Total synthesis and absolute configuration of minalemine A, a guanidine peptide from the marine tunicate Didemnum rodriguesi

Article abstract of DOI:10.1021/jo010076t

The total synthesis of the 3S,2S and 3R,2S diastereomers (1a and 1b) of minalemine A and the identification of the natural compound as the 3R,2S isomer is described. The key step in the synthesis is the preparation of the two enantiomers of the β-amino diacid 3-(N-carboxymethyl)-aminodecanoic acid (Ncma), which were obtained by stereoselective alkylation with allyl bromide of two nonanoic acid imides bearing chiral oxazolidinones as chiral auxiliaries. Natural minalemine A shows identical ¹H NMR and very similar ¹³C NMR spectra compared to the two synthetic diastereomers. Sufficient differences in their chromatographic behavior to allow conclusive identification were not found. However, the corresponding N-2-naphthoyl amides presented quite distinct circular dichroism spectra (CD), and these confirmed the 3R,2S configuration for the natural minalemines and the R configuration for the constituent β-amino diacid, Ncma.

Full text of DOI:10.1021/jo010076t

4206
J . Org. Chem. 2001, 66, 4206-4213
Tota l Syn th esis a n d Absolu te Con figu r a tion of Min a lem in e A, a
Gu a n id in e P ep tid e fr om th e Ma r in e Tu n ica te Did em n u m
r od r igu esi
Angeles Expo´sito,† Miryam Ferna´ndez-Sua´rez,† Teresa Iglesias,† Luis Mun˜oz,*,† and
Ricardo Riguera‡
Departamento de Quı´mica Orga´nica, Facultad de Ciencias, Universidad de Vigo, E-36200 Vigo, Spain,
and Departamento de Quı´mica Orga´nica, Facultad de Qu´ımica, Universidad de Santiago de Compostela,
E-15706 Santiago de Compostela, Spain
lmunoz@uvigo.es
Received J anuary 22, 2001
The total synthesis of the 3S,2S and 3R,2S diastereomers (1a and 1b) of minalemine A and the
identification of the natural compound as the 3R,2S isomer is described. The key step in the
synthesis is the preparation of the two enantiomers of the â-amino diacid 3-(N-carboxymethyl)-
aminodecanoic acid (Ncma), which were obtained by stereoselective alkylation with allyl bromide
of two nonanoic acid imides bearing chiral oxazolidinones as chiral auxiliaries. Natural minalemine
A shows identical 1H NMR and very similar 13C NMR spectra compared to the two synthetic
diastereomers. Sufficient differences in their chromatographic behavior to allow conclusive
identification were not found. However, the corresponding N-2-naphthoyl amides presented quite
distinct circular dichroism spectra (CD), and these confirmed the 3R,2S configuration for the natural
minalemines and the R configuration for the constituent â-amino diacid, Ncma.
In tr od u ction
Marine tunicates belonging to the genus Didemnum
(Phylum Chordata, class Ascidiacea) have proven to be
a particularly rich source of structurally diverse and
biologically potent marine metabolites.1 Most of these
metabolites are nitrogen-containing compounds derived
from amino acids, such as cyclic and acyclic peptides, and
aromatic alkaloids. Some representative examples of the
first group are the hexapeptides comoramides A and B
and the heptapeptides mayotamides A and B,2 isolated
from D. molle. Recent examples of aromatic alkaloids are
the novel predator antideterrant didemnimides A-D3
and the aromatic alkaloids granulatimide and isogranu-
latimide,4 from the Brazilian ascidian D. granulata.
As a result of our studies on the biologically active
extracts from the tunicate D. rodriguesi, we reported
some time ago the guanidinic peptide caledonin A5 and,
more recently, the isolation of minalemines A-F,6 the
first examples in nature of sulfamic acid guanidine
derivatives.
F igu r e 1. Structure of minalemines.
diacid, 3-(N-carboxymethyl)aminodecanoic acid (Ncma),
which can be viewed either as an N-carboxymethyl long
chain â-amino acid or as a nitrogen-modified glycine. In
minalemines, these two amino acids are linked by an
amide bond between the L-Leu amino group and the
carboxyl group on the nitrogen side chain of Ncma. The
two remaining carboxyl groups are bonded to the amino
group of two different R,ω-aminoguanidines: the carboxyl
group of leucine is bonded to agmatine (Agma) and the
carboxyl group of the â-amino acid is bonded to homo-
agmatine (Hagma). The individual minalemines differ in
the chain length of the â-amino acid Ncma (10, 11, or 12
carbons) and in the presence or absence of a sulfamic acid
group on the nitrogen of the same residue (Figure 1).
Minalemines are relatively small peptide-like com-
pounds (Figure 1) that consist of L-leucine and an amino
* Tel: +34-(9)86-812283. Fax: +34-(9)86-812382.
† Universidad de Vigo.
‡ Universidad de Santiago de Compostela.
(1) (a) Faulkner, D. J . Nat. Prod. Rep. 2000, 17, 7-55 and previous
reports in this series. (b) Davidson, B. S. Chem. Rev. 1993, 93, 1771-
1791.
(2) Rudi, A.; Aknin, M.; Gaydou, E. M.; Kashman, Y. Tetrahedron
1998, 54, 13203-13210.
(3) Vervoort, H. C.; Richards-Gross, S. E.; Fenical, W.; Lee, A. Y.;
Clardy, J . J . Org. Chem. 1997, 62, 1486-1490.
(4) Berlinck, R. G. S.; Britton, R.; Piers, E.; Lim, L.; Roberge, M.;
Moreira da Rocha, R.; Andersen, R. J . J . Org. Chem. 1998, 63, 9850-
9856.
(5) Vazquez, M. J .; Quin˜oa´, E.; Riguera, R.; Ocampo, A.; Iglesias,
T.; Debitus, C. Tetrahedron Lett. 1995, 36, 8853-8856.
(6) Expo´sito, M. A.; Lo´pez, B.; Ferna´ndez, R.; Va´zquez, M. J .;
Debitus, C.; Iglesias, T.; J ime´nez, C.; Quin˜oa´, E.; Riguera, R. Tetra-
hedron 1998, 54, 7539-7550.
Their structures, elucidated by spectroscopic (1D and
2D NMR) and spectrometric (MS) methods, are charac-
terized by the presence of two asymmetric carbons. The
absolute stereochemistry at C-2 in the leucine residue
was determined to be S by acid hydrolysis of minalemine
A and comparison with authentic standards by chiral GC.
10.1021/jo010076t CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/17/2001

Total Synthesis and Configuration of Minalemine A
J . Org. Chem., Vol. 66, No. 12, 2001 4207
Sch em e 1
Sch em e 2
Sch em e 3
However, the absolute configuration at C-3 in the â-ami-
no acid Ncma could not be established and remains
unknown.
In this paper we present our results on the stereospe-
cific synthesis of the two enantiomers of the â-amino acid
Ncma and, subsequently, of the two diastereomers (3S,2S)-
1a and (3R,2S)-1b of minalemine A (Scheme 1). Com-
parison of the spectroscopic properties of the two syn-
thetic diastereomers with those of the natural compound
unambiguously proved that the absolute configuration
of natural minalemine A is 3R,2S.
