N-acylpyridinium trifluoromethanesulfonates and tetrafluoroborates: Shuttle reagents for the acylation of enantiopure secondary alcohols

Article abstract of DOI:10.1055/s-1998-2074

Several enanflopure alcohols are esterified with N-acyl-4- benzylpyfidinium trifluoromethanesulfonates 7b,d,f or tetrafluoroborates 7a,c,e. These reagents, which can be generated in situ, or isolated as stable salts, are synthesized from readily available 4-alkylpyridines 3, acyl chlorides 4 and strong protic acids 6. The acyl moiety is transferred under neutral conditions and in high yields. The heterocyclic component 3a can be re-isolated almost quantitatively. The acetate, benzoate and pivaloate groups were selected with regard to their application as frequently used protecting groups for hydroxy compounds. As shown by paramagnetic shift experiments with a chiral europium(III) complex, these acylations proceed without detectable racemization or epimerization.

Full text of DOI:10.1055/s-1998-2074

June 1998
SYNTHESIS
883
N-Acylpyridinium Trifluoromethanesulfonates and Tetrafluoroborates: Shuttle
Reagents for the Acylation of Enantiopure Secondary Alcohols
Rüdiger Wagner, Wolfgang Günther, Ernst Anders*
Institut für Organische Chemie und Makromolekulare Chemie, Friedrich-Schiller-Universität Jena, Humboldtstraße 10, D-07743 Jena, Germany
Tel +49(3641)948210; Fax +49(3641)948212; E-mail: c5eran@rz.uni-jena.de
Received 5 August 1997; revised 12 December 1997
Dedicated to Professor Alan R. Katritzky on the occasion of his 70th birthday
Abstract: Several enantiopure alcohols are esterified with N-acyl-
4-benzylpyridinium trifluoromethanesulfonates 7b,d,f or tetra-
fluoroborates 7a,c,e. These reagents, which can be generated in situ,
literature, and diastereomeric esters formed from racemic
or isolated as stable salts, are synthesized from readily available 4-
alkylpyridines 3, acyl chlorides 4 and strong protic acids 6. The acyl
these authors derived the optical purity of the products by
comparison with specific rotation values reported in the
acids and (S)-(–)-8a were analyzed indirectly with NMR
spectroscopy. It was impossible to decide whether the re-
action proceeded under complete inversion or retention of
configuration. Detailed information about the stereo-
chemistry is available for the well-established Mitsunobu
reaction8, 9 and by a method developed by Burkhard and
Effenberger.10 In both cases enantiomeric esters are pro-
duced with complete inversion of configuration. A very
unreliable stereochemical course occurs for secondary
chiral alcohols [for instance (S)-(–)-8a] if they are treated
with a mixture consisting of alkaline metal carboxylates,
PPh3 and CCl4.11 We report a new method for esterifica-
tion in which we have thoroughly investigated both the
chemical and optical purity of the resulting products.
moiety is transferred under neutral conditions and in high yields.
The heterocyclic component 3a can be re-isolated almost quantita-
tively. The acetate, benzoate and pivaloate groups were selected
with regard to their application as frequently used protecting groups
for hydroxy compounds. As shown by paramagnetic shift experi-
ments with a chiral europium(III) complex, these acylations pro-
ceed without detectable racemization or epimerization.
Key words: shuttle reagents, esterification, N-acyl-4-benzylpyri-
dinium trifluoromethanesulfonates or tetrafluoroborates, enan-
tiopure secondary alcohols
Although the formation of carboxylic esters is one of the
best studied organic reactions,1, 2 there still is the need for
versatile methods which allow the efficient transformation
of sensitive hydroxy compounds into carboxylic esters.
This is the conditio sine qua non in the case of such chiral
alcohols which reveal a significant tendency to undergo ra-
cemization in the presence of minor amounts of acids or
bases (both Brønstedt and Lewis). In such cases, the acylat-
ing reagent must be activated under very mild conditions, in
particular, the formation of protic acids HX or basic species
during the reaction course must be suppressed.
Previously, a method to isolate stable, but still reactive,
pyridinium salts 7 in preparative yields, not using 4-ami-
nopyridines 1 but 4-alkylpyridines 3 as starting materi-
als,12 has been developed by our group (Scheme 1).
Steglich and co-workers showed3 that 4-aminopyridines
1, especially 4-(dimethylamino)pyridine (1a) and 4-pyr-
rolidinopyridine (1b), greatly assist the acylation of steri-
cally hindered alcohols using carboxylic acid anhydrides
or the free acid in combination with DCC, respectively.
The strong catalytic activity of 1 is due to the intermediate
formation of N-acylpyridinium salts which are thought to
be highly activated carbonyl groups. The structures of
these cations were supported by NMR spectroscopy4 and
the synthesis of a stable N-tert-butoxycarbonyl derivative
2,5 and that of the latter was confirmed by X-ray analysis.6
Rao and Roth7 successfully applied the carbodiimide acy-
lation method to hydroxy-, dihydroxy-, and mercaptocar-
boxylic acids. No racemization was observed when ethyl
lactate (S)-(–)-8a was esterified with simple, and more
complex, pharmacologically important acids. However,
Scheme 1

