2,2,4,4-Tetrathio substituted 1,3-dithietanes

Article abstract of DOI:10.1016/j.tet.2012.12.053

The synthesis of 2,2,4,4-tetrathio substituted 1,3-dithietanes and their intermediates by reaction of thiophosgene with carbonotrithioates or O-ethyldithiocarbonate is reported. Their reactivity against thermolysis and aminolysis was investigated.

Full text of DOI:10.1016/j.tet.2012.12.053

Tetrahedron 69 (2013) 2017e2021
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2,2,4,4-Tetrathio substituted 1,3-dithietanes
*
Wolfgang Weber , Helen Chirowodza, Harald Pasch
Department of Chemistry and Polymer Science, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 August 2012
Received in revised form 13 December 2012
Accepted 21 December 2012
Available online 29 December 2012
The synthesis of 2,2,4,4-tetrathio substituted 1,3-dithietanes and their intermediates by reaction of
thiophosgene with carbonotrithioates or O-ethyldithiocarbonate is reported. Their reactivity against
thermolysis and aminolysis was investigated.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Thiophosgene
Carbonotrithioates
1,3-Dithietanes
Tetrafunctional RAFT agent
1. Introduction
2. Results and discussion
The reaction of alkyl mercaptides with carbon disulfide to alkyl
carbonotrithioates, followed by alkylation, is commonly used to
prepare unsymmetrical dialkyl carbonotrithioates,1,2 which can act
as RAFT agents in controlled polymerization reactions of styrene
and other monomers.3,4
In 2008 an extended structure to the carbonotrithioates with an
additional eSeCSe functionality was reported with the synthesis
of the red colored pentathiodicarbonates5 (Fig. 1).
Benzyl mercaptan was therefore reacted with carbon disulfide
in the presence of aqueous potassium hydroxide to afford the
potassium salt of benzyl carbonotrithioate 1a. Addition of
thiophosgene resulted in a yellow solid, which was isolated and
characterized. For such a conjugated structure we expected a red or
purple colored compound, not a yellow product. The 13C NMR
spectrum in carbon disulfide contains only a signal for one thio-
carbonyl group at 217.99 ppm, not for two with different values as
expected for 2a. The product, which is insoluble in most organic
solvents, was crystallized from carbon disulfide and its crystal
structure determined by X-ray diffractometry. The results show
clearly that the compound is a dimer of the envisaged compound
2a and contains a 1,3-dithietane ring in the central position. The 13C
NMR spectra of 3a and 3b contain further proof of the formation of
1,3-dithietanes by displaying small signals for a quaternary carbon
atom at 72.42 and 72.66 ppm, respectively, which is in good
agreement with literature data.6
S
S
S
S
S
dibenzyl pentathiodicarbonate
Fig. 1. Chemical structure of dibenzyl pentathiodicarbonate.
The four-membered 1,3-dithietane ring could be formed in two
ways, the reaction of dimeric thiophosgene (2,2,4,4-tetrachloro-
1,3-dithietane) with potassium benzyl carbonotrithioate or by
dimerization of the relevant thioketone7 under the selected
reaction conditions.
The first pathway is not likely. Dimeric thiophosgene is primarily
formed by UV radiation of thiophosgene8aec or by Lewis acid assisted
dimerization.6 Both conditions were not encountered here.
Kato et al.7 reported that aliphatic thioanhydrides decompose
easily at room temperature and result in 1,3-dithietanes and other
In continuation of that work we recently investigated if the
sulfur containing part of those compounds can be extended further
by joining two carbonotrithioates with the reactive thiophosgene
to form a thiocarbonyl bridge. The resulting substance should
contain seven sulfur atoms and be intensely colored.
* Corresponding author. Tel.: þ27 (21) 808 3176; fax: þ 27 (21) 808 4967; e-mail
address: wgweber@sun.ac.za (W. Weber).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.12.053

2018
W. Weber et al. / Tetrahedron 69 (2013) 2017e2021
products. Upon comparing their thioanhydrides with our structures
the pathway involving dimerization seems plausible. A further
indication for that pathway is the appearance of a red color while
adding thiophosgene to the trithiocarbonate salt and small quan-
tities of remaining red oils in the mother liquor. The red color
most probably originates from the intermediate dibenzyl hepta-
thiotricarbonate 2a.