Protected agmatine 7 and protected homoagmatine 5
were prepared by guanilation of the corresponding R,ω-
diamines with the thiourea derivative 8 (Scheme 2). N,N′-
Di-tert-butoxycarbonyl thiourea 8 was obtained by treat-
ment of thiourea with sodium hydride and a slight excess
of di-tert-butyl dicarbonate (BOC2O).8 Reaction of cadav-
erine and putrescine with reagent 8 smoothly afforded
the required protected R,ω-aminoguanidines Hagma (5)
and Agma (7) respectively, in high yields.
The strategy for the stereocontrolled synthesis of the
long chain â-amino acids9,10 4a /4b hinges on the stereo-
selective alkylation of a long chain acid derivatized with
an Evans’ chiral auxiliary (Scheme 3).11 This reaction
guarantees a high degree of optical purity in the two
Resu lts a n d Discu ssion
Leucine is commercially available, and protected ag-
matine and homoagmatine can be prepared by gua-
nilation of cadaverine and putrescine, respectively. For
this reason the main synthetic problem in this study
involved the stereoselective synthesis of the two enanti-
omers of the â-amino acid Ncma (4a /4b) and their
coupling to the other structural fragments, as shown in
Scheme 1.
According to this retrosynthetic analysis, the last
synthetic step is the connection of the chiral fragments
2a and 2b with the bromide 3 by N-alkylation of the
primary amino group of 2. Compounds 2a and 2b were
easily prepared by coupling of the two enantiomers of
Ncma, 4a and 4b, with protected Hagma 5. Compound
3 was synthesized by the sequential coupling of Cbz-
protected7 L-leucine 6 with protected Agma 7 followed by
treatment with bromoacetic acid.
(8) (a) Iwanovicz, E. J .; Poss, M. A.; Lin, J . Synth. Commun. 1993,
23, 1443-1445. (b) Poss, M. A.; Iwanowicz, E.; J oyce, A. R.; Lin, J .;
Gu, Z. Tetrahedron Lett. 1992, 33, 5933-5936.
(9) Reviews on the synthesis of â-amino acids: (a) Cole, D. C.
Tetrahedron 1994, 50, 9517. (b) J uaristi, E.; Quintana, D.; Escalante,
J . Aldrichimica Acta 1994, 50, 9517. (d) Cardillo, G.; Tomasini, C.
Chem. Soc. Rev. 1996, 117. (e) J uaristi, E. Enantioselective Synthesis
of â-Amino Acids; Wiley-VCH: New York, 1996. (f) J uaristi, E.; Lo´pez-
Ruiz, H. Curr. Med. Chem. 1999, 6, 983. (g) Abdel-Magid, A. F.; Cohen,
J . H.; Maryanoff, C. Curr. Med. Chem. 1999, 6, 955.
(10) Leading references on the synthesis of â-amino acids: (a)
Podlech, J .; Seebach, D. Liebigs Ann. 1995, 1217. (b) Seebach, D.;
Overhand, M.; Ku¨hnle, F. N. M.; Martinoni, B.; Oberer, L.; Hommel,
U.; Widmer, H. Helv. Chim. Acta 1996, 79, 913.
In this way, the synthesis is highly convergent since
both diastereomers 1a and 1b are prepared from a
common fragment 3. In addition, the coupling steps
involving leucine are performed at the N-terminus, thus
avoiding potential racemization at its chiral center.
(11) (a) Lago, C.; Ferna´ndez-Sua´rez, M.; Mun˜oz, L.; Riguera, R.
Enantiopure Synthesis of Long Chain â-Amino Acids from Marine
Peptides; Communication presented at the 1st Euroconference on
Marine Natural Products, Athens, November 1997. Further variations
of this method have since been disclosed: (b) Arvanitis, E.; Ernst, H.;
Ludwig (ne´e D’Souza), A. A.; Robinson, A. J .; Wyatt, P. B. J . Chem.
Soc., Perkin Trans. 1 1998, 3, 521-528. (c) Evans, D. A.; Wu, L. D.;
Wiener, J . J . M.; J ohnson, J . S.; Ripin D. H. B.; Tedrow, J . S. J . Org.
Chem. 1999, 64, 6411-6417. (d) Sibi, M. P.; Deshpande, P. K. J . Chem.
Soc., Perkin Trans. 1 2000, 1461-1466.
(7) Bodansky, M.; Bodansky, A. The Practice of Peptide Synthesis,
2nd ed.; Springer-Verlag: New York, 1994.

4208 J . Org. Chem., Vol. 66, No. 12, 2001
Expo´sito et al.
Sch em e 4
Sch em e 5
enantiomers. It also provides functional groups that can
be easily modified to the target compounds by suitable
reactions: the carboxyl group of 13 can be converted to
an amine group, as in 14, and thereafter the terminal
olefin can be converted to the acid, 4. The chiral auxil-
iaries selected for the preparation of each enantiomer are
shown in Scheme 3 and were prepared by reaction of
L-phenylalaninol and commercially available (1S,2R)-
norephedrine with diethyl carbonate in the usual way.12
L-Phenylalaninol was easily prepared by reduction of
L-phenylalanine with the iodine-borohydride system
proposed by Meyers.13 Thus, both oxazolidinones were
sequentially treated with n-butyllithium and nonanoyl
chloride to give the imides 9 and 10 in good yields.
Compounds 9 and 10 were alkylated by sequential
treatment with LDA and allyl bromide to give compounds
11 and 12.14 Their 1H NMR spectra showed no ap-
preciable signals of their diastereomers, thus ensuring
high optical purity at the new chiral center. Acids 13a
and 13b were obtained in high yield after cleavage of the
chiral auxiliaries with lithium hydroperoxide. The car-
boxylic acid group of 13 was converted into the protected
carboxybenzyl amino group through a Curtius-type rear-
rangement15 by treatment with diphenylphosphoryl azide
(DPPA) and subsequent trapping of the isocyanate
intermediate with benzyl alcohol. The protected amines
14a and 14b afforded the acids 4a /4b by oxidative
cleavage of the double bond using the conditions proposed
by Lemieux and Rudloff (NaIO4 and a catalytic amount
of KMnO4 in a basic medium).16 Cbz-N-protected-â-amino
acids 4a and 4b were obtained in an optically pure form
in an overall yield of 53%.
Sch em e 6
Having obtained all of the components required for the
final synthesis, we proceeded to their assembly in the
following way. Coupling of the two enantiomers of the
N-protected â-amino acid (4a and 4b) with protected
Hagma 5 (Scheme 4) using the DCC/DMAP protocol
afforded 15a and 15b in moderate yields. Hydrogenolysis
of 15a and 15b provided compounds 2a and 2b in nearly
quantitative yield.
On the other hand, Cbz-protected L-leucine 6 was
coupled with protected Agma 7 using the DCC/HOBt
protocol to provide 16 in moderate yield (Scheme 5).
