Convergent synthesis of (-)-mirabazole C using a chloroimidazolidium coupling reagent, CIP

Article abstract of DOI:10.1021/jo952285h

Convergent synthesis of (-)-mirabazole C (1), a tetra thiazoline/thiazole alkaloid isolated from blue-green alga, has been described. The successive thiazoline rings of (-)-mirabazole C were formed by a single-step cyclization mediated by TiCl₄ treatment of tripeptide amide 4. Convergent synthesis of the key intermediate 33 derived from three 2-methylcysteine residues was first achieved using a newly developed coupling reagent, 2-chloro-1,3-dimethylimidazolidium hexafluorophosphate (CIP). The effectiveness of CIP for the coupling of α,α-dialkyl amino acids and the reaction pathway of the activation were clarified by the syntheses of model peptides containing an α,α-dimethylamino acid. A practical method of asymmetric synthesis of 2-methylcysteine by alkylation of 2,4-cis-oxazolidinone 23 has also been described.

Full text of DOI:10.1021/jo952285h

3350
J . Org. Chem. 1996, 61, 3350-3357
Con ver gen t Syn th esis of (-)-Mir a ba zole C Usin g a
Ch lor oim id a zolid iu m Cou p lin g Rea gen t, CIP
Kenichi Akaji, Naohiro Kuriyama, and Yoshiaki Kiso*
Department of Medicinal Chemistry, Kyoto Pharmaceutical University, Yamashina-ku, Kyoto 607, J apan
Received December 28, 1995X
Convergent synthesis of (-)-mirabazole C (1), a tetra thiazoline/thiazole alkaloid isolated from blue-
green alga, has been described. The successive thiazoline rings of (-)-mirabazole C were formed
by a single-step cyclization mediated by TiCl4 treatment of tripeptide amide 4. Convergent synthesis
of the key intermediate 33 derived from three 2-methylcysteine residues was first achieved using
a newly developed coupling reagent, 2-chloro-1,3-dimethylimidazolidium hexafluorophosphate (CIP).
The effectiveness of CIP for the coupling of R,R-dialkyl amino acids and the reaction pathway of
the activation were clarified by the syntheses of model peptides containing an R,R-dimethylamino
acid. A practical method of asymmetric synthesis of 2-methylcysteine by alkylation of 2,4-cis-
oxazolidinone 23 has also been described.
(-)-Mirabazole C (1) was isolated by Moore and co-
considered to be biosynthesized from this unusual amino
acid. However, subsequent to the structural revision of
tantazole B 2 by Fukuyama and Xu,5 the structure of
mirabazole C was re-examined synthetically by Parsons
and Heathcock6 and revised to 1, in which the ring-A
stereocenter has the R- rather than the S-configuration.
workers1 from the terrestrial cyanophyte Scytonemea
mirabile (Dillwyn) Bornet (strain BY-8-1) following the
structure elucidation of tantazole2 2 from the same blue-
green alga. Mirabazole C and structurally related mira-
bazoles, tantazoles, and thiangazole3 (3) are a unique
group of thiazoline/thiazole-containing alkaloids. Several
members of this family of alkaloids have been shown to
exhibit selective cytotoxicity against murine solid tumors
or to exhibit unusual high inhibitory activity against
HIV-1 protease in vitro.4
Because of the novel structural features as well as
interesting biological activities, these alkaloids have
stimulated considerable interest. In the synthetic works
regarding these alkaloids, basically two different strate-
gies have been adopted to construct the successive thia-
zoline rings: one is the sequential formation of thiazoline
rings by cyclocondensation of 2-methylcysteine with
imino ether7 or by transformation of ammonium thioester
into thiazoline5 and the other is a simultaneous formation
of thiazoline rings by Lewis acid mediated cycloconden-
sation of an appropriate precursor peptide consisting of
2-methylcysteine.6,8 The latter procedure is more ef-
ficient than the former regarding the formation of suc-
cessive thiazoline rings, but efficient preparation of the
precursor peptide is often difficult because of the ex-
tremely low reactivity of 2-methylcysteine.9 To overcome
this difficulty, we have developed an efficient coupling
agent, chloroimidazolidium agent 2-chloro-1,3-dimeth-
ylimidazolidium hexafluorophosphate (CIP), suitable for
the condensation of an R,R-dialkylamino acid. Here we
describe a method of convergent synthesis of (-)-mira-
bazole C using a newly developed coupling agent as well
as the efficiency and reaction pathway of this new
coupling agent.
Resu lts a n d Discu ssion
Scheme 1 shows our retrosynthetic route of (-)-
mirabazole C. The four successive thiazoline rings are
simultaneously formed at the final step of the synthesis
from the appropriately derivatized tripeptide amide 4
containing three 2-methylcysteine residues. The simul-
taneous formation by TiCl4-mediated cyclodehydration
was shown to be effective by Heathcock and co-workers6,8
in the syntheses of mirabazoles and thiangazole. The
Originally, the three asymmetric carbons of mirabazole
C had been elucidated to be in an S-configuration, be-
cause of the strong structural resemblance of mirabazole
C to the tantazoles. The tantazoles had been degraded
to (R)-2-methylcysteine, and the natural products were
X Abstract published in Advance ACS Abstracts, May 1, 1996.
(1) Carmeli, S.; Moore, R. E.; Patterson, G. M. L. Tetrahedron Lett.
1991, 32, 2593.
(2) Carmeli, S.; Moore, R. E., Patterson, G. M. L.; Corbett, T. H.;
Valeriote, F. A. J . Am. Chem. Soc. 1990, 112, 8195.
(3) J ansen, R.; Kunze, B.; Reichenbach, H.; J urkiewicz, E.; Hun-
smann, G.; Ho¨fle, G. Liebigs Ann. Chem. 1992, 357.
(4) J urkiewicz, E.; J ansen, R.; Kunze, B.; Trowitzsch-Kienast, W.;
Forche, E.; Reichenbach, H.; Ho¨fle, G.; Hunsmann, G. Antiviral Chem.
Chemother. 1992, 3, 189.
(5) Fukuyama, T.; Xu, L. J . Am. Chem. Soc. 1993, 115, 8449.
(6) Parsons, R. L., J r.; Heathcock, C. H. Tetrahedron Lett. 1994, 35,
1379.
(7) Boyce, R. J .; Pattenden, G. Synlett 1994, 587.
(8) Parsons,R. L., J r; Heathcock, C. H. J . Org. Chem. 1994, 59, 4733.
(9) Walker, M. A.; Heathcock, C. H. J . Org. Chem. 1992, 57, 5566.
S0022-3263(95)02285-7 CCC: $12.00 © 1996 American Chemical Society

Convergent Synthesis of (-)-Mirabazole C
J . Org. Chem., Vol. 61, No. 10, 1996 3351
R ) Fmoc
Ta ble 1. Yield s (%) of d ip ep tid es obta in ed by th e d iffer en t cou p lin g m eth od s
R ) Boc
R ) Cbz
PyBroP
CIP
CIP/HOAt
PyBroP
CIP
CIP/HOAt
PyBroP
CIP
CIP/HOAt
R-Aib-Val-OMe
R-Aib-Aib-OMe
R-Val-Aib-OMe
37
4
41
43
6
44
82
80
80
60
17
60
59
11
60
92
82
85
57
12
50
59
10
51
93
90
87
Sch em e 1
or lower racemization level than the anchoring reaction
using the conventional esterification reagents.