884
Papers
SYNTHESIS
4-Benzylpyridine (3a) is the most convenient compound taken from the chiral pool and some of them are structural
subunits of natural products.15, 16 They and their esters
for the formation of 1-acyl-4-alkylidene-1,4-dihydropy-
have frequently been used as chiral auxiliaries in organic
synthesis,17–21 for the study of enantioselective chemical
processes22–26 and for investigating the capacity of micro-
organisms and enzymes.27–31 Generally, 8a–d have been
esterified with acyl halides or anhydrides in the presence
of nitrogen heterocycles, dominantly pyridine (known as
the Einhorn reaction32) or DMAP (1a).
ridines 5a–c which are obtained by the sequential treatment
of 3a with acyl chlorides 4a–c and triethylamine.13 Elec-
tron-withdrawing groups at the exocyclic carbon stabilize
the dihydropyridines 5, whereas alkyl substituents activate
5 and deactivate the corresponding pyridinium salts 7.
The dihydropyridines 5a–c reveal a dynamic behavior in
solution, which was not previously described in the litera-
ture.12 Due to the partial double bond character of the C–
N bond and according to their NMR spectra, 5a,b exist at
room temperature as two rotamer pairs, which we assign
cisoid and transoid structures.
The experimental results of the reaction according to
Scheme 2 are summarized in Table 1. Analytical data of
new compounds 9c,f,l and insufficiently described esters
9i,k are given in Table 2. Table 1 clearly shows that best
results are achieved using the trifluoromethanesulfonates
7b,d,f. The yields obtained from the reaction with tetra-
fluoroborates 7a,c,e are poor; the crude mixture consisted
of unreacted alcohols and the corresponding carboxylic
acid produced from a partial hydrolysis of the group-
transfer reagents 7a,c,e. However, there are NMR spectro-
–
scopic indications that the non-nucleophilic BF4 ion is
not as stable as expected but reacts with the acyl moiety
producing acyl fluorides (Scheme 3).
The precursors 5a–c are suitable for storage under inert
conditions, but slowly decompose when exposed to mois-
ture. They may be regarded as the deactivated forms of the
title compounds 7a–f from which the latter are obtained by
reaction with strong protic acids 6a–c. The properties of
7a–f are controlled by the nature of the counterion X–. In
–
–
general, we prefer non-nucleophilic anions (BF4 , CF3SO3 ,
–
FSO3 ). In such cases the synthesis of 7 from 5 is quantita-
tive. Since all salts 7 are very moisture sensitive, they
should be prepared just before use and strictly have to be
kept under an inert nitrogen or argon atmosphere. Thus, the
reactivity of the oily pyridinium salts 7g–i resulting from
protonation of 5a–c with fluorosulfonic acid (6c) was not
investigated further because it is easier and more effective
for our purpose to handle solid compounds.
Scheme 3
In view of the simplicity of the experimental procedure
the use of the trifluoromethanesulfonates 7b,d,f is an ex-
cellent preparative method for the esterification of alco-
hols compared with other methods using heterocycles as
auxiliaries or catalysts. There is no need for an external
proton acceptor, since 3a, which is split off in the course
of the reaction, plays this role. The separation of the pyri-
dinium salts 10a,b is quantitative due to their insolubility
in diethyl ether or benzene. No purification with diluted
mineral acids or sodium carbonate solution is necessary.
Furthermore, it is possible to regenerate the pyridine
source 3a by aqueous neutralization in 80–90% yield, and
in the case of more expensive pyridine derivatives this
would be very useful.
The dihydropyridines 5a–c react readily with various nu-
cleophiles, whereas little is known about the synthetic po-
tential of the pyridinium salts 7a–f, besides their reactivity
towards aldehydes and ketones.14 We therefore investigat-
ed a simple, but important, reaction; the preparation of
chiral esters 9a–l performed with a variety of optically
pure alcohols 8a–d (Scheme 2). These substrates 8a–d are
The salts 7a–f were isolated in bulk, analyzed prior to use
and dissolved in CH2Cl2 for further reactions (Method A).
This is advantageous compared with the properties of the
analogous N-acylpyridinium halides derived from 1a and
1b33 or from pyridine.34 Salts with chlorides or bromides
as anions are not suitable for preparative acylation due to
their poor solubility in aprotic solvents and because they
exist in equilibrium with the corresponding acyl halide
and N-heterocycle.
Scheme 2