Deprotection of 16 by hydrogenolysis and further cou-
pling of 17 with bromoacetic acid in the presence of DCC
(Scheme 5) afforded compound 3.
Subsequently, reaction of each enantiomer 2a /2b with
3 in the presence of Hunig’s base in DMF provided the
fully protected minalemine A derivatives 18a and 18b
in moderate yield (Scheme 6). When this reaction was
carried out with model compounds, such as â-alanine, the
dialkylation product was obtained in relative high yield.
However, the reaction with fragments 2a /2b only af-
(12) Gage, I. R.; Evans, D. A. Org. Synth. 1989, 68, 77-91.
(13) McKennon, J .; Meyers, A. I.; Drauz, K.; Schwarm, J . J . Org.
Chem. 1993, 58, 3568-3571.
(14) (a) Evans, D. A.; Ennis, M. D.; Mathre, D. J . J . Am. Chem. Soc.
1982, 104, 1737-1739. (b) Evans, D. A.; Dow, R.; Shih, T. L.; Takacs,
J . M.; Zahler, R. J . Am. Chem. Soc. 1990, 112, 5290-5313. (c) Evans,
D. A.; Bender, S. L.; Morris, J . J . Am. Chem. Soc. 1988, 110, 2506-
2526.
(15) Eaton, P. E.; Shankar, B. K. R. J . Org. Chem. 1984, 49, 185-
186.
(16) (a) Aristoff, P. A.; J ohnson, P. D.; Harrison, A. W. J . Am. Chem.
Soc. 1995, 107, 7067-7974. (b) Lemieux, R. U.; Rudloff, E. Can. J .
Chem. 1955, 33, 1701-1709.

Total Synthesis and Configuration of Minalemine A
J . Org. Chem., Vol. 66, No. 12, 2001 4209
forded the monoalkylation product in moderate yield. We
ascribed this behavior to the long chain â-amino acid
steric hindrance. Since we obtained enough material for
the final reaction, we did not attempt to optimize the
reaction yield. Finally, complete deprotection was achieved
by acidolysis of the tert-butyl carbamates in a TFA/
dichloromethane solution to afford the diastereomers 1a
and 1b, which were isolated and purified by HPLC and
handled as trifluoroacetates for all further work.
Com p a r ison betw een Syn th etic 1a a n d 1b a n d
Na tu r a l Min a lem in e A. Despite all our efforts to find
appropriate experimental conditions for the effective
chromatographic separation of diastereoisomers 1a and
1b for their comparison with natural minalemine A, we
could not distinguish the two compounds by chromato-
graphic methods. Thus, TLC and analytical reversed-
phase HPLC showed, in all attempts, only a single spot
(e.g., n-BuOH/AcOH/H2O, 12:3:5; Rf ) 0.5) and a single
peak (µ-Bondapak C-18 MeOH/H2O/TFA, 60:40:0.1; tR )
9.8 min), respectively, for both compounds. This behavior
is also coincident with that shown by natural minalemine
A. Normal-phase HPLC could not be performed ef-
fectively as a result of the low solubility of the salts in
normal organic solvent systems.
We also attempted to carry out the identification by
comparison of the specific optical rotations. Unfortu-
nately, however, the values measured for diastereoiso-
mers 1a and 1b (-2.4, c ) 0.56 and -9.9, c ) 1.23,
respectively) are significantly different from the value
reported for the natural product (-22.8, c ) 0.002), thus
rendering this method ineffective in this case.
F igu r e 2. Difference in 13C NMR chemical shifts between
minalemine A and compound 1a and between minalemine A
and compound 1b. The x-axis represents the carbon number
This difference in the optical rotation values warrants
further comment. In our opinion, the discrepancy does
not solely arise from the different concentrations of the
solutions used in the experiment but principally from the
nature and concentration of the anion accompanying the
natural minalemines.17 The synthetic diastereoisomers
were prepared as trifluoroacetate salts, but the counte-
rion of the natural sample is not known.
and the y-axis represents ∆δ (∆δ ) δminalemine
compound).
- δsynthetic
Α
CD spectra of natural minalemine A and the synthetic
samples (1a and 1b) are all very weak as a result of the
absence of a strong chromophore, and unfortunately this
makes meaningful comparisons impossible. To overcome
this drawback, and after several unsuccessful attempts,
we managed to introduce a naphthoic acid group as an
auxiliary chromophore on the secondary amine group
close to the C-3 chiral center of the three compounds. This
was achieved by dissolving the synthetic diastereomers
1a and 1b and minalemine A in pyridine and adding
2-naphthoyl chloride. The mixture was allowed to react
for 24 h (Scheme 7). Purification by reversed-phase HPLC
afforded derivatives 1c, 1d , and 1e, respectively, and
these showed identical retention times under a variety
of conditions.
The CD and UV spectra of these three derivatives were
found to be very useful for comparisons (Figure 3). The
CD curves of 1d and 1e are practically superimposable
and present a positive band centered at 231 nm, while
1c shows a rather different CD spectrum with a positive
band at 250 nm. These data confirm the coincidence of
minalemine A with 1b, the absolute configuration of the
natural minalemines as 3R,2S, and the absolute config-
uration of the Ncma residue as R.
1
A detailed comparison of the H and 13C NMR spectra
of 1a and 1b with those of natural minalemine A,
measured in the same spectrometer, showed only tiny
differences in a few chemical shifts. These differences are
1
smaller than 0.06 ppm in the H NMR spectrum and 0.2
ppm in the 13C NMR spectrum and are of limited use for
our purpose. Thus, the1H NMR spectra of minalemine A
are practically identical to those of the synthetic com-
pounds 1a and 1b. On the other hand, the profile of the
plots for the 13C NMR chemical shifts show some differ-
ences in the carbons close to the asymmetric carbon
under scrutiny (C-3 of Ncma), and this points to a greater
similarity between natural minalemine A and 1b (Figure
2). Diastereomer 1b displays chemical shifts for C-2, C-3,
and C-4 of Ncma that are virtually identical to those of
minalemine A, while 1a shows larger differences in these
signals. In fact, it is in this region of the spectra where
the differences in chemical shifts are most marked.
Therefore, if minalemine A is identical to 1b, its absolute
configuration should be 3R,2S and that of Nmca at C-3
should be R.
Exp er im en ta l Section
Nevertheless, the limited significance of these data
prompted us to confirm the proposed stereochemistry by
other means, particularly by circular dichroism (CD). The
Gen er a l P r oced u r es. THF was distilled from the Na/
benzophenone system. Flash chromatography was carried out
with silica gel 60 (230-400 mesh). TLC chromatography was
done with silica gel 60 F254 on coated aluminum or glass plates.
Compounds were visualized using UV light, iodine vapor,
KMnO4 aqueous solution, or a ninhydrin ethanolic solution.
(17) Ikuko, O.; Takenori, K.; Hiroshi, K.; Kashman, Y.; Sholamit,
H. J . Am. Chem. Soc. 1992, 114, 8472-8479.

4210 J . Org. Chem., Vol. 66, No. 12, 2001
Expo´sito et al.