To evaluate the coupling efficiency, three types of
model dipeptides containing Aib, the most simple R,R-
dialkylamino acid, were synthesized using CIP: R-Aib-
Val-OMe, R-Val-Aib-OMe, and R-Aib-Aib-OMe (R ) Boc,
Cbz, or Fmoc) (Table 1). Coupling proceeded at 25 °C
for 60 min without an additive. For comparison, the
same model peptide was prepared using PyBroP under
the same conditions. For the Aib-Val and Val-Aib
sequences, the yields were moderate when Cbz or Fmoc
was employed as the NR-protecting group, whereas NR-
Boc amino acids gave lower yields than the Cbz and Fmoc
dipeptides. These low yields can be explained by the easy
formation of the corresponding N-carboxy anhydride
(NCA) of Boc amino acid as described by Fre´rot et al.15
In the preparation of the Aib-Aib sequence, extremely
low yields were obtained for all three NR-protecting
groups. To improve the yield of CIP coupling, we then
added DMAP16 (13), HOBt17 (14), HODhbt18 (15), or
HOAt19 (16) to the reaction mixture. DMAP and HOBt
are common additives used to enhance the reactivity of
an active ester of an amino acid in peptide synthesis. The
HODhbt ester of Fmoc amino acid has been reported to
be an effective acylating agent in solid-phase peptide
synthesis. HOAt, developed recently by Carpino, has also
been reported to show pronounced reactivity enhance-
ment in the acylation reaction, probably because of the
neighboring group effect caused by the pyridine ring.
most difficult aspect in this scheme is an efficient
preparation of the precursor tripeptide amide 4, since the
coupling of an R,R-dialkylamino acid such as 2-methyl-
cysteine is one of the most difficult reactions in peptide
synthesis.9,10 The convergent routes for preparation of
the tripeptide amide were unsuccessful even with PyBroP
which had been the best one among the coupling reagents
available in the previous synthesis of mirabazole C.9
Thus, prior to the synthesis of (-)-mirabazole C according
to the scheme, we developed an efficient coupling agent
and examined its efficiency as well as the reaction
pathway using model peptides.
Cou p lin g Agen t. For the coupling of an R,R-dialkyl
amino acid such as R,R-dimethyl amino acid (Aib, R-ami-
noisobutyric acid), a preactivated form of NR-protected
Aib (fluoride11 10 or N-carboxy anhydride12 12) and a
coupling reagent (PyBroP,13 11) have been developed and
applied for synthesis of Aib-containing peptaibols. Con-
sidering that a coupling reagent would be more conve-
nient and applicable to the coupling of general R,R-
dialkylamino acids than the preactivated form, we
examined the use of CIP (9), originally developed by us
for an efficient esterification reaction, as a coupling
reagent.14 We had demonstrated that the esterification
of the Fmoc amino acid derivative to 4-alkoxybenzyl
alcohol resin using CIP proceeded rapidly with the same
(10) Slomczynska, U.; Beusen, D. D.; Zabrocki, J .; Kociolek, K.;
Redlinski, A.; Reusser, F.; Hutton, W. C.; Leplawy, M. T.; Marshall,
G. R. J . Am. Chem. Soc. 1992, 114, 4095.
(11) Wenschuh, H.; Beyermann, M.; Krause, E.; Carpino, L. A.;
Bienert, M. Tetrahedron Lett. 1993, 34, 3733.
(12) Spencer, J . R.; Antonenko, V. V.; Delaet, N. G. J .; Goodman,
M. Int. J . Pept. Protein Res. 1992, 40, 282.
(13) Fre´rot, E.; Coste, J .; Pantaloni, A.; Defour, M-N.; J ouin P.
Tetrahedron 1991, 47, 259 and references cited therein.
(14) (a) Akaji, K.; Kuriyama, N.; Kimura, T.; Fujiwara, Y.; Kiso, Y.
Tetrahedron Lett. 1992, 33, 3177. (b) Akaji, K.; Kuriyama, N.; Kiso, Y.
Tetrahedron Lett. 1994, 35, 3315.
(15) Fre´rot, E.; Coste, J .; Poncet, J .; J ouin, P.; Castro, B. Tetrahedron
Lett. 1992, 33, 2815.
(16) Hassner, A.; Krepski, R.; Alexanian, V. Tetrahedron 1978, 34,
2069.
(17) Ko¨nig, W.; Geiger, R. Chem. Ber. 1973, 106, 3626.
(18) Atherton, E.; Cameron, L.; Meldal, M.; Sheppard, R. J . Chem.
Soc., Chem. Commun. 1986, 1763.
(19) Carpino, L A. J . Am. Chem. Soc. 1993, 115, 4397.

3352 J . Org. Chem., Vol. 61, No. 10, 1996
Akaji et al.
F igu r e 1. 1H-NMR spectra (4-5 ppm) of Fmoc-Aib-OH (17), its anhydride (18), and oxazolone 19.
All additives examined markedly enhanced the reac-
tivity in catalytic amount. Table 1 shows the yields
obtained by addition of HOAt in the coupling using CIP.
The dipeptides including NR-Boc peptides were obtained
in excellent isolation yields. The amount of D-Val in the
dipeptides prepared by the CIP/HOAt procedure was less
than 0.5% when the derivatized hydrolysate of R-Val-
Aib-OMe was examined by chiral GC analysis.20 This
shows that the coupling can be conducted without detect-
able racemization.
To estimate the reaction mechanism of the CIP cou-
pling, two expected intermediates, Fmoc-Aib anhydride
(18) and 2-[(9-fluorenylmethyl)oxy]-4-dimethyl-5-oxazolo-
ne (19), were prepared by the treatment of Fmoc-Aib-
OH with DIPCDI. Both compounds were isolated and
purified by silica gel column chromatography and char-
F igu r e 2. Time course of the activation. The reaction mixture
was examined periodically by H-NMR.
acterized. From the comparison of 1H-NMR chemical
shifts of the two compounds (Figure 1), we could identify
the upfield shift in δ of the methyleneoxy protons of the
oxazolone 19 as described earlier for an oxazolone derived
from Fmoc-Val-OH.21 This upfield shift makes it possible
to estimate each amount of Fmoc-Aib-OH, its anhydride
18, and the oxazolone 19 in the reaction mixture using
1H NMR.
1
For comparison, activation with DIPCDI and PyBroP was
also examined. In the activation with DIPCDI, slow
formation of the anhydride (60% after 19 h) and no
significant formation of the oxazolone were detected. By
contrast, immediate transformation to the oxazolone (ca.
90% after 15 min) was detected by CIP activation; 60%
transformation to the oxazolone was detected when
PyBroP was used for activation. The kinds of intermedi-
ates and their formation rates were clearly different in
these three coupling reagents (Figure 2). In the prepara-
tion of Fmoc-Aib-Val-OMe and Fmoc-Aib-Aib-OMe using
CIP in CDCl3, we could follow the gradual disappearance
Thus, Fmoc-Aib-OH in CDCl3 was treated with CIP
1
and the mixture was examined periodically by H NMR.
(20) Frank, H.; Woiwode, W.; Nicholson, G.; Bayer, E. Liebigs Ann.