Depending on the solvent system and mode of addition of
thiophosgene, the yield of the product 3a is between 30% and 65%.
The highest yields of product are isolated if the reaction is carried
out in water, thiophosgene added without a solvent and the black
oily product stirred with THF to separate the yellow product from
other more soluble by-products (Procedure B). At some stage we
suspected, that the possible peroxide content in THF led to the
isolation of the yellow dimer from the dark oil, but that could not be
confirmed. Even after using purified and freshly distilled THF the
dimer was formed. Solvents other than THF act similarly. A number
of by-products are formed. If the thiophosgene is dissolved in
THF and added dropwise, carbon disulfide is eliminated and
dibenzyldisulfide 49aec (Fig. 2) can be isolated in a yield of 21%.
Dissolving thiophosgene in carbon disulfide or chloroform also led
to a decreased yield of 3a (30e40%).
hydroxide and one drop of Aliquat 336 led to a recovery rate of 58%
3a after 2 days stirring at room temperature. 29% of dibenzyldi-
sulfide 6 was also isolated and identified.
Furthermore, we also investigated the decomposition of 3a in
inert solvents under elevated temperatures. Two solvents were
used under reflux conditions, bromobenzene (bp 156 ꢀC) and
chlorobenzene (bp 132 ꢀC). The reaction products and their yields
results were comparable. Heating 3a in chlorobenzene leads to
several red and yellow colored products, of which the two main
components were identified after column chromatography as
bis(benzylsulfanyl thiocarbonyl)disulfide 510 and dibenzyl trithio-
carbonate 611 (Fig. 2). Bis(benzylsulfanyl thiocarbonyl)disulfide
5 itself is stable under the reaction conditions (2 h reflux in chlo-
robenzene) and was fully recovered.
The prepared 1,3-dithietane 3a was tested as a multi-arm RAFT
agent in the polymerization reaction of styrene and other monomers
and the results will be published separately in due course.
To be able to characterize the polystyrene chains, the sulfur
containing end groups, originating from the RAFT agents, are
preferably removed by aminolysis.
As a preliminary experiment 3a was therefore treated with 4 or
5 mol-equiv of n-propylamine in THF. Within 15e20 min the solid 3a
S
S
S
S
S
S
S
S
S
S
S
4
5
6
Fig. 2. Structures of 4, 5, and 6.
To decrease the chances of by-product formation via hydrolysis,
a different approach was taken. The potassium alkyl carbon-
otrithioates 1a and 1b were prepared beforehand and dry benzene
used as solvent in the condensation step with thiophosgene,
which led to slightly improved yields of 70% compared to the water
based reaction (Procedure A). When the reaction was carried out
in dry diethylether, 3a was isolated in 65% yield. Stopping the
reaction after 75 min afforded 3a in 51% yield and a red oil as
by-product. The oil contained several compounds, amidst them
more of 3a (12%) and the assumed intermediate dibenzyl hepta-
thiotricarbonate 2a. Column chromatography on silica with
pentane/ethylacetate (2:1), followed by a second separation with
pentane led to the isolation of 2a. Other yellow products remained
on the column under the conditions used. In contrast to the column
purification of 7 we were not able to isolate pure dibenzyl hepta-
thiotricarbonate 2a. The NMR spectroscopic data of 2a are reported
in the Experimental section. All data are consistent with the pro-
posed structure. The data for the unidentified impurity are omitted.
Compound 3a is quite stable against hydrolysis. It does not
react with 32% hydrochloric acid in carbon disulfide at room tem-
perature. Stirring of 3a in benzene with an excess of 5% sodium
dissolved and the solution turned orange. The main product of that
aminolysis was identified as dibenzyldisulfide 4 (Fig. 2), which was
isolated in a yield of 78%. The residual orange oil contains several
colorless, yellow and red products. Based on the small amount of the
oil (1.25 g of 6.25 g possible as suggested by the desired structure)
one has to assume that under the reaction conditions most of 3a is
decomposed further into small volatile molecules, such as carbon
disulfide and propyl isothiocyanate, which were not recovered under
the work-up conditions. Elemental sulfur is a further product and
separates out of the THF solution.
