First total synthesis of the marine alkaloids (±)-fasicularin and (±)-lepadiformine based on stereocontrolled intramolecular acylnitroso-diels-alder reaction

Article abstract of DOI:10.1021/ja9939284

The first total synthesis of tricyclic marine alkaloids (±)-fasicularin (2) and (±)-lepadiformine (5) was accomplished. The key common strategic element for the synthesis is the stereocontrolled intramolecular hetero-Diels-Alder reaction of an N-acylnitroso moiety to an exocyclic diene with or without bromine substitution to control the syn-facial or anti-facial selectivity, respectively, leading to the trans- or cis-fused decahydroquinoline ring systems 21 or 39 involving the simultaneous introduction of the nitrogenated quaternary center in a single step. On further elaboration of the six-membered or five-membered ring A, the trans-fused adduct 21 provided either (±)-fasicularin (2) or (±)-lepadiformine (5). The hydrochloride salt of synthetic (±)-5 was found to be identical with the isolated natural sample of lepadiformine; however, the tricyclic amino alcohol 4 having the proposed structure of lepadiformine in a nonzwitterionic form, derived from the cis-fused adduct 39, was found to be different from lepadiformine by spectral comparison. These results thus unambiguously established the relative stereochemistry of lepadiformine, formerly assigned incorrectly, to be 3R*,5S*,7aR*,11aR* shown by 5.

Full text of DOI:10.1021/ja9939284

J. Am. Chem. Soc. 2000, 122, 4583-4592
4583
First Total Synthesis of the Marine Alkaloids (()-Fasicularin and
(()-Lepadiformine Based on Stereocontrolled Intramolecular
Acylnitroso-Diels-Alder Reaction
Hideki Abe, Sakae Aoyagi, and Chihiro Kibayashi*
Contribution from the School of Pharmacy, Tokyo UniVersity of Pharmacy & Life Science,
Horinouchi, Hachioji, Tokyo 192-0392, Japan
ReceiVed NoVember 8, 1999
Abstract: The first total synthesis of tricyclic marine alkaloids (()-fasicularin (2) and (()-lepadiformine (5)
was accomplished. The key common strategic element for the synthesis is the stereocontrolled intramolecular
hetero-Diels-Alder reaction of an N-acylnitroso moiety to an exocyclic diene with or without bromine
substitution to control the syn-facial or anti-facial selectivity, respectively, leading to the trans- or cis-fused
decahydroquinoline ring systems 21 or 39 involving the simultaneous introduction of the nitrogenated quaternary
center in a single step. On further elaboration of the six-membered or five-membered ring A, the trans-fused
adduct 21 provided either (()-fasicularin (2) or (()-lepadiformine (5). The hydrochloride salt of synthetic
(()-5 was found to be identical with the isolated natural sample of lepadiformine; however, the tricyclic amino
alcohol 4 having the proposed structure of lepadiformine in a nonzwitterionic form, derived from the cis-fused
adduct 39, was found to be different from lepadiformine by spectral comparison. These results thus
unambiguously established the relative stereochemistry of lepadiformine, formerly assigned incorrectly, to be
3R*,5S*,7aR*,11aR* shown by 5.
Introduction
Tunicates (ascidians) have been proven to be a particularly
rich source of a variety of structurally fascinating and bioactive
nitrogen compounds.1 Since the first members were reported
in 1993, 11 cylindricines A-K have been identified from the
Tasmanian ascidians ClaVelina cylindrica as new marine
alkaloids2 with a tricyclic ring system unprecedented among
natural products, consisting of the perhydropyrrolo[2,1-j]quino-
line or the perhydropyrido[2,1-j]quinoline. Shortly after the first
isolation of cylindricines A (1a) and B (1b)2a the isolation and
structure elucidation of a closely related marine alkaloid, named
lepadiformine, from the ascidian ClaVelina lepadiformis col-
lected in Tunisia was reported by Biard and co-workers in 1994.3
It was found to be moderately cytotoxic toward various tumor
cell lines in vitro. On the basis of extensive NMR studies, this
alkaloid was given structure 3 including a unique zwitterionic
form. Although its observed specific rotation value (in a CHCl3
solution) is zero, and the absolute stereochemistry is still
unknown, it is believed that lepadiformine is not racemic.
In addition to these tricyclic alkaloids, fasicularin (2) was
recently discovered by Patil and co-workers4 from the Micro-
(1) For recent reviews on marine natural products, see: (a) Davidson,
B. S. Chem. ReV. 1993, 93, 1771-1791. (b) Faulkner, D. J. Nat. Prod.
Rep. 1994, 11, 355-394. (c) Faulkner, D. J. Nat. Prod. Rep. 1995, 12,
223-269. (d) Faulkner, D. J. Nat. Prod. Rep. 1996, 13, 75-125. (e)
Faulkner, D. J. Nat. Prod. Rep. 1998, 15, 113-158. (f) Faulkner, D. J.
Nat. Prod. Rep. 1999, 16, 155-198.
(2) (a) Blackman, A. J.; Li, C.; Hockless, D. C. R.; Skelton, B. W.; White,
A. H. Tetrahedron 1993, 49, 8645-8656. (b) Li, C.; Blackman, A. J. Aust.
J. Chem. 1994, 47, 1355-1361. (c) Li, C.; Blackman, A. J. Aust. J. Chem.
1995, 48, 955-965.
(3) Biard, J. F.; Guyot, S.; Roussakis, C.; Verbist, J. F.; Vercauteren, J.;
Weber, J. F.; Boukef, K. Tetrahedron Lett. 1994, 35, 2691-2694.
(4) Patil, A. D.; Freyer, A. J.; Reichwein, R.; Carte, B.; Killmer, L. B.;
Faucette, L.; Johnson, R. K.; Faulkner, D. J. Tetrahedron Lett. 1997, 38,
363-364.
nesian ascidian Nephteis fasicularis, which has selective activity
against a DNA repair-deficient organism and is cytotoxic to
Vero cells with an IC50 of 14 µg/mL. The structure and relative
stereochemistry of 2 were deduced on the basis of extensive
NMR studies, though the absolute configuration is still unknown.
The novel structural features and biological significance of these
tricyclic alkaloids represent a promising new class of nitrogen
heterocycles biosynthesized by ascidians (such as indolizidines,
quinolizidines, and decahydroquinolines), and have attracted the
increasing attention of organic chemists during the past few
years. Thus, several approaches involving construction of the
10.1021/ja9939284 CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/26/2000

4584 J. Am. Chem. Soc., Vol. 122, No. 19, 2000
Abe et al.
azatricyclic ring systems have been developed, which led to
the total syntheses of cylindricines A-E (1a-e).5 In another
approach, Weinreb and co-workers6 reported the synthesis of
the putative structure 4 of lepadiformine, and they found that
the synthetic material is clearly different from natural lepadi-
formine and exists in a nonzwitterionic form as 4 instead of 3.