Chem. 1981, 354.
(21) (a) Paquet, A.; Chen, F. M. F.; Benoiton, N. L.; Can. J . Chem.
1984, 62, 1335. (b) Perlow, D. S.; Erb, J . M.; Gould, N. P.; Tung, R. D.;
Freidinger, R. M.; Williams, P. D.; Veber, D. F. J . Org. Chem. 1992,
57, 4394.
1
of the oxazolone methyleneoxy peaks on H NMR. The

Convergent Synthesis of (-)-Mirabazole C
J . Org. Chem., Vol. 61, No. 10, 1996 3353
Sch em e 2
Fmoc-Aib-Aib-OMe (21). The direct reaction of the
oxazolone with an amine component is slow to moderate
depending on the reactivity of the amine.
Syn th esis of (-)-Mir a ba zole C. The model studies
described above suggest that the coupling of 2-methyl-
cysteine would be feasible using CIP/HOAt as a coupling
agent. We then examined a practical route for stereo-
selective synthesis of (R)- or (S)-2-methylcysteine neces-
sary for the preparation of the precursor tripeptide amide
33.
Few routes have been successfully applied for the
asymmetric synthesis of 2-methylcysteine because of the
labile nature of the sulfhydryl group. Attempts to
prepare 2-methylcysteine by asymmetric alkylation of
chiral thiazolidine have been unsuccessful due to the
predominant â-elimination under usual reaction condi-
tions.22 An extremely low reaction temperature (below
-90 °C) and a unique starting thiazolidine derivative
were neccesarry to avoid this side reaction.23 A new
asymmetric route based on chiral aziridine synthons24
or an alternative route based on chiral bislactim syn-
thons25 contains multistep reactions to give the desired
product with moderate yield. These reports prompt us
to disclose an access for 2-methylcysteine from simple
R-amino acids.
Figure 4 shows our synthetic route for (R)-2-methyl-
cysteine. Application of the BF3‚OEt2-mediated conden-
sation of Cbz-D-Ala-OH (22) with benzaldehyde dimethyl
acetal was a modification of the method of Karady et al.26
to give the oxazolidinone 23. Under this condition, a
major isomer crystallized easily had the 2,4-cis configu-
ration as reported before.26 The lower reaction temper-
ature and fewer equivalents of BF3‚OEt2 caused increase
of the 2,4-trans isomer 26 ratio. Alkylation of 23 with
bromomethyl benzyl sulfide proceeded with a reasonable
yield using lithium diethylamide (LDEA) as a base.
F igu r e 3. Time course of the coupling reaction. Fmoc-Aib-
Aib-OMe was prepared by CIP coupling with or without an
additive. The reaction mixture was examined periodically by
1H-NMR.
rates of the peak disappearance corresponded well with
the isolation yields shown in Table 1.
1
By the same monitoring method using H NMR, the
catalytic enhancement effects of the additives for the CIP
coupling were also examined; the order of enhancement
was HOAt > HODhbt > DMAP > HOBt (Figure 3). In
the preparation of Fmoc-Aib-Aib-OMe, the coupling rate
obtained by the addition of 0.5 equiv of HOBt was ca.
nine times faster and that obtained by the addition of
0.25 equiv of HOAt was ca. 33 times faster than the rate
without additives. The oxazolone-derived peaks disap-
peared completely within 20 min in CIP coupling in the
presence of HOAt, HODhbt, or DMAP. In the CIP/HOBt
coupling, fast formation of another intermediate and its
gradual disappearance were detected on the 1H-NMR
spectrum. The intermediate would be the oxybenzotria-
zole ester of Fmoc-Aib.
These findings suggest that the CIP coupling proceeds
according to the pathway shown in Scheme 2. The
oxazolone of Fmoc-Aib formed through CIP activation is
transformed to a highly active intermediate (20) by a
catalytic amount of the additive; then the active inter-
mediate ester quickly reacts with H-Aib-OMe to give
(22) J eanguenat, A.; Seebach, D. J . Chem. Soc., Perkin Trans. I
1991, 2291.
(23) Pattenden, G.; Thom, S. M.; J ones, M. F. Tetrahedron 1993,
49, 2131.
(24) Shao, H.; Zhu, Q.; Goodman, M. J . Org. Chem. 1995, 60, 790.
(25) Groth, U.; Scho¨llkopf, U. Synthesis 1983, 37.
(26) Karady, S.; Amato, J . S.; Weinstock, L. M. Tetrahedron Lett.
1984, 25, 4337.

3354 J . Org. Chem., Vol. 61, No. 10, 1996
Akaji et al.
F igu r e 4. Synthetic route for (R)-2-methylcysteine.
F igu r e 6. Synthetic route for (-)-mirabazole C (1).
reasonable yields (60% and 55%, respectively) with CIP/
HOAt being the only coupling reagent, although the
reaction rates were somewhat lower than those between
the simple R,R-dimethylamino acids. The Cbz group of
32 was then removed with HBr-AcOH, and the resulting
amine was acylated with isobutyryl chloride to obtain 33.
Our synthetic route shown in Figure 5 is one of the
convergent routes which were unsuccessful with PyBroP
in the previous synthetic work.9
Finally, the key intermediate 33 was converted to (-)-
mirabazole C according to the route shown in Figure 6.
Four benzyl groups, which were thiol protecting groups,
were removed by treatment of 33 with sodium in am-
monia. Without further purification, the isolated crude
tetrathiol product 4 is immediately treated with titanium
tetrachloride in CH2Cl2 to obtain dihydromirabazole C
(34). During this cyclization reaction, a side product
containing three but not the terminal unsubstituted
thiazoline ring 35 was also formed. This side product
was easily removed by flash chromatography. The
terminal thiazoline ring of the purified dihydromiraba-
zole C (34) was then oxidized by nickel peroxide to (-)-
mirabazole C without difficulty. The synthetic material
had the same spectroscopic properties as those reported
for the natural product.
F igu r e 5. Synthetic route for the key intermediate tripeptide
amide 33.
Alkylated product 24 was obtained with an extremely
poor yield when lithium diisopropylamide (LDA) or
lithium hexamethyldisilazide (LHMDS) was employed as
the base with or without dimethylpropyleneurea (DMPU).
The same phenomena have been described in the related
oxazolidinone by Seebach and Fadel.27 Subsequent hy-
drolysis of 24 proceeded without difficulty. N-(Carboben-
zyloxy)-S-benzyl-(R)-2-methylcysteine (25) was prepared
by a three-step reaction from Cbz-D-Ala-OH in 28%
overall yield; the same or superior yield than the reported
values. Protected (S)-2-methylcysteine derivative 29 was
similarly prepared starting from Cbz-L-Ala-OH.
Figure 5 shows the synthetic route for the key inter-
mediate 33. N-(Carbobenzyloxy)-S-benzyl-(R)-2-methyl-
cysteine (25) was coupled with S-benzyl-2-aminoethaneth-
iol by a 60 min reaction using CIP/HOAt to obtain
S-protected-2-methylcysteine amide 30 quantitatively.
