Efficient dehydrocyanation of hindered 1-substituted olefins

Article abstract of DOI:10.1016/S0040-4039(02)00516-6

The chlorosulfides 7 which formed quantitatively by reaction of olefins 5 with PhSCl under neutral conditions could be converted into the unsaturated nitriles 6 in good yields by sequential treatment with alkaline cyanides and MCPBA, a similar result being observed by reversing this order.

Full text of DOI:10.1016/S0040-4039(02)00516-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 3175–3179
Efficient dehydrocyanation of hindered 1-substituted olefins
O. Temmem,a D. Uguen,a,* A. De Cianb and N. Gruberb
aLaboratoire de Synthe`se Organique, associe´ au CNRS, Ecole Europe´enne de Chimie, Polyme`res et Mate´riaux,
Universite´ Louis Pasteur, 25, rue Becquerel, 67087 Strasbourg, France
bLaboratoire de Cristallochimie et Chimie Structurale, associe´ au CNRS, Universite´ Louis Pasteur, 67070 Strasbourg, France
Received 13 February 2002; accepted 7 March 2002
Abstract—The chlorosulfides 7 which formed quantitatively by reaction of olefins 5 with PhSCl under neutral conditions could be
converted into the unsaturated nitriles 6 in good yields by sequential treatment with alkaline cyanides and MCPBA, a similar
result being observed by reversing this order. © 2002 Elsevier Science Ltd. All rights reserved.
With the aim of synthesising the plant-growth promo-
tor brassinolide 1 according to the approach sum-
marised in Scheme 1, we have previously shown that
using appropriate conditions the acid 2 could be
obtained with an acceptable stereoselectivity by Ire-
land–Claisen rearrangement of a neryl ester. Accord-
ingly, our next concern was to elaborate the acid 2 to
the compound 3, a potential precursor of the perhy-
drindane 4.1
Examination of the literature revealed that straightfor-
ward conversion of a 1-alkene into the corresponding
a,b-unsaturated nitrile could be performed using either
of the two methods summarised in Scheme 2.2
However, investigation of these procedures with
neohexene 5a to form 6a proved disappointing.
Whereas attempted cross-metathesis of 5a with acryl-
onitrile under recently-described conditions2b gave only
the homocoupling product of the electrophilic partner,
its treatment with the Caserio reagent, then KCN and
HBF4 and ensuing oxidative elimination as described
by Trost2c produced complex mixtures of rearranged
products, probably a result of the acidic conditions
required. This led us to consider the condensation of 5a
with PhSCl as a possible means of mediating the
planned dehydrocyanation process, with the hope being
as indicated that the chlorosulfide 7a, claimed to be the
kinetic product of this reaction, could be transformed
into 8a by treatment with cyanide under non-acidic
Due to the presence of two differently-substituted car-
bonꢀcarbon bonds in the structure 2 and, moreover, to
the neohexenyl-like substitution of the less-substituted
one, the cumbersome step of this planned conversion
would undoubtedly be the implementation of a cyano
group at the terminus of the vinyl residue. We describe
in this letter how this problem can be solved in the case
of the model olefins 5a–c, the application of the results
of this study to the 2–4 conversion being reported in an
accompanying letter.
HO
OPG
OPG
CO2H
OPG
OH
HO
HO
I
PG=Ptrotecting group
CN
CN
PhS
O
1
4
3
2
O
Scheme 1.
* Corresponding author.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00516-6

3176
O. Temmem et al. / Tetrahedron Letters 43 (2002) 3175–3179
MeS
Caserio reagent,
then KCN, HBF4,
Acrylonitrile
Schrock catalyst
CN
CN
CN
E/Z= 1:1
R
MeO2C
n-Bu
R=n-Bu (from Ref. 2a)
R=(CH2)8CO2Me (from Ref. 2c)
MCPBA, base
MeO2C
E/Z= 1:1
Scheme 2.
conditions (Scheme 3).3 In the event, a subsequent
oxidative elimination reaction would provide 6a.
DBU and NMR analysis of the unsaturated sulfone 13
thus produced.