Di-Boc-a gm a tin e (7). To a vigorously stirred solution of
1,4-diaminobutane (880 mg, 10 mmol) in DMF (30 mL) was
added a solution of 8 (276 mg, 1 mmol) in DMF (25 mL)
dropwise using an addition funnel. After 30 min the addition
had finished. The mixture was diluted with H2O and extracted
with a mixture of EtOAc/diethyl ether 1:1 (10 × 30 mL). The
organic phase was dried over MgSO4, filtered, and concen-
trated in a vacuum to give 7 as a colorless oil (300 mg, 91%
yield). 1H NMR (CDCl3) 1.22-1.56 (m, 22H), 2.71 (brs, 1H),
3.17-3.41(m, 4H), 8.02 (brs, 1H), 8.30 (brs, 1H). 13C NMR
(CDCl3) 26.2, 28.1, 30.5, 40.6, 41.5, 79.0, 82.9, 153.1, 156.0,
163.4. EI-HRMS calcd for [C15H30N4O4]+ 330.2267, found
330.2266.
Sch em e 7
(4R,5S)-5-P h en yl-4-m eth yl-3-(1′-oxon on a n oyl)-2-oxa zo-
lid in on e (9). To a stirred solution of (4R,5S)-5-phenyl-4-
methyl-2-oxazolidinone (0.53 g, 3 mmol) in dry THF (30 mL)
under argon atmosphere at -78 °C was added n-BuLi (1.6 M
in hexanes, 2.15 mL, 3.6 mmol) dropwise. After 10 min,
nonanoyl chloride (0.65 mL, 3.6 mmol) was added, and the
reaction mixture was stirred for another 30 min. Then, the
mixture was allowed to warm to room temperature, a satu-
rated solution of NH4Cl was added, and the stirring was
continued for another 10 min. The reaction mixture was made
basic with 1 M NaOH and extracted with CH2Cl2 (30 × 10
mL). The combined organic layers were washed with brine,
dried over MgSO4, filtered, and concentrated in vacuo. The
residue was purified by flash chromatography (EtOAc/hexanes
1:3) to give 9 as a colorless oil (789.3 mg, 83% yield). 1H NMR
(CDCl3) 0.85-0.89 (m, 6H), 1.25-1.36 (m, 10H), 1.62-1.70 (m,
2H), 2.84-3.01 (m, 2H), 4.76 (q, J ) 6.9, 1H), 5.66 (d, J ) 7.3,
1H), 7.27-7.42 (m, 5 H). 13C NMR (CDCl3) 14.0, 14.4, 22.5,
24.2, 29.0, 29.2, 31.7, 35.5, 78.8, 125.5, 128.5, 128.6, 133.3,
152.9, 173.0. EI-HRMS calcd for [C19H27NO3]+ 317.1991, found
Semipreparative HPLC purifications were done using a
system with a µ-Porasil (7.8 mm × 300 mm) column for
normal-phase separations, while the reversed-phase separa-
tions were performed either with a µ-Bondapack C18 (7.8 mm
× 300 mm) or a Nova-Pack HR-C18 column. A differential
refractometer and/or a diode array were used as detectors.
1H NMR and 13C NMR were recorded on 500, 400, or 300
MHz spectrometers. All chemical shifts (δ) are reported in ppm
relative to TMS using the residual solvent signal as internal
reference: δ 7.26 and 77.0 ppm for the CHCl3 1H and 13C
signals, respectively, and δ 3.30 and 49.05 ppm for the MeOH
1H and 13C signals, respectively. Coupling constants (J ) are
given in Hz and are apparent. Optical rotations were measured
on a polarimeter using a cell of 1 dm path length.
N,N′-Bis-ter t-bu toxyca r bon ylth iou r ea (8). To a stirred
solution of thiourea (571 mg, 7.50 mmol) in THF (150 mL)
under argon at 0 °C were added hexane and sodium hydride
(1.35 g, 33.8 mmol, 60% in mineral oil). After 5 min, the ice
bath was removed, and the reaction mixture was stirred at
room temperature for 10 min. The mixture was cooled to 0 °C
again, di-tert-butyl dicarbonate (3.60 g, 16.5 mmol) was added,
and the ice bath was removed after 30 min of stirring at that
temperature. The resulting slurry was stirred for another 2 h
at room temperature. Then the reaction was quenched with
an aqueous solution of saturated NaHCO3 (10 mL). The
reaction mixture was poured into water (250 mL) and ex-
tracted with EtOAc (3 × 70 mL). The combined organic layer
was dried over Na2SO4, filtered, and concentrated in vacuo to
give 8 as a white solid (1.78 g, 86% yield). Mp 127-129 °C. 1H
NMR (CDCl3) 1.50 (brs, 18 H). 13C NMR 27.4, 83.5, 149.8,
177.3. EI-HRMS calcd for [C11H20N2O4S]+ 276.1144, found
276.1136. Anal. Calcd for C11H20N2O4S: C, 47.81; H, 7.29; N,
10.13; S, 11.60. Found: C, 47.94; H, 7.63; N, 9.84; S, 11.51.
Di-Boc-h om oa gm a tin e (5). To a vigorously stirred solu-
tion of 1,5-diaminopentane (1.02 g, 10 mmol) in DMF (30 mL)
at room temperature was added a solution of 8 (273 mg, 1
mmol) in DMF (30 mL) dropwise through an addition funnel.
Once the addition was finished, the reaction mixture was
poured into water and extracted with a mixture of hexanes/
diethyl ether 1:1 (10 × 50 mL). The organic phase was dried
over MgSO4, filtered, and concentrated in vacuo, to give 5 as
a colorless oil (318 mg, 92% yield). 1H NMR (CDCl3) 1.36-
1.48 (m, 4H), 1.50 (brs, 18 H), 1.53-1.61 (m, 2H), 2.70 (t, J )
6.9, 2H), 3.42 (q, J ) 7.1, 2H). 13C NMR 26.7, 30.6, 30.8, 31.4,
36.0, 43.3, 44.5, 81.7, 85.5, 155.8, 158.6, 166.1. EI-HRMS calcd
for [C16H32N4O4]+ 344.2423, found 344.2410.
317.1990. [R]23 ) +38.1 (c 1.3, CHCl3).
D
(S)-4-P h en ylm eth yl-3-(1′-oxon on a n oyl)-2-oxa zolid in o-
n e (10). This compound was obtained following the same
procedure as for compound 9. Thus, (S)-4-phenylmethyl-2-
oxazolidinone (1.14 g, 6.45 mmol) and nonanoyl chloride (1.4
mL, 7.7 mmol) afforded after workup and purification by flash
chromatography (EtOAc/hexanes 1:30) compound 10 as a
1
colorless oil (1.87 g, 92% yield). H NMR (CDCl3) 0.89 (t, J )
6.9, 3H), 1.29-1.40 (m, 10H), 1.65-1.71 (m, 2H), 2.77 (dd, J
) 13.4 and 9.5, 1H), 2.85-3.01 (m, 2H), 3.29 (dd, J ) 13.4
and 3.3, 1H), 4.14-4.21 (m, 2H), 4.67 (m, 1H), 7.20-7.35 (m,
5H). 13C NMR (CDCl3) 14.0, 22.6, 24.2, 29.0, 29.1, 29.3, 31.7,
35.4, 37.8, 55.0, 66.0, 127.2, 128.8, 129.3, 135.3, 153.4, 173.3.