The Cbz group of 30 was removed by treatment with
HBr-AcOH, and the product was coupled with 25 using
CIP/HOAt. The same deprotection by HBr-AcOH and
the coupling by CIP/HOAt were repeated for the conden-
sation of 31 and N-(carbobenzyloxy)-S-benzyl-(S)-2-me-
thylcysteine (29) to give tripeptide amide 32. The desired
di- and tripeptide amides (31 and 32) were obtained in
Con clu sion
CIP in the presence of additive was found to be a
suitable agent for the coupling of R,R-dialkylamino acid.
All model dipeptides containing Aib have been obtained
with quantitative yields using CIP/HOAt as a coupling
agent. In the model studies, we found that the first
intermediate, 2-[(9-fluorenylmethyl)oxy]-4-dimethyl-5-ox-
azolone formed by CIP activation, is transformed to the
highly active ester by the catalytic additive to give the
desired peptide without detectable racemization.
Convergent synthesis of (-)-mirabazole C was first
achieved using CIP/HOAt. For the simultaneous forma-
tion of successive thiazoline rings of (-)-mirabazole C,
the TiCl4-mediated cyclization of a tripeptide amide (4)
derived from 2-methylcysteine was successfully achieved.
A convergent route for the key intermediate 33 by the
successive coupling of 2-methylcysteine was first achieved
using CIP/HOAt as a condensation agent. We consider
that the combination of the TiCl4-mediated single-step
(27) Seebach, D.; Fadel, A. Helv. Chim. Acta 1985, 68, 1243.

Convergent Synthesis of (-)-Mirabazole C
J . Org. Chem., Vol. 61, No. 10, 1996 3355
F m oc-Aib An h yd r id e (18) a n d 2-[(9-F lu or en ylm eth yl)-
oxy]-4-d im eth yl-5-oxa zolon e (19). To a stirred solution of
Fmoc-Aib-OH (0.2 g, 0.61 mmol) in dry CH2Cl2 (5 mL) was
added DIPCDI (97 µL, 0.61 mmol). After being stirred for 3 h
at 25 °C, the mixture was directly applied to a silica gel column
which was eluted with CHCl3:MeOH ) 40:1. The fast eluting
fraction (Rf ) 0.91 CHCl3:MeOH ) 40:1, oxazolone 19) and
the slow eluting fraction (Rf ) 0.62 CHCl3:MeOH ) 40:1,
cyclization and the efficient coupling of 2-methylcysteine
using CIP/HOAt should be generally applicable for the
syntheses of other members of the tantazole-mirabazole
family of alkaloids.
Exp er im en ta l Section
Gen er a l. All reactions involving air or moisture sensitive
reagents were conducted under an atmosphere of N2 in
septum-stoppered flasks. Solvents were reagent grade and
dried prior to use. All flash chromatographic separations were
carried out on Wakogel FC-40 obtained from WAKO Pure
Chemical Ind. Ltd. Organometallic reagents and DIPCDI
were obtained from Aldrich Chemical Co. D- and L-alanine
were purchased from Peptide Institute (Osaka). Fmoc amino
acid derivatives and PyBroP were obtained from CALBIO-
CHEM NOVABIOCHEM and used without further purifica-
tion.
1
anhydride 18) were separated. Oxazolone: yield 30 mg; H
NMR (270 MHz, CDCl3) δ 1.43 (s, 6H), 4.38 (t, J ) 7.3 Hz,
1H), 4.56 (d, J ) 7.3 Hz, 2H), 7.26-7.46 (m, 4H), 7.64 (d, J )
7.6 Hz, 2H), 7.78 (d, J ) 7.6 Hz, 2H); 13C NMR (68 MHz,
CDCl3) δ 25.3, 46.4, 66.8, 71.9, 120.2, 125.3, 127.2, 128.1, 141.4,
1
142.8, 157.0, 178.8. Anhydride: yield 40 mg; H NMR (270
MHz, CDCl3) δ 1.54 (s, 6H), 4.22 (t, J ) 6.8 Hz, 1H), 4.39 (d,
J ) 6.8 Hz, 2H), 7.26-7.43 (m, 4H), 7.60 (d, J ) 7.3 Hz, 2H),
7.76 (d, J ) 7.6 Hz, 2H); 13C NMR (68 MHz, CDCl3) δ 25.1,
47.2, 56.5, 66.5, 120.0, 125.0, 127.1, 127.7, 141.3, 144.0, 154.9,
175.1.
Mon itor in g of th e Activa tion Step Usin g 1H NMR
(F igu r e 2). To a solution of 25 mg of Fmoc-Aib-OH in 0.8
mL of CDCl3 were added DIEA (27 µL) and a coupling reagent
(1 equiv). The mixture was examined immediately on a 1H-
NMR instrument. The methyleneoxy doublet peaks at 4.29
ppm due to the starting Fmoc-Aib-OH were gradually replaced
by doublet peaks at 4.39 ppm due to the anhydride 18 or by
those at 4.56 ppm due to the oxazolone 19. Figure 2 shows
the time course of the activation.
Melting points were uncorrected. The 1H- and 13C-NMR
spectra were recorded on a 270 or 300 MHz spectrometer with
TMS as an internal standard. Optical rotations were mea-
sured at ambient temperature using a 1 mL cell. Analytical
HPLC was carried out on a reverse phase column (4.6 × 150
mm), which was eluted with a linear gradient of CH3CN (40-
90%, 30 min) in 0.1% aqueous TFA at a flow rate of 1.0 mL/
min.
Syn th eses of Aib-Con ta in in g Mod el Dip ep tid es (Ta ble
1). To the suspension of NR-protected amino acid (0.5 mmol)
with or without additive (0.25 equiv) in CH2Cl2 (4 mL) was
added DIEA (4 equiv) at an ice-bath temperature. Then, the
addition of a coupling agent (1 equiv) to the mixture was
followed by that of 1.1 equiv of HCl salt of amino acid methyl
ester. The mixture was stirred for 60 min at 25 °C and then
taken into AcOEt (20 mL). The solution was washed with 5%
citric acid and H2O and then dried over MgSO4. The solvent
was removed in vacuo, and the residue was purified by silica
gel column chromatography which was eluted with n-hexane:
AcOEt ) 6:4. The purified product was crystallized from
n-hexane. Table 1 summarizes each isolated yield. All known
dipeptides, NR-Boc and NR-Cbz dipeptide methyl esters, had
the same characterization data as the published values.13 The
characterization data of newly synthesized NR-Fmoc dipeptide
methyl esters are as follows. Fmoc-Aib-Val-OMe: mp 99-101
°C, 1H NMR (270 MHz, CDCl3) δ 0.86 (d, J ) 6.9 Hz, 3H),
0.92 (d, J ) 6.9 Hz, 3H), 1.52 (s, 3H), 1.55 (s, 3H), 2.17 (d of
heptet, J ) 8.6 Hz, 6.9 Hz, 1H), 3.70 (s, 3H), 4.20 (t, J ) 6.8
Hz, 1H), 4.41 (d, J ) 6.9 Hz, 2H), 4.52 (dd, J ) 8.6 Hz, 4.6 Hz,
1H), 5.45 (s, 1H), 6.82 (d, J ) 4.6 Hz, 1H), 7.26-7.78 (m, 8H).
Anal. Calcd for C25H30N2O5: C, 68.47; H, 6.90; N, 6.39.