Treating 5a with freshly distilled PhSCl at ca −10°C in
CH2Cl2 as described3 resulted in the formation of a
chlorosulfide having NMR features similar to those
reported for 7a. Confirmation of this structure was
secured by treatment of this sulfide with excess
MCPBA to obtain a crystalline sulfone to which the
structure 9a was assigned by X-ray analysis (Scheme
4).4 Furthermore, treatment of 9a by DBU gave the
sulfone 10a, conclusively identified by NMR analysis.
Cyanation of the chlorosulfide 7a was first tried by
using standard conditions (KCN, DMSO; Scheme 5).
Reacting the cyanosulfide 8a thus formed with excess
MCPBA under basic conditions (Na2CO3) gave the
desired nitrile 6a in good yield (80% overall, from 7a)
and with a perfect E selectivity (NMR).
3
1 or 2
Cl
CN
CN
Additional evidence for the structure 7a came from the
observation of a partial conversion of 7a into the
isomeric chlorosulfide 11 on attempted purification by
column chromatography, an isomerisation which was
rendered quantitative by stirring 7a with silica gel in
CH2Cl2 overnight at rt. Oxidation of this rearranged
product with MCPBA gave a chlorosulfone having the
structure 12 as clearly established by treatment with
SPh
SPh
7a
6a
8a
Scheme 5. Reagents and conditions: 1. KCN (5 equiv.),
DMSO (5 ml/mmol); rt, 12 h (81%); 2. n-Bu4NCN (5 equiv.),
CH2Cl2 (4 ml/mmol); rt, overnight (86%); 3. MCPBA (3
equiv.), Na2CO3 (6 equiv.), CH2Cl2 (10 ml/mmol); 0°C, 15
min, then rt, 5 h (99%).
R1
R1
R1
R1
R2
R3
R2
R3
R2
R3
R2
R3
Cl
CN
CN
SPh
7a-d
SPh
8a-d
6a-d
5a-d
5a, R1=R2=R2=Me
5b, R1=R2=Me; R3=CH2OTBDMS
5c, R1=R2=Me; R3=CH2CO2Me
5d, R1=Me; R2, R3=CH2OC(Me2)OCH2
Scheme 3.
Scheme 4. Reagents and conditions: 1. PhSCl (1 equiv.), CH2Cl2 (1.3 ml/mmol); −10°C, 10 min, then rt, 10 min; 2. MCPBA (3
equiv.), NaHCO3 (6 equiv.), CH2Cl2 (3 ml/mmol); 0°C to rt, 3 h; 3. DBU (1.5 equiv.), CH2Cl2 (2 ml/mmol); rt, 5 h; 4. silica gel
(3 g/mmol), CH2Cl2 (10 ml/mmol); rt, overnight (100%).

O. Temmem et al. / Tetrahedron Letters 43 (2002) 3175–3179
3177
The possibility of performing both the chlorosulfanyl-
ation and the cyanation steps in the same flask was
next examined. To this end excess N-tetra-
butylammonium cyanide (TBACN) was added to the
chlorosulfanylation mixture, without isolation of the
chlorosulfide 7a. After a few hours, removal of unre-
acted TBACN with water, followed by treatment of
the formed cyanosulfide 8a with the MCPBA/Na2CO3
reagent afforded the pure nitrile 6a in 81% yield after
purification on silica gel. This two-step process
proved capable of being extended to more elaborated
substrates (Table 1).5
That basic elimination of a sulfinate ion in 14 would
proceed with the desired E selectivity was confirmed
by first reacting 10a with NaCN in presence of
AcOH (Scheme 6). Treatment of the resulting
cyanosulfone 14 with t-BuOK in THF gave the pure
(NMR) nitrile 6a (99%; 87% overall).
The chlorosulfone 9a was next reacted with KCN
(twofold excess) in DMSO and after 24 h the nitrile
6a was isolated in high yield.7 Attempts to use a
protic solvent proved less rewarding however. In t-
BuOH, with added crown ether as recommended,6a
the 3:7 mixture of the vinylic sulfone 10a and the
cyanosulfone 14 which formed initially (Table 2, entry
2) evoluted to give ultimately a 4:1 mixture of 14 and
6a (entry 3). Interestingly, the same 4:1 mixture
resulted by submitting 10a to these conditions (entry
4), lending credence to the elimination–addition path-
way presented above.