In addition, Pearson et al.7 have synthesized the three other
possible diastereomers of 4 at C(3) and C(5) and have found
that none of these stereoisomers correspond to lepadiformine.8
These results called into question the validity of the published
structure of lepadiformine. However, at the outset of our project,
since these arguments pertaining to the lepadiformine structure
were not published, we planned to synthesize lepadiformine
based on structure 4 having the stereochemistry originally
assigned. Also, we became interested in fasicularin (2) as a target
for synthesis. Since these compounds 2 and 4 comprise a
perhydroquinoline ring system, which is a common core unit
present in this class of alkaloids, with trans and cis ring fusions,
respectively, it presented an opportunity for the diastereoselec-
tive construction of the perhydroquinoline skeleton. Accordingly,
the route for the synthesis of 2 and 4 we have envisaged is
shown in Scheme 1. Conceptually, this approach involves as
the key feature an intramolecular hetero-Diels-Alder reaction
using an N-acylnitroso compound 7, generated from a hydrox-
amic acid 6, wherein anti and syn facial additions are formally
possible through endo transition states 7a and 7b to form B/C
trans- and B/C cis-fused cycloadducts 8a and 8b, respectively,
which correspond to fasicularin (2) and compound 4.
Scheme 1
In this paper, we report the synthesis of (()-fasicularin (2)
and the tricyclic compound 49 having the stereochemistry
originally proposed for lepadiformine via diastereocontrolled
routes by employing an intramolecular hetero-Diels-Alder
methodology. Furthermore, we describe the application of this
methodology to the synthesis of (()-lepadiformine (5), which
led to revision of the proposed structure 3 for the natural product,
thus achieving the first total synthesis of lepadiformine (5) as
well as fasicularin (2).
system.12 In this case, syn-facial stereochemistry was predomi-
nant as a result of the approach of the acylnitroso moiety to the
exocyclic diene moiety from the same face to the anchor position
of the tether, though in low selectivity. These findings suggest
that the nitroso-diene intermediate generated from 6 prefers a
syn-facial approach via 7b to form the B/C cis-fused adduct
8b rather than the B/C trans-fused adduct 8a. Inspection of
molecular models, however, suggests that the anti-facial transi-
tion state conformation 7a leading to trans selectivity is
energetically most favorable among the possible conformations
(vide infra).
We thus began our investigation by preparing the hydroxamic
acid 19 needed for the cycloaddition to examine the facial
selectivity. As shown in Scheme 2, the Horner-Emmons
reaction of ketone 913 was carried out to give the unsaturated
nitrile 10 in 97% yield as a 13:1 inseparable mixture of the E
and Z isomers, which was without separation subjected to
DIBALH reduction to form the unsaturated aldehyde 11 (77%).
Results and Discussion
Diastereofacial Intramolecular Hetero-Diels-Alder Reac-
tion of Acylnitroso Compounds. The hetero-Diels-Alder
reaction of N-acylnitroso compounds has been widely investi-
gated and proved to be useful for natural products synthesis
because of its potential for further structural elaboration.10,11
However, there was only one example of the use of an exocyclic
diene as a diene component in the intramolecular acylnitroso-
Diels-Alder reaction for the formation of a fused 7/6 ring
(5) (a) For synthesis of (()-cylindricines A, D, and E, see: Snider, B.
B.; Liu, T. J. Org. Chem. 1997, 62, 5630-5633. (b) For synthesis of (-)-
cylindricine C, see: Molander, G. A.; Ro¨nn, M. J. Org. Chem. 1999, 64,
5183-5187. (c) For synthesis of (()-cylindricines A and B, see: Liu, J.
F.; Heathcock, C. H. J. Org. Chem. 1999, 64, 8263-8266.
(6) (a) Werner, K. M.; De los Santos, J. M.; Weinreb, S. M.; Shang, M.
J. Org. Chem. 1999, 64, 686-687. (b) Werner, K. M.; De los Santos, J.
M.; Weinreb, S. M.; Shang, M. J. Org. Chem. 1999, 64, 4865-4873.
(7) (a) Pearson, W. H.; Barta, N. S.; Kampf, J. W. Tetrahedron Lett.
1997, 38, 3369-3372. (b) Pearson, W. H.; Ren, Y. J. Org. Chem. 1999,
64, 688-689.
(8) For other approaches to the perhydropyrrolo[2,1-j]quinoline ring
system, see: (a) Aube´, J.; Millligan, G. L. J. Am. Chem. Soc. 1991, 113,
8965-8966. (b) Ollero, L.; Mentink, G.; Rutjes, F. P. J. T.; Speckamp, W.
N.; Hiemstra, H. Org. Lett. 1999, 1, 1331-1334.
(9) For part of a preliminary account of this work on the synthesis of
compound 4, see: Abe, H.; Aoyagi, S.; Kibayashi, C. Tetrahedron Lett.
2000, 41, 1205-1208.
(10) For our own achievements in the development of the intramolecular
acylnitroso-Diels-Alder approach for the synthesis of nitrogen-containing
natural products, see: Kibayashi, C.; Aoyagi, S. Synlett 1995, 873-879.
(11) For reviews on the acylnitroso-Diels-Alder reaction, see: (a) Kirby,
G. W. Chem. Soc. ReV. 1977, 6, 1-24. (b) Weinreb, S. M.; Staib, R. P.
Tetrahedron 1982, 38, 3087-3128. (c) Boger, D. L.; Weinreb, S. M. Hetero
Diels-Alder Methodology in Organic Synthesis; Academic Press: San
Diego, 1987. (d) Weinreb, S. M. In ComprehensiVe Organic Synthesis; Trost,
B. M., Fleming, I., Paquette, L. A., Eds.; Pergamon Press: Oxford, 1991;
Vol. 5, pp 401-449. (e) Streith, J.; Defoin, A. Synthesis 1994, 1107-1117.
(f) Orena, M. In Methods of Organic Chemistry (Houben-Weyl), 4th ed.;
Helmchen, G., Hoffmann, R. W., Mulzer, J., Schaumann, E., Eds.;
Thieme: Stuttgart, 1995; Vol. E21e, pp 5547-5587.
(12) (a) Burkholder, T. P.; Fuchs, P. L. J. Am. Chem. Soc. 1988, 110,
2341-2342. (b) Burkholder, T. P.; Fuchs, P. L. J. Am. Chem. Soc. 1990,
112, 9601-9613.
(13) Prepared according to: Corey, E. J.; Boger, D. L. Tetrahedron Lett.
1978, 2461-2464.