Found: C, 68.06; H, 6.91; N, 6.36. Fmoc-Aib-Aib-OMe: mp
Mon itor in g of th e Cou p lin g Rea ction Usin g 1H NMR
(F igu r e 3). To a mixture of Fmoc-Aib-OH (25 mg) and DIEA
(74 µL) with or without an additive (0.25 equiv) in CDCl3 (0.8
mL) were added a coupling reagent (1 equiv) and HCl‚ H-Aib-
OMe (3 equiv). The mixture was similarly examined as above.
The methyleneoxy doublet peaks at 4.56 ppm due to the
oxazolone 19 were gradually replaced by those at 4.41 ppm
due to Fmoc-Aib-Aib-OMe (21), the desired product. Figure 3
shows the time course of the coupling reaction. The rate
constant of CIP coupling without additive (first 90 min), 1.87
× 10-3 min-1. The rate constant of CIP/HOBt (0.5 equiv) (first
90 min), 17 × 10-3 min-1. The rate constant of CIP/HOAt (0.1
equiv) (first 90 min), 17 × 10-3 min-1. The rate constant of
CIP/HOAt (0.25 equiv) (first 40 min), 61 × 10-3 min-1
.
(2R,4R)-2-P h en yl-3-(ca r boben zyloxy)-4-m eth yloxa zo-
lid in -5-on e (23). To a stirred solution of N-Cbz-D-alanine
(10.0 g, 45 mmol) and benzaldehyde dimethyl acetal (6.8 mL,
45 mmol) in Et2O (90 mL) was added 33 mL (270 mmol) of
BF3‚Et2O at -78 °C. The mixture was stirred at -15 °C for 4
days. The reaction mixture was slowly added to ice-cooled,
saturated aqueous NaHCO3 (200 mL), and the mixture was
stirred for 30 min. After settling, the separated organic layer
was washed with 5% NaHCO3 and H2O and then dried
(MgSO4). The solvent was removed in vacuo. The residue was
dissolved in 140 mL of Et2O/hexane (3:4), and the solution was
kept standing at 4 °C for 2 h to precipitate white crystals with
a yield of 10.1 g (72%): mp 60-61 °C; HPLC, tR 15.92 min;
1
123-125 °C; H NMR (270 MHz, CDCl3) δ 1.51 (s, 6H), 1.54
(s, 6H), 3.72 (s, 3H), 4.21 (t, J ) 6.9 Hz, 1H), 4.41 (d, J ) 6.9
Hz, 2H), 5.40 (s, 1H), 6.87 (s, 1H), 7.26-7.78 (m, 8H). Anal.
Calcd for C24H28N2O5: C, 67.90; H, 6.65; N, 6.60. Found: C,
67.76; H, 6.62; N, 6.64. Fmoc-Val-Aib-OMe: mp 139-141 °C;
1H NMR (270 MHz, CDCl3) δ 0.94 (d, J ) 6.6 Hz, 3H), 0.97 (d,
J ) 6.6 Hz, 3H), 1.53 (s, 6H), 2.10 (m, 1H), 3.71 (s, 3H), 3.97
(m, 1H), 4.22 (t, J ) 6.9 Hz, 1H), 4.39 (m, 2H), 5.47 (br d, 1H),
[R]20 ) +27.02 (c ) 0.9, CHCl3); IR (KBr) 1794, 1709, 1697,
D
1460, 1452, 1417 cm-1; H NMR (270 MHz, CDCl3) δ 1.57 (d,
1
6.54 (br s, 1H), 7.26-7.77 (m, 8H). Anal. Calcd for C25
-
J ) 6.9 Hz, 3H), 4.48 (q, J ) 6.9 Hz, 1H), 5.16 (br s, 2H), 6.64
(s,1H), 7.25-7.40 (m, 10H); 13C NMR (68 MHz, CDCl3) δ 18.22,
52.15, 67.98, 89.06, 126.25, 128.12, 128.52, 128.64, 128.77,
129.79, 135.38, 136.94, 153.40, 172.52; EI-MS, 311.117 for M+
(calcd 311.116 for C18H17NO4).
H30N2O5: C, 68.47; H, 6.90; N, 6.39. Found: C, 68.44; H, 6.97;
N, 6.38.
Estim a tion of D-Va l Con ten t. R-Val-Aib-OMe (ca. 2 mg)
obtained above was hydrolyzed with 6 N HCl at 110 °C in a
sealed tube. After 24 h, the solvent was removed in vacuo.
To the residue was added 10% HCl in n-BuOH (500 µL), and
the mixture was heated at 110 °C for 2 h. The solvent was
removed in vacuo, and the resulting residue was taken into
pentafluoropropionic anhydride (50 µL). The mixture was
heated at 110 °C for 30 min, and then the excess reagent was
removed under an N2 stream. The residue was dissolved in
CH2Cl2 (100 µL), and an aliquot (1 µL) of the solution was
applied to a GC capillary Chirasil-Val-L column (0.26 mm ×
25 m; elution condition, 95 °C for 2 min and then temperature
gradient 4 °C/min): tR; Aib 3.785 min, D-Val 6.045 min, L-Val
6.585 min. The D-Val content of all dipeptides examined was
less than 0.5%, the value due to hydrolysis.
A small amount of (2S,4R)-oxazolidinone 26 was recovered
from the mother liquor when the reaction was performed at
-20 °C with 2 equiv of BF3‚Et2O for 1 day: mp 77-78 °C;
HPLC, tR 15.25 min; [R]24D ) -95.91 (c ) 0.7, CHCl3); IR (KBr)
1801, 1713, 1460, 1448, 1416 cm-1; 1H NMR (270 MHz, CDCl3)
δ 1.69 (br d, 3H), 4.52 (br q, 1H), 5.00 (br m, 2H), 6.48 (s, 1H),
7.26-7.41 (m, 10H); 13C NMR (68 MHz, CDCl3) δ 16.41, 16.69,
52.04, 67.65, 89.43, 126.56, 128.10, 128.48, 128.88, 130.10,
135.18, 136.42, 152.00, 172.27; EI-MS, 311.115 for M+ (calcd
311.116 for C18H17NO4).
(2S,4S)-Oxazolidinone 27 was prepared from Cbz-L-Ala-OH
using the same procedure described for (2R,4R)-oxazolidinone

3356 J . Org. Chem., Vol. 61, No. 10, 1996
Akaji et al.
23: [R]21 ) -27.11 (c ) 1.1, CHCl3); EI-MS, 311.117 for M+
N-(Ca r b oben zyloxy)-S-b en zyl-(R)-m et h ylcyst ein yl-S-
ben zyl-(R)-m eth ylcystein e N-((Ben zylth io)eth yl)a m id e
[Cbz-MeCys(Bn )-MeCys(Bn )-NHCH2CH2SBn ] (31). To a
stirred solution of Cbz-MeCys(Bn)-NHCH2CH2SBn (30, 0.85
g, 1.67 mmol) in CH2Cl2 (1 mL) was added HBr/AcOH (3 mL)
at 4 °C, and the mixture was stirred for 60 min at 25 °C. Ice-
cooled CH2Cl2 (20 mL) and 5% NaHCO3 (20 mL) were added
to the mixture at 4 °C. The organic layer was washed with
5% NaHCO3 and H2O, and dried (MgSO4). The solution was
rotary evaporated at 4 °C to give HBr‚H-MeCys(Bn)-NHCH2-
CH2SBn as an oil.
D
(calcd 311.116 for C18H17NO4).