EI-HRMS calcd for [C19H27NO3]+ 317.1990, found 317.1991.
[R]23 ) +45.3 (c 1.37, CHCl3).
D
(4R,5S)-3-((2′S)-2′-Allyl-1′-oxon on a n oyl)-5-p h en yl-4-
m eth yl-2-oxa zolid in on e (11). A freshly prepared solution of
LDA (0.6 M, 2.96 mL, 1.87 mmol) in anhydrous THF under
argon atmosphere was cooled to -78 °C. A solution of
compound 9 (0.54 g, 1.7 mmol) in dry THF (0.34 mL) was
added dropwise, and once the addition was finished the stirring
was continued for 1 h. Then, allyl bromide (0.44 mL, 5.1 mmol)
was slowly added to the yellow solution. The temperature was
allowed to rise from -60 to -40 °C, and the solution turned
to colorless. The CO2/acetone bath was removed, and the
mixture was stirred for 1 h at 0 °C. Saturated NH4Cl solution
was added, and the mixture was stirred for another 10 min.
The crude reaction was extracted with diethyl ether (3 × 5
mL), and the organic phase was dried over MgSO4, filtered,
and concentrated in a vacuum. The remaining yellow oil was
purified by flash chromatography (EtOAc/hexanes 1:30) to give
11 as a colorless oil (398 mg, 66% yield). 1H NMR (CDCl3) 0.86
(d, J ) 6.5, 3H), 0.89 (d, J ) 6.1, 3H), 1.19-1.32 (m, 10H),
1.51 (m, 1 H), 2.32 (m, 1H), 2.42 (m, 1H), 3.93 (m, 1H), 4.81
(q, J ) 6.7, 1H), 5.01-5.09 (m, 2H), 5.65 (d, J ) 7.3, 1H), 5.82
(m, 1H) 7.28-7.44 (m, 5H). 13C NMR (CDCl3) 14.0, 14.5, 22.6,
27.2, 29.1, 29.6, 31.6, 31.7, 36.7, 42.3, 54.9, 78.6, 116.9, 125.2,
125.6, 125.8, 128.6, 133.3, 135.2, 152.68, 175.8. EI-HRMS calcd
for [C22H31NO3]+ 357.2304, found 357.2308. [R]23 ) +12.0 (c
D
1.0, CHCl3).
(S)-3-((2′R)-Allyl-1′-oxon on a n oyl)-4-p h en yl-m et h yl-2-
oxa zolid in on e (12). Compound 12 was synthesized following

Total Synthesis and Configuration of Minalemine A
J . Org. Chem., Vol. 66, No. 12, 2001 4211
F igu r e 3. UV and CD spectra of compounds 1c, 1d , and 1e.
the same procedure as for 11. Thus, compound 10 (0.8 g, 2.54
mmol) and allyl bromide (0.7 mL, 7.62 mmol) afforded after
workup and purification by flash chromatography (EtOAc/
hexanes 1:30) compound 12 as a colorless oil (709 mg, 78%
2.45 (m, 1H), 5.02-5.10 (m, 2H), 5.78 (m, 1H). 13C NMR
(CDCl3) 14.0, 22.6, 27.2, 29.1, 29.4, 31.5, 31.8, 36.1, 45.2, 116.8,
135.2, 182.5. EI-HRMS calcd [C12H22O2]+ 198.1620, found
198.1621. [R]21 ) -8.1 (c 2.78, CHCl3).
D
1
yield). H NMR (CDCl3) 0.88 (t, J ) 6.8, 3H), 1.28 (brs, 10H),
(2R)-2-Allyln on a n oic Acid (13b). Compound 10 (2.25 g,
6.3 mmol) was submitted to the same procedure as for 11 to
give 13b as a yellowish oil (1.14 g, 91% yield). NMR and EI-
1.51 (m, 1H), 1.42 (m, 1H), 2.34 (m, 1H), 2.46 (m, 1H), 2.67
(dd, J ) 13.1 and 10.0, 1H), 3.92 (m, 1H), 4.10-4.20 (m, 2H),
4.69 (m, 1H), 5.03-5.12 (m, 2H), 5.83 (m, 1H), 7.22-7.35 (m,
5H). 13C NMR (CDCl3) 14.0, 22.5, 27.2, 29.1, 29.6, 31.5, 31.7,
36.7, 38.0, 42.3, 55.4, 65.8, 117.0, 127.2, 128.8, 129.3, 135.3,
135.4, 153.1, 176.1. EI-HRMS calcd for [C22H31NO3]+ 357.2303,
HRMS data are identical to that of its enantiomer 13a . [R]21
) +8.6 (c 3.02, CHCl3).
D
(4S)-4-Ben zyloxycar bon yl-u n dec-1-en e (14a). To a stirred
solution of 13a (197 mg, 1 mmol) in dry toluene (10 mL) under
argon atmosphere were added Et3N (0.14 mL, 1 mmol) and
diphenyl phosphoryl azide (DPPA) (0.2 mL, 1 mmol). The
mixture was refluxed for 8 h. Benzyl alcohol (0.5 mL) was then
added, and the reflux was continued for another 15 h. Finally,
the mixture was allowed to reach room temperature, it was
dissolved in EtOAc, and then it was sequentially washed with
H2O (3 × 20 mL) and brine (2 × 20 mL). The organic phase
was dried over MgSO4, filtered, and concentrated in a vacuum.
The residue was purified by flash chromatography (EtOAc/
hexanes 1:30) to give 14a as a white crystalline solid (251 mg,
83% yield). Mp 71-72 °C. 1H NMR (CDCl3) 0.86 (t, J ) 6.7,
3H), 1.23-1.31 (m, 10H), 1.46 (brs, 1H), 1.61 (brs, 1H), 2.13-
2.27 (m, 2H), 3.69 (d, J ) 6.1, 1H), 4.53 (d, J ) 8.5, 1H), 5.03-
5.08 (m, 4H), 5.73 (m, 1H), 7.27-7.36 (m, 5H). 13C NMR
(CDCl3) 14.1, 22.6, 25.8, 29.2, 29.4, 31.8, 34.6, 39.4, 50.7, 66.5,
117.7, 128.0, 128.4, 134.3, 136.7, 156.0. EI-HRMS calcd for
found 357.2302. [R]26 ) +47.8 (c 1.09, CHCl3).
