TBAF promoted formation of symmetrical trisulfides

Article abstract of DOI:10.1002/hc.21129

We have developed a new method for the synthesis of functionalized symmetrical trisulfides based on (5,5-dimethyl-2-thioxo-1,3,2-dioxa-phosphorin- 2-yl)disulfanyl derivatives prepared from readily available 5,5-dimethyl-2-sul- fanyl-2-thioxo-1,3,2-dioxaph

Full text of DOI:10.1002/hc.21129

Heteroatom Chemistry
Volume 25, Number 1, 2014
T
BAF Promoted Formation of Symmetrical
Trisulfides
Slawomir Lach and Dariusz Witt
Department of Organic Chemistry, Chemical Faculty, Gdansk University of Technology,
Narutowicza 11/12, 80–233, Gdansk, Poland
Received 30 April 2013; revised 30 September 2013
methods for their preparation are based on reaction
ABSTRACT: We have developed a new method for
of thiols with sulfur [6], sulfur dichloride [7], or cou-
the synthesis of functionalized symmetrical trisul-
pling of alkyl halides with sodium trisulfide [8]. Most
fides based on (5,5-dimethyl-2-thioxo-1,3,2-dioxa-
suitable substrates used in the synthesis of trisulfides
phosphorin-2-yl)disulfanyl derivatives prepared from
include Bunte salts [9], metal sulfides [10], and thio-
readily available 5,5-dimethyl-2-sul-fanyl-2-thioxo-
sulfenyl chloride [11]; the latter can also be used for
1
,3,2-dioxaphosphorinane or bis(5,5-dimethyl-2-
the preparation of unsymmetrical trisulfides. Other
practical procedures involve the reduction of thiosul-
fonates and disulfonyl sulfides with phosphines [12],
sulfur insertion reactions into thiosulfinates, thio-
sulfonates [13], and disulfides [14], alkoxide decom-
position of sulfenyl thiocarbonates [15], and reac-
thioxo-1,3,2-dioxaphosphorinan-2-yl) disulfide. The
symmetrical trisulfides can be obtained from
aliphatic and aromatic thiols and L-cysteine deriva-
tives under mild conditions with high yield and
purity. ꢀC 2013 Wiley Periodicals, Inc. Heteroatom
Chem. 25:10–14, 2014; View this article online at
wileyonlinelibrary.com. DOI 10.1002/hc.21129
ꢁ
tions of thiols with 1,1 -thiobis(benzimidazole) [16]
or bis(imidazolyl) sulfide [17].
Although the synthesis of symmetrical trisulfides
is well documented, their preparation is in fact more
complicated. Most of the abovementioned methods
suffer either from moderate yields or formation of
undesired polysulfide side products. The most con-
venient removal of these impurities can be accom-
plished by crystallization. However, it can be applied
only to solid trisulfides. Other methods may require
multistep synthesis of appropriate precursors or us-
ing freshly distilled sulfur dichloride. Very often, the
presence of additional functional groups, especially
unprotected, limits the scope of these methods.
We have previously demonstrated the prepara-
tion of functionalized unsymmetrical compounds,
such as dialkyl disulfides, alkyl aryl disulfides
INTRODUCTION
The biological importance of the sulfur–sulfur bond
also comprises organic trisulfides. Trisulfide func-
tionality was observed in the compounds isolated
from plants in the onion family [1] (genus Allium),
proteins [2], in the tumor inhibitors calicheamicin
[3], esperamicin [4], members of the enediyne group
of antibiotics. From this point of view, the trisulfide
functionality has been recognized as an important
target in organic synthesis. Preparation of symmetri-
cal, acyclic trisulfides without additional functional
groups is very well documented [5]. Most common
[18], diaryl disulfides [19], “bioresistant” disulfides
20], and unsymmetrical disulfides based on L-
[
Correspondence to: Dariusz Witt; e-mail: dwitt@chem.pg.gda.pl.
Contract grant sponsor: Polish Ministry of Science and Higher
Education.
cysteine and L-cystine derivatives [21]. We were
also able to obtain functionalized symmetrical [22]
and unsymmetrical [23] trisulfides bearing alkyl,
aryl groups and L-cysteine derivatives based on
Contract grant number: N N204 208440.