(2R,4R)-2-P h en yl-3-(car boben zyloxy)-4-m eth yl-4-(((ph e-
n ylm eth yl)th io)m eth yl)-oxazolidin -5-on e (24). To a stirred
solution of diethylamine (1.2 mL, 11.6 mmol) in dry THF (15
mL) at -78 °C was added 7.2 mL (11.6 mmol) of n-BuLi (1.6
M in hexane), and the mixture was stirred at -5 °C for 10
min. The solution was cooled to -78 °C, and a solution of 3.0
g (9.6 mmol) of 23 in THF (14.5 mL) was added at -78 °C.
After the solution was stirred for 40 min, a solution of 3.1 g
(14.3 mmol) of bromomethyl benzyl sulfide9 in 2 mL of THF
was added at -78 °C and the mixture was stirred overnight
at 2 °C. The reaction was quenched with saturated aqueous
NH4Cl, and the product was extracted with AcOEt (150 mL).
The organic layer was washed with 5% NaHCO3 and H2O,
dried (MgSO4), and rotary evaporated. The crude product was
purified by silica gel column chromatography using CHCl3
followed by flash chromatography using hexane:AcOEt (10:1)
to yield 1.67 g (39%) of 24 as an oil: [R]23D ) +129.11 (c ) 2.8,
CHCl3); IR (CHCl3) 1794, 1713, 1497, 1456, 1408, 1371, 1348
cm-1; 1H NMR (300 MHz, 80 °C, DMSO-d6) δ 1.72 (s, 3H), 2.95
(d, J ) 16.2 Hz, 1H), 3.51 (d, J ) 16.2 Hz, 1H), 3.69 (d, J )
12.3 Hz, 1H), 3.76 (d, J ) 12.3 Hz, 1H), 5.02 (s, 2H), 6.62 (s,
1H), 7.21-7.50 (m, 15H); 13C NMR (68 MHz, CDCl3) δ 23.54,
36.69, 36.98, 63.95, 67.38, 90.49, 126.97, 127.28, 127.69,
128.32, 128.50, 128.60, 128.73, 129.02, 135.18, 136.87, 137.64,
151.82, 173.87; FAB-MS, 448.158 for [M + H]+ (calcd 448.158
for C26H26NO4S).
To the solution of Cbz-MeCys(Bn)-OH (25, 0.81 g, 2.26
mmol) in CH2Cl2 (5 mL) were added DIEA (1.16 mL, 6.68
mmol) and HOAt (0.16 g, 1.17 mmol). CIP (0.70 g, 2.50 mmol)
and the above HBr‚H-MeCys(Bn)-NHCH2CH2SBn in CH2Cl2
(3 mL) were then added to the mixture. The reaction mixture
was stirred for 72 h at 25 °C. The solvent was removed in
vacuo, and the residue was extracted with AcOEt (50 mL). The
organic layer was washed with 5% citric acid, 5% NaHCO3,
and H2O and dried (MgSO4). The crude product was purified
by silica gel column chromatography using CHCl3 followed by
flash chromatography using hexane/AcOEt (5:2) to yield 0.72
g (60 %) of the desired dipeptide amide 31 as an oil: [R]23
)
D
+24.05 (c ) 0.4, EtOH); IR (CHCl3) 1714, 1685, 1664, 1535,
1
1495, 1454 cm-1; H NMR (270 MHz, CDCl3) δ 1.46 (s, 6H),
2.55 (t, J ) 7.1 Hz, 2H), 2.73 (d, J ) 13.5 Hz, 1H), 2.89 (d, J
) 13.5 Hz, 1H), 2.96 (d, J ) 13.5 Hz, 1H), 3.33 (d, J ) 13.5
Hz, 1H), 3.38 (q, J ) 6.7 Hz, 2H), 3.59 (s, 2H), 3.66 (s, 2H),
3.70 (s, 2H), 5.06 (s, 2H), 5.42 (s, 1H), 6.62 (s, 1H), 7.18-7.31
(m, 21H); 13C NMR (68 MHz, CDCl3) δ 22.79, 23.59, 30.39,
35.81, 37.27, 37.77, 39.01, 39.10, 59.53, 60.16, 67.38, 126.92,
127.22, 127.44, 128.23, 128.46, 128.61, 128.73, 128.84, 128.88,
155.68, 171.48, 172.36; FAB-MS, 716.269 for [M + H]+ (calcd
716.265 for C39H46N3O4S3).
N-(Ca r b ob en zyloxy)-S-b en zyl-(S)-m et h ylcyst ein yl-S-
b e n zyl-(R )-m e t h ylcyst e in yl-S -b e n zyl-(R )-m e t h ylcys-
tein e N-((Ben zylth io)eth yl)a m id e [Cbz-MeCys(Bn )-Me-
Cys(Bn )-MeCys(Bn )-NHCH 2CH2SBn ] (32). To a stirred
solution of Cbz-MeCys(Bn)-MeCys(Bn)-NHCH2CH2SBn (31,
0.67 g, 0.94 mmol) in CH2Cl2 (1.5 mL) was added HBr/AcOH
(4 mL) at 4 °C, and the mixture was stirred for 90 min at 25
°C. The deprotected HBr salt of the dipeptide amide HBr‚
H-MeCys(Bn)-MeCys(Bn)-NHCH2CH2SBn was isolated as de-
scribed above for 31.
(2S,4S)-Oxazolidinone 28 was similarly prepared from 27.
28: [R]26 ) -128.93 (c ) 0.6, CHCl3); FAB-MS, 448.159 for
D
[M + H]+ (calcd 448.158 for C26H26NO4S).
N-(Ca r b ob e n zyloxy)-S-b e n zyl-(R)-2-m e t h ylcyst e in e
[Cbz-MeCys(Bn )-OH] (25). To a stirred solution of 24 (1.24
g, 2.77 mmol) in THF/H2O (3:1, 40 mL) was added LiOH‚H2O
(0.23 g, 5.49 mmol) at 4 °C. The mixture was stirred overnight
at 25 °C, and then 5% NaHCO3 (100 mL) was added to the
reaction mixture. The aqueous layer was washed with n-
hexane and then acidified to pH 2 using 2 N HCl. The mixture
was extracted with AcOEt, and the organic layer was washed
with H2O and dried (MgSO4). The solvent was removed in
vacuo to yield 1.0 g (quantitative) of 25 as an oil. The product
was used in subsequent reactions without further purifica-
tion: [R]24 ) +28.44 (c ) 0.5, EtOH); IR (CHCl3) 1720, 1713,
D
1502, 1454 cm-1; H NMR (270 MHz, CDCl3) δ 1.60 (s, 3H),
1
3.06 (d, J ) 13.9 Hz, 1H), 3.23 (d, J ) 13.9 Hz, 1H), 3.64 (s,
2H), 5.09 (s, 2H), 5.78 (br s, 1H), 7.20-7.32 (m, 10H); 13C NMR
(68 MHz, CDCl3) δ 23.28, 37.43, 37.86, 60.32, 66.88, 127.15,
128.10, 128.21, 128.53, 128.89, 136.12, 137.93, 154.97, 177.54;
FAB-MS, 360.129 for [M + H]+ (calcd 360.127 for C19H22NO4S).