D
(2S)-2-Allyln on a n oic Acid (13a ). To a vigorously stirred
solution of 11 (0.57 g, 1.6 mmol) in a mixture of THF/H2O 4:1
(8 mL) at 0 °C were added H2O2 (30%, 1.45 mL, 12.8 mmol)
and LiOH (76.7 mg, 3.2 mmol) dissolved in H2O (5 mL). After
10 min, the ice bath was removed, and the mixture was stirred
for 1 h. The mixture was treated with a solution of Na2SO3
(1.61 g, 12.8 mmol) in water, and then it was stirred for
another 10 min. The aqueous residue was basified with a
solution of NaHCO3 (5%) and extracted with CH2Cl2 (3 × 25
mL). From the organic phase the chiral auxiliary was recov-
ered. The aqueous phase was acidified with HCl (6 M) until
pH 1, and the mixture was extracted with diethyl ether (4 ×
25 mL). The combined organic phase was washed with brine,
dried over MgSO4, filtered, and concentrated in a vacuum.
Compound 13a was obtained as a yellowish oil (305 mg, 96%
yield). 1H NMR (CDCl3) 0.88 (t, J ) 6.7, 3H), 1.23-1.32 (m,
10H), 1.50 (m, 1H), 1.62 (m, 1H), 2.27 (m, 1H), 2.39 (m, 1H),
[C19H29NO2]+ 303.2198, found 303.2202. [R]23 ) -18.1 (c 1.0,
D

4212 J . Org. Chem., Vol. 66, No. 12, 2001
Expo´sito et al.
CHCl3). Anal. Calcd for C19H29NO2: C, 75.21; H, 9.63; N, 4.61.
Found: 75.26; H, 9.53; N, 4.62.
P ep tid e (16). N-Cbz-L-leucine was obtained using literature
procedures7 as a yellow oil. A solution of N-Cbz-L-leucine in
toluene was prepared and titrated with NaOH 0.1 M using
phenolphthalein as indicator. To a stirred solution of compound
7 (881 mg, 2 mmol) in CH2Cl2 at 0 °C were sequentially added
a solution of N-Cbz-L-leucine (0.7 M, 2.86 mL, 2 mmol), HOBt
(270.3 mg, 2 mmol), and DCC (433.3 mg, 2.1 mmol). After 10
min the ice bath was removed, and the stirring was continued
overnight at room temperature. The mixture was filtered, and
the filtrate was concentrated in vacuo. The residue was
purified by flash chromatography (EtOAc/hexane 1:3) to afford
16 as a colorless oil that solidified on standing (84 mg, 42%
(4R)-4-Ben zyloxyca r b on yl-u n d ec-1-en e (14b ). Com-
pound 13b (580 mg, 2.95 mmol) was submitted to the same
Curtius-like reaction, giving 14b as a white crystalline solid
(742 mg, 83% yield). Mp, NMR, and EI-HRMS data are
identical to those of its enantiomer. [R]23 ) +19.5 (c 1.10,
D
CHCl3).
(3S)-3-Ben zyloxyca r bon yla m in o-d eca n oic Acid (4a ).
To a vigorously stirred solution of NaIO4 (1.42 g, 6.7 mmol) in
H2O (30 mL) was added KMnO4 (237 mg, 0.15 mmol). K2CO3
(112.5 mg, 0.81 mmol), t-BuOH (7.5 mL), and compound 14a
(0.23 g, 0.74 mmol) in t-BuOH (7.5 mL) were added to the
purple solution. After 2 h of stirring at room temperature the
solution turned to pink. Ethylenglycol (0.2 mL, 3.25 mmol) was
added, and the stirring was maintained for another 2 h. Finally
the solution was treated with HCl (1.5 M) until pH 1 and
extracted with EtOAc (4 × 30 mL). The combined organic layer
was washed with brine (40 mL), dried over MgSO4, filtered,
and concentrated in a vacuum. Compound 4a was obtained
as white solid (235 mg, 99% yield). Mp 89-92 °C. 1H NMR
(CDCl3) 0.81 (t, J ) 6.8, 3H), 1.19 (brs, 10H), 1.48 (brs, 10H),
2.53 (brs, 2H), 3.90 (brs, 1H), 5.03-5.13 (m, 2H), 7.23-7.31-
(m, 5H). 13C NMR (CDCl3) 14.0, 22.6, 26.1, 29.1, 29.2, 31.7,
34.3, 38.8, 48.0, 66.7, 128.1, 128.5, 136.4, 155.9, 176.5. EI-
1
yield). Mp 56-60 °C. H NMR (CDCl3) 0.90 (d, J ) 5.9, 6H),
1.46-1.67 (m, 25H), 3.21-3.37 (m, 4H), 4.12 (m, 1H), 5.06 (brs,
2H), 5.26 (d, J ) 8.1, 1H), 6.34 (brs, 1H), 7.27-7.35 (m, 5H),
8.31 (brs, 1H). 13C NMR 22.0, 22.9, 24.7, 26.4, 26.6, 28.0, 39.1,
40.3, 41.5, 53.6, 67.0, 79.4,83.2, 128.0, 128.2, 128.5, 136.2,
153.2, 156.2, 163.3, 172.1. FAB-HRMS calcd for [C29H47N5O7
+ H]+ 578.3509, found 578.3528. [R]27D ) -14.5 (c 1.15, CHCl3).
P ep tid e 17. To a stirred solution of compound 16 (84 mg,
0.14 mmol) in MeOH (15 mL) was added Pd (10% on activated
carbon). The mixture was stirred under hydrogen atmosphere
at 1 atm for 5 h. The crude was filtered through Celite, and
the filtrate was concentrated in vacuo. Compound 17 was
obtained as a colorless oil (62 mg, 100% yield). 1H NMR
(CDCl3) 0.90 (d, J ) 6.1, 3H), 0.93 (d, J ) 6.2, 3H), 1.25-1.70
(m, 25H), 3.19-3.48 (m, 5H), 7.39 (s, 1H), 8.30 (d, J ) 4.6,
1H). 13C NMR (CDCl3) 21.2, 23.4, 24.8, 26.5, 26.9, 28.0, 28.2,
38.5, 40.4, 43.9, 53.4, 79.2, 83.1, 153.2, 156.1, 163.5, 175.6. EI-
HRMS calcd for [C18H27NO4]+ 321.1940, found 321.1931. [R]23
D
) -12.3 (c 1.0, CHCl3). Anal. Calcd for C18H27NO4: C, 67.27;
H, 8.47; N, 4.36. Found: C, 67.26; H, 8.67; N, 4.49.
(3R)-3-Ben zyloxyca r bon yla m in o-d eca n oic Acid (4b).
Compound 14b (349 mg, 1.15 mmol) was submitted to the
same procedure as for 14a to give 4b as a white solid (343
mg, 93% yield). Mp, NMR, and EI-HRMS data are identical
HRMS calcd for [C21H41N5O5]+ 443.3108, found 443.3122. [R]23
) -14.7 (c 4.97, CHCl3).
D
Com p ou n d 3. To a stirred solution of bromoacetic acid (66
mg, 0.47 mmol) in CH2Cl2 (1 mL) cooled at 0 °C were
sequentially added compound 17 (150 mg, 0.34 mmol), DCC
(76.3 mg, 0.37 mmol), and a catalytic amount of DMAP. The
ice bath was removed after 10 min, and the mixture was
concentrated in vacuo. The residue was purified by flash
chromatography (EtOAc/hexanes 1:1). Compound 3 was ob-
tained as a white solid (129 mg, 67% yield). Mp 115-118 °C.
