Synthesis of spiroindoline phytoalexin (S)-(-)-spirobrassinin and its unnatural (R)-(+)-enantiomer

Article abstract of DOI:10.1016/j.tet.2012.11.067

The stereoselective syntheses of the cruciferous indole phytoalexin (S)-(-)-spirobrassinin and its unnatural (R)-(+)-enantiomer were achieved by bromine-induced spirocyclization of (-)- and (+)-1-(8-phenylmenthoxycarbonyl) brassinin in the presence of water to give the corresponding spirobrasinol derivatives, followed by oxidation to the derivatives of spirobrassinin and finally removal of the chiral auxiliary.

Full text of DOI:10.1016/j.tet.2012.11.067

Tetrahedron 69 (2013) 1092e1104
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of spiroindoline phytoalexin (S)-(ꢀ)-spirobrassinin
and its unnatural (R)-(þ)-enantiomer
,
a
b
c
ꢀ a
ꢁꢀ
ꢀ c
ꢀ
*
ꢁ
ꢀ
Mariana Budovska , Peter Kutschy , Tibor Kozar , Tatana Gondova , Jan Petrovaj
a
ꢁ
ꢀ
ꢁ
Department of Organic Chemistry, Institute of Chemical Sciences, Faculty of Science, P.J. Safarik University, Moyzesova 11, 040 01 Kosice, Slovak Republic
b
c
ꢁ
Department of Biophysics, Institute of Experimental Physics, Slovak Academy of Sciences, Watsonova 47, 040 11 Kosice, Slovak Republic
Department of Analytical Chemistry, Institute of Chemical Sciences, Faculty of Science, P.J. Safarik University, Moyzesova 11, 040 01 Kosice, Slovak Republic
ꢁ
ꢀ
ꢁ
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 July 2012
Received in revised form 30 October 2012
Accepted 20 November 2012
Available online 29 November 2012
The stereoselective syntheses of the cruciferous indole phytoalexin (S)-(ꢀ)-spirobrassinin and its un-
natural (R)-(þ)-enantiomer were achieved by bromine-induced spirocyclization of (ꢀ)- and (þ)-1-(8-
phenylmenthoxycarbonyl)brassinin in the presence of water to give the corresponding spirobrasinol
derivatives, followed by oxidation to the derivatives of spirobrassinin and finally removal of the chiral
auxiliary.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Indole phytoalexins
Oxindoles
Spirobrassinin
Stereoselectivity
Spirocyclization
1. Introduction
SCH3
SCH3
SCH3
SCH3
N
N
N
N
N
N
N
S
S
S
S
Phytoalexins include a chemically diverse group of low molecular
weight secondary metabolites biosynthesized de novo by plants as
a defence response to several forms of stress (microbes, UV light,
heavy metal salt). They are not present in naturally healthy plant
tissues.1,2 Cruciferous phytoalexins represent a particular group of
these stress metabolites specifically produced by economically and
dietary important plants of the family Cruciferae (syn. Brassicaceae),
cultivated worldwide (e.g., cabbage, turnip, Chinese cabbage, Japa-
nese radish, wasabi, broccoli, rapeseed and arabidopsis). The
phytoalexins of crucifers are indole alkaloids derived from (S)-tryp-
tophan, mostof whichcontainasulfuratomderived fromcysteine.2,3
Amongst the cruciferous phytoalexins, several spiroindoline[3,50]
thiazolidine-type compounds have been described including (S)-
(ꢀ)-spirobrassinin [(ꢀ)-1, from Japanese radish (Raphanus sativus)],4
(R)-(þ)-1-methoxyspirobrassinin [(þ)-2, from kohlrabi (Brassica
oleracea var. gongylodes)],5 1-methoxyspirobrassinol (3) and trans-
(2R,3R)-(ꢀ)-1-methoxyspirobrassinol methyl ether [(ꢀ)-4, from
Japanese radish (R. sativus), Fig. 1].6
O
O
OCH3
OH
N
H
OCH3
OCH3
trans-(2R,3R)-(−)-4
OCH3
(S)-(−)-1
(R)-(+)-2
3
S
S
SCH3
SCH3
NH
N
NH
SCH3
HO
S
O
N
N
R
N
H
H
6 R = H
7 R = OCH3
8
(S)-(−)-5
Fig. 1. Indole phytoalexins.
methanesulfonyl chloride. Synthetic (ꢁ)-spirobrassinin [(ꢁ)-1] was
enantioresolved by (S)-(ꢀ)-phenylethyl isocyanate giving natural
(S)-(ꢀ)-1 and (R)-(þ)-1. The absolute configuration of (S)-(ꢀ)-spi-
robrassinin [(ꢀ)-1] was determined by calculated electronic CD
methods and X-ray crystallographic analysis after derivatization
with (ꢀ)-camphanoyl chloride.7a,b A one-pot synthesis of (ꢁ)-spi-
robrassinin [(ꢁ)-1] was achieved by direct oxidative cyclization of
brassinin (6) using CrO3.7c The first enantioselective synthesis of
(S)-(ꢀ)-spirobrassinin [(ꢀ)-1] is based on a MsCl-mediated cycli-
zation of unnatural (R)-(þ)-dioxibrassinin [(þ)-5] prepared by
a catalytic asymmetric Henry reaction of isatin with nitromethane,
Racemic (ꢁ)-spirobrassinin [(ꢁ)-1] was first synthesized by
treatment of (ꢁ)-dioxibrassinin (5) with either thionyl chloride or
* Correspondingauthor. Tel.:þ421552341196; e-mail address: mariana.budovska@
ꢀ
upjs.sk (M. Budovska).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.11.067

ꢀ
M. Budovska et al. / Tetrahedron 69 (2013) 1092e1104
1093
followed by hydrogenation and reaction with carbon disulfide and
methyl iodide.7d,e (ꢁ)-1-Methoxyspirobrassinol [(ꢁ)-3] and (ꢁ)-1-
methoxyspirobrassinol methyl ether [(ꢁ)-4] were synthesized by
a dioxane dibromide-mediated cyclization of 1-methoxybrassinin
(7) in dioxane in the presence of water or methanol as a nucleo-
phile.8a The racemate of (ꢁ)-1-methoxyspirobrassinin [(ꢁ)-2] was
obtained by oxidation of (ꢁ)-3 and (ꢁ)-4.8b The stereoselective
syntheses of naturally occurring (R)-(þ)-1-methoxyspirobrassinin
[(þ)-2] and its unnatural (S)-(ꢀ)-enantiomer (ꢀ)-2 were achieved
by spirocyclization of 1-methoxybrassinin (7) using (ꢀ)- and
(þ)-menthol as a nucleophile and subsequent oxidation of the
obtained menthyl ethers. TFA-catalyzed methanolysis of chiral 1-
methoxyspirobrassinol menthyl ethers afforded trans-(2R,3R)-
(ꢀ)-1-methoxyspirobrassinol methyl ether [(2R,3R)-(ꢀ)-4] and its
(2S,3S)-, (2S,3R)-, (2R,3S)-isomers.8c The phytoalexin spirobrassinin
(1) is known to exhibit the unusual phenomenon of enantiomeric
self-disproportionation on achiral-phase chromatography (ESDAC)
whereby the enantiomeric composition of a non-racemic mixture
varies across the eluting chromatographic peak. This behaviour is
rationalized by the formation of two different complexes, both of
which are in dynamic equilibrium with the free molecules.7a,b
Complexation was also evident by NMR spectroscopy including
the observation of distinct NMR signals for each enantiomer in the
non-racemic mixturedthe phenomenon of self-induced di-
astereomeric anisochromism (SIDA).9 The 1H, 13C and 15N spectro-
scopic enantioresolution of chiral phytoalexins spirobrassinin (1),
1-methoxyspirobrassinin (2) and 1-methoxyspirobrassinol methyl
ether (4) was induced by application of the chiral solvating agents
[2,2,2-trifluoro-1-(9-anthryl)ethanol, (R)-(þ)-1,10-bi-2-naphthol,
(R)-(ꢀ)-1-phenyl-2,2,2-trifluoroethanol] in benzene.10,11
of spirobrassinol would be the major product. Oxidation with
pyridinium chlorochromate (PCC) afforded diastereoisomers of
1-substituted spirobrassinin 10 and removal of the chiral group by
treatment with sodium methoxide provided the enantiomers of
(ꢀ)-1 and (þ)-1. Oxidative rearrangement of 9-substituted de-
rivatives of cyclobrassinin 11 to 1-substituted spirobrassinin 10 was
achieved by treatment with oxone and sodium chloride (Scheme 1).
For the experiments we selected the (1R,2S,5R)-menthox-
ycarbonyl, (1R,2S,5R)-8-phenylmenthoxycarbonyl, (1S,2R,5S)-8-
phenylmenthoxycarbonyl, (1S)-phenylethoxycarbonyl and (1S)-
endo-borneoxycarbonyl group as a chiral auxiliary.
Indole-3-carboxaldehyde (12), which was used as a starting
compound, was transformed by reaction with (1R,2S,5R)-
(ꢀ)-menthyl chloroformate (13) in tetrahydrofuran in the pres-
ence of triethylamine to give 1-[(1R,2S,5R)-menthoxycarbonyl]
indole-3-carboxaldehyde (14) (Scheme 2). The starting point in
the synthesis of 1-[(1R,2S,5R)-8-phenylmenthoxycarbonyl]indole-
3-carboxaldehyde (26) was to develop a method for attaching
a (1R,2S,5R)-8-phenylmenthoxycarbonyl group onto the indole
nitrogen without working with toxic phosgene or triphosgene. An
effective and simple solution was to acylate the indole-3-
carboxaldehyde (12) with chiral carbonate 22, prepared by the
reaction of (1R,2S,5R)-(ꢀ)-8-phenylmenthol (16) with phenyl
chloroformate (20) in pyridine according to the procedure
published for (1R,2S,5R)-(ꢀ)-menthol (15).16 [(1S,2R,5S)-8-
Phenylmenthyl]phenyl carbonate (23) was synthesized from
(1S,2R,5S)-8-(þ)-phenylmenthol (17) (Scheme 2) in an analogous
way. Reaction of chiral alcohol (1S)-(ꢀ)-1-phenylethanol (18) and
[(1S)-endo]-(ꢀ)-borneol (19) with phenyl chloroformate (20) in
pyridine afforded carbonates 24 and 25. Aldehyde 26 and its en-
antiomer 27 were prepared in good yields by the reaction of in-
dole-3-carboxaldehyde (12) with carbonate 22 or 23 using sodium
hydride. The required starting chiral compounds 28e31 were
obtained by acylation of cyclobrassinin (8) with carbonates 21e25
using sodium hydride in acetonitrile.
The key intermediates, chiral derivatives of brassinins 38e40,
were prepared by the modified procedure used previously for 1-
Boc-brassinin.17 Treatment of aldehydes 14, 26 and 27 with hy-
droxylamine hydrochloride provided the corresponding oximes
32e34 as mixtures of Z- and E-isomers in high yields. The reduction
of oximes 32e34 with sodium borohydride in the presence of
catalytic nickel boride afforded the unstable amines 35e37, which
decomposed within a minute to unidentified material. Therefore no
useful spectra of these compounds could be recorded and they
were used as crude products immediately after isolation. Treatment
of amines 35e37 with CS2 and CH3I in methanol in the presence of
triethylamine afforded 1-substituted derivatives of brassinin
38e40 in 51% yield (Scheme 3).
With 38e40 in hand, the stereoselectivity of the key bromo-
spirocyclizationwas investigated.1-[(1R,2S,5R)-Menthoxycarbonyl]
brassinin (38) was subjected tothe conditions of the spirocyclization
in dichloromethane using bromine in the presence of water as the
nucleophile(Scheme 4). Theresultof this reactionwas the formation
of trans-diastereoisomers 41a and 41b in 51% yield in a 50:50 ratio.
cis-Diastereoisomers 41c and 41d wereisolated as minorproducts in
3% yield (Table 1).
In addition to their moderate antimicrobial activities,3 indole
phytoalexins from cruciferous plants display significant anticancer
activity. Brassinin (6), cyclobrassinin (8) and (ꢁ)-spirobrassinin
[(ꢁ)-1] have been shown to exhibit cancer chemopreventive ac-
tivity in 7,12-dimethylbenz[a]anthracene (DMBA)-induced pre-
cancerous lesions in mouse mammary gland organ culture.12
Various indole phytoalexins demonstrate in vitro cytostatic/cyto-
toxic effects against cultured human solid tumour and leukaemia
cell lines.13,14 Indole phytoalexins produced by cruciferous plants
show significant antiproliferative effects on intracellular amasti-
gotes of Trypanosoma cruzi and may be prospective candidates for
antichagasic drug design.15
2. Results and discussion
In the present paper we report a new stereoselective approach
to the spirooxindole phytoalexin (S)-(ꢀ)-spirobrassinin [(ꢀ)-1] as
well as its unnatural enantiomer (R)-(þ)-spirobrassinin [(þ)-1]. For
this purpose we investigated the stereoselectivity of spirocycliza-
tion of brassinin derivatives possessing chiral groups on the indole
nitrogen (9, Scheme 1). Our further aim was the study of the oxi-
dative rearrangement of chiral 9-substituted derivatives of cyclo-
brassinin 11 in the stereoselective synthesis of spirobrassinin (1).
The spirocyclization was performed with bromine in dichloro-
methane in the presence of water as a nucleophile. It was assumed
that one of the four possible diastereoisomers of chiral derivatives
S
SCH3
N
SCH3
N
S
SCH3
NH
S
O
N
N
Spirocyclization
N
COOR*
Oxidative
COOR*
COOR*
rearrangement
11
9
10
Scheme 1.

ꢀ
1094
M. Budovska et al. / Tetrahedron 69 (2013) 1092e1104
CHO
CHO
+
Cl
O
i
N
R*O
N
H
COOR*
14 *R1 (64%)
12
13 *R1
*R1 =
Cl
O
O
O
ii
*R-OH
+
R*
O
O
15 *R1
16 *R2
17 *R3
18 *R4
19 *R5
21 *R1 (87%)
22 *R2 (96%)
23 *R3 (96%)
24 *R4 (89%)
25 *R5 (92%)
20
*R2
=
CHO
CHO
O
O
R*
iii
+
*R3
=
O
N
N
H
COOR*
22 *R2
23 *R3
12
26 *R2 (77%)
27 *R3 (77%)
N
N
S
*R4
*R5
=
=
O
O
SCH3
SCH3
R*
iv
S
+
O
N
H
N
COOR*
28 *R1 (68%)
29 *R2 (62%)
30 *R4 (60%)
31 *R5 (64%)
21 *R1
22 *R2
24 *R4
25 *R5
8
H
i) Et3N, THF, 0 °C, 20 min, rt, 30 min; ii) Py, 0 °C - rt, 12 h; iii) NaH, CH3CN, 0 °C, 1.5 h;
iv) NaH, CH3CN, 0 °C, 1.5 h.
Scheme 2.
S
SCH3
NH2
N OH
CHO
NH
ii
iii
COOR*
i
N
N
N
N
COOR*
COOR*
COOR*
14 *R1
32 *R1 (99%)
33 *R2 (96%)
34 *R4 (96%)
38 *R1 (51%)
39 *R2 (51%)
40 *R4 (51%)
35 *R1
36 *R2
37 *R4
*R1 = (−)-menthyl
26 *R2
27 *R4
*R2 = (−)-8-phenylmenthyl
*R3 = (+)-8-phenylmenthyl
i) NH2OH.HCl, Na2CO3, EtOH, H2O, rt, 30 min; ii) NaBH4, NiCl6 6H2O, MeOH;
ii) Et3N, CS2, MeI, MeOH, rt, 1 h.
Scheme 3.
Above-mentioned findings are in accordance with the previous
evidence to suggest they were the cis-diastereoisomers. In the case
of 41a and 41b the NOESY spectrum did not show the interaction
between the Hb and OH protons required to confirming their trans-
configuration. The interaction between the Hb and H-2 protons was
also not observed. Thus, the structure of trans-diastereoisomers
was assigned to compounds 41a and 41b.
spirocyclization reactions of 1-Boc-, 1-acetyl-, 1-benzoyl- and
1-methoxycarbonylbrassinin using bromine in the presence of
water that always afforded trans-diastereoisomers as the major
products.8a,18
The trans-diastereoisomer is regarded as the one in which the
sulfur of the thiazoline ring and the hydroxy group in position-2
are located on the opposite sides of indoline ring, while the cis-
diastereoisomer has the sulfur and 2-hydroxy group on the same
side of indoline ring.
trans-Diastereoisomers 41a and 41b were not separable there-
fore their NMR spectrum was recorded as a mixture. The 1H NMR
spectra of 41a and 41b were almost identical except for a 0.01 ppm
difference in the shifts of the Ha protons of the thiazoline ring. The
structures of the diastereoisomers were determined by NMR
spectroscopic methods, including 2D HSQC, HMBC and NOESY
experiments (Fig. 2). Products 41c and 41d exhibited cross peaks
between the H-2 and Hb protons in the NOESY spectra providing
1
Examination of the H NMR spectra of spiroindoline[3,50]thia-
zolidine-type compounds revealed a significant difference in the
chemical shifts between the H-2 protons of trans- and cis-
diastereoisomers. In all cases the
d
(H-2)trans appeared at lower
8a,19
field compared to d(H-2)cis.
This relation is also observed for
trans-diastereoisomers 41a and 41b and cis-diastereoisomers 41c
and 41d. Diastereoisomers 41a and 41b show a chemical shift for
the H-2 proton at 5.97 ppm and so the trans-configuration was
assigned, whilst the cis-diastereoisomers 41c and 41d show a signal
for the H-2 proton at 5.65 ppm.
For the synthesis of the target spirobrassinin (1), trans-
diastereoisomers 41a and 41b, as major products, were used.