Surprisingly, replacing one sulfur atom in the alkyl carbon-
otrithioates by oxygen (potassium O-ethyldithiocarbonate instead
of potassium ethoxy(thioxo)methanethiolate) and then carrying
out the reaction under similar conditions did not afford
a 1,3-dithietane, but the originally targeted linear structure of
O,O0-diethyl pentathiotricarbonate 7 as a dark red oil, as this color is
expected from such a conjugated system (Scheme 1). The crude
product decomposed rapidly at room temperature and therefore
had to be purified as soon as possible. The 13C NMR spectrum in
CDCl3 of the purified 7 shows two signals at 202.97 ppm and
209.12 ppm for the two different C]S functionalities.
X
S
S
X
R
R
X
S
S
S
S
S
S
S
S
Cl
Cl
S
S
R
R
R
2
X
SK
X
S
S
X
S
S
S
R
X
R
2a, 2b, 7
Scheme 1. Reaction pathway for 2, 3, 7, and 8.
1a, 1b, 1c
3a, 3b, 8

W. Weber et al. / Tetrahedron 69 (2013) 2017e2021
2019
Zbirovsky and Ettel12 reported in 1957 the isolation of
4.3. Experimental procedures
O,O0-dialkyl pentathiotricarbonates as dark red oils. In their case,
thiophosgene was added to the potassium O-alkyl dithiocarbonate
in benzene. Under basic conditions the oil is transformed into the
dimeric structure with a melting point of 161 ꢀC for alkyl¼ethyl.
When they changed the solvent system to diethylether, they
isolated the dimeric structure in 89% yield. We modified their
procedure and isolated the 1,3-dithietane 8 in 95% yield.
We identified the red oily O,O0-diethyl pentathiocarbonate 7 as
by-product in the mother liquor. That compound decomposed
overnight into a yellow, multi-component oil, which also contained
traces of 8. Synthetic procedures and spectroscopic data are given
under the Experimental section.
The replacement of one sulfur atom (X¼S) in the alkyl
carbonotrithioates with nitrogen led to the formation of the
dialkyldithiocarbamates, which react with thiophosgene to form
different types of products by elimination of carbon disulfide, such
as the dialkylthiuram monosulfides (N,N,N0,N0-tetraalkyldicarbono
trithioic diamides).12 The procedure was similar to the reaction of
O-ethyldithiocarbonate with thiophosgene in benzene. The elimi-
nation of carbon disulfide as a side reaction also took place dur-
ing the synthesis of 3a, whereby dibenzyldisulfide 6 was formed
(Table 1).
4.3.1. Procedure A: synthesis of 3a and 3b in benzene
4.3.1.1. Tetrabenzyl(1,3-dithietane-2,2,4,4-tetrayl)tetracarbono-
trithioate 3a. Potassium benzyl carbonotrithioate 1a (4.75 g,
19.9 mmol) was suspended in dry benzene (40 mL) in a flask and
stirred in ice-water. Thiophosgene (1.30 g, 11.3 mmol, 90%) in dry
benzene (25 mL) was added dropwise over within 20 min. An in-
tense red color developed and the solid dissolved. After stirring for
a further 15 min, the cooling bath was removed and the mixture
stirred overnight. The formed yellow solid was filtered off, washed
several times with pentane until the solvent remained colorless,
washed twice with water, and again with pentane. The yellow
product, 3a was dried at 50 ꢀC (yield 2.55 g). Evaporation of the
solvents afforded a red oil, which was treated with THF to result in
a further 0.55 g of product. Yield: 3.10 g (70%) as yellow crystals, mp
160e161 ꢀC. 3a can be crystallized from carbon disulfide.
IR (neat) 3026, 2932, 1643, 1507, 1452, 1056, 816, 730, 696 cmꢁ1
.
1H NMR (300 MHz, CS2, CDCl3):
d
(ppm) 7.26e7.20 (20H, m, Ar),
4.44 (8 H, s, CH2).
13C NMR (75.4 MHz, CS2, CDCl3):
d (ppm): 217.99 (C]S), 133.70,
129.16, 128.58, 127.75 (Ar), 72.42 (quart. C), 41.21 (CH2).
Anal. calcd for C34H28S14þCS2 (961.66): C, 43.7%; H, 2.9%; S,
53.4%. Found: C, 43.8%; H, 3.0%; S 53.8%. 3a contains one mol carbon
disulfide if crystallized from CS2.