Total Synthesis of Fasicularin and Lepadiformine
J. Am. Chem. Soc., Vol. 122, No. 19, 2000 4585
Scheme 2a
Table 1. Intramolecular Diels-Alder Reaction of the
N-Acylnitroso Compounda
entry
periodate
solvent
CHCl3
H2O-MeOH (5:1)
H2O-MeOH (5:1)
H2O-DMF (5:1)
H2O-DMSO (5:1)
21:22b
yield (%)c
1
2
3
4
5
Pr4NIO4
Pr4NIO4
Bu4NIO4
Bu4NIO4
Bu4NIO4
2.1:1
4.5:1
4.5:1
4.7:1
4.8:1
58
80
84
75
77
a All reactions were carried out by treatment of the hydroxamic acid
19 with the periodate at 0 °C for 30-45 min. b The ratios were
determined by HPLC analysis. c Isolated combined yield.
cycloaddition of 19 was carried out in aqueous media; the results
obtained are collected in Table 1 (entries 2-5). As can be seen,
employing these aqueous conditions significantly enhanced the
desired trans selectivity (4.5:1 to 4.8:1) as well as yields (75-
84%). These isomers 21 and 22 could be separated chromato-
graphically. The minor isomer 22, which is crystalline, was
subjected to X-ray crystallographic analysis,16 unequivocally
confirming its relative stereochemistry as shown.
The trans facial preference observed in the cycloaddition of
19 can be rationalized in terms of endo transition states 20. The
syn-facial transition state conformer 20B leading to the cis-
fused adduct 22 would produce repulsive interactions between
the ring methylene group at C(3) and the nitroso-containing
tethering side chain and also between the C(5)-methylene group
and the nitrogen atom. The other possible syn-facial conformer
20C leading to the cis-fused adduct 22 would be disfavored
due to the tethering side chain placed into an axial position.
Thus, avoidance of these unfavorable steric interactions should
lead to the most favored anti-facial conformer 20A, in which
the tethering side chain is equatorially disposed, affording the
trans-fused adduct 21. The hydrophobic effect in this case may
have contributed to stabilize the more compact endo transition
state conformer 20A rather than 20B and 20C on enhancement
of the trans selectivity as well as the yield.
a Reagents and conditions: (a) (EtO)2P(O)CH2CN, BuLi, THF, -50
°C; (b) DIBALH, CH2Cl2, -30 °C; (c) (EtO)2P(O)CH2CO2Et, NaH,
THF, 0 °C; (d) DIBALH, CH2Cl2; (e) MOMCl, i-Pr2NEt, CH2Cl2; (f)
TsOH, MeOH, room temperature, then separation of the geometric
isomers; (g) (COCl)2, DMSO, Et3N, CH2Cl2, -78f0 °C; (h) NaClO2,
2-methyl-2-butene, NaH2PO4, t-BuOH-H2O; (i) CH2N2, Et2O; (j)
NH2OH‚HCl, KOH, MeOH.
Horner-Emmons olefination of 11 yielded the (E,E)-ester 12
along with a small amount of a (Z,E)-isomer (combined yield:
91%), which underwent sequentially DIBALH reduction, MOM
protection of the hydroxyl group, deprotection of the THP ether,
and then separation of the geometric isomers, affording the
(E,E)-alcohol 15 in 78% overall yield. Compound 15 was
converted by Swern oxidation and subsequent oxidation with
sodium chlorite to the corresponding carboxylic acid 17 (66%),
which was transformed into the hydroxamic acid 19 (81%) by
diazomethane esterification followed by treatment with hy-
droxylamine under alkaline conditions.
Upon oxidation of 19 with Pr4NIO4 under the conventional
nonaqueous conditions using CHCl3 at 0 °C, the in situ generated
acylnitroso compound 20 was subjected to intramolecular [4 +
2] cycloaddition to yield the B/C trans-fused and cis-fused
tricyclic lactams, 21 and 22, with low diastereoselection of 2.1:1
in 58% combined yield (see Table 1, entry 1).
It has been demonstrated that the use of water as the solvent
for Diels-Alder reaction improves rate, yield, and selectivity
owing to the hydrophobic effect on a reactant encapsulated in
a cavity surrounded by a hydrogen-bonding network of water
molecules.14 Indeed, our previous studies revealed15 that the use
of aqueous conditions for the intramolecular acylnitroso-Diels-
Alder reaction effects significant enhancement of the diaste-
reoselectivity due to the hydrophobic effect. Accordingly, the
To the best of our knowledge, the predominant formation of
the trans-adduct via such an anti-facial transition state with the
dienophile approaching from the face opposite to the anchor
position of the tether has not been previously recognized in an
intramolecular Diels-Alder reaction.17
From these results, we expected that the cycloaddition using
the nitroso-diene compound with bromine substitution at the
(14) For a recent review, see: Lubineau, A.; Auge´, J.; Queneau, Y.
Synthesis 1994, 741-760.
(15) Naruse, M.; Aoyagi, S.; Kibayashi, C. Tetrahedron Lett. 1994, 35,
595-598.
(16) See Supporting Information.

4586 J. Am. Chem. Soc., Vol. 122, No. 19, 2000
Abe et al.
Scheme 3a
diene tether would lead to the preferential formation of the B/C
cis-fused adduct 24a, since a syn-facial transition state 23A with
the tethering 2-alkyl side chain adopting an axial position to
avoid the 1,3-allylic strain18 with the bromine atom is predicted
to be energetically more favorable than an anti-facial one 23B
leading to a B/C trans-fused adduct 24b. We thus next turned
our attention to the cycloaddition using the bromine-substituted
nitroso-diene compound.
a Reagents and conditions: (a) (EtO)2P(O)CH2CN, BuLi, THF, -50
°C; (b) DIBALH, CH2Cl2, -30 °C; (c) (i) Br2‚dioxane, -50 °C; (ii)
pyridine, room temperature; (d) (EtO)2P(O)CH2CO2Et, NaH, THF, (e)
DIBALH, CH2Cl2, 0 °C, then separation of the geometric isomers; (f)
BnCl, Bu4N‚HSO4, NaOH, benzene; (g) PPTS, t-BuOH; (h) (COCl)2,
DMSO, Et3N, CH2Cl2; (i) NaClO2, 2-methyl-2-butene, NaH2PO4,
t-BuOH-H2O; (j) CH2N2, Et2O; (k) NH2OH‚HCl, KOH.
condensation, DIBALH reduction, and then separation of the
mixture of the geometric isomers to provide the (Z,E)-alcohol
30 as a single isomer in 47% overall yield. After O-benzylation
followed by removal of the MOM protecting group, the resulting
alcohol 32 was converted to the carboxylic acid 34 via two-
step oxidation (Swern oxidation followed by NaClO2 oxidation)
in 56% overall yield for four steps from 30. Esterification
followed by treatment with hydroxylamine provided the hy-
droxamic acid 36 (66%).