ꢀ
M. Budovska et al. / Tetrahedron 69 (2013) 1092e1104
1095
S
Ha
N
SCH3
SCH3
i
SCH3
H4
N
Hb
SCH3
SBr
N
NH
S
S
OH
H2
N
O
N
N
N
O
COOR*
Br
B
COOR*
COOR*
A
1
38
*R
2
39
*R
3
41c
NOESY
40 *
R
SCH3
N
N
SCH3
N
N
SCH3
S
OH
SCH3
N
N
S
OH
Fig. 2. NOESY correlations of compound 41c.
S
S
OH
OH
+
N
+
+
N
COOR*
COOR*
COOR*
cis
COOR*
diastereoisomers 44a and 44b was 50:50, which was determined
by deconvolution of the partially overlapping signals of the Hb
protons in the 1H NMR spectrum of the crude mixture using Mes-
tReNova software.20 The 1H NMR spectra of 44a and 44b were al-
most identical except for a 0.01 ppm difference in the shifts of the
Hb protons of the thiazoline ring. Diastereoisomers 44a and 44b
showed very close Rf values in various eluents, which prevented
their separation by column chromatography. Diastereoisomer 44b
was separated by crystallization from a solution of dichloro-
methane/n-hexane. The pure diastereoisomer 44b was isolated as
white crystals in 24% yield. Removal of the (1R,2S,5R)-menthox-
ycarbonyl group from pure diastereoisomer 44b was achieved by
treatment with sodium methoxide in methanol giving (S)-
(ꢀ)-spirobrassinin [(S)-(ꢀ)-1] in 82% yield (Scheme 5). Its absolute
trans
41c *R1
41b *R1
41d *R1
41a *R1
2
2
42b
*R
42a
*R
3
3
ii
43b *
R
43a *
R
SCH3
SCH3
N
N
N
S
S
+
O
O
N
COOR*
COOR*
*R1 = ( )-menthyl
–
1
1
*R2 = ( )-8-phenylmenthyl
–
44b
44a
*R
*R
45b *R2
46b *R3
45a *R2
46a *R3
*R3 = (+)-8-phenylmenthyl
i) Br2, H2O, CH2Cl2, rt, 20 min, Et3N; ii) PCC, MgSO4, CH2Cl2, rt, 24 h.
Scheme 4.
SCH3
SCH3
N
N
Their oxidation using Jones reagent afforded the mixture of di-
astereoisomers 44a and 44b in a moderate 47% yield. With the aim
of improving the yield of oxidation, Jones reagent was substituted
with PCC, which increased the yield to 71%. The ratio of
S
S
i
i
44b
44a
O
O
N
H
N
H
(S)-(−)-1 (82%)
(R)-(+)-1 (82%)
92% ee
65% ee
Table 1
Stereochemistry, ratio and yields of diastereoisomers 1-[(1R,2S,5R)-menthox-
ycarbonyl] (41), 1-[(1R,2S,5R)-8-phenylmenthoxycarbonyl] (42) and 1-[(1S,2R,5S)-8-
phenylmenthoxycarbonyl]spirobrassinol (43)
SCH3
SCH3
N
N
S
S
ii
ii
O
O
N
45b
46a
N
H
*
R
Compound/ratioa (%)/yield (%)
H
SCH3
SCH3
SCH3
SCH3
(S)-(−)-1 (85%)
(R)-(+)-1 (85%)
S
S
S
S
91% ee
94% ee
H
OH
OH
H
OH
COOR*
H
H
OH
COOR*
N
N
N
N
i) CH3ONa, CH3OH, rt, 20 min; ii) CH3ONa, CH3OH, rt, 1.5 h.
COOR*
COOR*
Scheme 5.
trans
cis
*
R1
41a
40b
30c
51d
41b
40b
58c
41c
10b
4c
41d
10b
8c
configuration was determined by direct comparison of the ECD
spectrum with the published data of the natural product.7a The
enantiomeric excess of the synthesized enantiomer (S)-(ꢀ)-1, de-
termined by chiral HPLC, was 92%. Diastereoisomer 44a was ob-
tained by evaporation of the solvent from the filtrate after filtration
of crystals of diastereoisomer 44b. Deprotection resulted in for-
mation of (R)-(þ)-spirobrassinin [(R)-(þ)-1]. Most major ECD sig-
nals showed opposing signs in comparison to ECD signals of (S)-
(ꢀ)-1 obtained from 44b. The optical purity of R-enantiomer (R)-
(þ)-1 was 65% ee (Scheme 5).
3d
*
*
R2
42a
50e
63f
42b
nd
nd
50e
R3
43a
50e
63f
43b
50e
*R1¼(ꢀ)-menthyl.
*R2¼(ꢀ)-8-phenylmenthyl.
*R3¼(þ)-8-phenylmenthyl.
With the aim to improve the stereoselectivity, spirocyclization
of compound 38 was performed at lower temperatures from
ꢀ110 ꢂC up to ꢀ70 ꢂC leading to a moderate increase in the ster-
eoselectivity. The ratio of diastereoisomers 44a and 44b obtained
after subsequent oxidation of crude product 41 with PCC is listed in
Table 2.
a
The ratio of diastereoisomers 41ae41d was determined by deconvolution of the
partially overlapping signals of the Ha protons for 41a and 41b and the Hb protons
for 41c and 41d in the 1H NMR spectrum of the crude mixture. The ratio of products
42a,42b and 43a,43b was determined by integration of the non-overlapping signals
of the H-2 protons in the 1H NMR spectrum of the crude product mixture.
b
Ratio of diastereoisomers 41ae41d obtained by spirocyclization of 38 at room
temperature.
Low diastereoselectivity in synthesis of derivatives 41a and 41b
prompted us to examine the impact of the (1R,2S,5R)-8-
phenylmenthoxycarbonyl group on the cyclization of derivate 39.
c
Ratio of diastereoisomers 41ae41d obtained by spirocyclization of 38 at ꢀ70 ꢂC.
d
Yields of the mixture of trans-isomers 41a, 41b and cis-isomers 41c, 41d.
e
Ratio of diastereoisomers 42a, 42b and 43a, 43b obtained by spirocyclization of
We expected the
p stacking effect to lock reactant 39 preferentially
39 and 40 at room temperature.
f
in one conformation.21
Yields of the mixture of trans-isomers 42a, 42b and trans-isomers 43a, 43b.

ꢀ
M. Budovska et al. / Tetrahedron 69 (2013) 1092e1104
1096
Table 2
the (1S,2R,5S)-8-phenylmenthoxycarbonyl group from di-
astereoisomer 46a with sodium methoxide in 85% yield (Scheme 5).
The ECD spectrum of prepared (R)-(þ)-spirobrassinin [(R)-(þ)-1]
was the mirror image of the ECD spectrum of (S)-(ꢀ)-spirobrassinin
[(ꢀ)-1] obtained from 45b.
Influence of temperature on the stereoselectivity of spirocyclization of brassinin
derivatives 38e40
Solvent
Temperature of
spirocyclization
Ratio
44a/44ba
45a/45bb
46a/46bb
For the explanation of the stereoselectivity of the spirocycliza-
tion, the conformational analysis of brassinin 40 within (1S,2R,5S)-
8-phenylmenthoxy group was done using DFT methods at the
B3LYP 6-31** level in the gas-phase using the JAGUAR program.22
In Fig. 3 there are depicted structures of the five most stable
conformations represented by their relative energies (kJ/mol) and
the percentage of their abundance (Table 3). Conformers C1eC5
differ in the orientation of the (1S,2R,5S)-8-phenylmenthoxy group
and indole core. The energy of conformer C5 is lower by more than
4 kJ/mol relative to the energy of other unentered conformers. In
the most stable conformer C1, as well as in the conformer C3, the
phenyl nucleus of the (1S,2R,5S)-8-phenylmenthoxycarbonyl
group spatially restricts the access to the C-3 atom of indole
moiety from the Re face. In the case of conformers C2 and C4,
neither the Re nor the Si face is shielded. Conformation C5 has
more restraints Si face. The spatial arrangement of the energeti-
cally favourable conformers C1 and C3, in comparison with con-
former C5, probably allows effective interaction of the O]CeO
CH2Cl2
THF
Et2O
CH2Cl2
CH2Cl2
CH2Cl2
THF
rt
rt
rt
50:50
48:52
48:52
d
50:50
50:50
50:50
40:60
27:73
18:82
25:75
12:88
50:50
d
d
ꢀ20 ꢂC
ꢀ70 ꢂC
ꢀ90 ꢂC
ꢀ100 ꢂC
ꢀ110 ꢂC
60:40
d
36:64
36:64
42:58
42:58
82:18
d
Et2O
86:14
a
The ratio of diastereoisomers 44a and 44b was determined by deconvolution of
the partially overlapping signals of the Hb protons in the 1H NMR spectrum of the
crude mixture.
b
The ratio of products 45a/45b and 46a/46b was determined by integration of
the non-overlapping signals of the H-7 protons in the 1H NMR spectrum of the crude
product mixture.
Spirocyclization of 1-[(1R,2S,5R)-8-phenylmenthoxycarbonyl]
brassinin (39) using bromine in the presence of water at room
temperature resulted in the formation of trans-diastereoisomers
42a and 42b in 63% yield that were not separable (Table 1). Oxi-
dation of their mixture with PCC provided (1R,2S,5R)-8-
phenylmenthoxycarbonyl derivatives of spirobrassinin 45a and
45b in a 50:50 ratio isolated in 34% and 32% yields after chroma-
tography on silica gel (Scheme 4). Performing spirocyclization at
lower temperatures led to preferential formation of di-
astereoisomer 45b (Table 2). Thus spirocyclization in dichloro-
methane at ꢀ90 ꢂC and subsequent oxidation gave diastereoisomer
group with the phenyl core
(p
-stacking).21 Assuming equal
reactivity of individual conformers in the spirocyclization
reaction, our experimentally found stereoselectivity supports the
prediction that the dominant product will be (3R)-diastereoisomer
of 1-[(1S,2R,5S)-8-phenylmenthoxycarbonyl]spirobrassinol (43a)
as a result of sulfur atom attacking from the Si face.
Fig. 3. Molecular geometries of the most stable conformers C1eC5 of derivate 40 resulting from DFT B3LYP 6-31** minimalization.
45b in 54% yield. The ratio of products 45a and 45b was determined
by integration of the non-overlapping signals of the H-7 protons in
the 1H NMR spectrum of the crude product mixture. With the pure
diastereoisomer 45b in hand, the S-enantiomer of indole phyto-
alexin spirobrassinin [(S)-(ꢀ)-1] was synthesized by removing the
chiral group with sodium methoxide (Scheme 5). The absolute
configuration of the enantiomer was identified by comparison of its
ECD spectrum with the published data.7a The enantiomeric excess
of the prepared (S)-enantiomer was 91%.
Table 3
Relative energies and the percentage abundance of the five most stable conformers
C1eC5 of 1-[(1S,2R,5S)-8-phenylmenthoxycarbonyl]brassinin (40)
Conformer
C1
0.00
79.5
C2
C3
C4
C5
D
E (kJ/mol)
5.19
9.7
6.82
5.1
7.95
3.1
8.37
2.6
Percentage of abundance (%)a
a
The variety of conformations was accounted for by population-weight averaged
assessment of energies.23
Cyclization of 1-[(1S,2R,5S)-8-phenylmenthoxycarbonyl]brassi-
nin (40) in dichloromethane using bromine in the presence of
water at room temperature afforded trans-diastereoisomers 43a
and 43b in 63% yield also as a non-separable mixture (Scheme 4).
Subsequent oxidation of this mixture with PCC led to epimers 46a
and 46b in a 50:50 ratio. As can be seen from Table 2, the spi-
rocyclization of 40 at the lower reaction temperatures led to the
preferential formation of diastereoisomer 46a. Spirocyclization
performed at ꢀ90 ꢂC in dichloromethane and subsequent oxidation
afforded diastereoisomer 46a as the major product in 54% yield.
(R)-(þ)-Spirobrassinin [(R)-(þ)-1] was obtained after removal of
Oxidative rearrangement using halogenated electrophilic agents
(N-bromosuccinimide, tert-butyl hypochlorite) or epoxidation with
dimethyldioxirane allows the conversion of condensed indole de-
rivatives in to spirooxindoles.24aec This fact led us to study the ep-
oxidation and oxidative rearrangement of 9-substituted derivatives
of cyclobrasinin 28e31 in the stereoselective synthesis of spiro-
brassinin (1). Chiral derivatives of cruciferous phytoalexins cyclo-
brassinin 28e31 were selected as starting compounds with the aim
of studying the stereoselectivity of the oxidative rearrangement. The
oxidative rearrangement was explored with chiral cyclobrassinin