N-(Carbobenzyloxy)-S-benzyl-(S)-2-methylcysteine (29) was
To the solution of Cbz-MeCys(Bn)-OH (29, 0.47 g, 1.31
mmol) in CH2Cl2 (5 mL) were added DIEA (0.65 mL, 3.74
mmol) and HOAt (0.089g, 0.66 mmol). CIP (0.39 g, 1.40 mmol)
and the HBr‚H-MeCys(Bn)-MeCys(Bn)-NHCH2CH2SBn in CH2-
Cl2 (3 mL) were then added to the mixture. The reaction
mixture was stirred for 72 h at 25 °C. The product was
isolated as described for above dipeptide amide 31 and purified
by silica gel column chromatography using CHCl3 followed by
flash chromatography using hexane/AcOEt (5:2) to give the
similarly prepared from 28. 29: [R]23 ) -34.08 (c ) 0.5,
D
EtOH); FAB-MS, 360.131 for [M + H]+ (calcd 360.127 for
C19H22NO4S).
Cbz tripeptide amide 32 (0.48 g, 55%) as an oil: [R]25
)
N-(Ca r boben zyloxy)-S-ben zyl-(R)-2-m eth ylcystein e N-
((Ben zylth io)eth yl)a m id e [Cbz-MeCys(Bn )-NHCH 2CH2-
SBn ] (30). To a stirred solution of Cbz-MeCys(Bn)-OH (0.60
g, 1.67 mmol) and diisopropylethylamine (DIEA, 1.45 mL) in
CH2Cl2 (8 mL) at 4 °C were added HOAt (0.16 g, 1.17 mmol)
and CIP (0.70 g, 2.51 mmol). Then, S-benzyl-2-aminoet-
hanethiol hydrochloride9 (0.51 g, 2.51 mM) was added and the
resulting solution was stirred for 60 min at 25 °C. The solvent
was removed in vacuo, and the residue was extracted with
AcOEt. The organic layer was washed with 5% citric acid and
H2O and dried (MgSO4). The solvent was removed in vacuo,
and the resulting oil was purified by silica gel column chro-
matography using CHCl3 to yield 0.84 g (99%) of the product
D
-16.39 (c ) 1.3, EtOH); IR (CHCl3) 1713, 1678, 1524, 1495,
1454 cm-1; 1H NMR (270 MHz, CDCl3) δ 1.24 (s, 3H), 1.35 (s,
3H), 1.46 (s, 3H), 2.53 (m, 2H), 2.76 (d, J ) 13.9 Hz, 1H), 2.85
(d, J ) 12.5 Hz, 1H), 2.89 (d, J ) 13.2 Hz, 1H), 2.97 (d, J )
12.9 Hz, 1H), 3.39 (m, 2H), 3.42 (d, J ) 13.2 Hz, 1H), 3.69
(overlapping m, 9H), 4.89 (d, J ) 11.9 Hz, 1H), 5.22 (d, J )
12.2 Hz, 1H), 5.40 (s, 1H), 6.22 (s, 1H), 7.14-7.35 (m, 22H);
13C NMR (68 MHz, CDCl3) δ 21.08, 23.77, 24.08, 30.39, 35.72,
37.30, 37.77, 37.83, 38.92, 39.93, 59.33, 59.66, 60.16, 67.40,
126.83, 126.93, 127.24, 127.62, 128.43, 128.57, 128.61, 128.79,
128.88, 128.93, 129.00, 135.74, 137.64, 138.06, 138.43, 138.52,
155.61, 171.64, 172.85, 173.69; FAB-MS, 923.343 for [M + H]+
(calcd 923.337 for C50H59N4O5S4).
as an oil: [R]24 ) +4.76 (c ) 0.2, CHCl3); IR (CHCl3) 1720,
D
1672, 1495, 1454 cm-1; H NMR (270 MHz, CDCl3) δ 1.52 (s,
N-(2-Meth ylp r op ion yl)-S-ben zyl-(S)-m eth ylcystein yl-
S-b en zyl-(R)-m et h ylcyst ein yl-S-b en zyl-(R)-m et h ylcys-
tein e N-((Ben zylth io)eth yl)a m id e [iP r CO-MeCys(Bn )-
MeCys(Bn )-MeCys(Bn )-NHCH2CH2SBn ] (33). To a stirred
solution of Cbz tripeptide amide 32 (0.38 g, 0.41 mmol) in CH2-
Cl2 (1 mL) was added HBr/AcOH (3 mL) at 4 °C, and the
mixture was stirred for 90 min at 25 °C. The deprotected HBr
salt of the tripeptide amide HBr‚H-MeCys(Bn)-MeCys(Bn)-
MeCys(Bn)-NHCH2CH2SBn was isolated as described for 32,
1
3H), 2.51 (t, J ) 6.6 Hz, 2H), 2.95 (d, J ) 13.9 Hz, 1H), 3.14
(d, J ) 13.5 Hz, 1H), 3.34 (q, J ) 6.3 Hz, 2H), 3.65 (s, 2H),
3.67 (s, 2H), 5.07 (s, 2H), 5.58 (br s, 1H), 6.63 (br t, 1H), 7.21-
7.35 (m, 15H); 13C NMR (68 MHz, CDCl3) δ 28.36, 30.89, 35.63,
37.59, 38.13, 38.98, 59.87, 66.86, 127.13, 127.24, 128.14,
128.23, 128.53, 128.61, 128.82, 128.88, 136.10, 137.91, 138.02,
154.88, 172.77; FAB-MS, 509.196 for [M + H]+ (calcd 509.193
for C28H33N2O3S2).

Convergent Synthesis of (-)-Mirabazole C
J . Org. Chem., Vol. 61, No. 10, 1996 3357
and the product was dissolved in CH2Cl2 (3 mL). To this
solution were added triethylamine (0.23 mL, 1.64 mmol) and
(CH3)2CHCOCl (0.13 mL, 1.24 mmol) at 4 °C, and the mixture
was stirred for 120 min at 25 °C. CH2Cl2 (20 mL) was added
to the reaction mixture. The organic layer was washed with
H2O, dried (MgSO4),and rotary evaporated. The residue was
purified by silica gel column chromatography using CHCl3 to
yield 0.28 g (80%) of the tripeptide amide 33 as a solid: mp
(d, J ) 11.6 Hz, 1H), 3.71 (d, J ) 11.5 Hz, 1H), 3.79 (d, J )
11.2 Hz, 1H), 4.29 (t, J ) 8.6 Hz, 1H), 4.30 (t, J ) 8.6 Hz, 1H);
13C NMR (68 MHz, CDCl3) δ 21.00, 21.19, 25.73, 26.11, 26.16,
33.10, 34.02, 42.80, 42.88, 64.33, 83.40, 83.70, 177.88, 179.12;
FAB-MS, 427.113 for [M + H]+ (calcd 427.112 for C18H27N4S4).