ꢀ
2013 Wiley Periodicals, Inc.
C
10

TBAF Promoted Formation of Symmetrical Trisulfides 11
SCHEME 1 Preparation of symmetrical trisulfides 5.
TABLE 1 Preparation of Symmetrical Trisulfides 5
Entry
R
5
Yield (%)
1
2
3
4
5
6
–(CH
–(CH
–(CH
–(CH
2
2
2
2
)
)
)
)
11CH
11OH
10COOH
OH
CH(NHBoc)CO
-CH
3
5a
5b
5c
5d
5e
5f
94
97
92
91
85
87
2
–(R)-CH
–4-C
2
2
Et
6
H
4
3
the phosphorodithioic acid derivatives. These ex-
cellent results encouraged us to extend the strat-
egy to the preparation of symmetrical trisulfides
based on readily available (5,5-dimethyl-2-thioxo-
SCHEME 2 Proposed mechanism for the formation of trisul-
fide 5a.
1
4
,3,2-dioxaphosphorinan-2-yl)disulfanyl derivatives
. Compounds 4 can be obtained from either phos-
products. The formation of the trisulfide product can
be explained by the nucleophilic attack of the fluo-
ride anion from TBAF on the phosphorus atom of
the disulfanyl derivative 4a. The driving force in this
step seems to be the high affinity of fluoride to phos-
phorus (bond dissociation energy for the P–F bond
= 439 kJ/mol) [25]. This leads to the formation of
a phosphorothioic acid fluoride 6 and dodecyldisul-
fide anion 7. In the next step, anion 7 reacts with still
present in the reaction mixture disulfanyl derivative
4a to produce the symmetrical didodecyl trisulfide
5a (Scheme 2).
phorodithioic acid 1 or its disulfide 2 by treatment
with bromine to quantitatively afford sulfenyl bro-
mide 3, which subsequently, without isolation, re-
acts with thiols [22].
The purification of starting material, phospho-
rodithioic acid 1, has been previously accomplished
by vacuum distillation [24]. The vacuum must be
kept below 1.5 mmHg upon heating; otherwise con-
tent of the flask can decompose and sometimes ex-
plode. We have found that crude phosphorodithioic
acid 1 can also be purified by crystallization from
carbon tetrachloride with 60% yield. Moreover, fil-
trate after crystallization can be used for the prepa-
ration of ammonium salt required for synthesis [20]
of phosphorodithioic acid disulfide 2. The modified
procedures (see Experimental) for the preparation of
phosphorodithioic acid 1 and its disulfide 2 make
the developed method of symmetrical trisulfide syn-
thesis more common and versatile.
As can be seen from the proposed mechanism,
the reaction should also be accomplished with 0.5
equivalent of TBAF. Unfortunately, in this case, the
reaction was very sluggish and the product was con-
taminated with symmetrical di- and tetrasulfide. It
looks like the first step of the reaction of fluoride ion
at the phosphorus atom is the slowest. Moreover,
disulfide anion 7 should be consumed very quickly,
so the sulfur extrusion can be avoided and forma-
tion of corresponding symmetrical disulfide may be
excluded. The increased rate of the first step of re-
action and higher concentration of disulfide anion
7 (in this case, this anion is also consumed faster
in the reaction with derivative 4) can be achieved
by using excess of TBAF. Although di-, tri-, and
tetrasulfides cannot be separated by column chro-
matography, the presence of these impurities can be
RESULTS AND DISCUSSION
We report here an extension of our previously re-
ported procedure [22] for the synthesis of sym-
metrical trisulfides. Treatment of 1.0 equivalent
of 5,5-dimethyl-2-thioxo-1,3,2-dioxaphosphorinan-
2-yl)disulfanyl derivative 4 with 1.05 equivalent of
1
tetra n-butylammonium fluoride (TBAF) in the mix-
ture of THF and CH2Cl2 afforded symmetrical trisul-
fides 5 (Scheme 1, Table 1).
Although the yields are very high, much more
important is the exclusive formation of the trisulfide
without the formation of undesired polysulfide side
easily confirmed by H nuclear magnetic resonance
(NMR). The chemical shift of the methylene group
(triplet in CDCl3) connected with sulfur for di-, tri-,
and tetrasulfide is 2.66, 2.87, and 3.05, respectively.