A minor shortcoming of this otherwise satisfactory
dehydrocyanation process came into light when we
noticed that treatment by TBACN of pure 7d induced
partial reversion of the sulfanylation process as evi-
denced by the detection in GLC of PhSCN alongside
5d. Efforts to suppress this side reaction proved inef-
fective, which led us to examine the condensation of
the chlorosulfone 9a with cyanides since no reversion
to the olefin 5a had been expected in this case. A few
vinylic sulfones have been shown to give the corre-
sponding nitriles by treatment with KCN: due to the
basicity of this reagent, the cyanosulfone thus formed
eliminating a sulfinate to give the corresponding a,b-
unsaturated nitrile.6 It thus could be hoped that 9a
would react with excess cyanide, acting both as a
base and a nucleophile, to give successively the vinylic
sulfone 10a (and a chloride ion), the cyanosulfone 14,
and then the nitrile 6a.
In conclusion, one-carbon homologation of hindered
olefins into the corresponding a,b-unsaturated nitriles
has been realised by a very simple procedure which
usefully complements methods based on prior degra-
dation of olefins into aldehydes and ensuing
cyanomethylenation condensations.8
Table 2. Incidence of solvent conditions on the 9a–6a
conversion
Substrate
Conditionsa
Product composition
(%)
Table 1. Dehydrocyanation of the olefins 5a–d by the
‘two-step process’
Entry 1
Entry 2
Entry 3
Entry 4
9a
9a
9a
10a
A, 24 h
B, 4 h
B, 15 h
B, 15 h
6a (95)
10a (30), 14 (70)
14 (80), 6a (20)
14 (80), 6a (20)
Starting Olefin
Product (Yield)*
CN
a Conditions A: KCN (2 equiv.) in DMSO (2 ml/mmol); rt.
Conditions B: KCN (10 equiv.), 18-crown-6 (0.1 equiv.) in t-BuOH
(2 ml/mmol); reflux.
5a
6a (81 %)
TBDMSO
5b
5c
6b (80 %)
CN
MeO2C
6c (67 %)
CN
Acknowledgements
O
O
5d
6d (80 %)
CN
Thanks are due to ‘Le Ministe`re de la Recherche et
de la Technologie’ for a grant (to O.T.).
* Overall, from compound 5
1
2
3
CN
Cl
SO2Ph
CN
14 SO2Ph
9a
6a
SO2Ph
10a
Scheme 6. Reagents and conditions: 1. NaCN (20 equiv.), 15-crown-5 (0.2 equiv.), AcOH (1.2 equiv.), H2O (1 ml/mmol), CH2Cl2
(1 ml/mmol); rt, 15 h (88%); 2. t-BuOK (1 equiv.), THF (5 ml/mmol); rt, 0 min (99%); 3. KCN (2 equiv.), DMSO; rt, 1 day (95%).

3178
O. Temmem et al. / Tetrahedron Letters 43 (2002) 3175–3179
solvents was then purified by chromatography on silica gel
References
(hexane/ether) to give pure nitrile 6a (882 mg; 81%).
6. (a) Taber, D. F.; Saleh, S. A. J. Org. Chem. 1981, 46,
4817–4819; (b) Bailey, P. L.; Jackson, R. F. W. Tetra-
hedron Lett. 1991, 32, 3119–3122.
1. Temmem, O.; Uguen, D.; De Cian, A.; Gruber, N. Tetra-
hedron Lett. 2002, 43, 3169–3173.
2. (a) Crowe, W. E.; Goldberg, D. R. J. Am. Chem. Soc.
1995, 117, 5162–5163; (b) Blanco, O. M.; Castedo, L.
Synthesis 1999, 557–558; (c) Trost, B. M.; Shibata, T.;
Martin, S. J. Am. Chem. Soc. 1982, 104, 3228–3230.
3. Mueller, W. H.; Butler, P. E. J. Am. Chem. Soc. 1968, 90,
2075–2081. For reviews on the chlorosulfanylation of
olefins and i. a. the reactivity of halosulfides with nucle-
ophilic reagents, see for instance: (a) Lucchini, V.; Mod-
ena, G.; Pasquato, L. Gazz. Chim. Ital. 1997, 127,
178–188; (b) Koval, I. V. Russian Chem. Rev. 1995, 64,
731–751.
