr. C. Kibayashi, H. Abe, S. Aoyagi
School of Pharmacy, Tokyo University of Pharmacy & Life Science
1432-1 Horinouchi, Hachioji, Tokyo, 192-0392 (Japan)
Fax : (þ 81)426-76-4475
E-mail: kibayasi@ps.toyaku.ac.jp
[**] This work was supported in part by a Grant for Private Universities
provided by the Ministry of Education, Sports, and Culture of Japan
and the Promotion and Mutual Aid Corporation for Private Schools of
Japan. We are grateful to Professor J. F. Biard for a sample of natural
lepadiformine.
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A crucial element in our approach to the target
compound was the N-acyliminium-ion-initiated olefin
azacyclization[9] to elaborate the azaspirocyclic core of
lepadiformine (3). Such cyclizations, which lead to
spirocyclic compounds, were initially developed by
Speckamp and co-workers[10] and Evans et al.[11] Re-
cently, this reaction was applied successfully by Wein-
reb and co-workers[6] in the total synthesis of (Æ )-3.
Herein we describe the first total synthesis of (ꢀ)-
lepadiformine (3) by using a new variant of the N-
acyliminium-ion-initiated intramolecular spirocycliza-
tion in which a conjugated diene was exploited as a
p nucleophile and has been proven to be quite effective
for the highly stereoselective and extremely short
approach to 3. Furthermore, the synthesis of 3 allowed
the absolute configuration of natural lepadiformine to
be established as 3S,5R,7aS,11aS (Scheme 1).
MgBr
a
BnO
+
O
N
Boc
5
4
O
OBn
BnO
N
HNBoc
b
H
Boc
HCOO
3'
6
7
Scheme 2. Reagents and conditions: a) THF, 08C, 79%; b) HCOOH, toluene/THF
(95:5), 08C, 2 h, 88%.
Our synthesis commenced with the known (S)-N-
Boc-2-pyrrolidinone 4,[12] which upon treatment with
the (5E,7E)-tetradeca-5,7-dienyl Grignard reagent 5 under-
went selective attack at the endocyclic (ring) carbonyl
group[13] to yield ketone 6 (Scheme 2). When a solution of 6
in toluene/THF (95:5) was treated with formic acid at 08C for
2 h and neutralized with NH4OH, in situ generation of the N-
acyliminium ion followed by spirocyclization proceeded with
formate substitution at C3’ occurred with low diastereoselec-
tivity (1.6:1 favoring the 3’b-formate) as determined by HPLC
analysis (see below).
Exclusive preferential formation of (6S)-7 can be under-
stood by comparison of the stabilities of the configurations of
the six-membered chairlike p complexes A D, which arise
from the in situ generation of the N-acyliminium ion 8 as
shown in Scheme 3. In the transition states C and D, which
lead to (6R)-10, the N-Boc group displays unfavorable
nonbonding interactions with the axially oriented diene
moiety and the 1,3-diaxial hydrogen atoms of the newly
formed cyclohexane ring, respectively. The latter steric
interaction between the N-Boc group and the 1,3-diaxial
hydrogen atoms is also present in the transition state B, which
leads to (6S)-7. Neither of these interactions are present in the
transition state A and therefore, of the four possible chairlike
transition states A D, A is the least disfavored and leads to
the observed (6S)-7.