3
1
When the P NMR spectrum of reaction mixture
was recorded then the presence of phosphorothioic
Heteroatom Chemistry DOI 10.1002/hc

12 Lach and Witt
acid fluoride 6 (52.1 ppm) and phosphorodithioic
acid 1 anion (110.1 ppm) was observed what sup-
ports proposed mechanism. We have also noticed
that the formation of trisulfide is effected by steric
hindrance. When disulfanyl derivatives 4 (R =
tert-butyl or triphenylmethyl) were treated with
TBAF, then trisulfides were not formed and start-
ing materials were recovered with 85% and 90%
yield, respectively. It seems that the attack of fluo-
ride ion at the phosphorus atom of compound 4 is
very slow and the subsequent reaction of generated
disulfide anion with these compounds is suppressed
by the steric congestion of the R group (R = tert-
butyl or triphenylmethyl) (Scheme 2). We have also
noticed that the developed method cannot be applied
for the effective formation of unsymmetrical trisul-
fides. When the mixture of 4a and 4b (1:1 ratio) was
treated with TBAF, the corresponding unsymmet-
rical trisulfide (CH3(CH2)11S3(CH2)11OH) [23] was
isolated with 45% yield. The symmetrical trisulfide
chromatography was performed using silica gel 60
(230–400 mesh, Merck). Thin layer chromatogra-
phy was performed with silica gel Polygram SIL
G/UV254 (Macherey–Nagel, D u¨ ren, Germany). Melt-
ing points were measured with a Gallenkamp 7936B
(Gallenkamp, Loughborough, England) apparatus
and are uncorrected. NMR spectra were recorded
on Varian Gemini 500 or 200 MHz (Palo Alto, CA,
USA) spectrometers. The residual solvent peak was
used as the internal reference (CDCl3: δ = 7.26 ppm
1
13
for H, δ = 77.0 ppm for C). An external standard
(85% H3PO4: δ = 0 ppm) was used as the reference
3
1
for recording the P NMR spectra. Mass spectra of
ESI (electrospray ionization) of positive ions were
registered on an Agilent 6230 Accurate Mass TOF
instrument (Agilent Technologies, Santa Clara, CA,
USA).
5, 5-Dimethyl-2-sulfanyl-2-thioxo-1, 3, 2-dioxa-
phosphorinane 1
5a (40%) and 5b (42%) were also isolated. However,
the scope of that approach is strongly limited by
the formation of symmetrical products and possibil-
ity of their separation from unsymmetrical trisulfide
by column chromatography. The above limitations
have encouraged us to develop a new synthetic strat-
egy for the preparation of unsymmetrical trisulfides
bearing the additional functionalities at both sides.
These results will be reported in due course.
In conclusion, presented reactions took place
under mild conditions without any additional cat-
alysts or reagents, except for commercially avail-
able TBAF. The symmetrical trisulfides can be ob-
tained from aliphatic and aromatic thiols and also
L-cysteine derivatives. The presence of functional
groups: amino, hydroxy, or carboxy did not disturb
the course of the reaction. The advantages of this
new method are the easily accessible starting mate-
rials, convenient manipulation, short reaction times,
and very high purities and yields. From this point of
view, the above-presented method for the synthe-
sis of symmetrical trisulfides seems to be currently
one of the most versatile and convenient approaches
to preparation of functionalized symmetrical
trisulfides.
To a suspension of P4S10 (44.8 g, 0.1 mol) in dry
toluene (260 mL), a 2,2-dimethylpropane-1,3-diol
(
41.6 g, 0.4 mol) was added [24]. The reaction mix-
◦
ture was stirred at 60–80 C for 15 h under nitrogen,
then traces amount of unreacted P4S10 were filtered
off. Solvent was evaporated under reduced pressure
and the residue was kept under vacuum at room tem-
perature for 30 min. The obtained sticky solid was
dissolved in hot CCl4 (25 mL for each 10 g of crude
◦
product) and placed in the freezer (–15 C) for 6 h.
Product was filtered off and dried under vacuum at
room temperature to yield 47.6 g (0.24 mol, 60%); the
residue from filtrate after evaporation of CCl4 under
reduced pressure can be used for the preparation of
phosphorodithioic acid ammonium salt.
◦
◦
Melting point (mp) 81–82 C (Lit. [24c] 81–82 C),
P NMR (CDCl3) = 77.68.