ꢀ
M. Budovska et al. / Tetrahedron 69 (2013) 1092e1104
1097
28. The use of several oxidants (N-bromosuccinimide, N-chlor-
osuccinimide, dioxane dibromide, H2O2, tert-butyl hydroperoxide,
m-chloroperoxybenzoic acid, dimethyldioxirane) did not afford
1-[(1R,2S,5R)-menthoxycarbonyl]spirobrassinin (44). The use of
PCC, CrO3, 2-iodoxybenzoic acid or Davis oxaziridine resulted in
the oxidation of the CH2 and SCH3 groups leading to the formation
of 9-[(1R,2S,5R)-menthoxycarbonyl]-2H-3,4-dihydro-[1,3]thiazino
[6,5-b]indole-2,4-dione (47) (Scheme 6). Conversion of chiral
cyclobrassinins 28e31 into the desired spirobrassinin derivatives
44, 46, 48 and 49 was achieved only by oxidative chlorination using
oxone and sodium chloride, however, the reaction proceeded with
no stereoselectivity. The diastereoselectivity of the rearrangement
was not influenced probably due to the big distance between the
reaction centre and the chiral auxiliary on the indole nitrogen.
G/UV254. Preparative column chromatography was performed on
Kieselgel 60 Merck Type 9385 (0.040e0.063). Optical rotations
were measured at room temperature in a 10 cm cell with a polar-
imeter JASCO P-2000 and P3002 (Kreuss) at the sodium D-line. CD
spectra were obtained in a 1 mm quartz cell on a JASCO J-810
spectrometer. The enantiomeric excess of synthesized enantiomers
was determined by analytical chiral HPLC, using a CHIROBIOTIC T
(Astec, 25ꢃ0.46 cm ID) column, using the eluent n-hexane/ethanol
(8:2) at flow rate 0.6 mL/min. (1R,2S,5R)-(ꢀ)-Menthyl chlor-
oformate (ee>95.0%, Fluka), (1R,2S,5R)-(ꢀ)-menthol (ee 96%,
Aldrich), (1R,2S,5R)-(ꢀ)-8-phenylmenthol (ee>98%, Fluka),
(1S,2R,5S)-(þ)-8-phenylmenthol (ee>98%, Fluka), (1S)-(ꢀ)-1-
phenylethanol (ee>98,5%, Fluka) and [(1S)-endo]-(ꢀ)-borneol (ee
98%, Aldrich) were used as received.
SCH3
O
N
S
SCH3
N
N
N
N
NH
S
SCH3
S
O
S
i
ii
O
N
O
+
N
COOR*
COOR*
COOR*
COOR*
50 : 50
47 *R1 (71%)
28 *R1
44b *R1
*R1 = (−)-menthyl
(68%)
44a
45a
48a
49a
29 *R2
30 *R3
31 *R4
45b *R2
48b *R3
49b *R4
*R2 = (−)-8-phenylmenthyl
*R3 = (−)-1-phenylethyl
*R4 = (−)-endo-bornyl
(68%)
(63%)
(72%)
i) PCC, CH2Cl2, 24 h; ii) oxone, NaCl, EtOAc, H2O, rt, 24 h.
Scheme 6.
3. Conclusion
4.1.1. 1-[(1R,2S,5R)-Menthoxycarbonyl]indole-3-carboxaldehyde
(14). To solution of indole-3-carboxaldehyde (12) (1.45 g,
a
In summary, we have developed a stereoselective approach to the
cruciferous spirooxindole phytoalexin (S)-(ꢀ)-spirobrassinin [(S)-
(ꢀ)-1] anditsunnatural (R)-(þ)-enantiomer [(R)-(þ)-1]. The keystep
of the synthesis involves a bromine-mediated cyclization of chiral
(ꢀ)-8-phenylmenthoxycarbonyl or (þ)-8-phenylmenthoxycarbonyl
derivatives of brassinin in the presence of water with the forma-
tion of non-separable diastereoisomers of chiral derivatives of
spirobrassinol. Spirocyclization of 39 or 40 performed in dichloro-
methane at ꢀ90 ꢂC ꢂC followed by oxidation with PCC provided di-
astereoisomers 45a,45b or 46a,46b in 66% yield and 82:18
diastereomeric ratio (dr). Removal of thechiralauxiliaryafforded(S)-
(ꢀ)-spirobrassinin [(S)-(ꢀ)-1] and its unnatural (R)-(þ)-enantiomer
[(R)-(þ)-1] in 10% overall yield, with enantiomeric excesses of 91%
and 94%, respectively.
10.0 mmol) in THF (40 mL) at 0 ꢂC was added triethylamine (1.06 g,
1.46 mL, 10.5 mmol). The reaction mixture was stirred at 0 ꢂC for
10 min. After that (1R,2S,5R)-(ꢀ)-menthyl chloroformate (13)
(2.30 g, 2.23 mL, 10.5 mmol) was added and the reaction mixture
was stirred at 0 ꢂC for 20 min and for 30 min at room temperature.
After the reaction was finished, THF was evaporated. The residue
obtained after evaporation of the solvent was subjected to column
chromatography (15 g silica gel, cyclohexane/ethyl acetate 8:1). The
obtained compound was further crystallized from hot ethanol to
afford aldehyde 14. Yield: 2.11 g (64%), white crystals, mp 62e65 ꢂC
20
(ethanol), [
a]
ꢀ10.0 (c 0.70, CHCl3), Rf 0.56 (cyclohexane/ethyl
D
acetate 8:1). Anal. Calcd for C20H25NO3 requires: C, 73.37; H, 7.70; N,
4.28. Found: C, 73.01; H, 7.98; N, 4.56. 1H NMR (400 MHz, CDCl3)
d
10.12 (s, 1H, CHO), 8.30 (dd, 1H, J 7.7, 0.6, H-4), 8.27 (s, 1H, H-2),
8.18 (d, 1H, J 8.1, H-7), 7.46e7.36 (m, 2H, H-6, H-5), 5.01 (dt, 1H, J
10.9, 4.2, H-10), 2.26e2.22 (m, 1H, H-80), 2.04e1.97 (m, 1H, H-60),
1.83e1.75 (m, 2H, H-30, H-40), 1.69e1.54 (m, 2H, H-20, H-50),
1.28e0.94 (m, 3H, H-30, H-40, H-60), 0.97 (d, 3H, J 6.6), 0.96 (d, 3H, J
6.9) [H-90, H-100], 0.84 (d, 3H, J 6.9, H-70). 13C NMR (100 MHz, CDCl3)
4. Experimental
4.1. Chemistry
Melting points were determined on a Koffler micro melting
point apparatus and are uncorrected. IR spectra were recorded on
an IR-75 spectrometer (Zess Jena). 1H NMR (400 MHz) and 13C NMR
(100 MHz) spectra were measured on a Varian Mercury Plus
d 185.8 (CHO), 150.0 (C]O), 136.1 (C-2), 136.0 (C-3a), 126.3 (C-5),
126.1 (C-7a), 124.8 (C-6), 122.2 (C-4), 122.0 (C-3), 115.2 (C-7), 79.3
(C-10), 47.3 (C-20), 40.9 (C-60), 34.0 (C-40), 31.5 (C-50), 26.5 (C-80),
23.4 (C-30), 22.0, 20.8 (C-90, C-100), 16.4 (C-70). IR (CHCl3) nmax 2960,
2927, 2880, 1740 and 1667 (C]O), 1540, 1447, 1347, 1213, 1093,
946 cm1. MS (MALDI-TOF), m/z (%): 350.72 [MþNa]þ (17), 328.02
[MþH]þ (41).
spectrometer. Chemical shifts (d) are reported in parts per million
downfield from TMS as internal standard and coupling constants (J)
are given in hertz. Microanalyses were performed with a Per-
kineElmer, Model 2400 analyzer. The EI mass spectra were recor-
ded on a GS-MS Trio 1000 (Fisons Instruments) spectrometer at an
ionization energy of 70 eV, whereas MALDI-TOF mass spectra of 14,
32, 41 and 44 were measured on a MALDI IV (Shimazu, Kratos
4.1.2. General procedure for the synthesis of carbonates 21e25. To
a solution of chiral alcohol 15e19 (4.30 mmol) in dry pyridine
(1 mL) cooled in an ice-bath, phenyl chloroformate (20) (0.741 g,
0.600 mL, 4.73 mmol) was added slowly with vigorous stirring. A
white precipitate was formed. The mixture was stirred at room
temperature for 12 h and then was poured into water (50 mL). The
Analytical). The samples were ionized with a N2-laser (
The progress of chemical reactions was monitoring by thin layer
chromatography, using Macherey-Nagel plates AlugramÒ Sil
l
¼337 nm).

ꢀ
M. Budovska et al. / Tetrahedron 69 (2013) 1092e1104
1098
product was extracted with diethyl ether (1ꢃ50 mL and 2ꢃ25 mL).
The extract was washed with 10% aqueous citric acid (2ꢃ10 mL),
aqueous sodium hydrogen carbonate (1ꢃ10 mL) and finally water
(1ꢃ10 mL). The extract was dried over Na2SO4 and the solvent was
evaporated.
60% suspension in mineral oil) in dry acetonitrile (5 mL) was added
indole-3-carboxaldehyde (12) (0.383 g, 2.64 mmol). After stirring
for 10 min at room temperature, the mixture was cooled in an ice-
bath and a solution of carbonate 22 or 23 (0.847 g, 2.40 mmol) in
dry acetonitrile (5 mL) was added. The reaction mixture was stirred
in an ice cooling bath for 1.5 h, then poured into cold water (200 mL)
and the product was extracted with ethyl acetate (1ꢃ100 mL and
1ꢃ50 mL). The extract was dried over Na2SO4 and the residue ob-
tained after evaporation of the solvent was subjected to chroma-
4.1.2.1. [(1R,2S,5R)-Menthyl]phenyl carbonate (21).16 Following
the general procedure, product 21 was obtained using (1R,2S,5R)-
(ꢀ)-menthol (15) (0.675 g, 4.30 mmol). Yield: 1.03 g (87%), lit.16
95%, colourless oil, Rf 0.88 (n-hexane/ethyl acetate 5:1). Spectro-
scopic data were consistent with those reported in the literature.16
tography on silica gel (30 g, cyclohexane/ethyl acetate 8:1). Data for
20
26: yield: 0.742 g (77%), white crystals, mp 53e55 ꢂC (ethanol), [
a]
D
ꢀ56.3 (c 0.46, CHCl3), Rf 0.51 (cyclohexane/ethyl acetate 8:1). Anal.
4.1.2.2. [(1R,2S,5R)-8-Phenylmenthyl]phenyl carbonate (22).
Following the general procedure, product 22 was obtained using
Calcd for C26H29NO3 requires: C, 77.39; H, 7.24; N, 3.47. Found: C,
77.53; H, 7.01; N, 3.64. 1H NMR (400 MHz, CDCl3)
d 9.85 (s,1H, CHO),
(1R,2S,5R)-(ꢀ)-8-phenylmenthol (16) (1.00 g, 4.30 mmol). Yield:
8.21e8.19 (m,1H, H-7), 8.11 (d,1H, J 7.2, H-4), 7.39e7.31 (m, 2H, H-5,
H-6), 7.28e7.25 (m, 2H, H-200, H-600), 7.08e7.04 (m, 2H, H-300, H-500),
6.95e6.91 (m, 1H, H-400), 6.73 (s, 1H, H-2), 5.08 (dt, 1H, J 10.7, 4.3, H-
10), 2.37e2.31 (m,1H, H-20), 2.09e2.06 (m, 2H, H-30, H-60),1.84e1.80
(m, 1H, H-40), 1.71e1.54 (m, 1H, H-50), 1.36 (s, 3H), 1.21 (s, 3H) [H-90,
H-100], 1.33e0.94 (m, 3H, H-30, H-40, H-60), 0.94 (d, 3H, J 6.5, H-70).
20
1.46 g (96%), colourless oil, [
a
]
ꢀ63.4 (c 0.81, CHCl3), Rf 0.81 (n-
D
hexane/ethyl acetate 5:1). Anal. Calcd for C23H28O3 requires: C,
78.38; H, 8.01. Found: C, 78.61; H, 7.83. 1H NMR (400 MHz, CDCl3)
d
7.44e7.09 (m, 10H, H-arom), 4.72 (dt, 1H, J 10.7, 4.5, H-10),
2.06e2.00 (m, 2H, H-20, H-60), 1.65e1.61 (m, 2H, H-30, H-40),
1.50e1.46 (m, 1H, H-50), 1.41 (s, 3H), 1.32 (s, 3H) [H-90, H-100],
1.25e0.81 (m, 3H, H-30, H-40, H-60), 0.89 (d, 3H, J 6.5, H-70). 13C
13C NMR (100 MHz, CDCl3) 186.1 (CHO), 152.5 (C-100), 149.1 (C]O),
d
136.8 (C-2), 136.1 (C-3a), 128.4 (C-300, C-500), 126.1 (C-5), 125.9 (C-7a),
125.4 (C-400), 125.1 (C-200, C-600), 124.8 (C-6), 122.1 (C-7), 121.3 (C-3),
115.2 (C-4), 78.8 (C-10), 50.8 (C-20), 41.8 (C-60), 39.5 (C-80), 34.7
(C-40), 31.6 (C-50), 31.1 (C-90), 26.5 (C-30), 21.9 (C-70), 21.8 (C-100). IR
(CHCl3) nmax 2927,1723 (C]O),1673 (C]O),1393,1093 cm1. MS (EI),
m/z (%): 404 [MþH]þ (11), 403 [M]þ (28), 119 (94), 105 (100), 91
NMR (100 MHz, CDCl3)
d 152.9 (C]O), 151.4, 150.9, 129.5, 128.3,
125.9, 125.7, 125.6, 121.3 (C-arom), 79.8 (C-10), 51.0 (C-20), 41.8 (C-
60), 40.2 (C-80), 34.6 (C-40), 31.6 (C-50), 27.0, 26.9 (C-90, C-100), 26.8
(C-30), 21.9 (C-70). IR (CHCl3) nmax 2953, 1733 (C]O), 1227 cm1. MS
(EI), m/z (%): 216 (38), 214 (64), 119 (100), 105 (40), 91 (46), 77 (29).
(62). Data for 27: yield: 0.742 g (77%), white crystals, mp 53e55 ꢂC
20
4.1.2.3. [(1S,2R,5S)-8-Phenylmenthyl]phenyl carbonate (23).
Following the general procedure, product 23 was obtained using
(ethanol), [
a]
þ54.3 (c 0.50, CHCl3), Rf 0.66 (cyclohexane/ethyl
D
acetate 8:1). The spectral data were fully identical with those of
(1S,2R,5S)-(þ)-8-phenylmenthol (17) (1.00 g, 4.30 mmol). Yield:
enantiomeric product 26.
20
1.46 g (96%), colourless oil, [
a
]
þ59.8 (c 0.56, CHCl3), Rf 0.81 (n-
D
hexane/ethyl acetate 5:1). The spectral data were fully identical
4.1.4. General procedure for the preparation of 9-substituted de-
rivatives of cyclobrassinin 28e31. To a suspension of NaH (0.176 g,
4.40 mmol, 60% suspension in mineral oil) in dry acetonitrile
(12 mL) was added cyclobrassinin (8)25 (0.264 g, 1.10 mmol). After
stirring for 10 min at room temperature, the mixture was cooled in
an ice-bath and a solution of carbonate 21, 22, 24 or 25 (1.00 mmol)
in dry acetonitrile (12 mL) was added. The reaction mixture was
stirred with cooling for 1.5 h, then poured into cold water (30 mL)
and the product was extracted with ethyl acetate (1ꢃ30 mL and
1ꢃ20 mL). The extract was dried over Na2SO4 and the residue ob-
tained after evaporation of the solvent subjected to chromatogra-
phy on silica gel (50 g, cyclohexane/ethyl acetate 8:1).
with those of enantiomeric product 22.
4.1.2.4. [(1S)-Phenylethyl]phenyl carbonate (24). Following the
general procedure, product 24 was obtained using (1S)-(ꢀ)-1-
phenylethanol (18) (0.525 g. 0.460 mL, 4.30 mmol). Yield: 0.927 g
(89%), colourless oil, [
a
]
20 ꢀ78.6 (c 0.31, CHCl3), Rf 0.64 (n-hexane/
D
ethyl acetate 5:1). Anal. Calcd for C15H14O3 requires: C, 74.36; H,
5.82. Found: C, 74.02; H, 5.46. 1H NMR (400 MHz, CDCl3)
(m,10H, H-arom), 5.82 (quartet,1H, J 6.6, CH),1.67 (d, 3H, J 6.6, CH3).
13C NMR (100 MHz, CDCl3)
153.3 (C]O), 151.2, 140.8, 129.8, 129.6,
d 7.43e7.14
d
128.8, 128.6, 126.8, 126.4, 126.2, 121.3, 121.2 (C-arom), 77.6 (CH),
22.5 (CH3). IR (CHCl3) nmax 2967, 1760 (C]O), 1587, 1233 cm1. MS
(EI), m/z (%): 247 [M]þ (1), 214 (45), 105 (100), 94 (16), 77 (97).
4.1.4.1. 9-[(1R,2S,5R)-Menthoxycarbonyl]cyclobrassinin (28).
Following the general procedure, product 28 was obtained using
4.1.2.5. [(1S)-endo-Bornyl]phenyl carbonate (25). Following the
general procedure, product 25 was obtained using [(1S)-endo]-
carbonate 21 (0.276 g, 1.00 mmol). Yield: 0.283 g (68%), light
20
yellow oil, [
a
]
ꢀ20.1 (c 0.24, CHCl3), Rf 0.66 (cyclohexane/ethyl
D
(ꢀ)-borneol (19) (0.622 g, 4.30 mmol). Yield: 1.09 g (92%), colour-
acetate 8:1). Anal. Calcd for C22H28N2O2S2 requires: C, 63.43; H,
20
less oil, [
a
]
ꢀ14.4 (c 0.32, CHCl3), Rf 0.8 (n-hexane/ethyl acetate
6.77; N, 6.72. Found: C, 63.18; H, 7.10; N, 6.65. 1H NMR (400 MHz,
D
5:1). Anal. Calcd for C17H22O3 requires: C, 74.42; H, 8.08. Found: C,
CDCl3) d 8.10 (d, 1H, J 7.6, H-8), 7.42e7.40 (m, 1H, H-5), 7.31e7.24
74.73; H, 7.74. 1H NMR (400 MHz, CDCl3) 7.44e7.35 (m, 2H, H-300,
d
(m, 2H, H-7, H-6), 5.02 (s, 2H, CH2), 4.91 (dt, 1H, J 10.9, 4.4, H-10),
2.53 (s, 3H, SCH3), 2.25e2.22 (m, 1H, H-80), 2.11e2.00 (m, 1H, H-
60), 1.80e1.67 (m, 2H, H-30, H-40), 1.61e1.53 (m, 2H, H-20, H-50),
1.29e0.82 (m, 3H, H-30, H-40, H-60), 0.97 (d, 3H, J 6.4), 0.96 (d, 3H, J
6.9) [H-90, H-100], 0.82 (d, 3H, J 6.9, H-70). 13C NMR (100 MHz,
H-500), 7.29e7.17 (m, 3H, H-200, H-600, H-400), 4.49 (dd, 1H, J 10.0, 5.4,
H-20), 2.46e2.38 (m, 1H, H-30), 2.01e1.95 (m, 1H, H-60), 1.83e1.71
(m, 2H, H-50, H-40), 1.39e1.28 (m, 2H, H-60, H-50), 1.21e1.17 (m, 1H,
H-30), 0.93 (s, 3H), 0.92 (s, 3H) [H-80, H-90], 0.89 (s, 3H, H-100). 13C
NMR (100 MHz, CDCl3)
d
154.2 (C]O), 151.4 (C-100), 129.8, 129.6 (C-
CDCl3) d 155.5 (C]N), 150.9 (C]O), 136.3 (C-8a), 127.7 (C-4b),
300, C-500), 126.1 (C-400), 121.3 (C-200, C-600), 85.1 (C-20), 49.2 (C-70),
48.2 (C-10), 44.9 (C-40), 36.6 (C-30), 28.2 (C-50), 27.1 (C-60), 19.9, 19.1
(C-80, C-90), 13.7 (C-100). IR (CHCl3) nmax 2958, 2885, 1731 (C]O),
1591, 1265 cm1. MS (EI), m/z (%): 274 [M]þ (1), 137 (100), 95 (84).
124.3 (C-9a), 124.2 (C-7), 123.3 (C-6), 117.1 (C-5), 115.1 (C-8), 108.8
(C-4a), 78.8 (C-10), 47.9 (CH2), 47.1 (C-20), 40.9 (C-60), 34.0 (C-40),
31.5 (C-50), 26.2 (C-80), 23.2 (C-30), 21.9, 20.9 (C-90, C-100), 16.2 (C-
70), 15.1 (SCH3). IR (CHCl3) nmax 2958, 2872, 1732 (C]O), 1664,
1541, 1317 cm1. MS (EI), m/z (%): 416 [M]þ (22), 161 (100).
4.1.3. 1-[(1R,2S,5R)-8-Phenylmenthoxycarbonyl]indole-3-carboxalde-
hyde (26) and 1-[(1S,2R,5S)-8-phenylmenthoxycarbonyl]indole-3-
carboxaldehyde (27). To a suspension of NaH (0.384 g, 9.60 mmol,
4.1.4.2. 9-[(1R,2S,5R)-8-Phenylmenthoxycarbonyl]cyclobrassinin
(29). Following the general procedure, product 29 was obtained