ꢀ
synchronous formation of the new C O bond at C3’ to furnish
the 1-azaspirocyclic formate ester 7 in 88% yield. Notably, the
spirocyclization of the N-acyliminium ion generated from 6,
which bears a conjugated diene, proceeded quite smoothly
and was completed in a short time, in marked contrast to the
case with the reported spirocyclization of N-acyliminium ions
that bear nonconjugated olefins which require long reaction
times.[10,11]
The 6-exo-trig ring closure[14] gave 7 with complete regio-
control, as predicted by considering the initially formed
carbocation, which is stabilized through resonance with the
olefin p bond in the alkene side chain. In this manner, the
nucleophilic attack of the olefin moiety occurred exclusively
at the sterically less hindered b face (opposite to the 5-
benzyloxymethyl group) of the 1-pyrroline ring, with exclu-
sive introduction of the desired 6S chirality. The concomitant
The formate ester 7, which is epimeric at C3’, underwent
basic hydrolysis to give the allylic alcohols 11a/11b, whose
diastereomeric ratio was determined to be 1:1.6 by HPLC.[15]
This diastereomeric mixture was oxidized with MnO2 to give
Boc
BnO
N
+
BnO
BnO
BnO
+
6S
HCOOH
+
H
N
N
N
6
Boc
C6H13
Boc
C6H13
Boc
C6H13
8
HCOO
C6H13
–
A
9
HCOO
7
Boc
N
BnO
C6H13
C6H13
+
BnO
6R
N
+
H
+
BnO
BnO
N
Boc
N
H
H
Boc
H
H
Boc
HCOO
C6H13
C6H13
B
D
10
C
Scheme 3. The selective formation of 7 from 6 occurs via transition state A. Transition states B D are disfavored.
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(colorless gum). However, it was not possible to
determine the absolute stereochemistry of natural
lepadiformine by comparison of the optical rotations,
since, as described above, the natural product (actually
the hydrochloride salt) had been reported to have no
optical rotation in chloroform.
BnO
BnO
N
N
a
b
H
H
Boc
Boc
7
O
HO
The absolute stereochemistry was assigned success-
fully by comparison of synthetic and natural lepadifor-
mine on a chiral HPLC column. In the chiral HPLC
analysis, the authentic sample of racemic lepadiformine
(Æ )-3[5b] gave peaks at different retention times [(þ)-3,
45 min; (ꢀ)-3, 48 min] (Figure 1). Comparison of reten-
tion times and co-injection revealed that the synthetic
(ꢀ)-enantiomer corresponds to the natural product, thus
allowing the absolute stereochemistry of natural lep-
adiformine to be assigned as 3S,5R,7aS,11aS.
12
11a (α-OH)/11b (β-OH)
(11a:11b = 1:1.6)
BnO
BnO
N
N
R
d
c
H
H
Boc
HO
HO
In summary, we have reported the first total synthesis
of the natural enantiomer of lepadiformine (3) by
employing an intramolecular conjugated diene cycliza-
tion of an N-acyliminium ion. The 1-azaspiro formate
ester 7 is obtained in a single step from the simple keto
amide 6, which bears the conjugated diene. The total
synthesis is highly stereoselective and is completed in
nine steps with an overall yield of 31.4% starting from
the (S)-N-Boc-2-pyrrolidinone 4 and provides a highly
efficient and extremely short route to 3. Furthermore,
the present synthesis allowed us to establish the
3S,5R,7aS,11aS absolute configuration of natural lepa-
diformine.
11a 97% de
13, R = Boc
14, R = H
e
RO
N
f
H
H
15, R = Bn
3, R = H
g
free base: [α]2D8 = –15.0 (c = 0.37 in MeOH)
HCl salt: [α]2D6 = +2.6 (c = 0.54 in CHCl3)
Received: March 21, 2002 [Z18953]
Scheme 4. Reagents and conditions: a) K2CO3, MeOH/H2O, 98%;
b) MnO2, CH2Cl2, 91%; c) (S)-BINAL-H, THF, ꢀ788C, 92%; d) H2, PtO2,
EtOAc, 86%; e) TFA, CH2Cl2, 91%; f) CBr4, PPh3, Et3N, CH2Cl2, 82%;
g) H2, Pd(OH)2, MeOH, 87%; 3 (free base): [a]2D8 ¼ ꢀ15.0 (c ¼ 0.37 in
MeOH), 3¥HCl: [a]2D6 ¼ þ 2.6 (c ¼ 0.54in CHCl 3). (S)-BINAL-H ¼ 2,2’-
binaphthoxyaluminum hydride, TFA ¼ trifluoroacetic acid.
the a,b-conjugated ketone 12 (Scheme 4). Reduction of 12 to
the corresponding alcohol with BH3¥THF (THF, 08C) was
almost nonstereoselective (11a/11b 1.15:1). (S)-BINAL-H
diastereoselective reduction[16] provided the alcohol (3’S)-11a
in 92% yield with 97% de. Catalytic hydrogenation of 11a
over palladium on carbon (10%) in methanol was carried out
in the presence of diethylamine (1 equiv) to prevent hydro-
genolysis of the benzyl ether[17] and afforded 13 in 64% yield.