3
1
Bis-(5,5-dimethyl-2-thioxo-1,3,2-dioxaphospho-
rinanyl) disulfide 2
A dry ammonia gas was passed through the
solution of 5,5-dimethyl-2-sulfanyl-2-thioxo-1,3,2-
dioxaphosphorinane 1 (47.6 g, 0.24 mol) [or the
residue from filtrate after evaporation of CCl4 (32 g,
EXPERIMENTAL
0.16 mol)] in the mixture of toluene (350 mL) and
Tetrabutylammonium fluoride (1
is commercially available from Sigma-Aldrich
Milwaukee, WI, USA). (5,5-Dimethyl-2-thioxo-1,
, 2-dioxaphosphorinan-2-yl)disulfanyl derivatives
a–f [22] were synthesized by described pro-
M
in THF)
diethyl ether (50 mL) (ether is required to produce
precipitate that is easier to filtered off) cooled in an
ice bath for 30 min [24]. White precipitate was fil-
tered off and washed with toluene (50 mL) and ether
(50 mL). After filtration, ammonium salt was dried
under vacuum to yield 49.5 g (0.23 mol, 96%) (or 28 g
0.13 mol, 81% from the residue after evaporation
(
3
4
cedures. Dichloromethane and toluene (POCh,
Gliwice, Poland) were dried before use. Column
Heteroatom Chemistry DOI 10.1002/hc

TBAF Promoted Formation of Symmetrical Trisulfides 13
◦
31
filtrate) of white powder; mp 263–265 C, ( P NMR
Bis(10-carboxydecyl) Trisulfide (5c)
(
D2O) = 110.22).
◦
◦
Melting point 79–81 C (Lit.[22] 79–81 C), R
=
f
A solution of the ammonium salt of phospho-
0.3 (CHCl3–MeOH, 25:1), white solid; yield: 92%
rodithioic acid (43 g, 0.2 mol) in water (300 mL)
was stirred at room temperature and a solution of
I2 (25.4 g, 0.1 mol) and KI (50 g, 0.31 mol) in wa-
ter (200 mL) was dropwise added. The brown solid
was filtered off, washed with water (400 mL), and
dissolved in ethyl acetate (1 L). Solution was washed
with 10% Na2S2O3 aqueous solution (150 mL), dried
over anhydrous Na2SO4, and evaporated under re-
duced pressure. The residue was recrystallized from
ethanol to yield disulfide 2 (33.5 g 0.085 mol, 85 %);
(
215 mg, 0.46 mmol).
1
H NMR (200 MHz, CDCl3): δ = 1.24–1.50 (m,
2
7
4H, CH2), 1.54–1.84 (m, 8H, CH2), 2.36 (t, J =
.3 Hz, 4H, CH2COO), 2.88 (t, J = 7.3 Hz, 4H, SCH2),
and 8.90 (br s, 2H, COOH).
1
3
C NMR (50 MHz, CDCl3): δ = 180.2, 38.8, 34.0,
2
9.3, 29.3, 29.1,29.1, 29.0, 28.8, 28.4, and 24.6. Sig-
nals: expected, 11; observed, 11.
High-resolution mass spectrometry (ESI): m/z
+
[
4
M + Na] calcd for C22H42NaO4S3: 489.2143; found:
◦
◦
31
mp 133–134 C (Lit. [24c] 133.5–134 C), P NMR
CDCl3) = 80.87.
89.2149.
(
Trisulfides 5a–f (Typical Procedure)
Bis(2-hydroxyethyl) Trisulfide (5d)
A solution of TBAF (0.274 g, 1.05 mmol) in THF
R = 0.25 (CHCl3–MeOH, 25:1), colorless oil; yield:
f
(
10 mL) was added to the solution of 4 (1.0 mmol) in
91% (85 mg, 0.46 mmol)[16,26].
◦
1
dry CH2Cl2 (10 mL) at 0 C under N2 atmosphere. The
mixture was stirred for 30 min and then evaporated
under reduced pressure. The residue was purified by
column chromatography.
H NMR (200 MHz, CDCl3): δ = 2.16 (s, 2H, OH),
3.09 (t, J = 5.7 Hz, 4H, SCH2), 3.99 (t, J = 5.7 Hz,
4H, OCH2).
1
3
C NMR (50 MHz, CDCl3): δ = 59.5, 41.7. Sig-
nals: expected, 2;observed, 2.