ꢀ
M. Budovska et al. / Tetrahedron 69 (2013) 1092e1104
1099
using carbonate 22 (0.352 g, 1.00 mmol). Yield: 0.305 g (62%), light
d 8.69 (s, 0.3H, CH]N min.), 8.32 (s, 0.7H, CH]N maj.), 8.24 (d,
20
yellow oil, [
a
]
ꢀ21.2 (c 0.26, CHCl3), Rf 0.73 (cyclohexane/ethyl
0.3H, J 8.1, H-4 min.), 8.19 (d, 0.7H, J 8.1, H-4 maj.), 8.11 (d, 0.7H, J 7.9,
H-7 maj.), 7.86 (br s,1H, OH), 7.81(s, 0.7H, H-2 maj.), 7.79 (s, 0.3H, H-
2 min.), 7.73 (d, 0.3H, J 7.8, H-7 min.), 7.40 (dt,1H, J 7.3,1.4, H-6), 7.34
(dt, 0.3H, J 8.2, 0.9, H-5 min.), 7.32 (dt, 0.7H, J 7.9, 0.8, H-5 maj.), 4.99
(dt, 0.3H, J 10.8, 4.4, H-10 min.), 4.96 (dt, 0.7H, J 10.8, 4.4, H-10 maj.),
2.23e2.20 (m, 1H, H-80), 2.05e1.95 (m, 1H, H-60), 1.80e1.74 (m, 2H,
H-30, H-40), 1.68e1.53 (m, 2H, H-20, H-50), 1.27e0.93 (m, 3H, H-30, H-
40, H-60), 0.96 (d, 3H, J 6.4), 0.95 (d, 3H, J 7.1) [H-90, H-100], 0.84 (d,
0.9H, J 7.0, H-70 min.), 0.83 (d, 2.1H, J 7.0, H-70 maj.). 13C NMR
D
acetate 8:1). Anal. Calcd for C28H32N2O2S2 requires: C, 68.26; H,
6.55; N, 5.69. Found: C, 68.01; H, 6.37; N, 5.90. 1H NMR (400 MHz,
CDCl3)
d 7.81 (br s, 1H, H-8), 7.36e7.34 (m, 1H, H-5), 7.22e7.20 (m,
4H, H-200, H-600, H-7, H-6), 6.90e6.86 (m, 2H, H-300, H-500), 6.69e6.61
(m, 1H, H-400), 5.17 (dt, 1H, J 10.8, 4.7, H-10), 5.15 (d, 1H, J 17.7, Hb),
4.77 (d, 1H, J 17.7, Ha), 2.54 (s, 3H, SCH3), 2.25e2.22 (m, 1H, H-20),
2.04e1.81 (m, 2H, H-30, H-60), 1.73e1.69 (m, 1H, H-40), 1.56e1.47
(m, 1H, H-50), 1.38 (s, 3H), 1.27 (s, 3H) [H-90, H-100], 1.33e0.84 (m,
3H, H-30, H-60, H-40), 0.93 (d, 3H, J 6.5, H-70). 13C NMR (100 MHz,
(100 MHz, CDCl3)
d 150.6 (C]O min.), 150.3 (C]O maj.), 145.0
CDCl3)
d
156.3 (C]N), 150.7 (C]O), 150.3 (C-100), 136.4 (C-8a), 127.9
(CH]N maj.), 138.8 (CH]N min.), 136.0 (C-7a maj.), 134.5 (C-7a
min.), 131.1 (C-3a maj.), 128.8 (C-3a min.), 127.6 (C-2 maj.), 127.1
(C-2 min.), 125.6 (C-6 maj.), 125.2 (C-6 min.), 123.8 (C-5 maj.), 123.5
(C-5 min.), 122.4 (C-4 maj.), 118.3 (C-4 min.), 115.5 (C-7 min.), 115.2
(C-7 maj.), 114.7 (C-3 maj.), 110.0 (C-3 min.), 78.5 (C-10 min.), 78.4
(C-10 maj.), 47.3 (C-20 maj.), 47.2 (C-20 min.), 41.0 (C-60 maj.), 40.9
(C-60 min.), 34.2 (C-40 min.), 34.1 (C-40 maj.), 31.6 (C-50 min.), 31.5
(C-50 maj.), 26.5 (C-80 maj.), 25.9 (C-80 min.), 23.6 (C-30 min.), 23.5
(C-30 maj.), 22.2 (min.), 22.0 (maj.), 21.0 (min.), 20.8 (maj.) (C-90,
C-100), 16.5 (C-70 min.), 16.4 (C-70 maj.). IR (CHCl3) nmax 3580 (OH),
2960, 2873, 1707 (C]O), 1440, 1333, 1226, 1080, 1020, 940 cm1. MS
(MALDI-TOF), m/z (%): 365.91 [MþNa]þ (5), 343.45 [MþH]þ (35),
327.3 (100).
(C-300, C-500), 127.7 (C-4b), 127.6 (C-9a), 125.3 (C-200, C-600), 124.9
(C-400), 124.0 (C-7), 123.2 (C-6), 117.0 (C-5), 115.5 (C-8), 108.8 (C-4a),
78.3 (C-10), 51.5 (C-20), 48.0 (CH2), 42.4 (C-60), 39.9 (C-80), 34.5
(C-40), 31.7 (C-50), 28.4 (C-90), 26.8 (C-30), 24.8 (C-100), 21.9 (C-70),
15.4 (SCH3). IR (CHCl3) nmax 2960, 2856, 1720 (C]O), 1662, 1450,
1321 cm1. MS (EI), m/z (%): 492 [M]þ (25), 161 (100).
4.1.4.3. 9-[(1S)-Phenylethoxycarbonyl]cyclobrassinin (30). Follo-
wing the general procedure, product 30 was obtained using car-
bonate 24 (0.242 g, 1.00 mmol). Yield: 0.229 g (60%), light yellow
20
oil, [
a]
ꢀ53.7 (c 0.17, CHCl3), Rf 0.55 (cyclohexane/ethyl acetate
D
8:1). Anal. Calcd for C20H18N2O2S2 requires: C, 62.80; H, 4.74; N,
7.32. Found: C, 62.48; H, 4.98; N. 7.47. 1H NMR (400 MHz, CDCl3)
d
8.08 (d, 1H, J 7.1, H-8), 7.50e7.48 (m, 2H, H-20, H-60), 7.43e7.29 (m,
4.1.6. 1-[(1R,2S,5R)-8-Phenylmenthoxycarbonyl]indole-3-carboxalde-
hyde oxime (33) and 1-[(1S,2R,5S)-8-phenylmenthoxycarbonyl]in-
dole-3-carboxaldehyde oxime (34). To a stirred solution of aldehyde
26 or 27 (0.706 g, 1.75 mmol) in ethanol (14 mL) was added a so-
lution of hydroxylammonium chloride (0.190 g, 2.73 mmol) and
sodium carbonate (0.135 g, 1.28 mmol) in water (1.7 mL) and the
mixture was stirred for 30 min at room temperature. After evapo-
ration of ethanol and addition of water (8 mL), the oxime was
extracted with diethyl ether (2ꢃ30 mL) and after drying with
Na2SO4 the solvent evaporated. Data for 33: yield: 0.703 g (96%) of
a mixture of E- and Z-isomer in a 70:30 ratio, white crystals, mp
4H, H-5, H-30, H-50, H-40), 7.28e7.25 (m, 2H, H-7, H-6), 6.13 (quartet,
1H, J 6.6, CH), 5.00 (s, 2H, CH2), 2.52 (s, 3H, SCH3), 1.81 (d, 3H, J 6.6,
CH3). 13C NMR (100 MHz, CDCl3)
d 155.6 (C]N), 150.7 (C]O), 140.4
(C-10), 136.5 (C-8a), 129.0 (C-30, C-50), 128.3 (C-4b), 127.9 (C-9a),
126.6 (C-20, C-60), 126.3 (C-40), 124.5 (C-7), 123.6 (C-6), 117.4 (C-5),
115.4 (C-8), 109.3 (C-4a), 77.2 (CH), 48.1 (CH2), 22.5 (CH3), 15.4
(SCH3). IR (CHCl3) nmax 2986, 1720 (C]O), 1587, 1373 cm1. MS (EI),
m/z (%): 382 [M]þ (19), 161 (100), 105 (82).
4.1.4.4. 9-[(1S)-endo-Borneoxycarbonyl]cyclobrassinin (31). Follo-
wing the general procedure product 31 was obtained using car-
56e58 ꢂC (diethyl ether), [
a
]
D
20 ꢀ62.9 (c 0.56, CHCl3), Rf 0.50 and 0.41
bonate 25 (0.274 g, 1.00 mmol). Yield: 0.265 g (64%), light yellow oil,
(n-hexane/acetone 3:1). Anal. Calcd for C26H30N2O3 requires: C,
20
[a]
ꢀ35.4 (c 0.24, CHCl3), Rf 0.63 (cyclohexane/ethyl acetate 8:1).
74.61; H, 7.22; N, 6.69. Found: C, 74.90; H, 6.93; N, 6.45. 1H NMR
D
Anal. Calcd for C22H26N2O2S2 requires: C, 63.74; H, 6.32; N, 6.76.
(400 MHz, CDCl3) d 10.08 (br s, 0.3H, OH min.), 9.86 (br s, 0.7H, OH
Found: C, 63.52; H, 6.45; N, 6.49. 1H NMR (400 MHz, CDCl3)
d
8.13 (d,
maj.), 8.20e8.08 (m, 0.7H, H-7 maj.), 8.12 (s, 0.7H, CH]N maj.), 8.03
(ddd, 0.7H, J 7.7, 0.8, 0.6, H-4 maj.), 8.01e7.92 (m, 0.3H, H-7 min.),
7.72 (s, 0.3H, CH]N min.), 7.69e7.67 (m, 0.3H, H-4 min.), 7.37e7.28
(m, 2H, H-5, H-6), 7.26e7.24 (m, 2.4H, H-2, H-200, H-600 maj.),
7.22e7.20 (m, 0.6H, H-200, H-600 min.), 7.10e7.06 (m, 1.4H, H-300, H-500
maj.), 6.99e6.96 (m, 1.3H, H-300, H-500 min., H-400 maj.), 6.82e6.79
(m, 0.3H, H-400 min.), 5.15 (dt, 0.3H, J 10.7, 4.5, H-10 min.), 5.05 (dt,
0.7H, J 10.7, 4.3, H-10 maj.), 2.29 (dt, 0.7H, J 12.6, 3.5, H-20 maj.), 2.21
(dt, 0.3H, J 12.1, 3.5, H-20 min.), 2.08e1.97 (m, 2H, H-30, H-60),
1.88e1.69 (m, 1H, H-40), 1.65e1.51 (m, 1H, H-50), 1.39 (s, 0.9H, H-90
min.), 1.36 (s, 2.1H, H-90 maj.), 1.22 (s, 0.9H, H-100 min.), 1.21 (s, 2.1H,
H-100 maj.), 1.33e0.83 (m, 3H, H-30, H-40, H-60), 0.92 (d, 2.1H, J 6.5,
H-70 maj.), 0.91 (d, 0.9H, J 5.4, H-70 min.). 13C NMR (100 MHz, CDCl3)
1H, J 7.9, H-8), 7.42e7.40 (m, 1H, H-5), 7.32e7.25 (m, 2H, H-7, H-6),
5.23 (dd, 1H, J 10.0, 5.4, H-20), 5.00 (s, 2H, CH2), 2.58e2.48 (m, 1H, H-
30), 2.53 (s, 3H, SCH3), 2.21e2.14 (m,1H, H-60),1.90e1.78 (m, 2H, H-50,
H-40), 1.54e1.36 (m, 2H, H-60, H-50), 1.35e1.31 (m, 1H, H-30), 1.08 (s,
3H), 0.98 (s, 3H) [H-80, H-90], 0.94 (s, 3H, H-100). 13C NMR (100 MHz,
CDCl3)
d 155.7 (C]N), 151.8 (C]O), 136.6 (C-8a), 127.9 (C-4b), 124.6
(C-9a), 124.4 (C-7), 123.5 (C-6), 117.4 (C-5), 115.3 (C-8), 109.2 (C-4a),
85.6 (C-20), 49.4 (C-70), 48.4 (C-10), 48.1 (CH2), 44.9 (C-40), 36.8 (C-30),
28.3 (C-50), 28.0 (C-60), 20.0, 19.1 (C-80, C-90), 15.4 (SCH3),14.7 (C-100).
IR (CHCl3) nmax 2956, 2881, 1734 (C]O), 1614, 1450, 1352 cm1. MS
(EI), m/z (%): 414 [M]þ (24), 161 (75), 81 (100).
4.1.5. 1-[(1R,2S,5R)-Menthoxycarbonyl]indole-3-carboxaldehyde ox-
ime (32). To a stirred solution of aldehyde 14 (1.60 g, 4.50 mmol) in
ethanol (28 mL) was added a solution of hydroxylammonium
chloride (0.533 g, 7.70 mmol) and sodium carbonate (0.370 g,
3.50 mmol) in water (2.4 mL) and the mixture was stirred for
30 min at room temperature. After evaporation of ethanol and
addition of water (12 mL), the oxime was extracted with diethyl
ether (1ꢃ40 mL, 1ꢃ30 mL and 1ꢃ20 mL) and after drying with
Na2SO4 the solvent evaporated. Yield: 1.65 g (99%) of a mixture of E-
d
152.2 (C-100 maj.), 151.1 (C-100 min.), 150.0 (C]O min.), 149.5 (C]O
maj.), 145.4 (CH]N maj.), 139.1 (CH]N min.), 136.2 (C-7a maj.),
134.6 (C-7a min.), 131.4 (C-3a min.), 128.7 (C-300, C-500 min.), 128.4
(C-300, C-500 maj.), 128.3 (C-2 min.), 128.2 (C-2 maj.), 126.9 (C-3a
maj.),125.5 (C-6),125.4 (C-400 min.),125.3 (C-400 maj.),125.2 (C-200, C-
600 maj.), 125.1 (C-200, C-600 min.), 123.9 (C-5 maj.), 123.5 (C-5 min.),
122.4 (C-4 maj.), 118.4 (C-4 min.), 115.7 (C-7 min.), 115.3 (C-7 maj.),
113.9 (C-3 maj.), 109.8 (C-3 min.), 78.5 (C-10), 51.3 (C-20 min.), 50.9
(C-20 maj.), 42.2 (C-60 min.), 41.9 (C-60 maj.), 39.8 (C-80 min.), 39.5
(C-80 maj.), 34.7 (C-40 maj.), 34.6 (C-40 min.), 31.7 (C-50 min.), 31.0 (C-
50 maj.), 30.4 (C-90 min.), 29.8 (C-90 maj.), 26.8 (C-30 min.), 26.6 (C-30
maj.), 24.2 (C-100 maj.), 22.7 (C-100 min.), 22.6 (C-70 min.), 21.9 (C-70
maj.). IR (CHCl3) nmax 3560 (OH), 2953, 1713 (C]O), 1367, 1220,
and Z-isomer in a 70:30 ratio, white crystals, mp 56e59 ꢂC (diethyl
20
ether), [
a
]
ꢀ40.1 (c 0.52, CHCl3), Rf 0.49 and 0.53 (cyclohexane/
D
acetone 2:1). Anal. Calcd for C20H26N2O3 requires: C, 70.15; H, 7.65;
N, 8.18. Found: C, 69.82; H, 7.79; N, 8.02. 1H NMR (400 MHz, CDCl3)