The structure of 3a was further confirmed by X-ray diffrac-
tometry.14 The remaining oil consisted of several by-products,
which were not characterized.
Table 1
Substituents in 1,3-dithietanes
Compound
R
X
1a, 2a, 3a
1b, 2b, 3b
1c, 7, 8
C6H5eCH2e
n-Bu
Et
S
S
O
4.3.1.2. Tetrabutyl(1,3-dithietane-2,2,4,4-tetrayl)tetracarbono-
trithioate 3b. Thiophosgene (1.30 g, 11.3 mmol, w90%) in dry
benzene (25 mL) was added dropwise over 20 min to an ice-water
cooled suspension of 1b (4.10 g, 20.1 mmol) in dry benzene (40 mL).
After a further 15 min the cooling bath was removed and the red
solution stirred overnight at room temperature. The solvent was
removed by evaporation and the red residue washed thoroughly
with water. The solid was stirred in pentane (50 mL). The resulting
yellow powder was isolated and washed with pentane and acetone
until all red traces were gone. Concentration of the wash solvents
led to a red, crystallizing oil, from which more product could be
recovered by treatment with a few mL of pentane.
3. Conclusions
In the present study, we have demonstrated that sulfur-
containing compounds can be extended further by joining two
carbonotrithioates with the reactive thiophosgene to form a thio-
carbonyl bridge. The resulting heptathiocarbonate dimerizes
to form a heterocyclic sulfur compound (with a four-membered
1,3-dietane ring), which can be used as a RAFT agent.
Yield: 2.45 g (66%) as yellow crystals, mp 126e127 ꢀC (CHCl3).
IR (neat) 2950, 2923, 2864, 1459, 1419, 1269, 1057, 820, 729 cmꢁ1
.
4. Experimental
1H NMR (300 MHz, CDCl3):
d (ppm) 3.27 (8H, t, CH2CH2S,
4.1. General remarks
J¼7.5 Hz), 1.66 (8H, quintet, CH2CH2S, J¼7.5 Hz), 1.42 (8H, sextet,
CH3CH2, J¼7.5 Hz), 0.93 (12H, t, CH3CH2, J¼7.5 Hz).
13C NMR (75.4 MHz, CDCl3)
d (ppm): 220.03 (C]S), 72.66 (quart.
C), 36.29 (CH2), 29.84 (CH2), 22.12 (CH2), 13.57 (CH3).
The residual oil contained several intense red or yellow
by-products, which were not further identified.
All NMR spectra were acquired using a Varian VNMRS 300 MHz
spectrometer. IR spectra were scanned neat using a Nicolet iS10
(Thermo Scientific) FTIR spectrophotometer (Transmission mode).
Caution: Thiophosgene and carbon disulfide are very dangerous
chemicals and should be handled with extra caution in a well
ventilated fumehood.
4.3.2. Procedure B: synthesis of 3a in water
4.3.2.1. Synthesis of tetrabenzyl(1,3-dithietane-2,2,4,4-tetrayl)
tetracarbonotrithioate 3a. Potassium hydroxide (7.30 g, 130.1 mmol)
was dissolved in water (50 mL). Benzyl mercaptan (12.40 g,
100.0 mmol) was added dropwise and the solution stirred for
30 min. After adding carbon disulfide (7.60 g, 99.8 mmol) and one
drop of Aliquat 336 the solution was stirred for another 30 min. The
clear orange solution was cooled with ice water and thiophosgene
(5.85 g, 50.9 mmol, 98%) added within 30 min. The solution turned
dark red and a black, oily substance separated and solidified at the
walls of the flask. Stirring was continued at room temperature for
another 30 min. THF (75 mL) was added and the mixture stirred for
another hour. The yellow solid was filtered off, washed with water
and THF and dried at 50 ꢀC. The product, 3a was very slightly soluble
4.2. Reagents
Thiophosgene (Fluka, techn., 90% and 98%, respectively),
potassium O-ethyldithiocarbonate 1c (Merck) and sodium diethyl
dithiocarbamate trihydrate (Aldrich) were used as received.