Treatment of 36 with Pr4NIO4 in a CHCl3 solution at room
temperature allowed the acylnitroso-diene 37 generated in situ
to undergo intramolecular cycloaddition; however, although the
TLC analysis revealed that the starting material disappeared
within 20 min, the reaction produced only a poor yield (8%) of
the cycloadducts 39 and 40 in a 2.6:1 ratio, though the desired
cis-fused cycloadduct 39 was predominantly formed as expected.
Neither prolonging the reaction time at room temperature nor
heating at reflux temperature improved the yield of the cy-
cloadducts; in the latter case, rapid decomposition of the
substrate resulted. The poor yield in this cyclization would be
attributed to a significant decrease of the reactivity of the
acylnitroso compound 37 due to the attachment of an electron-
The ketone 2513 was subjected to Horner-Emmons olefina-
tion to give the unsaturated nitrile 26 in 99% yield as a 16:1
E/Z mixture, which was converted in 87% yield to the
unsaturated aldehyde 27 by DIBALH reduction (Scheme 3).
Bromination of 27 led to a 1:1.4 E/Z mixture of the bromide
28 (72%), which was subjected sequentially to Horner-Emmons
(17) For reviews on intramolecular Diels-Alder reaction, see: (a)
Oppolzer, W. Angew. Chem., Int. Ed. Engl. 1977, 16, 10-23. (b) Brieger,
G.; Bennett, J. N. Chem. ReV. 1980, 80, 63-97. (c) Fallis, A. G. Can. J.
Chem. 1984, 62, 183. (d) Ciganek, E. Org. React. 1984, 32, 1-374. (e)
Craig, D. Chem. Soc. ReV. 1987, 16, 187-238. (f) Roush, W. R. In
ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, I., Paquette, L.
A., Eds.; Pergamon Press: Oxford, 1991; Vol. 5, pp 513-550. (g) Craig,
D. In Methods of Organic Chemistry (Houben-Weyl), 4th ed.; Helmchen,
G., Hoffmann, R. W., Mulzer, J., Schaumann, E., Eds.; Thieme: Stuttgart,
1995; Vol. E21c, pp 2872-2904.
(18) (a) Johnson, F. Chem. ReV. 1968, 68, 375-413. (b) Hoffmann, R.
W. Chem. ReV. 1989, 89, 1841-1860.

Total Synthesis of Fasicularin and Lepadiformine
J. Am. Chem. Soc., Vol. 122, No. 19, 2000 4587
Scheme 4
Scheme 5a
a Reagents and conditions: (a) H2, Pd-C, MeOH; (b) concentrated,
HCl, MeOH, reflux; (c) BnBr, NaH, Bu4NI, DMF, room temperature;
(d) Na(Hg), Na2HPO4, EtOH; (e) TBDPSCl, imidazole, DMF; (f) H2,
Pd-C, MeOH; (g) TsCl, Et3N, DMAP, CH2Cl2; (h) NaH, THF, reflux;
(i) (i) LiNH2BH3, THF; (ii) CbzCl, Na2CO3, CHCl3-H2O; (j) (COCl)2,
DMSO, Et3N, CH2Cl2, -78f 0 °C; (k) C6H13MgBr, Et2O, 0 °C, then
PCC, CH2Cl2.
withdrawing bromine atom to the diene moiety, which would
lead to decomposition of the in situ generated 37 with properties
associated with the RCO-NdO species, namely, they are
extremely labile and short-lived.11a To overcome these problems
including the inherent disadvantage of the acylnitroso com-
pound, we sought to utilize the 9,10-dimethylanthracene adduct
38 which was considered to be a stable acylnitroso equivalent.19
Thus, upon exposure of 36 to the same oxidation conditions
(Pr4NIO4, CHCl3, room temperature) in the presence of 9,10-
dimethylanthracene, intermolecular cycloaddition reaction
smoothly proceeded to form the adduct 38 in 84% yield (Scheme
4). Thermolysis of 38 in refluxing benzene caused a retro-Diels-
Alder reaction to regenerate the intermediate acylnitroso diene
37, which immediately underwent intramolecular cycloaddition
under the reaction conditions, affording the cycloadducts 39 and
40 in 75% yield and in a 5.5:1 ratio favoring the B/C cis-fused
adducts 39 in contrast to the facial selectivity (anti) observed
in the cycloaddition with the nonbrominated acylnitroso-dine
20 described above.
Synthesis of (()-Fasicularin. Having obtained the desired
trans-fused cycloadduct 21 through the face-selective anti-
addition described above, we initially focused our efforts on
the total synthesis of fasicularin (2). Catalytic hydrogenation
of the double bond of 21 and removal of the MOM protecting
group produced 42 (87%), the stereochemistry of which was
confirmed by X-ray crystallography.16 After protection of the
hydroxyl group as the benzyl ether, the N-O bond of 43 was
cleaved by treatment with sodium amalgam and the resulting
secondary alcohol 44 was protected as the tert-butyldiphenylsilyl
(TBDPS) ether to afford 45 in 84% overall yield from 42.
Compound 45 was converted to the tosylate 47 in 82% yield
by hydrogenolytic removal of the benzyl group followed by
tosylation. Cyclization of 47 to the tricyclic lactam 48 was
accomplished using sodium hydride in THF at reflux to give a
virtually quantitative yield. Since attempts to introduce the hexyl
side chain into the lactam ring in 48 were unsuccessful, ring-
opening of the lactone ring was deemed necessary for the
attachment of the hexyl side chain. Thus, 48 was exposed to
LiNH2BH3, prepared from BH3‚NH3 and BuLi,20 at room
temperature, leading to reductive ring-opening,21 and it under-
went subsequent N-protection to give the azaspiro[5,5]undecane
derivative 49 in 97% yield. Swern oxidation of 49 and sub-
sequent addition of the hexyl Grignard reagent followed by PCC
oxidation provided the ketone 51 in 87% overall yield.
Palladium-catalyzed hydrogenation of 51 resulted in ring
closing via hydrogenolytic cleavage of the Cbz group followed
by hydrogenation of the in situ generated iminium ion 52 to
form the tricyclic products 53 and 54 in slight preference for
undesired 53 (1.3:1, 86% combined yield) with the configura-
tion at the hexyl group inconsistent with that of fasicularin
(Scheme 6).
The formation of desired 54 was thus found to be less favored
probably due to the steric hindrance in an intermediary iminium
ion 52 that disturbs hydrogen delivery from the sterically
congested â face. To circumvent this problem associated with
the face selectivity of the hydrogenation, we envisaged the use
(19) (a) Kirby, G. W.; Sweeny, J. G. J. Chem. Soc., Chem. Commun.