ꢀ
M. Budovska et al. / Tetrahedron 69 (2013) 1092e1104
1100
1087 cm1. MS (EI), m/z (%): 420 [MþH]þ (4), 419 [M]þ (14), 204 (34),
160 (61), 119 (95), 105 (100). Data for 34: yield: 0.703 g (96%) of
a mixture of E- and Z-isomer in a 70:30 ratio, white crystals, mp
stirring was continued for 1 h. The solvent was evaporated and the
residue obtained after evaporation of the solvent was subjected to
chromatography on silica gel (50 g, n-hexane/ethyl acetate 5:1).
56e58 ꢂC (diethyl ether), [
a]
D
20 þ66.2 (c 0.54, CHCl3), Rf 0.50 and 0.41
Data for 39: yield: 0.252 g (51%), colourless oil, [
a]
20 ꢀ63.5 (c 0.79,
D
(n-hexane/acetone 3:1). The spectral data were fully identical with
those of enantiomeric product 33.
CHCl3), Rf 0.60 (n-hexane/ethyl acetate 5:1). Anal. Calcd for
C
28H34N2O2S2 requires: C, 67.98; H, 6.93; N, 5.66. Found: C, 68.25;
H, 6.74; N, 5.41. 1H NMR (400 MHz, CDCl3)
d
8.14 (s,1H, H-7), 7.49 (d,
4.1.7. 1-[(1R,2S,5R)-Menthoxycarbonyl]indol-3-ylmethyl
amine
1H, J 7.6, H-4), 7.35e7.23 (m, 4H, H-6, H-5, H-200, H-600), 7.12 (t, 2H, J
7.3, H-300, H500), 7.02e7.00 (m, 1H, H-400), 6.87 (s, 1H, H-2), 6.22 (br s,
1H, NH), 5.03 (dt, 1H, J 10.6, 4.2, H-10), 4.88 (s, 2H, CH2), 2.69 (s, 3H,
SCH3), 2.29 (dt, 1H, J 12.1, 3.5, H-20), 2.08e2.04 (m, 1H, H-60),
1.92e1.96 (m, 1H, H-30), 1.80e1.76 (m, 1H, H-40), 1.63e1.59 (m, 1H,
H-50), 1.36 (s, 3H), 1.20 (s, 3H) [H-90, H-100], 1.32e0.87 (m, 3H, H-30,
H-40, H-60), 0.92 (d, 3H, J 6.5, H-70). 13C NMR (100 MHz, CDCl3)
(35). To a solution of NiCl2$6H2O (0.784 g, 3.30 mmol) in
methanol (30 mL) was added oxime 32 (1.03 g, 3.00 mmol) in
methanol (42 mL) followed by NaBH4 (0.908 g, 24.0 mmol) in
one portion with stirring and cooling with flowing cold water.
After 5 min, methanol in the mixture was evaporated to ¼ of its
original volume and mixture was poured into a saturated solu-
tion of NH4Cl (150 mL). After extraction with ethyl acetate
(1ꢃ100 mL and 1ꢃ60 mL), drying the extract with Na2SO4 and
evaporation of the solvent, the crude amine 35 (0.985 g) was
obtained. The crude amine 35 was employed in the next reaction
without purification.
d
198.9 (C]S), 152.5 (C-100), 149.7 (C]O), 135.8 (C-7a), 129.0 (C-3a),
128.4 (C-300, C-500), 125.3 (C-200, C-600), 125.1 (C-6), 125.0 (C-2), 124.9
(C-400), 123.3 (C-5), 118.9 (C-4), 115.6 (C-7), 114.9 (C-3), 77.9 (C-10),
50.8 (C-20), 42.7 (CH2), 41.9 (C-60), 39.6 (C-80), 34.7 (C-40), 31.6 (C-50),
30.3 (C-90), 26.6 (C-30), 22.9 (C-100), 21.9 (C-70), 18.5 (SCH3). IR
(CHCl3) nmax 3233 (NH), 2947, 1773 (C]O), 1387, 1087 cm1. MS (EI),
4.1.8. 1-[(1R,2S,5R)-8-Phenylmenthoxycarbonyl]indol-3-ylmethyl
amine (36) and 1-[(1S,2R,5S)-8-phenylmenthoxycarbonyl]indol-3-
ylmethyl amine (37). To a solution of NiCl2$$6H2O (0.238 g,
1.00 mmol) in methanol (20 mL) was added oxime 33 or 34 (0.419 g,
1.00 mmol) in methanol (20 mL) followed by NaBH4 (0.378 g,
10.0mmol)inoneportionwithstirringandcoolingwithflowingcold
water. After 5 min, methanol in the mixture was evaporated to ¼ of
its original volume and mixture was poured into a saturated solution
of NH4Cl (50 mL). After extraction with ethyl acetate (2ꢃ40 mL),
drying the extract with Na2SO4 and evaporation of the solvent, the
crude amine 36 or 37 (0.404 g) was obtained. The crude amine 36 or
37 was employed in the next reaction without purification.
m/z (%): 495 [MþH]þ (2), 447 (10), 119 (100), 105 (96). Data for 40:
20
yield: 0.252 g (51%), colourless oil, [
a]
þ61.5 (c 0.62, CHCl3), Rf
D
0.60 (n-hexane/ethyl acetate 5:1). The spectral data were fully
identical with those of enantiomeric product 39.
4.1.11. Spirocyclization of 1-[(1R,2S,5R)-menthoxycarbonyl]brassinin
(38). To a stirred solution of 1-[(1R,2S,5R)-menthoxycarbonyl]
brassinin (38) (0.418 g, 1.00 mmol) in a mixture of dichloro-
methane/water (9 mL/1 mL) at room temperature was added
a freshly prepared solution of Br2 (2.57 mL, 1.10 mmol). The stock
solution was obtained by dissolving of bromine (0.060 mL) in
dichloromethane (2.64 mL). The reaction mixture was stirred for
20 min at room temperature, then triethylamine (0.223 g, 0.305 mL,
2.20 mmol) was added. Stirring was continued for 10 min, and the
reaction mixture was diluted with dichloromethane (50 mL),
washed with brine (2ꢃ50 mL). The organic layer was dried over
Na2SO4 and the residue obtained after evaporation of the solvent
subjected to chromatography on silica gel (35 g, cyclohexane/ethyl
acetate 8:1), affording a mixture of trans-diastereoisomers 41a and
41b and mixture of cis-diastereoisomers 41c and 41d.
4.1.9. 1-[(1R,2S,5R)-Menthoxycarbonyl]brassinin (38). To a stirred
solution of crude freshly prepared amine 35 (0.985, 3.00 mmol) in
methanol (21 mL) were added triethylamine (0.910 g, 1.25 mL,
9.00 mmol) and carbon disulphide (0.685 g, 0.541 mL, 9.00 mmol).
After stirring for 5 min at room temperature, methyl iodide (1.28 g,
0.560 mL, 9.00 mmol) was added and stirring was continued for 1 h.
The solvent was evaporated and the residue obtained after evap-
oration of the solvent was subjected to chromatography on silica
gel (90 g, cyclohexane/ethyl acetate 8:1). Yield: 0.64 g (51%), col-
4.1.11.1. trans-1-[(1R,2S,5R)-Menthoxycarbonyl]spirobrassinol
(41a, 41b). Yield: 0.224 g (51%) of a mixture of diastereoisomers
41a and 41b in a 50:50 ratio, colourless oil, Rf 0.18 (cyclohexane/
ethyl acetate 8:1). Anal. Calcd for C22H30N2O3S2 requires: C, 60.80;
H, 6.96; N, 6.45. Found: C, 61.09; H, 7.18; N, 6.22. IR (CHCl3) nmax
3437 (OH), 2958, 2872, 1713 (C]O), 1568 (C]N), 1483, 1281,
955 cm1. MS (MALDI-TOF), m/z (%): 460.17 [MþNa]þ (26), 435.26
[MþH]þ (90).
20
ourless oil, [
a
]
ꢀ61.9 (c 0.73, CHCl3), Rf 0.53 (cyclohexane/ethyl
D
acetate 8:1). Anal. Calcd for C22H30N2O2S2 requires: C, 63.12; H,
7.22; N, 6.69. Found: C, 63.27; H, 7.51; N, 6.42. 1H NMR (400 MHz,
CDCl3) d 8.18 (d, 1H, J 8.0, H-7), 7.66 (s, 1H, H-2), 7.58 (d, 1H, J 7.6, H-
4), 7.38e7.34 (m, 1H, H-6), 7.29e7.23 (m, 1H, H-5), 7.03 (br s, 1H,
NH), 5.05 (d, 2H, J 4.7, CH2), 4.95 (dt,1H, J 10.9, 4.4, H-10), 2.66 (s, 3H,
SCH3), 2.27e2.20 (m, 1H, H-80), 2.05e1.95 (m, 1H, H-60), 1.83e1.74
(m, 2H, H-30, H-40), 1.69e1.53 (m, 2H, H-20, H-50), 1.28e0.94 (m, 3H,
H-30, H-40, H-60), 0.96 (d, 3H, J 6.5), 0.95 (d, 3H, J 6.7) [H-90, H-100],
Compound 41a: 1H NMR (400 MHz, CDCl3)
d 7.77 (br s, 1H, H-7),
7.39 (d, 1H, J 7.5, H-4), 7.29 (dd, 1H, J 7.5, 7.6, H-6), 7.07 (dd, 1H, J 7.5,
7.5, H-5), 5.97 (s, 1H, H-2), 5.29 (s, 1H, OH), 5.07 (d, 1H, J 15.5, Hb),
4.84 (ddd, 1H, J 10.5, 10.5, 4.3, H-10), 4.30 (d, 1H, J 15.5, Ha), 2.60 (s,
3H, SCH3), 2.17e2.16 (m, 1H, H-80), 1.97e1.94 (m, 1H, H-60),
1.76e1.72 (m, 2H, H-30, H-40), 1.57e1.51 (m, 2H, H-20, H-50),
1.17e0.95 (m, 3H, H-30, H-40, H-60), 0.94 (d, 3H, J 7.0), 0.92 (d, 3H, J
7.0) [H-90, H-100], 0.82 (d, 3H, J 6.9, H-70). 13C NMR (100 MHz, CDCl3)
0.83 (d, 3H, J 6.8, H-70). 13C NMR (100 MHz, CDCl3)
d 199.0 (C]S),
150.5 (C]O), 135.6 (C-7a), 129.1 (C-3a), 125.2 (C-6), 124.8 (C-2),
123.2 (C-5), 119.1 (C-4), 115.9 (C-3), 115.5 (C-7), 78.4 (C-10), 47.2
(C-20), 42.4 (CH2), 40.9 (C-60), 34.1 (C-40), 31.5 (C-50), 26.5 (C-80),
23.5 (C-30), 22.0, 20.8 (C-90, C-100), 18.2 (SCH3), 16.4 (C-70). IR
(CHCl3) nmax 3366 (NH), 2960, 2926, 1700 (C]O), 1440, 1373, 1286,
1073 cm1. MS (EI), m/z (%): 418 [MþH]þ (6), 130 (100).
d
164.7 (C]N), 150.5 (C]O), 140.9 (C-7a), 129.9 (C-6), 129.5 (C-3a),
124.1 (C-4), 123.8 (C-5), 114.9 (C-7), 91.7 (C-2), 77.2 (C-10), 70.5 (C-
3), 67.7 (CH2), 47.3 (C-20), 40.9 (C-60), 34.1 (C-40), 31.5 (C-50), 26.4 (C-
80), 23.4 (C-30), 21.9, 20.8 (C-90, C-100), 16.3 (C-70), 15.4 (SCH3).
NOESY correlation (400 MHz, CDCl3): Ha/H-4, Ha/Hb, H-7/H-6, H-4/
H-5.
4.1.10. 1-[(1R,2S,5R)-8-Phenylmenthoxycarbonyl]brassinin (39) and
1-[(1S,2R,5S)-8-phenylmenthoxycarbonyl]brassinin (40). To a stirred
solution of crude freshly prepared amine 36 or 37 (0.404 g,
1.00 mmol) in methanol (10 mL) were added triethylamine
(0.304 g, 0.418 mL, 3.00 mmol) and carbon disulphide (0.229 g,
0.181 mL, 3.00 mmol). After stirring for 5 min at room temperature,
methyl iodide (0.424 g, 0.187 mL, 3.00 mmol) was added and
Compound 41b: 1H NMR (400 MHz, CDCl3)
d 7.77 (s, 1H, H-7),
7.39 (d, 1H, J 7.5, H-4), 7.29 (dd, 1H, J 7.5, 7.6, H-6), 7.07 (dd, 1H, J 7.5,
7.5, H-5), 5.97 (s, 1H, H-2), 5.29 (s, 1H, OH), 5.07 (d, 1H, J 15.5, Hb),