June 1998
SYNTHESIS
885
Furthermore, it is possible to generate the group-transfer to]europium(III).41 A LIS experiment with racemic (±)-9e
reagents 7a–f in situ, by dissolving the precursors 5a–c in using a fourfold excess of this LSR led to a small, but
CH2Cl2 and adding equimolar amounts of acids 6a or b clearly observable, pseudocontact shift difference DDd =
(Method B). An excess of acid suppresses the esterifica- 0.02 which was never detected for the enantiopure carbox-
tion completely and, therefore, has to be strictly avoided. ylic esters 9a–l under identical or similar conditions41
This can be done easily, because the course of the proto- (limit of detection ~2%). Since the absolute configuration
nation can be monitored by means of the color of the so- of the enantiopure alcohols 8a–d and most of the products
lution. The first drop of 6a,b turns the yellow or orange 9 is known (by comparing the sign of optical rotation with
solution of the dihydropyridines 5a–c brown, and an ex- literature values), we draw the conclusion that the de-
cess of acid produces a yellow color again. This last scribed acylation reaction proceeds with complete reten-
change must be avoided.
tion of configuration. We, therefore, assign each product 9
an ee value of ≥96%. A comparison with the DMAP/an-
hydride method reveals that this well-defined stereochem-
ical course is not self-evident (Table 3).
The optical purity of all products 9a–l was determined
with chiral lanthanide induced shift (LIS) experiments.35–40
This method is “absolute” in the sense that it is capable of
providing a direct measure of the optical purity without re- Although chemical yields are similar to our method and
course to a standard whose optical purity is already the synthesis of 9i proceeds without racemization, stereo-
known. Among many others, a well-described and com- chemically labile mandelic acid derivative 8b is partially
mercially available chiral lanthanide shift reagent (LSR) inverted when it is esterified with DMAP/anhydride under
is tris[3-(trifluoromethylhydroxymethylene)-d-camphora- typical conditions which are taken from the original liter-

886
Papers
SYNTHESIS
Table 3. Esters Prepared with DMAP/Anhydride
workup. Standard syringe techniques were used to transfer solvents
and to add liquid reagents. Glassware was flame-dried and flushed
with N2 before use. The reagents 3a, 4a–c, 8a–d were of commercial
quality (Aldrich) and freshly distilled or recrystallized. All alcohols
8a–d were 99% optical pure. The acids 6a,b were taken from original
containers (Fluka). Et2O, toluene, and benzene were distilled from so-
dium benzophenone ketyl; CH2Cl2 was purified by column chroma-
tography (alumina). Mps were taken using a copper block apparatus
(Linström) and are uncorrected. Microanalyses were obtained with a
LECO CHNS-932 element analyzer. Optical rotations were measured
with an Eloptron Polarotronic E polarimeter (l = 589 nm). IR spectra
were recorded using a Nicolet Impact 400 spectrophotometer and
22
Compound
Yielda) (%)
[a] (c = 1, CH2Cl2)
ee (%)
589
9d
9e
9f
9i
84
69
80
70
–85.5
0
–81.0
+73.0
54
0
82
≥96b
a Isolated and purified substances, identical analytical data.
b de.
ature.3 Complete racemization occurred in the course of
the formation of compound 9e with DMAP/anhydride.
This might be the result of the workup procedure, since it
is necessary to hydrolyze excessive benzoic acid anhy-
dride and to remove the resulting acid under strong basic
conditions (2 N NaOH).
1
wavelength (n) are reported in cm–1.13C and H NMR spectra were
determined on a Bruker DRX400 instrument operating at 100 and
400 MHz. Chemical shifts are given in ppm downfield from TMS or
using the solvent signal of CDCl3 (dH = 7.24, dC = 77.0) as an internal
standard. The degree of substitution of C-atoms was determined with
the aid of DEPT-90 and DEPT-135 spectra. All 1H and 13C resonances
of alicyclic compounds 9g–l were assigned using 2D NMR tech-
niques (COSY, NOESY, HMBC, HMQC, Bruker standard pulse pro-
grams, Bruker software XwinNMR).
All reactions were carried out under purified N2 atmosphere by apply-
ing a positive pressure of the protecting gas, followed by non-inert