1H NMR (CDCl3) 1.00 (brs, 6H), 1.45 (brs, 18H), 1.47-1.65
(m, 7 H), 3.14-3.36 (m, 4H), 3.76 (s, 2H), 4.36 (dd, J ) 8.3
and 5.8, 1H), 6.62 (t, J ) 5.7, 1H), 7.13 (d, J ) 8.3, 1H), 8.27
(t, J ) 5.1, 1H). 13C NMR (CDCl3) 22.2, 22.8, 24.7, 26.4, 26.6,
28.0, 28.2, 28.6, 39.2, 40.3, 41.1, 52.3, 79.4, 83.2, 153.2, 156.1,
163.3, 166.0, 171.4. FAB-HRMS calcd for [C23H42N5O6Br + H]+
to those of its enantiomer 4a . [R]25 ) +10.0 (c 0.48, CHCl3).
D
P ep tid e 15a . To a stirred solution of 4a (321 mg, 1 mmol)
in CH2Cl2 cooled at 0 °C with an ice bath were sequentially
added amine 5 (434 mg, 1.26 mmol), DCC (227 mg, 1.1 mmol),
and DMAP (cat.). The mixture was stirred at 0 °C for 10 min
and then at room temperature for 19 h. The DCU was filtered
off, and the solution was concentrated in vacuo. The residue
was purified by flash chromatography (EtOAc/hexane 1:1) to
give compound 15a as a white solid (404 mg, 62% yield). Mp
72-75 °C. 1H NMR (CDCl3) 0.84 (t, J ) 6.8, 3H), 1.19-1.63
(m, 34H), 2.35 (dd, J ) 14.8 and 6.1, 2H), 2.44 (dd, J ) 14.9
and 4.4, 1H), 3.19 (dd, J ) 13.1 and 6.3, 2H), 3.37 (dd, J )
12.4 and 6.7, 2H), 3.84 (d, J ) 5.4, 1H), 5.05 (brs, 2H), 5.50 (d,
J ) 8.3, 1H), 5.88 (brs, 1H), 7.25-7.32 (m, 5H), 8.31 (brs, 1H).
13C NMR (CDCl3) 14.0, 22.6, 24.0, 24.9, 25.6, 26.2, 28.0, 28.2,
28.5, 29.0, 29.1, 29.3, 31.7, 33.9, 34.6, 39.1, 40.6, 41.0, 49.0,
66.4, 127.8, 127.9, 128.4, 136.6, 153.3, 156.1, 156.2, 163.5,
170.8. FAB-HRMS calcd for [C34H58N5O7 + H]+ 648.4336,
566.2376, found 566.2394. [R]23 ) -21.4 (c 1.59, CHCl3).
D
P ep tid e 18a . To a stirred solution of 2a (250 mg, 0.5 mmol)
in DMF (0.5 mL) was added DIEA (0.13 mL, 0.75 mmol)
dropwise, and the mixture was cooled to 0 °C. Then compound
3 (282 mg, 0.5 mmol) was added, and the mixture was stirred
for 2 h at room temperature. The crude was poured into water,
and the resulting mixture was extracted with EtOAc. The
organic phase was dried over MgSO4, filtered, and concen-
trated in a vacuum. The residue was purified by normal-phase
HPLC, column µ-Porasil 7.8 mm × 300 mm; mobile phase
OEtAc/MeOH 9.5:0.5; isocratic; flow 2 mL/min. Compound 18a
found 648.4339. [R]25 ) -5.7 (c 2.10, CHCl3).
D
P ep tid e 15b. Using the same procedure as for 15a , but
starting from 4b (96.3 mg, 0.3 mmol) and 5 (100 mg, 0.3 mmol),
isomer 15b (105 mg, 54% yield) was obtained as a white solid.
Mp, NMR, and FAB-HRMS data are identical to those of its
enantiomer 15a . [R]24 ) +4.8 (c 1.02, CHCl3).
1
D
was obtained as a light orange solid (215 mg, 43% yield). H
P ep tid e 2a . To a stirred solution of 15a (375 mg, 0.6 mmol)
in MeOH (15 mL) was added Pd (10% on activated charcoal,
250 mg), and the mixture was stirred under hydrogen atmo-
sphere at normal pressure for 5 h. The crude was filtered
through Celite, and the filtrate was concentrated in a vacuum.
Compound 2a was obtained as a colorless oil (303 mg, 98%
yield) and was used without further purification. 1H NMR
(CDCl3) 0.88 (t, J ) 6.9, 3H), 1.26-1.62 (m, 34H), 2.16 (d, J )
11.8, 1H), 2.37 (d, J ) 15.5, 1H), 3.12 (brs, 1H), 3.25 (dd, J )
12.8 and 6.6, 2H), 3.41 (dd, J ) 12.4 and 7.4, 2H), 8.30 (brs,
1H). 13C NMR (CDCl3) 14.0, 22.5, 24.2, 24.9, 25.6, 25.9, 28.0,
28.2, 28.5, 29.0, 29.1, 29.4, 31.7, 33.9, 38.4, 38.8, 40.6, 43.0,
48.8, 50.3, 79.1, 83.0, 153.2, 156.0, 163.5, 172.0. FAB-HRMS
NMR (CDCl3) 0.83 (t, J ) 6.8, 3H), 0.87 (d, J ) 6.1, 3H), 0.89
(d, J ) 6.2, 3H), 1.20-1.66 (m, 51H), 2.13 (dd, J ) 14.8 and
8.0, 1H), 2.33 (dd, J ) 14.9 and 3.5, 1H), 2.92 (brs, 1H), 3.13-
3.35 (m, 10H), 4.38 (dd, J ) 13.7 and 8.2, 1H), 6.65 (t, J ) 5.3,
1H), 6.86 (t, J ) 5.3, 1H), 7.73 (d, J ) 8.3, 1H), 8.28 (brs, 1H).
13C NMR (CDCl3) 13.9, 22.00, 22.5, 22.8, 24.1, 24.7, 25.9, 26.4,
26.6, 27.9, 28.2, 28.3, 28.5, 29.0, 29.1, 29.5, 31.6, 33.8, 38.9,-
39.0, 40.2, 40.3, 40.6, 48.7, 51.5, 55.1, 79.1, 83.0, 153.1, 156.0,
163.4, 172.1, 171.6, 171.9. FAB-HRMS calcd for [C49H92N10O11
+ H]+ 997.7025, found 997.7073. [R]24D ) -8.3 (c 0.98, CHCl3).