ꢀ
M. Budovska et al. / Tetrahedron 69 (2013) 1092e1104
1101
4.84 (ddd, 1H, J 10.5, 10.5, 4.3, H-10), 4.29 (d, 1H, J 15.5, Ha), 2.60 (s,
3H, SCH3), 2.17e2.16 (m, 1H, H-80), 1.97e1.94 (m, 1H, H-60),
1.76e1.72 (m, 2H, H-30, H-40), 1.57e1.51 (m, 2H, H-20, H-50),
1.17e0.95 (m, 3H, H-30, H-40, H-60), 0.94 (d, 3H, J 7.0), 0.92 (d, 3H, J
7.0) [H-90, H-100], 0.82 (d, 3H, J 6.9, H-70). 13C NMR (100 MHz, CDCl3)
(n-hexane/ethyl acetate 5:1). Anal. Calcd for C28H34N2O3S2 re-
quires: C, 65.85; H, 6.71; N, 5.49. Found: C, 65.93; H, 6.49; N, 5.70.
MS (EI), m/z (%): 511 [M]þ (2), 251 (47), 119 (100), 105 (99), 91 (64).
Compound 42a: 1H NMR (400 MHz, CDCl3)
d 7.79 (d, 1H, J 8.1, H-
7), 7.38e7.18 (m, 5H, H-4, H-6, H-200, H-600, H-5), 7.04e7.01 (m, 2H,
H-300, H-500), 6.91e6.86 (m, 1H, H-400), 5.95 (s, 1H, H-2), 5.03 (dt, 1H, J
10.8, 4.3, H-10), 4.89 (d, 1H, J 15.0, Hb), 4.15 (d, 1H, J 15.0, Ha), 2.60 (s,
3H, SCH3), 2.19e2.12 (m, 1H, H-20), 2.05e1.82 (m, 2H, H-30, H-60),
1.73e1.71 (m, 1H, H-40), 1.53e1.47 (m, 1H, H-50), 1.38 (s, 3H), 1.21 (s,
3H) [H-90, H-100], 1.31e0.88 (m, 3H, H-30, H-60, H-40), 0.89 (d, 3H, J
d
164.7 (C]N), 150.5 (C]O), 140.9 (C-7a), 129.9 (C-6), 129.5 (C-3a),
124.1 (C-4), 123.8 (C-5), 114.9 (C-7), 91.7 (C-2), 77.2 (C-10), 70.5 (C-
3), 67.7 (CH2), 47.3 (C-20), 40.9 (C-60), 34.1 (C-40), 31.5 (C-50), 26.4 (C-
80), 23.4 (C-30), 21.9, 20.8 (C-90, C-100), 16.3 (C-70), 15.4 (SCH3).
NOESY correlation (400 MHz, CDCl3): Ha/H-4, Ha/Hb, H-7/H-6, H-4/
H-5.
6.5, H-70). 13C NMR (100 MHz, CDCl3)
d
164.4 (C]N), 152.3 (C-100),
148.9 (C]O), 140.8 (C-7a), 130.1 (C-6), 128.6 (C-3a), 128.1 (C-300, C-
500),126.1 (C-5),125.5 (C-200, C-600),125.1 (C-400),123.8 (C-4),114.8 (C-
7), 90.1 (C-2), 77.4 (C-10), 70.8 (C-3), 68.5 (CH2), 50.8 (C-20), 42.5 (C-
60), 39.7 (C-80), 34.7 (C-40), 31.7 (C-50), 29.9 (C-90), 26.7 (C-30), 23.5
(C-100), 21.9 (C-70), 15.4 (SCH3). NOESY correlation (400 MHz,
CDCl3): Ha/H-4, Ha/Hb.
4.1.11.2. cis-1-[(1R,2S,5R)-Menthoxycarbonyl]spirobrassinol (41c,
41d). Yield: 0.014 g (3%) of a mixture of diastereoisomers 41c and
41d in a 50:50 ratio, colourless oil, Rf 0.59 (cyclohexane/ethyl ace-
tate 8:1). Anal. Calcd for C22H30N2O3S2 requires: C, 60.80; H, 6.96;
N, 6.45. Found: C, 61.05; H, 6.68; N, 6.22. MS (MALDI-TOF), m/z (%):
identical with spectrum of 41a and 41b.
Compound 42b: 1H NMR (400 MHz, CDCl3)
d 7.73 (s, 1H, H-7),
Compound 41c: 1H NMR (400 MHz, CDCl3)
d
7.67 (br s, 1H, H-7),
7.32e7.16 (m, 5H, H-4, H-6, H-200, H-600, H-5), 7.09e6.80 (m, 2H, H-
300, H-500), 6.74e6.61 (m, 1H, H-400), 5.84 (s, 1H, H-2), 5.03 (dt, 1H, J
10.8, 4.3, H-10), 4.23 (d, 1H, J 15.6, Ha), 4.23 (d, 2H, J 15.6, Hb), 2.58 (s,
3H, SCH3), 2.18e2.14 (m, 1H, H-20), 1.97e1.85 (m, 2H, H-30, H-60),
1.73e1.70 (m, 1H, H-40), 1.58e1.45 (m, 1H, H-50), 1.39 (s, 3H), 1.23 (s,
3H) [H-90, H-100], 1.26e0.88 (m, 3H, H-30, H-60, H-40), 0.90 (d, 3H, J
7.39 (d, 1H, J 7.5, H-4), 7.30e7.26 (m, 1H, H-6), 7.15e6.99 (m, 1H, H-
5), 5.65 (s, 1H, H-2), 4.97 (s, 1H, OH), 4.83 (ddd, 1H, J 11.1, 11.1, 4.3, H-
10), 4.38 (d, 1H, J 15.1, Hb), 4.02 (d, 1H, J 15.1, Ha), 2.57 (s, 3H, SCH3),
2.19e2.14 (m, 1H, H-80), 2.02e1.95 (m, 1H, H-60), 1.74e1.72 (m, 2H,
H-30, H-40), 1.64e1.50 (m, 2H, H-20, H-50), 1.27e0.86 (m, 3H, H-30, H-
40, H-60), 0.93 (d, 3H, J 7.1), 0.92 (d, 3H, J 7.1) [H-90, H-100], 0.81 (d,
6.5, H-70). 13C NMR (100 MHz, CDCl3)
d
164.4 (C]N), 153.0 (C-100),
3H, J 6.9, H-70). 13C NMR (100 MHz, CDCl3)
d
166.7 (C]N),153.5 (C]
151.3 (C]O), 139.4 (C-7a), 130.3 (C-6), 129.7 (C-3a), 128.2 (C-300, C-
500), 125.2 (C-200, C-600, C-5), 123.9 (C-400), 123.6 (C-4), 115.3 (C-7),
92.0 (C-2), 77.4 (C-10), 70.2 (C-3), 68.1 (CH2), 51.6 (C-20), 42.7 (C-60),
39.8 (C-80), 34.7 (C-40), 31.7 (C-50), 29.9 (C-90), 26.8 (C-30), 24.0 (C-
100), 21.9 (C-70), 15.4 (SCH3). NOESY correlation (400 MHz, CDCl3):
Ha/H-4, Ha/Hb.
O), 139.9 (C-7a), 130.0 (C-6), 129.7 (C-3a), 124.3 (C-4), 124.0 (C-5),
115.1 (C-7), 88.4 (C-2), 75.7 (C-10), 75.6 (C-3), 64.5 (CH2), 47.5 (C-20),
41.6 (C-60), 34.4 (C-40), 31.5 (C-50), 26.5 (C-80), 23.5 (C-30), 22.2, 21.1
(C-90, C-100), 16.4 (C-70), 15.4 (SCH3). NOESY correlation (400 MHz,
CDCl3): Ha/H-4, Hb/H-2, Ha/Hb.
Compound 41d: 1H NMR (400 MHz, CDCl3)
d 7.67 (br s, 1H, H-7),
7.39 (d, 1H, J 7.5, H-4), 7.30e7.26 (m, 1H, H-6), 7.15e6.99 (m, 1H, H-
5), 5.65 (s, 1H, H-2), 4.97 (s, 1H, OH), 4.83 (ddd, 1H, J 11.1, 11.1, 4.3, H-
10), 4.37 (d, 1H, J 15.1, Hb), 4.03 (d, 1H, J 15.1, Ha), 2.57 (s, 3H, SCH3),
2.19e2.14 (m, 1H, H-80), 2.02e1.95 (m, 1H, H-60), 1.74e1.72 (m, 2H,
H-30, H-40), 1.64e1.50 (m, 2H, H-20, H-50), 1.27e0.86 (m, 3H, H-30, H-
40, H-60), 0.93 (d, 3H, J 7.1), 0.92 (d, 3H, J 7.1) [H-90, H-100], 0.82 (d,
4.1.12.2. trans-1-[(1S,2R,5S)-8-Phenylmenthoxycarbonyl]spiro-
brassinol (43a, 43b). Yield: 0.129 g (63%) of a mixture of di-
astereoisomers 43a and 43b in an 82:18 ratio, colourless oil, Rf 0.21
(n-hexane/ethyl acetate 5:1). Anal. Calcd for C28H34N2O3S2 re-
quires: C, 65.85; H, 6.71; N, 5.49. Found: C, 65.99; H, 6.86; N, 5.76.
Properties and spectral data of compounds 43a and 43b were fully
identical with those of enantiomers 42a and 42b.
3H, J 6.9, H-70). 13C NMR (100 MHz, CDCl3)
d 166.7 (C]N),154.2 (C]
O), 139.9 (C-7a), 130.0 (C-6), 129.7 (C-3a), 124.3 (C-4), 124.0 (C-5),
115.1 (C-7), 88.4 (C-2), 75.7 (C-10), 75.6 (C-3), 60.6 (CH2), 47.5 (C-20),
41.5 (C-60), 34.3 (C-40), 31.5 (C-50), 26.7 (C-80), 23.6 (C-30), 22.2, 21.1
(C-90, C-100), 16.6 (C-70), 15.4 (SCH3). NOESY correlation (400 MHz,
CDCl3): Ha/H-4, Hb/H-2, Ha/Hb.
4.1.13. 1-[(1R,2S,5R)-Menthoxycarbonyl]spirobrassinin (44a, 44b). A
solution of a mixture of 41a and 41b (0.172 g, 0.400 mmol) in dry
dichloromethane (0.2 mL) was added to a vigorously stirred slurry
of pyridinium chlorochromate (0.256 g, 1.20 mmol) and anhydrous
magnesium sulfate (0.216 g, 1.80 mmol) in dry dichloromethane
(0.2 mL). The reaction mixture was stirred for 24 h at room tem-
perature and then diluted with dichloromethane (5 mL). After
adding a small amount of silica gel, the solvent was evaporated and
residue preabsorbed on silica was subjected to silica gel column
chromatography (12 g, n-hexane/ethyl acetate 5:1) affording
a mixture of 44a and 44b as a colourless oil. The obtained mixture
of 44a and 44b was further crystallized from dichloromethane/n-
hexane to afford pure diastereoisomer 44b. Diastereoisomer 44a
was obtained after filtration of crystals of diastereoisomer 44b and
4.1.12. Spirocyclization of 1-[(1R,2S,5R)-8-phenylmenthoxycarbonyl]
brassinin (39) and 1-[(1S,2R,5S)-8-phenylmenthoxycarbonyl]brassinin
(40). To
a stirred solution of 1-[(1R,2S,5R)-8-phenylmenth-
oxycarbonyl]brassinin (39) or-1-[(1S,2R,5S)-8-phenylmenthoxyca-
rbonyl]brassinin (40) (0.198 g, 0.400 mmol) in a mixture of
dichloromethane/water (3.6 mL/0.4 mL) cooled toꢀ90 ꢂC was added
a freshly prepared solution of Br2 (1.10 mL, 0.440 mmol). The stock
solution was obtained by dissolving of bromine (0.040 mL) in
dichloromethane (1.76 mL). The reaction mixture was stirred for
20 min at ꢀ90 ꢂC, then triethylamine (0.089 g, 0.124 mL,
0.880 mmol) was added. Stirring was continued for 10 min and the
reaction mixture was diluted with dichloromethane (20 mL), then
washed with brine (2ꢃ20 mL). The organic layer was dried over
Na2SO4 and the residue obtained after evaporation of the solvent
subjected to chromatography on silica gel (20 g, n-hexane/ethyl
acetate 5:1), affording a mixture of trans-diastereoisomers 42a and
42b in an 18: 82 ratio or 43a and 43b in an 82:18 ratio.
evaporation of the solvent.
20
Data for 44a: yield: 0.082 g (47%), colourless oil, [
a]
ꢀ51.3 (c
D
0.16, CHCl3), Rf 0.32 (n-hexane/ethyl acetate 5:1). Anal. Calcd for
22H28N2O3S2 requires: C, 61.08; H, 6.52; N, 6.48. Found: C, 61.39; H,
6.40; N, 6.27. 1H NMR (400 MHz, CDCl3)
7.88 (d, 1H, J 8.2, H-7),
C
d
7.42e7.34 (m, 2H, H-4, H-6), 7.23 (dd, 1H, J 7.6, 7.6, H-5), 4.91 (ddd,
1H, J 10.9, 10.9, 4.3, H-10), 4.73 (d, 1H, J 15.2, Hb), 4.51 (d, 1H, J 15.2,
Ha), 2.62 (s, 3H, SCH3), 2.21e2.18 (m, 1H, H-80), 2.15e2.09 (m, 1H, H-
60), 1.77e1.71 (m, 2H, H-30, H-40), 1.65e1.51 (m, 2H, H-20, H-50),
1.26e0.88 (m, 3H, H-30, H-40, H-60), 0.94 (d, 3H, J 6.4), 0.93 (d, 3H, J
6.9) [H-90, H-100], 0.81 (d, 3H, J 6.9, H-70). 13C NMR (100 MHz, CDCl3)
4.1.12.1. trans-1-[(1R,2S,5R)-8-Phenylmenthoxycarbonyl]spiro-
brassinol (42a, 42b). Yield: 0.129 g (63%) of a mixture of di-
astereoisomers 42a and 42b in an 18:82 ratio, colourless oil, Rf 0.21
d
174.4 (C]O), 163.7 (C]N), 150.3 (C]O), 138.3 (C-7a), 130.1 (C-6),