Potassium benzyl carbonotrithioate 1a and potassium n-butyl
carbonotrithioate 1b were prepared by modifying a procedure
described in literature.13 The modification included using dry THF
as solvent and a 1.05:1:1.25-ratio of KOH, mercaptan and CS2. The
potassium salts were isolated by removing the solvent under
reduced pressure and dried under vacuum. The yields of 1a and 1b
were 98% and 85%, respectively.

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W. Weber et al. / Tetrahedron 69 (2013) 2017e2021
in organic solvents besides carbon disulfide and toluene. The yield
was 12.90 g (58%) of yellow crystals, mp 161e162 ꢀC (CS2).
4.5. Synthesis of O,O0-diethyl pentathiotricarbonate 7
Potassium O-ethyldithiocarbonate 1c (6.40 g, 39.9 mmol)
was dissolved in dry benzene (30 mL). Thiophosgene (2.60 g,
22.6 mmol, w90%) in benzene (10 mL) was added over 20 min
under cooling with ice-water. The solution turned black and was
stirred overnight. The solvent was removed by evaporation,
the residue washed with water and the dark red oil extracted
with pentane. After drying over sodium sulfate, the solvent was
removed. The oil (5.40 g) was purified by column chromatography
over silica with pentane as solvent.
4.3.3. Synthesis of 3a and dibenzyl heptathiotricarbonate 2a in
diethylether. Potassium benzyl carbonotrithioate 1a (4.75 g,
19.9 mmol) was suspended in dry diethylether (30 mL) Thio-
phosgene (1.30 g, 11.3 mmol, 90%) in diethylether (20 mL) was
added dropwise to an ice-cooled suspension thereof. The solution
turned red. After 75 min the formed solid was filtered off, washed
successively with diethylether, water and diethylether, and dried
overnight at 50 ꢀC to afford 3a as a yellow powder. Yield: 2.25 g
(51%), mp 160e161 ꢀC (CS2).
The organic solvent was evaporated and the remaining red oil
(2.2 g) stored in the refrigerator overnight. The oil solidified and
a further 0.55 g (12%) of 3a was separated and washed with a few
mL of THF. The residual oil was then purified by column chroma-
tography on silica with pentane/ethylacetate (2:1), followed by
another column separation with pure pentane as eluent. The
isolated red oil/low melting solid, 2a still contained a colorless
impurity in all relevant fractions.
Yield: 2.3 g (40%) of a dark red oil.
IR (neat) 1600, 1493, 1451, 1230, 1198, 1057, 756, 692 cmꢁ1
.
1H NMR (300 MHz, CDCl3)
d (ppm): 4.68 (4 H, q, CH3CH2,
J¼6 Hz), 1.49 (6 H, t, CH3CH2, J¼6 Hz).
13C NMR (75.4 MHz, CDCl3)
71.69 (CH2), 13.39 (CH3).
d (ppm): 209.12 (C]S), 202.97 (C]S),
After two weeks storage at ꢁ5 ꢀC parts of the oil crystallized. The
separated solid was washed with diethylether until colorless and
1.3 g (22%) of the dimeric 8 was isolated.
Yield: 0.35 g (8%).
1H NMR (300 MHz, CS2/CDCl3):
d
(ppm) 7.27e7.18 (10 H, m, Ar),
(ppm) 219.42 (C]S), 213.19
4.6. Synthesis of S,S0,S00,S000-1,3-dithietane-2,2,4,4-tetrayl-
O,O0,O00,O000-tetraethyl tetrakis(dithiocarbonate) 812
4.44 (4 H, s, CH2).
13C NMR (75.4 MHz, CS2/CDCl3):
d
(C]S), 133.49, 129.21, 128.67, 127.90 (Ar), 43.32 (CH2).