P ep t id e 18b . Using the same procedure as for 18a ,
compound 2b (190 mg, 0.37 mmol) was treated with DIEA (0.1
mL, 0.5 mmol) and compound 3 (211 mg, 0.37 mmol). After
the same workup and purification, compound 18b was ob-
tained as a light orange solid (82 mg, 22% yield). 1H NMR
(CDCl3) 0.86 (t, J ) 6.9, 3H), 0.89 (d, J ) 6.1, 3H), 0.92 (d, J
) 6.0, 3H), 1.13-1.70 (m, 51 H), 2.17 (dd, J ) 10.7 and 5.5,
1H), 2.46 (dd, J ) 14.9 and 2.9, 1H), 2.95 (brs, 1H), 3.23-3.47
calcd for [C28H54N2O6 + H]+ 514.3982, found 514.3992. [R]22
D
) +7.6 (c 2.57, CHCl3).
P ep tid e 2b. Following the same previous procedure from
15b (256 mg, 0.4 mmol) compound 2b (197 mg, 96% yield) was
obtained. [R]23 ) -4.9 (c 1.05, CHCl3).
D

Total Synthesis and Configuration of Minalemine A
J . Org. Chem., Vol. 66, No. 12, 2001 4213
(m, 10H), 4.44 (m, 1H), 6.50(brs, 1H), 7.17 (brs, 1H), 7.85 (d,
J ) 8.4, 1H), 8.31 (brs, 1H). 13C NMR (CDCl3) 14.0, 21.8, 22.6,
23.0, 24.1, 24.8, 25.8, 26.6, 28.0, 28.3, 28.6, 29.0, 29.1, 29.6,
31.7, 39.1, 39.3, 40.5, 40.6, 41.1, 49.0, 52.0, 55.4, 79.3, 83.1,
153.2, 156.1, 163.5, 171.1, 171.6, 172.5. FAB-HRMS calcd for
[C49H92N10O11 + H]+ 997.7025, found 997.6988. [R]24D ) -21.3
(c 0.52, CHCl3).
172.4, 174.5. FAB-HRMS calcd for [C29H60N10O3 + H]+ 597.4928,
found 597.4917. [R]24 ) -9.9 (c 1.23, MeOH).
D
P r oced u r e for Der iva tiza tion of 1a , 1b, a n d Min a le-
m in e A. Syn th esis of Com p ou n d s 1c, 1d , a n d 1e. To a
stirred solution of 1a (1 mg, 0.002 mmol) in pyridine (0.15 mL)
was added an excess of 2-naphthoyl chloride. The mixture was
stirred for 48 h, concentrated in vacuo, and purified by
reversed-phase HPLC, column NovaPack HR-C18 7.8 mm ×
300 mm; mobile phase MeOH/H2O/TFA (70:30:0.1); isocratic;
flow 2 mL/min, affording 1c (0.51 mg, 43% yield); tR ) 11.0
min. ESI-LRMS m/z ) 865.6 [M + TFA]+.
Derivatization of 1b (1 mg, 0.002 mmol) by the same
procedure as for 1a and purification using the same conditions
as for 1c afforded 1d (0.54 mg, 45% yield). tR ) 11.0 min. ESI-
LRMS m/z ) 865.6 [M + TFA]+.
(S,S) Dia ster eoisom er (1a ). A solution of 18a (34 mg,
0.034 mmol) in a mixture of CH2Cl2/TFA 1:1 (3 mL) was stirred
for 3 h. The mixture was concentrated in vacuo several times
after adding several portions of diethyl ether. The residue was
purified by reversed-phase HPLC, µ-Bondapack C18 7.8 mm
× 300 mm; mobile phase MeOH/H2O/TFA (60:40:0.1); isocratic;
flow 2 mL/min. Compound 1a was obtained as a solid (21 mg,
55% yield). 1H NMR (MeOD) 0.93 (t, J ) 6.8, 3H), 0.97 (d, J )
6.6, 3H), 1.00 (d, J ) 6.6, 3H), 1.34-1.46 (m, 12H), 1.56-1.66
(m, 11H), 1.72 (m, 1H), 1.79 (m, 1H), 2.61 (dd, J ) 16.4 and
7.2, 1H), 2.73 (dd, J ) 16.2 and 4.2, 1H), 3.19-3.25 (m, 8H),
3.55 (m, 1H), 3.92 (d, J ) 15.7, 1H), 3.97 (d, J ) 15.7, 1H),
4.41 (dd, J ) 9.3 and 5.8, 1H). 13C NMR (MeOD) 14.3, 21.9,
23.3, 23.6, 25.0, 25.9, 26.3, 27.1, 27.5, 29.4, 29.8, 30.0, 30.3,
31.6, 32.8, 34.9, 39.7, 40.2, 42.1, 42.3, 42.4, 46.6, 53.7, 57.7,
158.7, 166.6, 172.4, 174.5. FAB-HRMS calcd for [C29H60N10O3
+ H]+ 597.4928, found 597.4918. [R]24D ) -2.4 (c 0.56, MeOH).
(R,S) Dia ster eoisom er (1b). Compound 18b (30 mg, 0.03
mmol) was also deprotected using CH2Cl2/TFA 1:1 (3 mL), and
the reaction product was submitted to the same purification
process as for 1a to afford compound 1b (80 mg, 45% yield).
1H NMR (MeOD) 0.94 (t, J ) 6.9, 3H), 0.97 (d, J ) 6.6, 3H),
1.00 (d, J ) 6.6, 3H), 1.34-1.46 (m, 12H), 1.56-1.66 (m, 1H),
1.72 (m, 1H), 1.79 (m, 1H), 2.61 (dd, J ) 16.5 and 7.1, 1H),
2.79 (dd, J ) 16.4 and 4.4, 11H), 3.19-3.26 (m, 8H), 3.55 (m,
1H), 3.92 (d, J ) 15.7, 1H), 3.98 (d, J ) 15.7, 1H), 4.41 (dd, J
) 8.2 and 6.6, 1H). 13C NMR (MeOH) 14.3, 21.9, 23.3, 23.6,
25.0, 25.9, 26.3, 27.1, 27.5, 29.4, 29.8, 30.1, 30.3, 31.7, 32.8,
35.0, 39.7, 40.2, 42.1, 42.2, 42.4, 46.5, 53.7, 57.5, 158.7, 166.6,
Min a lem in e A (1 mg, 0.002 mmol) was derivatized with
the same procedure as that followed for 1a . Purification using
the same conditions as for 1c gave 1e (0.48 mg, 40% yield). tR
) 11.0 min. ESI-LRMS m/z ) 865.6 [M + TFA]+.
Ack n ow led gm en t. This work was financially sup-
ported by grants from Xunta de Galicia (XUGA 30112A96
and PGIDT99PX130105A). A.E. acknowledges a fellow-
ship from Xunta de Galicia. We are grateful to Dr. Koji
Nakanishi and Dr. Nina Berova, Columbia University
(New York), for assistance with the comparative study
by circular dichroism (CD).
Su p p or tin g In for m a tion Ava ila ble: 1H NMR, 13C NMR,
and DEPT spectra of natural minalemine A, 1a , 1b, and all
the synthetic intermediates and mass spectra, UV, and CD
spectra of 1c, 1d , and 1e. This material is available free of
charge via the Internet at http://pubs.acs.org.
J O010076T