1973, 704-705. (b) Corrie, J. E. T.; Kirby, G. W.; Sharma, R. P. J. Chem.
Soc., Chem. Commun. 1975, 915-916. (c) Keck, G. E. Tetrahedron Lett.
1978, 4767-4770
(20) Myers, A. G.; Yang, B. H.; Kopecky, D. J. Tetrahedron Lett. 1996,
37, 3623-3626.
(21) For recent application of this reaction to lactam ring-opening, see:
Aoyagi, S.; Tanaka, R.; Naruse, M.; Kibayashi, C. J. Org. Chem. 1998,
63, 8397-8406.

4588 J. Am. Chem. Soc., Vol. 122, No. 19, 2000
Abe et al.
Table 2. Cyclization of 56 via Intramolecular Reductive
Amination
Scheme 6
entry
solvent
EtOH
AcOEt
benzene
cyclohexane
58:59a
yield (%)b
1
2
3
4
1:1.7
1:1.3
3.4:1
5.2:1
63
66
65
62
a The ratios were based on the isolated products. b Isolated combined
yield.
Scheme 7a
Figure 1. The X-ray structure (Chem3D representation) of 58‚HCl.
metal surface, which diminishes the directing effect of the hy-
droxyl group in the iminium intermediate 57. We envisaged
the hydrocarbons as nonpolar solvents, which do not compete
for binding sites of the catalytic surface, thus enforcing the hy-
droxyl group-catalyst association and thereby favoring formation
of the desired 6R-hexyl isomer 58.24 Therefore, the catalytic
hydrogenation of 56 was carried out using benzene and cyclo-
hexane, whereby the stereochemical outcome of the cyclization
was found to be reversed as expected, affording the desired 6R-
hexyl isomer 58 which predominated with ratios of 3.4:1 and
5.2:1, respectively, over its 6-epi isomer 59 (entries 3 and 4).
The 6R-hexyl side chain occupying the equatorial position
postulated for 58 was irrevocably confirmed by the X-ray crystal
structure analysis (Figure 1). We expected that the 6â-hexyl
isomer 59 would adopt the conformation 59b in a twist-boat
form for ring B leaving the hexyl group in the equatorial position
to avoid severe steric interaction between the â-hexyl group
and ring A bearing the â-hydroxyl group present in the chair
conformation 59a. This is in agreement with molecular mechan-
ics (MM3) calculations (Molecular Mechanics, Version 4.0,
CAChe system, Oxford Molecular Group, Inc.), which show
that the twist-boat conformation 59b is ca. 2.0 kcal/mol more
stable than the chair conformation 59a.
For the final thiocyanation step, transformation of the
hydroxyl group in 58 into an adequate leaving group in an SN2
fashion was carried out. Although triflation was difficult,
probably due to the sterically hindered nature of the hydroxyl
group function, the mesylate 60 was obtained in 76% yield,
and we expected to introduce the thiocyanyl group with the
correct stereochemistry via a nucleophilic displacement reaction.
However, all attempts using KSCN under various conditions
gave only elimination and decomposition products. To circum-
vent this problem in this thiocyanation strategy, an alternative
route to convert 58 directly into (()-fasicularin (2) was next
sought. Treatment of 58 with the isothiocyanatophosphonium
salt25 resulted in no reaction (at -45 °C to room temperature)
a Reagents and conditions: (a) Bu4NF, THF; (b) (i) DEAD, Ph3P,
p-NO2C6H4CO2H; (ii) 5% NaOH, THF, (c) H2, Pd-C, for the solvent,
see Table 2; (d) MsCl, Et3N, DMAP; (e) HSCN, Ph3P, DEAD, benzene.
of a substrate bearing a hydroxyl group to be bound to the
catalytic surface during hydrogenation so as to enforce addition
of hydrogen from the sterically hindered â face.22 For this
purpose, the silyl protecting group in 51 was removed and sub-
sequent inversion of configuration at the secondary alcohol
center in 55 using the Mitsunobu procedure23 led to the epi-
merized alcohol 56 in very high yield. We first examined the
reductive cyclization of 56 in ethanol, which proceeded under
the hydrogenolytic conditions with palladium on carbon to
provide a 1:1.7 mixture of the tricyclic products 58 and 59 in
63% combined yield favoring the undesired 6â-hexyl isomer
59 (Table 2, entry 1). The use of ethyl acetate as a solvent re-
sulted in the preferential formation of 59 as well with a slight
decrease of the 58/59 ratio (1:1.3) (entry 2). The results indicate
that the use of a polar solvent does not lead to the face selectivity
of the hydrogenation in the desired sense, presumably due to
the competitive association of the solvent molecule with the
(24) For a discussion of the solvent effect on the stereochemical outcome
of directed catalytic hydrogenation, see: Thompson, H. W.; McPherson,
E.; Lences, B. L. J. Org. Chem. 1976, 41, 2903-2906.
(25) (a) Tamura, Y.; Kawasaki, T.; Adachi, M.; Tanio, M.; Kita, Y.
Tetrahedron Lett. 1977, 4417-4420. (b) Burski, J.; Kieszkowski, J.;
Michalski, J.; Pakulski, M.; Skowronska, A. Tetrahedron 1983, 39, 4175-
4181.
(22) For a comprehensive review on heteroatom-directed organic reac-
tions, see: Hoveyda, A. H.; Evans, D. A.; Fu, G. C. Chem. ReV. 1993, 93,
1307-1370.
(23) Mitsunobu, O. Synthesis 1981, 1-28.

Total Synthesis of Fasicularin and Lepadiformine
J. Am. Chem. Soc., Vol. 122, No. 19, 2000 4589
Scheme 8a
Scheme 9a
a Reagents and conditions: (a) H2, Pd-C, Et3N, MeOH, then
separation of the cis/trans isomers; (b) Na(Hg), NaHPO4, EtOH; (c)
(i) MsCl, collidine; (ii) NaOH, THF-MeOH; (d) NaH, THF; (e)
MOMCl, i-Pr2NEt.
a Reagents and conditions: (a) (i) LiNH2BH3, THF; (ii) CbzCl,
Na2CO3, benzene-H2O; (iii) K2CO3, MeOH-H2O; (b) (COCl)2,
DMSO, Et3N, CH2Cl2; (c) (i) C6H13MgBr, Et2O; (ii) PCC, CH2Cl2; (d)
H2, Pd-C, EtOH; (e) concentrated HCl, MeOH.
or formation of a small amount of the isothiocyanate (at 60
°C); however, a Mitsunobu condensation (Ph3P, DEAD, ben-
zene)26 with thiocyano acid proceeded with complete inversion
of configuration at the reaction center to provide (()-fasicularin
(2), albeit in low yield (20%) and with concomitant formation
of an elimination product (54%) and an isothiocyanate (2%).