7. Protocol for the oxidation–cyanation process: NaHCO3 (5
g; 6 equiv.) was added with stirring to a solution of the
chlorosulfide 7a prepared as described previously.5 After
dilution with CH2Cl2 (30 ml), and cooling (ice/methanol),
80% MCPBA (6.5 g; 3 equiv.) was added and the resulting
mixture was stirred for 3 h at rt, then poured into 0.25 M
Na2S2O3. The resulting two-phase system was worked-up
as described above for the 8a–6a conversion to afford the
crystalline chlorosulfone 9a (2.55 g; 98%). KCN (1.63 g; 2
equiv.) was added to a stirred solution of 9a (1.3 g; 5
mmol) in DMSO (10 ml). After 24 h stirring at rt, the
reaction mixture was diluted with ether (300 ml), then
washed with water (4×50 ml), brine (2×50 ml), and dried
(MgSO4). The residue left by evaporation of the solvents
was chromatographed on silica gel (pentane/ether). After
removal of the solvents, the crude nitrile 6a was further
purified by bulb-to-bulb distillation to give 6a as a pale
yellow liquid (491 mg; 90%). It is worth noting that
2-t-butylsuccinonitrile, which was detected (NMR) in the
crude product and removed during the purification steps,
was the only isolated product (99%) using KCN in larger
4. Crystal data of 9a: C12H17O2SCl, MW=260.79, triclinic,
,
space group P1, a=8.003(2), b=7.971(2), c=21.018(6) A,
3
,
h=91.93(2), i=99.11(2), k=90.22(2)°, V=1323(1) A ,
Z=4, Dcalcd=1.31 g cm−3, v(Mo Ka)=0.422 mm−1. Data
were collected using a Nonius Mach3 diffractometer,
,
graphite monochromated Mo Ka radiation (u=0.7173 A)
at room temperature. 4978 reflections were collected using
a crystal of 0.35×0.30×0.20 mm3. The structure was solved
using direct methods and refined with 3076 reflections
having I>3|(I). Hydrogen atoms were introduced as fixed
contributors at their computed coordinates (d(CꢀH)=0.95
1
excess (5 equiv.). Selected data: 7a: H NMR: 1.13 (s, 9H),
,
A, B(H)=1.3). Full matrix refinements against ꢀFꢀ. Final
3.13 (dd, J=5, 7 Hz, 1H), 3.65 (dd, J=7, 12 Hz, 1H), 3.93
(dd, J=5, 12 Hz, 1H), 7.21–7.36 (m, 3H), 7.48–7.53 (m,
2H); 13C NMR: 28.4, 36.2, 46.3, 64.3, 126.9, 129.2, 131.6,
136.5; 11: 1H NMR: 1.04 (s, 9H), 3.06 (dd, J=10.5, 14 Hz,
1H), 3.45 (dd, J=2.5, 14 Hz, 1H), 3.79 (dd, J=2.5, 10.5
Hz, 1H), 7.22–7.42 (m, 5H); 13C NMR: 26.6, 36.2, 39.4,
72.6, 126.7, 129, 130.3, 136.5; 9a: Mp 60–63°C (hexane/
ether); 1H NMR: 1.3 (s, 9H), 3.22 (t, J=4 Hz, 1H), 3.7
(dd, J=4, 13 Hz, 1H), 3.78 (dd, J=4, 13 Hz, 1H), 7.57–7.7
(m, 3H), 7.9–7.95 (m, 2H); 13C NMR: 28.9, 36.5, 39.6,
,
results: R(F)=0.041 A, Rw(F)=0.063, GOF=1.27, largest
peak in final difference=0.45 e A−3. Crystallographic data
,
for the structural analysis have been deposited with the
Cambridge Crystallographic Data Centre, for compound
9a. Copies of this information may be obtained free of
charge from the Director, CCDC, 12 Union Road, Cam-
bridge, CN2 1EZ, UK (fax: +44-1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.
ac.uk).
5. In all cases, the chlorosulfide was independently character-
ised and treated successively with MCPBA, then DBU,
and each intermediate being identified by NMR. Protocol
for the two-step process: (N.B. All operations conducted
under an argon atmosphere and in using well-dried glass-
ware) Freshly prepared, and distilled (34°C at 1~), PhSCl
(1 equiv.) was added by syringe as a 3.5 M CH2Cl2
solution (2.86 ml) to a cooled (ice/methanol) and well-
stirred solution of the olefin 5a (10 mmol) in the same
solvent (10 ml). After the initial yellow coloration had
discharged (10 min), the resulting colourless solution was
further stirred at rt for 10 min. Subsequently a solution of
TBACN (13.4 g; 5 equiv.) in CH2Cl2 (40 ml) was added.