Alternatively, the use of PtO2 catalyst and ethyl acetate as
solvent without diethylamine remarkably improved the yield
of 13 to 86%. After removal of the Boc protecting group with
trifluoroacetic acid, the resulting amino alcohol 14 was treated
with CBr4 and PPh3 to form the tricyclic amine 15 in 82%
yield, with complete inversion of the configuration at C3’.
Hydrogenolytic removal of the benzyl protecting group
afforded lepadiformine (3) whose spectral properties were
identical in all respects with those of an authentic sample of
racemic lepadiformine (Æ )-3 previously prepared by us.[5b]
The optical rotation of synthetic alkaloid 3 was measured:
[a]2D8 ¼ ꢀ15.0 (c ¼ 0.37 in MeOH) for the free base (oil) and
[a]2D6 ¼ þ 2.6 (c ¼ 0.54in CHCl 3) for the hydrochloride salt
Figure 1. HPLC chromatogram of lepadiformine on a Daicel Chir-
alpak OD: a) racemate; b) mixture of racemate and natural; c) mixture
of synthetic (ꢀ)-enantiomer and natural. Conditions: mobile phase,
hexane/2-propanol/Et2NH (500:10:1); flow rate: 0.1 mLminꢀ1; detec-
tion: RI (refractive index).
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[1] J. F. Biard, S. Guyot, C. Roussakis, J. F. Verbist, J. Vercauteren, J. F.
Weber, K. Boukef, Tetrahedron Lett. 1994, 35, 2691 2694.
[2] M. Jugÿ, N. Grimaud, J. F. Biard, M. P. Sauviat, M. Nabil, J. F. Verbist,
J. Y. Petit, Toxicon 2001, 39, 1231 1237.
[3] a) K. M. Werner, J. M. de los Santos, S. M. Weinreb, J. Org. Chem.
1999, 64, 686 687; b) K. M. Werner, J. M. de los Santos, S. M.
Weinreb, J. Org. Chem. 1999, 64, 4865 4873.
™Molecular∫ Molecular Sieves: Lid-Free
Decamethylcucurbit[5]uril Absorbs and
Desorbs Gases Selectively**
Yuji Miyahara,* Kazuaki Abe, and Takahiko Inazu
[4] a) W. H. Pearson, N. S. Barta, J. W. Kampf, Tetrahedron Lett. 1997, 38,
3369 3372; b) W. H. Pearson, Y. Ren, J. Org. Chem. 1999, 64, 688
689.
[5] a) H. Abe, S. Aoyagi, C. Kibayashi, Tetrahedron Lett. 2000, 41, 1205
1208; b) H. Abe, S. Aoyagi, C. Kibayashi, J. Am. Chem. Soc. 2000, 122,
4583 4592.
Several types of inorganic crystalline materials and organic
polymers function as molecular sieves, the most common of
which are numerous types of zeolites.[1] While some organic
host molecules encapsulate small molecules in solution,[2]
practical applications have never been attempted because of
their inaccessibility. Here we report that the title compound,
which can be readily synthesized from simple starting
materials, shows potential for use as molecular sieves in the
solid state by utilizing its molecular cavity.
In 1981 Mock and co-workers reported[3] that the com-
pound which Behrend prepared from glycoluril and formalin
in 1905[4] has a beautiful barrel-like macrocyclic structure
composed of six glycoluril units; they named it cucurbituril
(Cuc6). Since then extensive studies have been conducted on
this unique host compound.[5]
[6] P. Sun, C. Sun, S. M. Weinreb, Org. Lett. 2001, 3, 3507 3510.
[7] T. J. Greshock, R. L. Funk, Org. Lett. 2001, 3, 3511 3514.
[8] Many attempts at obtaining
a crystalline derivative of natural
lepadiformine (provided by Professor Biard as an oil) suitable for
X-ray crystallography were unsuccessful (W. Pearson, personal
communication).