77% yield. The preferential formation of this diastereomer can
be explained by invoking an iminium intermediate 70 in which
hydrogenation should occur on the less hindered R face. Finally,
removal of the MOM protecting group from 71 under metha-
nolic HCl conditions provided in 95% yield the tricyclic amino
alcohol 4, possessing the stereostructure proposed for lepadi-
formine, which was found to exist in a nonzwitterionic form as
shown rather than the proposed zwitterionic structure 3. The
spectral properties (1H and 13C NMR) for both the free base
and the hydrochloride salt of the synthetic material, however,
were clearly different from those reported3 for natural lepadi-
formine.
Total Synthesis of Lepadiformine. While our work was in
progress, Weinreb et al.6 reported the synthesis of the putative
structure 4 of lepadiformine and found their synthetic material
to be different from natural lepadiformine. At the same time,
Pearson et al.7 described the syntheses of the remaining three
of the four diastereoisomers of 4 at C(3) and C(5); however,
none of these compounds was found to be compatible with
lepadiformine. From these results, they suggested that the
originally proposed structure 3 (or 4 in a nonzwitterionic form)
of lepadiformine must be revised to be epimeric at C(11a), thus
constituting the trans-fused perhydroquinoline ring system as
in fasicularin. Furthermore, the cis relationship between the
hydroxymethyl group of the pyrrolidine ring and the C(11)
methylene group has previously been defined by NOESY
correlation.3 Considering these findings and the NMR spectral
evidence, two structures 72 and 5 both constituting the trans-
fused decahydroquinoline framework can be reasonably pro-
posed for lepadiformine. It was envisioned that these compounds
72 and 5 could be constructed by utilizing the above-described
The synthetic material of (()-2 so obtained showed 1H and 13
C
NMR spectra in full agreement with those of natural fasicularin,
which verified the structure and relative stereochemistry pro-
posed in the literature4 for the natural product.
Synthesis of the Putative Structure of Lepadiformine. The
next object of our research was the synthesis of the putative
structure 4 of lepadiformine in the nonzwitterionic form utilizing
the above-described B/C cis-fused cycloadduct 39 including a
small amount of the trans-fused isomer 40 which were obtained
as an inseparable 5.5:1 diastereomeric mixture (Scheme 8).
Reduction of the double bond, debenzylation, and debromination
of this mixture were accomplished by hydrogenation over
palladium on carbon in the presence of Et3N in a single
operation, affording the cis-fused isomer 61 in 63% yield after
chromatographic separation. Reductive N-O bond cleavage of
61 using sodium amalgam gave the 1,2-diol 62 in excellent
yield, which was converted to the epoxide 63 in 65% yield via
selective mesylation of the primary alcohol function followed
by alkaline treatment. Treatment of 63 with NaH in refluxing
THF caused selective intramolecular 5-exo-epoxy ring-opening
to form the tricyclic lactam 64 in 87% yield in preference to
the 6-endo-epoxy ring-opening (leading to 65 in 10% yield).
After protection of the hydroxyl group as the MOM ether,
reductive lactam ring-opening of 66 was effected using LiNH2-
BH3, leading to the azaspiro alcohol which underwent N-
protection to give 67 in 78% overall yield (Scheme 9). Swern
oxidation of 67 and subsequent addition of hexylmagnesium
bromide followed by PCC oxidation afforded the ketone 69 in
66% overall yield. Cyclization of 69 could be successfully
performed via intramolecular reductive amination under catalytic
hydrogenation conditions to produce 71 as a single isomer in
(26) This procedure was based upon the following: Sliwska, H.-R.;
Liaaen-Jensen, S. Tetrahedron: Asymmetry 1993, 4, 361-368.

4590 J. Am. Chem. Soc., Vol. 122, No. 19, 2000
Abe et al.
Scheme 10a
Scheme 11a
a Reagents and conditions: (a) PCC, CH2Cl2; (b) H2, Pd-C, MeOH;
(c) concentrated HCl, MeOH.
Scheme 12a
a Reagents and conditions: (a) Na(Hg), Na2HPO4, EtOH; (b) MsCl,
Et3N, DMAP; (c) t-BuOK, THF, room temperature; (d) (i) LiNH2BH3,
THF, room temperature; (ii) CbzCl, Na2CO3; (e) (COCl)2, DMSO, Et3N,
-78f 0 °C; (f) C6H13MgBr, Et2O, 0 °C; (g) (i) p-NO2C6H4CO2H, Ph3P,
DEAD; (ii) 10% NaOH, THF-MeOH.
a Reagents and conditions: (a) H2, Pd-C, MeOH; (b) Ph3P, CBr4,
Et3N, room temperature; (c) concentrated HCl, MeOH, room temper-
ature, then 5% K2CO3; (d) 1 N HCl-MeOH.
ring closure upon catalytic hydrogenation to form the tricyclic
compound 82 as a single isomer in 74% overall yield (Scheme
11). Removal of the MOM protecting group under the standard
acidic conditions afforded the tricyclic amino alcohol 72 in 96%
yield. The spectra (1H and 13C NMR) of both the free base and
the hydrochloride salt of 72 were found to be different from
those reported3 for natural lepadiformine.
tricyclic lactam 41, the intermediate for the synthesis of
fasicularin, as the starting material having the trans-fused
octahydroquinolinone unit. To investigate this approach, 41 was
subjected to reductive cleavage of the N-O bond to give the
alcohol 73, which was converted to the tricyclic lactam 75 via
mesylation followed by base treatment (Scheme 10). Reductive
lactam ring-opening of 75 using LiNH2BH3 followed by
N-protection provided the azaspiro compound 76 in 63% yield.
Swern oxidation of 76 and subsequent addition of the hexyl
Grignard reagent to the aldehyde 77 afforded the epimeric
secondary alcohols 78 and 79 with a 2.0:1 ratio favoring the
R-isomer 78 in 68% combined yield from 76. The stereochem-
ical outcome of the preferential formation of 78 could be
accounted for by a transition-state model 80 involving the
chelation27 of the carbonyl groups of the formyl and N-Cbz
functions with Mg. The minor â-isomer 79 could be converted
to the R-isomer 78 by inversion of the hydroxyl configuration
using the Mitsunobu procedure in 90% yield; thus, the total
yield of 78 from the aldehyde 77 was 81%.
Our next concern was the synthesis of another target 5 by
utilizing the R-alcohol 78. Thus, after hydrogenolytic removal
of the Cbz group, the resulting amino alcohol 83 was exposed
to CBr4 and Ph3P, which led to smooth dehydrocyclization28
with complete inversion of the configuration at C(3′) to form
84 in 74% yield from 78 (Scheme 12). Deprotection of the
MOM protecting group with concentrated HCl in methanol and
subsequent basic treatment provided 5 as an oil in 96% yield.