ꢀ
M. Budovska et al. / Tetrahedron 69 (2013) 1092e1104
1102
129.9 (C-3a), 125.5 (C-5), 124.1 (C-4), 115.2 (C-7), 78.7 (C-10), 75.9
(CH2), 65.0 (C-3), 46.8 (C-20), 40.7 (C-60), 34.0 (C-40), 31.5 (C-50), 25.9
(C-80), 23.1 (C-30), 21.9, 20.9 (C-90, C-100), 16.0 (C-70), 15.6 (SCH3). IR
(CHCl3) nmax 2958, 2872, 1774 (C]O), 1734 (C]O), 1589 (C]N),
1290 cm1. MS (MALDI-TOF), m/z (%): 455.66 [MþNa]þ (34), 433.61
[MþH]þ (18), 177.83 (100).
0.92 (d, 3H, J 6.5, H-70). 13C NMR (100 MHz, CDCl3)
d 173.9 (C]O),
164.1 (C]N), 151.4 (C-100), 149.4 (C]O), 138.3 (C-7a), 129.9 (C-6),
129.1 (C-3a), 127.9 (C-300, C-500), 125.5 (C-200, C-600), 125.4 (C-5), 124.9
(C-400), 124.0 (C-4), 115.4 (C-7), 77.6 (C-10), 76.2 (CH2), 65.1 (C-3),
50.9 (C-20), 42.0 (C-60), 39.8 (C-80), 34.5 (C-40), 31.7 (C-50), 29.5 (C-
90), 26.6 (C-30), 23.5 (C-100), 21.9 (C-70), 15.9 (SCH3). MS (EI), m/z (%):
509 [M]þ (4), 251 (49), 119 (100), 105 (76), 91 (57).
Data for 44b: yield: 0.041 g (24%), colourless crystals, mp
143e145 ꢂC (dichloromethane/n-hexane), [
Rf 0.4 (n-hexane/ethyl acetate 5:1). Anal. Calcd for C22H28N2O3S2
a
]
D
20 ꢀ10.6(c 0.60, CHCl3),
4.1.14.2. 1-[(1S,2R,5S)-8-Phenylmenthoxycarbonyl]spirobrassinin
requires: C, 61.08; H, 6.52; N, 6.48. Found: C, 61.29; H, 6.40; N, 6.27.1H
(46a, 46b). Data for 46a: yield: 0.068 g (54%), colourless oil, [a]
20
D
NMR(400 MHz, CDCl3)
d
7.88(d,1H,J 8.2, H-7), 7.42e7.39(m, 2H,H-4,
þ100.0 (c 0.49, CHCl3), Rf 0.64 (dichloromethane/ethyl acetate
30:1). Anal. Calcd for C28H32N2O3S2 requires: C, 66.11; H, 6.34; N,
5.51. Found: C, 65.91; H, 6.09; N, 5.20. Properties and spectral data
of compound 46a were fully identical with those of enantiomer
H-6), 7.23 (dd,1H, J 7.5, 9.1, H-5), 4.91 (ddd,1H, J 10.9, 10.9, 4.3, H-10),
4.74 (d, 1H, J 15.3, Hb), 4.51 (d, 1H, J 15.3, Ha), 2.62 (s, 3H, SCH3),
2.21e2.18 (m,1H, H-80), 2.15e2.09 (m,1H, H-60),1.77e1,71 (m, 2H, H-
30, H-40), 1.65e1.51 (m, 2H, H-20, H-50), 1.26e0.88 (m, 3H, H-30, H-40,
H-60), 0.94 (d, 3H, J 6.4), 0.93 (d, 3H, J 6.9) [H-90, H-100], 0.81 (d, 3H, J
45b.
20
Data for 46b: yield: 0.015 g (12%), colourless oil, [
a]
þ31.7 (c
D
6.9, H-70). 13C NMR (100 MHz, CDCl3)
d
174.4 (C]O), 163.7 (C]N),
0.26, CHCl3), Rf 0.78 (dichloromethane/ethyl acetate 30:1). Anal.
Calcd for C28H32N2O3S2 requires: C, 66.11; H, 6.34; N, 5.51. Found: C,
66.31; H, 6.07; N, 5.28. Properties and spectral data of compound
46b were fully identical with those of enantiomer 45a.
150.3 (C]O),138.3 (C-7a),130.1 (C-6),129.2 (C-3a),125.4 (C-5),124.1
(C-4), 115.2 (C-7), 78.7 (C-10), 75.9 (CH2), 65.0 (C-3), 46.8 (C-20), 40.7
(C-60), 34.0 (C-40), 31.5 (C-50), 25.1 (C-80), 23.1 (C-30), 21.9, 20.9 (C-90,
C-100), 16.0 (C-70), 15.6 (SCH3). IR (KBr) nmax 2947, 2922, 1774 (C]O),
1734 (C]O), 1597 (C]N), 1288 cm1. MS (MALDI-TOF), m/z (%):
455.66 [MþNa]þ (34), 433.61 [MþH]þ (18), 177.83 (100).
4.1.15. Enantiomers of spirobrassinin [(R)-(þ)-1, (S)-(ꢀ)-1] from 44a
and 44b. To a stirred solution of 44a or 44b (0.030 g, 0.070 mmol) in
dry methanol (1.0 mL) was added CH3ONa (0.006 g, 0.104 mmol).
The mixture was stirred at room temperature for 20 min, then the
solvent was evaporated and the residue was purified by silica gel
column flash chromatography (2.5 g, n-hexane/ethyl acetate 2:1)
affording (R)-(þ)-1 [0.014 g, 82% from 44a] and (S)-(ꢀ)-1 [0.014 g,
82% from 44b]. (R)-(þ)-Spirobrassinin [(R)-(þ)-1]: colourless nee-
4.1.14. 1-[(1R,2S,5R)-8-Phenylmenthoxycarbonyl]spirobrassinin (45a,
45b) and 1-[(1S,2R,5S)-8-phenylmenthoxycarbonyl]spirobrassinin
(46a, 46b). A solution of a mixture of 42a and 42b or 43a and 43b
(0.129 g, 0.250 mmol) in dry dichloromethane (0.5 mL) was added
to
a vigorously stirred slurry of pyridinium chlorochromate
(0.377 g, 1.75 mmol) and anhydrous magnesium sulfate (0.258 g,
1.25 mmol) in dry dichloromethane (0.5 mL). The reaction mixture
was stirred for 24 h at room temperature and then diluted with
dichloromethane (2 mL). After adding a small amount of silica gel,
the solvent was evaporated and residue preabsorbed on silica was
subjected to silica gel column chromatography (20 g, dichloro-
methane/ethyl acetate 30:1) and diastereoisomers 45a and 45b or
46a and 46b were separated.
dles, mp 156e158 ꢂC (n-hexane/acetone), [
a
]
D
20 þ29.2 (c 0.13, CHCl3),
65% ee, ECD (CH3OH, cw0.1 mM) lext
(
D
3
): 200 (þ5.7), 210 (þ0.80),
218 (ꢀ10.9), 237 (þ1.35), 260 (þ2.37), 278 (þ0.74), 305 (þ2.2) nm.1H
NMR (400 MHz, CDCl3) d 8.96 (s, 1H, NH), 7.36 (d, 1H, J 7.1, H-4), 7.26
(ddd,1H, J 7.8, J 7.7, J 1.2, H-6), 7.09 (ddd,1H, J 7.7, J 7.1, J 1.0, H-5), 6.94
(d,1H, J 7.8, H-7), 4.68 (d,1H, J 15.1, Hb), 4.51 (d,1H, J 15.1, Ha), 2.62 (s,
3H, SCH3). 13C NMR (100 MHz, CDCl3)
d 178.5 (C]O), 164.1 (C]N),
139.5 (C-7a), 131.2 (C-3a), 129.7 (C-6), 124.5 (C-4), 123.7 (C-5), 110.5
(C-7), 75.0 (CH2), 64.7 (C-3), 15.7 (SCH3). Spectral data of (R)-(þ)-1
were fully identical with those of natural product.4 (S)-(ꢀ)-Spiro-
4.1.14.1. 1-[(1R,2S,5R)-8-Phenylmenthoxycarbonyl]spirobrassinin
(45a, 45b). Data for 45a: yield: 0.015 g (12%), colourless oil, [
ꢀ17.5 (c 0.16, CHCl3), Rf 0.78 (dichloromethane/ethyl acetate 30:1).
Anal. Calcd for C28H32N2O3S2 requires: C, 66.11; H, 6.34; N, 5.51.
Found: C, 66.43; H, 6.07; N, 5.36. 1H NMR (400 MHz, CDCl3)
20
a]
brassinin [(S)-(ꢀ)-1]: colourless needles, mp 153e154 ꢂC (n-hexane/
D
20
acetone), [
a
]
D
ꢀ55.4 (c 0.13, CHCl3), 92% ee, ECD (CH3OH,
cw0.1 mM) lext
(D
3
): 200 (ꢀ11.7), 210 (ꢀ1.04), 218 (þ21.26), 237
(ꢀ1.62), 260 (ꢀ5.22), 278 (ꢀ1.40), 305 (ꢀ4.42) nm. Spectral data of
d
7.66e7.64 (m, 1H, H-7), 7.33e7.29 (m, 2H, H-4, H-6), 7.25e7.23 (m,
(S)-(ꢀ)-1 were fully identical with those of natural product.4
2H, H-200, H-600), 7.20e7.16 (m, 1H, H-5), 6.99e6.95 (m, 2H, H-300, H-
500), 6.80e6.77 (m,1H, H-400), 5.10(dt,1H, J10.8, 4.7, H-10), 4.59 (d,1H, J
15.1, Hb), 4.42 (d, 1H, J 15.1, Ha), 2.62 (s, 3H, SCH3), 2.24e2.16 (m, 1H,
H-20), 2.01e1.86 (m, 2H, H-30, H-60), 1.74e1.69 (m, 1H, H-40),
1.60e1.45 (m, 1H, H-50), 1.38 (s, 3H), 1.20 (s, 3H) [H-90, H-100],
1.32e0.84 (m, 3H, H-30, H-60, H-40), 0.92 (d, 3H, J 6.5, H-70). 13C NMR
4.1.16. Enantiomers of spirobrassinin [(S)-(ꢀ)-1, (R)-(þ)-1] from 45b
and 46a. To a stirred solution of 45b or 46a (0.028 g, 0.055 mmol)
in dry methanol (0.5 mL) was added CH3ONa (0.009 g, 0.165 mmol).
The mixture was stirred at room temperature for 1.5 h, then the
solvent was evaporated and the residue was purified by silica gel
column flash chromatography (2.5 g, n-hexane/ethyl acetate 2:1)
affording (S)-(ꢀ)-1 [0.012 g, 85% from 45b] and (R)-(þ)-1 [0.012 g,
(100 MHz, CDCl3) d
173.8(C]O),164.0(C]N),151.2(C-100),149.5(C]
O),138.2 (C-7a),129.9 (C-6),129.6 (C-3a),127.9 (C-300, C-500),125.5 (C-
200, C-600, C-5), 125.1 (C-400), 124.0 (C-4), 115.4 (C-7), 77.2 (C-10), 76.4
(CH2), 64.9 (C-3), 50.8 (C-20), 41.8 (C-60), 39.8 (C-80), 34.5 (C-40), 31.7
(C-50), 29.4 (C-90), 26.6 (C-30), 23.5 (C-100), 21.9 (C-70),15.9 (SCH3). MS
85% from 46a]. (S)-(ꢀ)-Spirobrassinin [(S)-(ꢀ)-1]: colourless nee-
20
dles, mp 153e154 ꢂC (n-hexane/acetone), [
CHCl3), 91% ee, ECD (CH3OH, cw 0.1 mM) lext
a
(
]
ꢀ55.1 (c 0.12,
3
D
D
): 200 (ꢀ15.57),
(EI), m/z (%): 509 [M]þ (4), 251 (54), 119 (100), 105 (68), 91 (55).
210 (ꢀ5.24), 218 (þ24.22), 237 (ꢀ0.38), 260 (ꢀ5.59), 278 (ꢀ1.63),
305 (ꢀ5.32) nm. Spectral data of (S)-(ꢀ)-1 were fully identical with
those of natural product.4 (R)-(þ)-Spirobrassinin [(R)-(þ)-1]: col-
20
Data for 45b: yield: 0.068 g (54%), colourless oil, [
0.47, CHCl3), Rf 0.64 (dichloromethane/ethyl acetate 30:1). Anal.
a
]
ꢀ89.3 (c
D
Calcd for C28H32N2O3S2 requires: C, 66.11; H, 6.34; N, 5.51. Found: C,
ourless needles, mp 152e153 ꢂC (n-hexane/acetone), [
a]
20 þ56.0 (c
D
66.27; H, 6.15; N, 5.79. 1H NMR (400 MHz, CDCl3)
d
7.60e7.58 (m,
0.12, CHCl3), 94% ee, ECD (CH3OH, cw 0.1 mM) lext (D 3 ): 200
1H, H-7), 7.35e7.23 (m, 4H, H-4, H-6, H-200, H-600), 7.19e7.16 (m, 1H,
H-5), 6.99e6.95 (m, 2H, H-300, H-500), 6.78e6.69 (m, 1H, H-400), 5.12
(dt, 1H, J 10.8, 4.7, H-10), 4.60 (d, 1H, J 15.2, Hb), 4.40 (d, 1H, J 15.2,
Ha), 2.62 (s, 3H, SCH3), 2.24e2.17 (m, 1H, H-20), 1.98e1.88 (m, 2H, H-
30, H-60), 1.74e1.68 (m, 1H, H-40), 1.59e1.45 (m, 1H, H-50), 1.39 (s,
3H), 1.24 (s, 3H) [H-90, H-100], 1.29e0.84 (m, 3H, H-30, H-60, H-40),
(þ15.57), 210 (þ5.24), 218 (ꢀ24.22), 237 (þ0.38), 260 (þ5.59), 278
(þ1.63), 305 (þ5.32) nm. Spectral data of (R)-(þ)-1 were fully
identical with those of natural product.4
4.1.17. General procedure of preparation of 1-substituted derivatives
of spirobrassinin 44, 45, 48, 49 from 9-substituted derivatives of