The resulting mixture was stirred overnight at rt. The
solvent was removed in vacuo and the residue taken up in
ether (500 ml). After washing with water (4×100 ml), and
drying (MgSO4), the solvents were removed in vacuo to
give the cyanosulfide 8a as a pale yellow oil, which was
immediately diluted with CH2Cl2 (100 ml). After cooling
(ice/methanol), Na2CO3 (6.5 g; 6 equiv.) and 80% MCPBA
(6.5 g; 3 equiv.) were sequentially added. After 15 min, the
cooling bath was removed and the mixture was stirred at
rt (5 h), before being poured into 0.25 M Na2SO3 (200 ml).
The aqueous layer was extracted with CH2Cl2 (3×100 ml)
and the combined organic phases were washed with satu-
rated NaHCO3 (100 ml), brine (100 ml), and dried
(MgSO4). The oily residue left by evaporation of the
1
76.1, 126.5, 129.4, 133.8, 140.4; 12: H NMR: 0.98 (s, 9H),
3.49 (dd, J=8.5, 15 Hz, 1H), 3.58 (dd, J=2, 15 Hz, 1H),
4.05 (dd, J=2, 8.5 Hz, 1H), 7.52–7.7 (m, 3H), 7.9–8 (m,
2H); 10a: Mp 136–139°C (hexane/ether); 1H NMR: 1.22 (s,
9H), 5.96 (d, J=1.1 Hz, 1H), 6.28 (d, J=1.1 Hz, 1H), 6.28
(d, J=1.1 Hz, 1H), 6.28 (d, J=1.1 Hz, 1H), 7.5–7.6 (m,
3H), 7.84–7.9 (m, 2H); 13C NMR: 30.3, 36.5, 124.9, 127.7,
129.1, 133.1, 140.4, 159.5; 13: Mp 54–56°C (hexane/ether);
1H NMR: 1.08 (s, 9H), 6.26 (d, J=15 Hz, 1H), 6.99 (d,
J=15 Hz, 1H), 7.5–7.61 (m, 3H), 7.84–7.89 (m, 2H); 13C
NMR: 28.3, 34.1, 126.6, 127.5, 129.2, 133.2, 140.8, 156.4;
1
8a: H NMR: 1.13 (s, 9H), 2.62 (dd, J=7.3, 17.1 Hz, 1H),
2.75 (dd, J=5.3, 17.1 Hz, 1H), 3.07 (dd, J=5.3, 7.3 Hz,
1
1H), 7.26–7.38 (m, 3H), 7.52–7.57 (m, 2H); 14: H NMR:
1.29 (s, 9H), 2.67 (dd, J=5.5, 18 Hz, 1H), 2.82 (dd, J=5.5,
18 Hz, 1H), 3.22 (t, J=5.5, 1H), 7.57–7.75 (m, 3H),
7.91–7.96 (m, 2H); 13C NMR: 26.9, 39.6, 70.7, 76.2, 121.3,
1
128.5, 129.4, 133.8, 140.4; 6a: H NMR: 1.08 (s, 9H), 5.23
(d, J=16.5 Hz, 1H), 6.63 (d, J=16.5 Hz, 1H); 6b: 1H
NMR: 0.03 (s, 6H), 0.89 (s, 9H), 1.02 (s, 6H), 3.35 (s, 2H),
1
5.29 (d, J=16.8 Hz, 1H), 6.75 (d, J=16.8 Hz, 1H); 6c: H
NMR: 1.21 (s, 6H), 2.37 (s, 2H), 3.66 (s, 3H), 5.29 (d,
1
J=16.5 Hz, 1H), 6.82 (d, J=16.5 Hz, 1H); 6d: H NMR:
0.99 (s, 3H), 1.4 (s, 3H), 1.45 (s, 3H), 3.49–3.93 (m, 4H),
5.58 (d, J=16 Hz, 1H), 6.89 (d, J=16 Hz, 1H); 8b: 1H