Further treatment of this material with methanolic HCl followed
by evaporation of the solvent resulted in the hydrochloride salt
of 5 as a solid. This provided single crystals from recrystalli-
zation in ether, thus allowing structural assignment to be
unambiguously secured by X-ray analysis,29 which revealed the
stereochemistry of 5‚HCl with the B ring in a somewhat unusual
boat (twist-boat) form with the preferred adoption of an
equatorial orientation of the hexyl side chain (Figure 2).
1
Although both H and 13C NMR spectral data for synthetic 5
as the free base were distinctly different from those published3
for natural lepadiformine, measurement of the 1H and 13C NMR
spectra of the synthetic hydrochloride salt 5‚HCl allowed direct
The 2.0:1 epimeric mixture of the alcohols 78 and 79
described above was converted to the ketone 81 that underwent
(28) Shishido, Y.; Kibayashi, C. J. Org. Chem. 1992, 57, 2876-2883.
(29) 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).
(27) Garner, P.; Ramakanth, S. J. Org. Chem. 1986, 51, 2609-2611.

Total Synthesis of Fasicularin and Lepadiformine
J. Am. Chem. Soc., Vol. 122, No. 19, 2000 4591
The second fractions afforded 22 (126 mg, 15%) as a white solid,
which was recrystallized from AcOEt-hexane to give colorless
needles: mp 101-102 °C; IR (KBr) 3319, 1732, 1669 cm-1; 1H NMR
(400 MHz, CDCl3) δ 1.18-1.97 (m, 9H), 2.12 (qd, J ) 12.9, 5.6 Hz,
1H), 2.26 (br d, J ) 13.2 Hz, 1H), 2.42 (A part of ABX, J ) 17.2,
12.5, 6.5 Hz, 1H), 2.54 (B part of ABX, J ) 17.2, 5.7, 1.7 Hz, 1H),
3.37 (s, 3H), 3.56 (A′ part of A′B′X, J ) 10.9, 4.5 Hz, 1H), 3.68 (B′
part of A′B′X, J ) 10.9, 7.3 Hz, 1H), 4.58 (m, 1H), 4.68 and 4.69
(ABq, J ) 6.5 Hz, 2H), 5.82 (A′′ part of A′′B′′X, J ) 10.4, 3.6 Hz,
1H), 6.35 (B′′ part of A′′B′′X, J ) 10.4, 1.8 Hz, 1H); 13C NMR (100.6
MHz, CDCl3) δ 20.6, 22.3, 22.5, 27.4, 32.1, 32.6, 38.3, 55.4, 61.5,
68.3, 78.3, 97.2, 123.6, 131.1, 164.7; EIMS m/z (relative intensity) 281
(M+, 1), 221 (12), 45 (100). Anal. Calcd for C15H23NO4: C, 64.04; H,
8.24; N, 4.98. Found: C, 64.07; H, 7.96; N, 4.99.
Figure 2. The X-ray structure (Chem3D representation) of synthetic
(()-lepadiformine hydrochloride [5‚HCl].
comparison with the spectra on natural lepadiformine kindly
provided by Professor Biard, revealing an exact match. This
finding strongly implies that the structure of natural lepadi-
formine reported in the literature3 was actually that of the
hydrochloride salt; this is understandable because the isolation
of the natural alkaloid was made by evaporation of an acidic
solution (MeOH-1 N HCl, 99:1) of the chromatography
fractions. These results therefore clearly indicate that structural
formula 3 involving the zwitterionic form originally assigned
to natural lepadiformine should be revised to 5 as shown.
In summary, the first total synthesis of tricyclic marine
alkaloids (()-fasicularin (2) and (()-lepadiformine (5) was
accomplished. The key common strategic element for the
synthesis is the stereocontrolled intramolecular hetero-Diels-
Alder reaction of an N-acylnitroso moiety to an exocyclic diene
with or without bromine substitution to control the syn-facial
or anti-facial selectivity, respectively, leading to the cis- or trans-
fused decahydroquinoline ring systems involving the simulta-
neous introduction of the nitrogenated quaternary center in a
single step. On further elaboration of the six-membered or five-
membered ring A, the trans-fused adduct provided either (()-
fasicularin (2) or (()-lepadiformine (5), and the cis-fused adduct
was converted into the originally proposed structure 4 of
lepadiformine in the nonzwitterionic form. The present inves-
tigation thus unambiguously established the relative stereo-
chemistry of lepadiformine, formerly assigned incorrectly, to
be 3R*,5S*,7aR*,11aR*.
Benzyl
(2E,4Z)-4-Bromo-4-{2-[3-(1,8-dimethyl-15-oxa-16-
szatetracyclo[6.6.2.02,7.09,14]hexadeca-2,4,6,9,11,13-hexaen-16-yl)-3-
oxopropyl]cyclohexylidene}-2-butenyl Ether (38). A solution of 36
(696 mg, 1.70 mmol) in CHCl3 (10 mL) was added dropwise to a
solution of 9,10-dimethylanthracene (897 mg, 4.26 mmol) and tetra-
propylammonium periodate (moistened with 10% water; 788 mg, 1.70
mmol) in CHCl3 (10 mL). The mixture was stirred for 1 h at room
temperature, quenched with 5% aqueous Na2S2O3 (5 mL), and extracted
with CHCl3 (2 × 50 mL). The organic phase was washed with brine,
dried (MgSO4), and concentrated in vacuo. Purification of the residue
by chromatography (hexane-AcOEt, 25:1) gave 38 (883 mg, 84%) as
a white solid, which was recrystallized from Et2O-hexane to give
colorless needles: mp 41-42 °C; IR (KBr) 1678 cm-1; 1H NMR (400
MHz, CDCl3) δ 1.36-1.80 (m, 9H), 1.88 (td, J ) 14.2, 4.4 Hz, 1H),
2.04 (ddd, J ) 16.3, 10.8, 4.9 Hz, 1H), 2.20 (s, 3H), 2.21 (m, 1H),
2.69 (s, 3H), 3.10 (m, 1H), 4.15 (dd, J ) 5.7, 1.3 Hz, 2H), 4.55 (s,
2H), 6.16 (dt, J ) 14.6, 5.7 Hz, 1H), 6.61 (d, J ) 14.6 Hz, 1H), 7.18-
7.50 (m, 13H).