By flash chromatography 10 mg (14%) of a side product (35)
was separated: [R]24 ) -32.70 (c ) 0.6, CHCl3); Rf 0.35
D
1
CHCl3:MeOH ) 100:1; H NMR (270 MHz, CDCl3) δ 1.24 (d,
106-108 °C; [R]25 ) +3.30 (c ) 0.5, EtOH); IR (KBr) 1676,
D
J ) 6.9 Hz, 6H), 1.47 (s, 3H), 1.60 (s, 3H), 1.66 (s, 3H), 2.69
(m, 2H), 2.84 (heptet, J ) 6.9 Hz, 1H), 3.18 (d, J ) 10.9 Hz,
1H), 3.23 (d, J ) 11.2 Hz, 1H), 3.28 (d, J ) 11.5 Hz, 1H), 3.47
(m, 2H), 3.60 (d, J ) 11.9 Hz, 1H), 3.66 (d, J ) 11.6 Hz, 1H),
3.78 (d, J ) 11.2 Hz, 1H), 7.12 (br t, 1H); 13C NMR (68 MHz,
CDCl3) δ 21.01, 21.19, 24.64, 24.91, 26.09, 34.00, 41.24, 42.03,
42.84, 42.95, 83.31, 83.57, 84.51, 174.82, 178.47, 179.46; FAB-
MS, 445.123 for [M + H]+ (calcd 445.122 for C18H29N4OS4).
(-)-Mir a ba zole C (1). To the solution of dihydromiraba-
zole C (6 mg, 0.01 mmol) in benzene (4 mL) was added 200
mg of crushed NiO2 (ca. 2.0 mequiv of O2/g), and the mixture
was refluxed for 3 h. The mixture was filtered, and the solvent
was rotary evaporated. The residue was purified by silica gel
column chromatography using CHCl3/CH3OH (100:1) to yield
4 mg (67%) of 1 as an oil: [R]24D ) -110.77 (c ) 0.065, CHCl3)
[lit.1 [R]D ) -112, c ) 0.03, CHCl3]; 1H NMR (270 MHz, CDCl3)
δ 1.25 (d, J ) 6.9 Hz, 3H), 1.26 (d, J ) 6.9 Hz, 3H), 1.64 (s,
3H), 1.70 (s, 3H), 1.75 (s, 3H), 2.91 (heptet, J ) 6.9 Hz, 1H),
3.31 (d, J ) 11.6 Hz, 2H), 3.45 (d, J ) 11.6 Hz, 1H), 3.74 (d,
J ) 11.5 Hz, 1H), 3.78 (d, J ) 11.2 Hz, 1H), 3.84 (d, J ) 11.2
Hz, 1H), 7.24 (d, J ) 3.3 Hz, 1H), 7.77 (d, J ) 3.3 Hz, 1H)
[these data are essentially the same as those reported for the
natural product1: 1H NMR (300 MHz, CDCl3) δ 1.23 (d, J )
6.7 Hz, 3H), 1.23 (d, J ) 6.7 Hz, 3H), 1.62 (s, 3H), 1.69 (s,
3H), 1.74 (s, 3H), 2.82 (qq, J ) 6.7 Hz, 1H), 3.28 (d, J ) 11.3
Hz, 1H), 3.28 (d, J ) 11.4 Hz, 1H), 3.43 (d, J ) 11.3 Hz, 1H),
3.73 (d, J ) 11.3 Hz, 1H), 3.75 (d, J ) 11.3 Hz, 1H), 3.80 (d,
J ) 11.3 Hz, 1H), 7.23 (d, J ) 3.3 Hz, 1H), 7.76 (d, J ) 3.3 Hz,
1H)]; FAB-MS, 425.096 for [M + H]+ (calcd 425.096 for
C18H25N4S4).
1649, 1519, 1495, 1452 cm-1
;
1H NMR (270 MHz, CDCl3) δ
1.03 (d, J ) 6.6 Hz, 3H), 1.05 (d, J ) 6.9 Hz, 3H), 1.32 (s, 3H),
1.34 (s, 3H), 1.47 (s, 3H), 2.12 (heptet, J ) 6.9 Hz, 1H), 2.54
(m, 2H), 2.84 (d, J ) 13.5 Hz, 1H), 2.88 (d, J ) 13.2 Hz, 1H),
2.96 (d, J ) 13.2 Hz, 2H), 3.38 (m, 2H), 3.45 (d, J ) 13.5 Hz,
1H), 3.69 (overlapping m, 9H), 6.00 (s, 1H), 6.11 (s, 1H), 7.19-
7.35 (m, 22H); 13C NMR (68 MHz, CDCl3) δ 19.30, 19.39, 21.11,
24.01, 30.48, 35.31, 35.85, 37.52, 37.71, 37.79, 39.08, 39.80,
59.05, 59.30, 60.20, 126.84, 126.90, 127.29, 127.76, 128.43,
128.64, 128.84, 128.89, 128.95, 129.05, 129.16, 138.11, 138.24,
138.49, 138.65, 171.70, 172.65, 173.87, 177.82; FAB-MS,
859.345 for [M + H]+ (calcd 859.342 for C46H59N4O4S4).
Dih yd r om ir a ba zole C (34). To the solution of iPrCO-
MeCys(Bn)-MeCys(Bn)-MeCys(Bn)-NHCH2CH2SBn (33, 0.145
g, 0.169 mmol) in dry THF (5 mL) was added 20 mL of liquid
NH3 at -78 °C. Na (0.3 g) was added to the solution, and the
resulting dark blue solution was stirred for 60 min at -78 °C.
NH4Cl was added in portions until the blue color disappeared.
The NH3 of the mixture was removed under a stream of N2,
and the residue was dried in vacuo. To the dried solid was
added CH2Cl2 (10 mL), and the mixture was filtered. The
solvent was rotary evaporated at 4 °C, and the resulting oily
residue was dried in vacuo to give the deprotected tripeptide
amide 4 as an oil.
The deprotected product was dissolved in CH2Cl2 (4 mL).
To this solution was added 2.7 mL of TiCl4 (1.0 M solution in
CH2Cl2) at -78 °C, and the mixture was stirred overnight at
25 °C. Saturated aqueous Na2CO3 (10 mL) was added, and
the mixture was filtered. The aqueous layer was extracted
with CH2Cl2 (20 mL × 3). The combined organic fractions were
washed with H2O and dried (MgSO4), and the solvent was
rotary evaporated. The residue was purified by silica gel
column chromatography using CHCl3/CH3OH (100:1) followed
by flash chromatography using hexane/AcOEt (2:1) to provide
25 mg (35%) of dihydromirabazole C as an oil: [R]24D ) -68.79
(c ) 0.5, CHCl3); Rf 0.43 CHCl3:MeOH ) 100:1; 1H NMR (270
MHz, CDCl3) δ 1.230 (d, J ) 6.9 Hz, 3H), 1.234 (d, J ) 6.9 Hz,
3H), 1.57 (s, 3H), 1.59 (s, 3H), 1.66 (s, 3H), 2.83 (heptet, J )
6.9 Hz, 1H), 3.21 (d, J ) 11.2 Hz, 1H), 3.24 (d, J ) 11.2Hz,
1H), 3.26 (dd, J ) 8.2 Hz, 2H), 3.28 (d, J ) 11.2 Hz, 1H), 3.68
Su p p or tin g In for m a tion Ava ila ble: GC analysis for the
1
estimation of D-Val content, H NMR data for the monitoring
of the activation and coupling, HPLC data of cis and trans
isomers of 23, and 1H and 13C NMR spectra for compounds 1
and 30-35 (17 pages). This material is contained in libraries
on microfiche, immediately follows this article in the microfilm
version of the journal, and can be ordered from the ACS; see
any current masthead page for ordering information.
J O952285H