[9] For reviews of the chemistry of N-acyliminium ions, see: a) W. N.
Speckamp, H. Hiemstra, Tetrahedron 1985, 41, 4367 4416; b) H.
Hiemstra, W. N. Speckamp in The Alkaloids, Vol. 32 (Ed.: A. Brossi),
Academic Press, San Diego, 1988, pp. 271 339; c) H. Hiemstra, W. N.
Speckamp in Comprehensive Organic Synthesis, Vol. 2 (Eds.: B. M.
Trost, I. Fleming, C. H. Heathcock), Pergamon, Oxford, 1991,
pp. 1047 1109; d) W. N. Speckamp, M. J. Moolenaar, Tetrahedron
2000, 56, 3817 3856.
[10] a) H. E. Schoemaker, W. N. Speckamp, Tetrahedron Lett. 1978, 1515
1518; b) H. E. Schoemaker, W. N. Speckamp, Tetrahedron Lett. 1978,
4841 4844; c) H. E. Schoemaker, W. N. Speckamp, Tetrahedron 1980,
36, 951 958.
[11] D. A. Evans, E. W. Thomas, Tetrahedron Lett. 1979, 411 414.
[12] a) T. Katoh, Y. Nagata, Y. Kobayashi, K. Arai, J. Minami, S.
Terashima, Tetrahedron Lett. 1993, 34, 5743 5746; b) T. Katoh, Y.
Nagata, Y. Kobayashi, K. Arai, J. Minami, S. Terashima, Tetrahedron
1994, 50, 6221 6238.
O
O
O
O
O
O
O
N
O
N
N
N
N
ON
N
O
O
O
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
O
N
N
N
N
N
N
N
N
N
N
N
O
O
O
N
N
O
O
O
O
O
O
[13] For the organometallic ring-opening reaction of N-alkoxycarbonyl
lactams, see: A. Giovannini, D. Savoia, A. Umani-Ronchi, J. Org.
Chem. 1989, 54, 228 234.
MeCuc5
Cuc6
[14] J. E. Baldwin, J. Chem. Soc. Chem. Commun. 1976, 734 736.
[15] The diastereomeric mixture of the formate ester 7 gave a combined
signal on a HPLC column. However, the alcohols 11a and 11b gave
well-separated signals on the HPLC column, and thus the ratio was
determined by HPLC integration.
In 1982, in his PhD thesis on cucurbituril, Shih also
described a compound, assumed to have a pentameric
structure, which was obtained by heating dimethylglycoluril
under reflux with formalin in dilute HCl;[6] Shih and Mock did
not study the compound further because of its inactivity as a
host molecule.[3b] In 1992, Stoddart and co-workers confirmed
its basic structure by X-ray structure analysis of a crystal
grown in dilute HNO3 and named it decamethylcucurbit[5]ur-
il (MeCuc5).[7] At the time when the paper by Stoddart and
co-workers appeared we had already found that the product
obtained under the reaction conditions contained two NH4Cl
[16] R. Noyori, I. Tomino, Y. Tanimoto, J. Am. Chem. Soc. 1979, 101, 3129
3131.
ꢀ
[17] For inhibition of hydrogenolysis of benzyl ether groups with Pd C in
the presence of ammonia and amines, see: H. Sajiki, Tetrahedron Lett.
1995, 36, 3465 3468.
[*] Dr. Y. Miyahara, K. Abe, Prof. Emer. Dr. T. Inazu
Department of Chemistry, Faculty of Sciences
Kyushu University
6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581 (Japan)
Fax : (þ 81)92-642-2607
E-mail: miyahscc@mbox.nc.kyushu-u.ac.jp
[**] This work was supported in part by a Grant-in-Aid for COE Research
™Design and Control of Advanced Molecular Assembly Systems∫
from the Ministry of Education, Science, Sports, and Culture, Japan
(#08CE2005). The authors also thank Prof. T. Shinmyozu of the
Institute for Fundamental Organic Chemistry in Kyushu University
for use of a Rigaku RAPID X-ray instrument.
Supporting information for this article is available on the WWW under
http://www.angewandte.org or from the author.
3020
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