(3R*,8aR*,12aR*)- and (3R*,8aS*,12aR*)-3-[(Benzyloxy)methyl]-
1-bromo-8,8a,9,10,11,12-hexahydro-3H-[1,2]oxazino[3,2-j]quinolin-
6(7H)-one (39 and 40). A solution of 38 (876 mg, 1.43 mmol) in
benzene (450 mL) was heated at reflux with stirring for 3 h. After
being cooled to room temperature, the solution was concentrated in
vacuo and subjected to chromatography eluting with hexane-AcOEt
(25:1) to give 437 mg (75%) of an inseparable 5.5:1 mixture (based
on 1H NMR analysis) of 39 and 40 as a colorless oil: IR (neat) 1790,
1
1683 cm-1; H NMR (400 MHz, CDCl3) δ 1.16-1.66 (m, 6H), 1.78
(br d, J ) 10.4 Hz, 1H), 1.87 (tdd, J ) 13.6, 6.2, 3.5 Hz, 2H), 2.29-
2.39 and 2.41 (m, and, d, J ) 6.1 Hz, respectively, total 2H in 5.5:1
ratio), 2.50 (m, 1H), 2.57 and 2.62 (dd, J ) 13.5, 6.8 Hz, each, total
1H in 5.5:1 ratio), 3.60 and 3.58 (A part of ABX, J ) 9.9, 7.0 Hz, and
A′ part of A′B′X, J ) 9.8, 7.8 Hz, respectively, total 1H in 5.5:1 ratio),
3.84 and 3.90 (B part of ABX, J ) 9.9, 5.8 Hz, and B′ part of A′B′X,
J ) 9.7, 5.8 Hz, respectively, total 1H in 5.5:1 ratio), 4.54 and 4.56
(1/2ABq, J ) 11.8 Hz, each, total 1H in 5.5:1 ratio), 4.60 and 4.62
(1/2ABq, J ) 11.8 Hz, each, total 1H in 5.5:1 ratio), 4.61 (m, 1H),
6.18 and 6.32 (d, J ) 3.2 Hz, and, d, J ) 2.6 Hz, respectively, total
1H in 5.5:1 ratio), 7.27-7.35 (m, 5H); 13C NMR (100.6 MHz, CDCl3)
δ 21.4 and 21.3 (1 carbon in 5.5:1 ratio), 22.5 and 24.5 (1 carbon
in 5.5:1 ratio), 25.0 and 25.2 (1 carbon in 5.5:1 ratio), 25.8 and 27.9
(1 carbon in 5.5:1 ratio), 29.9 and 33.9 (1 carbon in 5.5:1 ratio), 32.3
and 38.9 (1 carbon in 5.5:1 ratio), 37.2 and 44.7 (1 carbon in 5.5:1
ratio), 66.8 and 64.9 (1 carbon in 5.5:1 ratio), 71.7 and 69.5 (1 car-
bon in 5.5:1 ratio), 73.4 and 72.0 (1 carbon in 5.5:1 ratio), 81.7 and
80.9 (1 carbon in 5.5:1 ratio), 127.7, 127.8 (2C), 128.4 (2C), 129.5,
130.5, 138.0, 169.0 and 169.2 (1 carbon in 5.5:1 ratio); EIMS m/z
Experimental Section30
Preparation of (3R*,8aS*,12aR*)- and (3R*,8aR*,12R*)-2-
[(Methoxymethoxy)methyl]-8,8a,9,10,11,12-hexahydro-3H-[1,2]ox-
azino[3,2-j]quinolin-6(7H)-one (21 and 22): Typical Procedure. To
an ice-cooled, vigorously stirred emulsion of 19 (824 mg, 2.91 mmol)
dispersed in a 5:1 mixture (13 mL) of water and MeOH was added
solid tetrabutylammonium periodate (2.52 g, 5.11 mmol). After being
stirred at 0 °C for 20 min, the mixture was quenched with 10% aqueous
Na2S2O3 (50 mL) and extracted with CHCl3 (2 × 500 mL). The
combined organic extracts were washed with water, dried (MgSO4),
and concentrated in vacuo to give an oil. HPLC analysis (hexane-2-
propanol, 5:1) of the crude product showed the formation of a 4.5:1
mixture of the cycloadducts 21 and 22, which was separated by
chromatography (CHCl3-acetone, 9:1). The first fractions afforded 21
(565 mg, 69%) as a colorless oil, which solidified on storage in a
refrigerator. Recrystallization from hexane afforded colorless needles:
mp 28-29 °C; IR (KBr) 3445, 1679 cm-1; 1H NMR (400 MHz, CDCl3)
δ 1.23-1.88 (m, 10H), 2.41 (dt, J ) 12.9, 3.1 Hz, 1H), 2.45-2.51 (m,
2H), 3.39 (s, 3H), 3.58 (dd, J ) 10.6, 6.1 Hz, 1H), 3.89 (m, 1H), 4.62
(tt, J ) 6.6, 2.7 Hz, 1H), 4.68 and 4.74 (ABq, J ) 6.4 Hz, 2H), 5.88
(A part of ABX, J ) 10.6, 2.4 Hz, 1H), 6.36 (B part of ABX, J )
10.6, 2.5 Hz, 1H); 13C NMR (100.6 MHz, CDCl3) δ 22.1, 24.7, 25.6,
27.1, 34.3, 38.2, 43.7, 55.4, 62.4, 69.9, 79.3, 97.0, 125.8, 130.6, 168.3;
EIMS m/z (relative intensity) 281 (M+, 0.4), 45 (100); HRMS calcd
for C15H23NO4 (M+) 281.1627, found 281.1617.
(relative intensity) 408 (M+ + 3, 6), 406 (M+ + 1, 6), 326 (M+
-
Br, 11), 268 (12), 205 (10), 176 (13), 91 (100). Anal. Calcd for
C20H24NO3Br: C, 59.12; H, 5.95; N, 3.45. Found: C, 59.09; H, 6.08;
N, 3.33.
Acknowledgment. We are grateful to Professor J. F. Biard
(Universite´ de Nantes) for a sample of natural lepadiformine
and Professor D. J. Faulkner (Scripps Institution of Oceanog-
raphy, University of California at San Diego) and Dr. J. Chan
(SmithKline Beecham Pharmaceuticals) for proving authentic
(30) See Supporting Information for General Procedures.

4592 J. Am. Chem. Soc., Vol. 122, No. 19, 2000
Abe et al.
spectra of 1H and 13C NMR of facicularin. We also thank Mr.S.
Igarashi and Dr. K. Yoza (Bruker Japan Co., Ltd.) for X-ray
crystallographic analysis of synthetic (()-lepadiformine hydro-
chloride salt. This work was supported in part by a grant for
private universities provided by the Ministry of Education,
Science, Sports, and Culture and The Promotion and Mutual
Aid Corporation for Private Schools of Japan.
Supporting Information Available: Detailed experimental
procedures and spectral data for 2, 4, 5, 10-19, 26-36, 41-
51, 53-56, 58-69, 71-79, and 81-84; X-ray data and ORTEP
diagrams of 5‚HCl, 23, 41, and 58‚HCl (/PDF). This material
is available free of charge via Internet at http://pubs.acs.org.
JA9939284