Potassium O-ethyldithiocarbonate 1c (8.00 g, 49.9 mmol) was
suspended in dry diethylether (75 mL) at room temperature under
stirring. Thiophosgene (2.60 g, 22.6 mmol, 90%) in dry diethylether
(20 mL) was added dropwise over 20 min. The solvent turned red
(not black as with benzene) and the solid slowly dissolved and
a white precipitate separated out. After stirring for 75 min at room
temperature, the solid 8 was filtered off, washed successively with
water and diethylether. The solvent was evaporated and the red
residue extracted with pentane. The resulting red oil (0.2 g, 3%)
4.4. Reactions of 3a
4.4.1. Reaction of 3a with n-propylamine to dibenzyldisulfide
4 (aminolysis). Compound 3a (8.90 g, 10.1 mmol) was suspended in
THF (100 mL) and n-propylamine (2.50 g, 42.3 mmol) added
under stirring at room temperature. The stirring was continued
for 60 min, whereupon 3a dissolved and the solution turned
orange-red. Precipitated sulfur was filtered off and the solvent
removed. The residue was washed with water and the remaining
crystals/oils extracted several times with pentane. After the
removal of the solvent by evaporation a near colorless solid
remained, which was recrystallized from pentane. The remainder
from the extraction of 4 consisted of a multi-component orange oil.
Colorless crystals, Yield: 3.85 g (78%), mp 70e71 ꢀC (pentane)
[mp lit.9c 71e72 ꢀC].
consisted mostly of O,O0-diethyl pentathiotricarbonate
7 and
decomposed overnight at room temperature.
Yield: 5.45 g (95%) of a whitish powder, mp 163e164 ꢀC
(mp lit.12 161 ꢀC).
1H NMR (300 MHz, CDCl3):
d (ppm) 4.54 (8 H, q, CH3CH2,
J¼7.2 Hz), 1.43 (12 H, t, CH3CH2, J¼7.2 Hz).
13C NMR (75.4 MHz, CDCl3):
69.12 (CH2), 13.76 (CH3).
d (ppm) 209.85 (C]S), 192.10 (CS2),
1H NMR (300 MHz, CDCl3):
d
(ppm) 7.34e7.22 (10 H, m, Ar), 3.59
The 13C NMR signal for the quaternary carbon could not be
(4 H, s, CH2).
13C NMR (75.4 MHz, CDCl3)
127.40 (Ar), 43.26 (CH2).
d
(ppm): 137.34, 129.39, 128.45,
unequivocally identified.
Acknowledgements
4.4.2. Thermolysis of 3a in chlorobenzene to bis(benzylsulfanyl
thiocarbonyl)disulfide 5 and dibenzyl trithiocarbonate 6. Compound
3a 4.45 g (5.0 mmol) was suspended in dry chlorobenzene (20 mL)
and heated under reflux. The color of the solvent changed from
yellow over green to dark red, while 3a dissolved over time. After
2 h the reaction was stopped and the solvent removed. The dark red
residue was extracted with pentane until the solvent stayed
colorless. The remainder was partially soluble in diethylether and
fully soluble in chloroform. It contained several yellow and red
components, which were not further identified.
The authors acknowledge the financial support of Mpact Lim-
ited. Jan Gertenbach and Vincent Smith (Department of Chemistry
and Polymer Science, Stellenbosch University) are acknowledged
for carrying out the single crystal X-ray analysis of the selected
structure 3a.
References and notes
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The pentane was again removed and the orange residue sepa-
rated on silica by column chromatography with pentane as eluent.
The fractions with the first two yellow components were collected
and processed further.
The products were identified by their 1H and 13C NMR spectra,
melting points and TLC comparison with independently prepared
samples:
Bis(benzylsulfanyl thiocarbonyl)disulfide 5, yellow crystals
(0.20 g, 5%).
Dibenzyl trithiocarbonate 6, yellow oil/crystals (0.30 g, 10%).

W. Weber et al. / Tetrahedron 69 (2013) 2017e2021
2021
9. (a) Freeman, F.; Angeletakis, C. N. J. Org. Chem. 1982, 47, 4194e4200; (b) Freeman,
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14. Note: The X-ray analysis was performed on a Bruker Apex-II DUO CCD (Charge
Coupled Device) Single Crystal X-ray Diffractometer, using graphite-
monochromated MoKa radiation. Crystallographic data (excluding structure
10. Weber, W.; McLeary, J. B.; Sanderson, R. D. Tetrahedron Lett. 2006, 47,
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factors) for the structure 3a in this paper have been deposited with the Cam-
bridge Crystallographic Data Centre as Supplementary data No. CCDC 891881.
Copies of the data can be obtained, free of charge, on application to CCDC,
12 Union Road, Cambridge CB2 1EZ, UK (fax: þ44-(0)1223-336033 or e-mail
deposit@ccdc.cam.ac.uk).