NMR: 0.03 (s, 3H), 0.05 (s, 3H), 0.88 (s, 9H), 1.04 (s, 3H),
1.07 (s, 3H), 2.67 (dd, J=7.3, 17.1 Hz, 1H), 3.4 (dd,

O. Temmem et al. / Tetrahedron Letters 43 (2002) 3175–3179
3179
1
J=5.2, 7.3 Hz, 1H, plus d, J=10 Hz, 1H), 3.65 (d, J=10
Hz, 1H), 3.7 (dd, J=5.2, 17.1 Hz, 1H), 7.25 (m, 3H),
7.51–7.57 (m, 2H); 8d: 1H NMR: 0.96 (s, 3H), 1.38 (s, 3H),
1.43 (s, 3H), 2.65–2.71 (m, 1H), 3.49–3.93 (m, 6H), 7.21–
7.35 (m, 3H), 7.4–7.45 (m, 2H); 9b: Mp 87–88°C; 1H NMR:
0.08 (s, 3H), 0.1 (s, 3H), 0.91 (s, 9H), 1.19 (s, 3H), 1.31 (s,
3H), 3.49 (d, J=10 Hz, 1H), 3.7 (t, J=4 Hz, 1H), 3.82 (s,
2H), 3.83–3.87 (m, 1H), 7.52–7.66 (m, 3H), 7.91–8.01 (m,
2H); 13C NMR: −5.8, −5.7, 18, 22.8, 23.7, 25.7, 39.2, 41.5,
69.9, 71, 128.4, 129.2, 133.9, 139; 9c: Mp 87–89°C; 1H
NMR: 1.33 (s, 3H), 1.4 (s, 3H), 2.41 (d, J=1.15 Hz, 1H),
3.44 (d, J=15 Hz, 1H), 3.7 (dd, J=5, 11 Hz, 1H), 3.72 (s,
3H), 3.85 (dd, J=5, 11 Hz, 1H), 4.38 (t, J=5 Hz, 1H),
7.53–7.73 (m, 3H), 7.91–8.01 (m, 2H); 13C NMR: 25.1,
27.4, 37.4, 38.9, 44.4, 51.2, 69.8, 128, 129.1, 133.6, 139.9,
172.2; 9d: Mp 88–92°C; 1H NMR: 1.3 (s, 3H), 1.36 (s, 3H),
1.45 (s, 3H), 3.56–3.79 (m, 3H), 3.94–4.04 (m, 3H), 4.55
(dd, J=2.8, 12.1 Hz, 1H), 7.57–7.69 (m, 3H), 7.95–8 (m,
2H); 10b: H NMR: 0 (s, 6H), 0.84 (s, 9H), 1.13 (s, 6H),
3.53 (s, 2H), 6 (s, 1H), 6.4 (s, 1H), 7.52–7.66 (m, 3H),
7.91–8.01 (m, 2H); 13C NMR: −5.5, 18.2, 24.7, 25.7, 41.7,
1
70.1, 127.6, 127.7, 129, 133, 141.8, 155.1; 10c: H NMR:
1.33 (s, 6H), 2.78 (s, 2H), 3.72 (s, 3H), 5.96 (d, J=1.6 Hz,
1H), 6.25 (d, J=1.6 Hz), 7.51–7.61 (m, 3H), 7.86–7.91 (m,
2H); 13C NMR: 26.3, 38.4, 45.1, 51.2, 126.4, 127.9, 129.1,
133.3, 141.4, 156.9, 172.2; 10d: 1H NMR: 1.23 (s, 3H), 1.29
(s, 3H), 1.38 (s, 3H), 3.6 (d, J=11.9 Hz, 2H), 4.14 (d,
J=11.9 Hz, 2H), 6.26 (d, J=1.4 Hz, 1H), 6.47 (d, J=1.4
Hz, 1H), 7.53–7.62 (m, 3H), 7.85–7.9 (m, 2H); 13C NMR:
20.1, 23.2, 24.2, 38.7, 87.8, 98.1, 127.8, 128.4, 129.2, 133.4,
141.2, 153.6. All H and 13C NMR spectra at 200 and 50
MHz, respectively, in CDCl3. The results presented in this
letter are taken in part from the thesis dissertation of
Olivier Temmem (Strasbourg, December 2000).
1
8. D’sa, B. A.; Kisanga, P.; Verkade, J. G. J. Org. Chem.
1998, 63, 3961–3967 and references cited therein.