High-resolution mass spectrometry (ESI): m/z
Didodecyl Trisulfide (5a)
+
[
2
M + Na] calcd for C4H10NaO2S3: 208.9741; found:
◦
◦
08.9746.
Melting point 15–16 C (Lit.[22] 15–16 C), R = 0.3
f
(
(
petroleum ether), waxy white solid; yield: 94%
204 mg, 0.47 mmol).
Bis[(R)-2-(tert-butoxycarbonylamino)-2-
ethoxycarbonyl)ethyl] Trisulfide (5e)
1
H NMR (200 MHz, CDCl3): δ = 0.89 (t, J =
(
6
.5 Hz, 6H, CH3), 1.15–1.45 (m, 36H, CH2), 1.64–1.84
◦
◦
(
m, 4H, CH2), and 2.88 (t, J = 7.3 Hz, 4H, SCH2).
Melting point 74–77 C (Lit. [22] 74–77 C), R =
f
1
3
C NMR (50 MHz, CDCl3): δ = 38.9, 31.9, 29.6,
0.32 (CH2Cl2–EtOAc, 25:1), white solid; yield: 85%
(225 mg, 0.43 mmol).
2
9.6, 29.5, 39.3,29.2, 28.8, 28.5, 22.7, and 14.1. Sig-
1
nals: expected, 12; observed, 11.
H NMR (200 MHz, CDCl3): δ = 1.30 (t, J =
High-resolution mass spectrometry (ESI): m/z
7.1 Hz, 6H, COOCH2CH3), 1.46 (s, 18H, Boc), 3.37
(d, J = 4.8 Hz, 4H, SCH2), 4.23 (q, J = 7.1 Hz, 4H,
COOCH2), 4.56–4.72 (m, 2H, CH), and 5.44(d, J =
+
[
4
M + H] calcd for C24H51S3: 435.3153; found:
35.3155.
7
.8 Hz, 2H, BocNH).
1
3
C NMR (50 MHz, CDCl3): δ = 170.3, 155.0,
Bis(11-hydroxyundecyl) Trisulfide (5b)
8
0.2, 61.9, 52.9,41.1, 28.3, and 14.1. Signals: ex-
◦
◦
pected, 8; observed, 8.
Melting point 61–63 C (Lit.[22] 61–63 C), R
=
f
High-resolution mass spectrometry (ESI): m/z
0.35 (CHCl3–MeOH, 25:1), white solid; yield: 97%
+
[
M + Na] calcd for C20H36N2NaO8S3: 551.1531;
(
213 mg, 0.49 mmol).
1
found: 551.1536.
H NMR (200 MHz, CDCl3): δ = 1.20–1.46 (m,
2
8H, CH2), 1.39 (s, 2H, OH), 1.46–1.65 (m, 4H, CH2),
1
.65–1.82 (m, 4H, CH2), 2.87 (t, J = 7.3 Hz, 4H,
Bis(4-tolyl) Trisulfide (5f)
SCH2), and 3.64 (t, J = 6.5 Hz, 4H, OCH2).
1
3
◦
◦
C NMR (50 MHz, CDCl3): δ = 63.0, 38.8, 32.7,
Melting point 76–77 C (Lit. [[7]a] 78–79 C), R = 0.35
f
29.5, 29.4, 29.1,28.8, 28.5, and 25.7. Signals: ex-
(CH2Cl2–petroleum ether, 1:2), bright yellow solid;
yield: 87% (121 mg, 0.44 mmol).
pected, 11; observed, 9.
1
High-resolution mass spectrometry (ESI): m/z
H NMR (200 MHz, CDCl3): δ = 2.36 (s, 6H,
+
[
4
M + Na] calcd for C22H46NaO2S3: 461.2558; found:
CH3), 7.12 (d, J = 7.9 Hz, 4H, Ar), and 7.45 (d, J =
61.2561.
7.9 Hz, 4H, Ar).
Heteroatom Chemistry DOI 10.1002/hc

14 Lach and Witt
1
3
[
[
8] Fuson, R. C.; Price, Ch. C.; Burness, D. M.; Foster,
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1
5
29.8, 128.5, and 21.2. Signals: expected, 5; observed,
1
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.
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High-resolution mass spectrometry (ESI): m/z
+
[
3
M + Na] calcd for C14H14NaS3: 301.0155; found:
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Heteroatom Chemistry DOI 10.1002/hc