ꢀ
M. Budovska et al. / Tetrahedron 69 (2013) 1092e1104
1103
cyclobrassinin 28e31. To a stirred solution of cyclobrassinin de-
rivatives 28e31 (0.075 mmol) in ethyl acetate (0.2 mL) was added
solution of NaCl (0.160 mL, 0.083 mmol; the stock solution was
obtained by dissolving of 0.030 g of NaCl in 1.00 mL of water) and
oxone (0.051 g, 0.083 mmol). The reaction mixture was stirred for
24 h. The reaction mixture was diluted with ethyl acetate (5 mL)
and water (5 mL), extracted with ethyl acetate (2ꢃ5 mL). The or-
ganic layer was washed with a saturated solution of Na2S2O3
(1ꢃ5 mL) and dried over Na2SO4 and the residue obtained after
evaporation of the solvent subjected to chromatography on silica
gel (2.5 g, n-hexane/ethyl acetate 5:1 or n-hexane/ethyl acetate
3:1), affording mixture of diastereoisomers 44a and 44b, 45a and
45b, 48a and 48b, 49a and 49b in a 50:50 ratio.
1290 cm1. MS (EI), m/z (%): 430 [M]þ (17), 250 (38), 137 (53), 81
(100).
4.1.18. 9-[(1R,2S,5R)-Menthoxycarbonyl]-2H-3,4-dihydro-[1,3]thia-
zino[6,5-b]indol-2,4-dione (47). A solution of compound 28
(0.021 g, 0.050 mmol) in dry dichloromethane (0.2 mL) was added
to
a vigorously stirred slurry of pyridinium chlorochromate
(0.028 g, 0.130 mmol) in dry dichloromethane (0.2 mL). The re-
action mixture was stirred for 24 h at room temperature and then
diluted with dichloromethane (5 mL). After adding a small amount
of silica gel, the solvent was evaporated and the residue pre-
absorbed on silica was subjected to silica gel column chromatog-
raphy (2 g, n-hexane/ethyl acetate 5:1). Yield: 0.014 g (71%), light
yellow oil, [
a
]
20 ꢀ19.4 (c 0.20, CHCl3), Rf 0.39 (n-hexane/ethyl ace-
D
4.1.17.1. 1-[(1R,2S,5R)-Menthoxycarbonyl]spirobrassinin
(44a,
tate 5:1). Anal. Calcd for C21H24N2O4S requires: C, 62.98; H, 6.04; N,
44b). Following the general procedure, product 44 was obtained
using cyclobrassinin derivative 28 (0.031 g, 0.075 mmol). Yield:
0.021 g (68%) of a mixture of diastereoisomers 44a and 44b in
a 50:50 ratio.
6.99. Found: C, 62.72; H, 6.04; N, 6.71. 1H NMR (400 MHz, CDCl3)
d
9.01 (s, 1H, NH), 8.35e8.33 (m, 1H, H-5), 8.11e8.08 (m, 1H, H-8),
7.44e7.42 (m, 2H, H-7, H-6), 5.01 (dt,1H, J 10.9, 4.5, H-10), 2.28e2.22
(m, 1H, H-80), 2.07e1.99 (m, 1H, H-60), 1.86e1.71 (m, 2H, H-30, H-40),
1.61e1.53 (m, 2H, H-20, H-50), 1.37e0.88 (m, 3H, H-30, H-40, H-60),
0.99 (d, 3H, J 6.5), 0.98 (d, 3H, J 6.8) [H-90, H-100], 0.84 (d, 3H, J 6.9,
4.1.17.2. 1-[(1R,2S,5R)-8-Phenylmenthoxycarbonyl]spirobrassinin
(45a, 45b). Following the general procedure, product 45 was ob-
tained using cyclobrassinin derivative 29 (0.037 g, 0.075 mmol).
Yield: 0.026 g (68%) of a mixture of diastereoisomers 45a and 45b
in a 50:50 ratio.
H-70). 13C NMR (100 MHz, CDCl3)
d 164.5 (C]O), 160.4 (C]O), 150.7
(C]O), 139.8 (C-8a), 136.4 (C-4b), 126.5 (C-9a), 126.2 (C-7), 125.5
(C-6), 121.5 (C-5), 115.2 (C-8), 107.9 (C-4a), 81.3 (C-10), 47.4 (C-20),
41.1 (C-60), 34.1 (C-40), 31.8 (C-50), 26.6 (C-80), 23.5 (C-30), 22.1, 21.0
(C-90, C-100), 16.4 (C-70). IR (CHCl3) nmax 3156, 3040, 2954, 2926,
2868, 1728, 1682, 1664, 1522, 1448, 1312 cm1. MS (EI), m/z (%): 400
[M]þ (16), 262 (28), 175 (44), 83 (100).
4.1.17.3. 1-[(1S)-Phenylethoxycarbonyl]spirobrassinin (48a, 48b).
Following the general procedure, product 48 was obtained using
cyclobrassinin derivative 30 (0.029 g, 0.075 mmol). Yield: 0.019 g
(63%) of a mixture of diastereoisomers 48a and 48b in a 50:50 ratio,
light yellow oil, Rf 0.42 (n-hexane/ethyl acetate 3:1). Anal. Calcd for
4.2. Molecular modelling computational protocols
C
20H18N2O3S2 requires: C, 60.28; H, 4.55; N, 7.03. Found: C, 60.52; H,
The 2D molecular structures drawn and saved in MDL mol for-
mat were imported into Accelrys Discovery Studio26 in order to
convert them into 3D form and add hydrogens. The ‘clean geome-
try’ option of the program was used to remove the unwanted short
contacts. The 3D structures thus obtained were saved in MDL mol
format, consecutively. The geometries obtained in this way were
imported into MAESTRO (Schrodinger, LLC) and submitted from
there for ab initio minimization runs using the JAGUAR program22
on DFT level with 6-31** basis sets. The geometries of conformers
were optimized in the gas-phase. The variety of conformations was
accounted for by population-weight averaged assessment of
energies.23
4.68; N, 7.19. 1H NMR (400 MHz, CDCl3)
d
7.90e7.88 (m, 2H, 2ꢃH-7),
7.53e7.49 (m, 4H, 2ꢃH-200, 2ꢃH-600), 7.43e7.29 (m, 10H, 2ꢃH-4,
2ꢃH-300, 2ꢃH-500, 2ꢃH-400, 2ꢃH-6), 7.24e7.18 (m, 2H, 2ꢃH-5), 6.09
(quartet, 2H, J 6.6, 2ꢃCH), 4.76 (d, 1H, J 15.3, Hb), 4.73 (d, 1H, J 15.3,
Hb), 4.51 (d, 1H, J 15.3, Ha), 4.49 (d, 1H, J 15.3, Ha), 2.63 (s, 3H, SCH3),
2.62 (s, 3H, SCH3), 1.72 (d, 3H, J 6.6, CH3), 1.71 (d, 3H, J 6.6, CH3). 13
C
NMR (100 MHz, CDCl3)
d 173.5, 173.4 (C]O), 163.1, 163.0 (C]N),
148.9, 148.8 (C]O), 139.5, 139.4 (C-10), 137.1, 137.0 (C-7a), 129.0 (C-
6), 127.7 (C-30, C-50), 128.6 (C-3a), 127.2 (C-40), 125.2 (C-20, C-60),
124.7 (C-5), 123.1 (C-4), 114.3 (C-7), 75.6 (CH), 75.0, 74.9 (CH2), 64.0,
63.9 (C-3), 21.7, 21.6 (CH3), 14.6 (SCH3). IR (CHCl3) nmax 3020, 1767
(C]O), 1727 (C]O), 1593 (C]N), 1207 cm1. MS (EI), m/z (%): 398
[M]þ (2), 249 (17), 105 (100).
Acknowledgements
4.1.17.4. 1-[(1S)-endo-Borneoxycarbonyl]spirobrassinin (49a,
49b). Following the general procedure, product 49 was obtained
using cyclobrassinin derivative 31 (0.031 g, 0.075 mmol). Yield:
0.023 g (72%) of a mixture of diastereoisomers 49a and 49b in
a 50:50 ratio, light yellow oil, Rf 0.64 (n-hexane/ethyl acetate
3:1). Anal. Calcd for C22H26N2O3S2 requires: C, 61.37; H, 6.09; N,
6.51. Found: C, 61.63; H, 5.83; N, 6.75. 1H NMR (400 MHz, CDCl3)
We would like to thank the Slovak Grant Agency for Science
(Grant No. 1/3553/06 and No. 1/0406/10 and No. 1/0954/12) and
the Slovak Research and Development Agency (Contract No. APVV-
0514-06) for financial support of this work. This work was also
supported by the State NMR Programme No. 2003SP200280203.
d
7.92 (d, 2H, J 8.1, 2ꢃH-7), 7.41 (dd, 2H, J 7.5, 1.0, 2ꢃH-4),
References and notes
7.36e7.34 (m, 2H, 2ꢃH-6), 7.24e7.20 (m, 2H, 2ꢃH-5), 5.10 (dd,
2H, J 10.0, 5.4, 2ꢃH-20), 4.76 (d, 1H, J 15.3, Hb), 4.75 (d, 1H, J 15.3,
Hb), 4.52 (d, 1H, J 15.3, Ha), 4.51 (d, 1H, J 15.3, Ha), 2.62 (s, 6H,
2ꢃSCH3), 2.51e2.44 (m, 2H, 2ꢃH-30), 2.31e2.21 (m, 2H, 2ꢃH-60),
1.79e1.73 (m, 4H, 2ꢃH-50, 2ꢃH-40), 1.42e1.39 (m, 4H, 2ꢃH-60,
2ꢃH-50), 1.26e1.20 (m, 2H, 2ꢃH-30), 0.95 (s, 12H) [2ꢃH-80, 2ꢃH-
1. Purkayastha, R. P. Handbook of Phytoalexin Metabolism and Action In Progress
in Phytoalexin Research During the Past 50 Years; Daniel, M., Purkayastha, R. P.,
Eds.; Marcel Dekker: New York, NY, 1995; pp 1e39.
2. Pedras, M. S. C.; Yaya, E. E.; Glawischnig, E. Nat. Prod. Rep. 2011, 28, 1381e1405.
3. Pedras, M. S. C.; Zheng, Q.; Sarma-Mamillapale, V. K. Nat. Prod. Commun. 2007,
2, 319e330.
90], 0.91 (s, 6H, 2ꢃH-100). 13C NMR (100 MHz, CDCl3)
d 174.6 (C]
4. Takasugi, M.; Monde, K.; Katsui, N.; Shirata, A. Chem. Lett. 1987, 1631e1638.
5. Gross, D.; Porzel, A.; Schmidt, J. Z. Naturforsch., C 1994, 49, 281e285.
6. Monde, K.; Takasugi, M.; Shirata, A. Phytochemistry 1995, 39, 581e586.
O), 163.9 (C]N), 151.2 (C]O), 138.5 (C-7a), 130.2 (C-6), 129.5 (C-
3a), 125.8 (C-5), 124.3 (C-4), 115.4 (C-7), 84.8 (C-20), 76.4 (CH2),
65.2 (C-3), 49.3 (C-70), 48.1 (C-10), 45.0 (C-40), 36.9 (C-30), 28.2 (C-
50), 27.5 (C-60), 19.9, 19.1 (C-80, C-90), 15.9 (SCH3), 13.7 (C-100). IR
(CHCl3) nmax 2958, 2879, 1772 (C]O), 1734 (C]O), 1589 (C]N),
ꢀ
7. (a) Suchy, M.; Kutschy, P.; Monde, K.; Goto, H.; Harada, N.; Takasugi, M.; Dzurilla,
ꢀ
M.; Balentova, E. J. Org. Chem. 2001, 66, 3940e3947; (b) Monde, K.; Osawa, S.;
ꢀ
ꢀ
Harada, N.; Takasugi, M.; Suchy, M.; Kutschy, P.; Dzurilla, M.; Balentova, E. Chem.
Lett. 2000, 886e887; (c) Pedras, M. S. C.; Suchy, M.; Ahiahonu, P. W. K. Org. Biomol.
Chem. 2006, 4, 691e701; (d) Liu, L.; Zhang, S.; Xue, F.; Lou, G.; Zhang, H.; Ma, S.;
ꢀ

ꢀ
M. Budovska et al. / Tetrahedron 69 (2013) 1092e1104
1104
Duan, W.; Wang, W. Chem.dEur. J. 2011, 17, 7791e7795; (e) Zhang, Y.; Li, Z. J.; Xu,
17. Kutschy, P.; Mezencev, R. Indole Phytoalexins from Brassicaceae: Synthesis and
Anticancer Activity. In. Targets in Heterocyclic SystemsdChemistry and Proper-
ties; Attanasi, O. A., Spinelli, D., Eds.; Italian Society of Chemistry: Urbino, Italy,
2009; Vol. 12, pp 120e148.
H. S.; Zhang, Y.; Wang, W. RSC Adv. 2011, 1, 389e392.
ꢁ
ꢀ
ꢁ
ꢀ
ꢀ
8. (a) Kutschy, P.; Suchy, M.; Monde, K.; Harada, N.; Maruskova, R.; Curillova, Z.;
ꢁ
ꢀ
ꢁ ꢁ
Dzurilla, M.; Miklosova, M.; Mezencev, R.; Mojzis, J. Tetrahedron Lett. 2002, 43,
ꢁ
ꢁ
ꢁ
ꢀ
ꢀ
ꢀ
ꢀ
9489e9492; (b) Monde, K.; Taniguchi, T.; Miura, N.; Kutschy, P.; Curillova, Z.;
18. Budovska, M.; Kutschy, P.; Curillova, Z. Diastereoselectivity of 1-methoxy and
1-acyl derivatives of indole phytoalexin brassinin. Unpublished results.
ꢀ
ꢀ
ꢁ ꢁ
Pilatova, M.; Mojzis, J. Bioorg. Med. Chem. 2005, 13, 5206e5212; (c) Curillova, Z.;
ꢁ
ꢀ
ꢀ
ꢀ
ꢀ
ꢁꢀ
ꢁ ꢁ
Kutschy, P.; Budovska, M.; Monde, K.; Nakahashi, A. Tetrahedron Lett. 2007, 48,
19. Kutschy, P.; Salayova, A.; Curillova, Z.; Kozar, T.; Mezencev, R.; Mojzis, J.;
ꢀ
ꢀ
ꢀ
8200e8204.
Pilatova, M.; Balentova, E.; Pazdera, P.; Sabol, M.; Zburova, M. Bioorg. Med. Chem.
2009, 17, 3698e3712.
20. MestReNova. Mestrelab Research S. L.: A Coruna 15706, Santiagode Compostela,
ꢀ
ꢀ
9. Klika, K.; Budovska, M.; Kutschy, P. J. Fluorine Chem. 2010, 131, 467e476.
10. Klika, K.; Budovska, M.; Kutschy, P. Tetrahedron: Asymmetry 2010, 21, 647e658.
~
11. Pedras, M. S. C.; Hossain, M.; Sarwar, M. G.; Montaut, S. Bioorg. Med. Chem. Lett.
2004, 14, 5469e5471.
12. Mehta, R. G.; Liu, J.; Constantinou, A.; Hawthorne, M.; Pezzuto, J. M.; Moon, R.
C.; Moriarty, R. M. Anticancer Res. 1994, 14, 1209e1213.
Spain (mestrec@usc.es).
21. Jones, G. B.; Chapman, B. J. Synthesis 1995, 5, 475e497.
22. Jaguar, Version 7.6; Schrodinger: New York, NY, USA, 2009.
23. Klika, K. D.; Kramer, M.; Kleinpeter, E. J. Mol. Struct. (Theochem) 2009, 913,
247e253.
ꢁ ꢁ
ꢀ
ꢀ
ꢀ
13. Mezencev, R.; Mojzis, J.; Pilatova, M.; Kutschy, P. Neoplasma 2003, 50, 239e245.
ꢁ
ꢁ
ꢀ
ꢀ
ꢁ
ꢁꢁ
ꢀ
14. Pilatova, M.; Sarissky, M.; Kutschy, P.; Mirossay, A.; Mezencev, R.; Curillova, Z.;
24. (a) Marti, C.; Carreira, E. M. Eur. J. Org. Chem. 2003, 2209e2219; (b) Pellegrini,
ꢀ
ꢁꢁ
ꢁ ꢁ
€
Suchy, M.; Monde, K.; Mirossay, L.; Mojzis, J. Leukemia Res. 2005, 29, 415e421.
C.; Strassler, C.; Weber, M.; Borschberg, H. J. Tetrahedron: Asymmetry 1994, 5,
15. Mezencev, R.; Galizzi, M.; Kutschy, P.; Docampo, R. Exp. Parasitol. 2009, 122,
1979e1992; (c) Pettersson, M.; Knueppel, D.; Martin, S. F. Org. Lett. 2007, 9,
4623e4626.
25. Csomos, P.; Fodor, I.; Sohar, P.; Bernath, G. Tetrahedron 2005, 61, 9257e9262.
66e69.
ꢀ
ꢀ
ꢀ
16. Dillen, J. L. M.; Meth-Cohn, O.; Moore, C.; van Rooyen, P. Tetrahedron 1988, 44,
3127e3138.
26. Discovery Studio Visualizer, v2.5; Accelrys Software: San Diego, CA, USA, 2010.