June 1998
SYNTHESIS
887
1-Acyl-4-alkylidene-1,4-dihydropyridines 5a–c; General Proce- 1-Acetyl-4-benzylpyridinium Trifluoromethanesulfonate (7b):
dure:
From dihydropyridine 5a (4.3 g); trifluoromethanesulfonic acid (6b)
This procedure was described in the literature,12 but we modified the (1.95 mL); yield: 6.90 g (96%); slightly yellowish powder.
synthesis slightly and added a new derivative 5c. A new interpretation 1H NMR (400 MHz, CD3OCD3): d = 3.19 (s, 3H, CH3), 4.70 (s, 2H,
of the 1H NMR and additional 13C NMR data are given:
CH2), 7.33 (m, 1H, Ph-p-H), 7.36–7.42 (m, 4H, Ph-o,o¢,m,m¢-H), 8.23
To a stirred solution of 4-benzylpyridine (3a) (40.0 mL, 0.25 mol) in (d, 2H, Py-3,5-H), 9.55 (d, 2H, Py-2,6-H).
toluene (600 mL) the acyl halide 4a–c (0.20 mol) was added with a 13C NMR (100.6 MHz, CD3OCD3): d = 21.26 (CH3), 41.38 (CH2),
syringe. The resulting yellow or orange suspension was stirred vigor- 127.44 (Py-3,5-C), 127.55 (Ph-p-C), 129.14, 129.46 (Ph-o,o¢,m,m¢-
ously for 30 min followed by dropwise addition of NEt3 (70 mL, C), 136.80 (Ph-i-C), 140.38 (Py-2,6-C), 168.15 (Py-4-C), 168.82
0.50 mol). The mixture was heated under reflux for 30 min and al- (CO).
lowed to stand overnight at r.t. The precipitated HNEt3Cl was filtered
off and the solvent was evaporated under reduced pressure to give the
crude product which was recrystallized.
1-Benzoyl-4-benzylpyridinium Trifluoromethanesulfonate (7d):
From dihydropyridine 5b (5.50 g); trifluoromethanesulfonic acid (6b)
(1.95 mL); yield: 7.96 g (94%); colorless powder.
1H NMR (400 MHz, CD3OCD3): d = 4.59 (s, 2H, CH2), 7.34 (m, 1H,
1-Acetyl-4-benzylidene-1,4-dihydropyridine (5a):
Pha-p-H), 7.39–7.47 (m, 4H, Ph-o,o¢,m,m¢-H), 7.73 (m, Bzb-m,m¢-H),
From acetyl chloride (4a) (14.2 mL); recrystallized from acetone;
yield: 35.6 g (84%); yellow crystals; mp 81–82 °C [ref12 mp 78–79°C
(Et2O)].
7.93 (m, 1H, Bz-p-H), 8.03 (m, 2H, Bz-o,o ¢-H), 8.30 (dd, 2H, Py-3,5-
H), 9.38 (dd, 2H, Py-2,6-H).
1H NMR (400 MHz, CDCl3, 227 K): d = transoid-5a: 2.28 (s, 3H,
CH3), 6.43 (d, 1H, H(5)), 6.54 (d, 1H, H(3)), 6.71 (d, 1H, H(6)), 7.36
(d, 1H, H(2)); cisoid-5a: 2.28 (s, 3H, CH3), 5.85 (d, 1H, H(3)), 5.95
(d, 1H, H(5)), 6.65 (d, 1H, H(2)), 7.31 (d, 1H, H(6)); other resonanc-
es: 5.86 (s, 1H, H(a)), 5.88 (s, 1H, H(a)), 7.10–7.38 (m, 10H, PhH).
13C NMR (100.6 MHz, CDCl3, 227 K): d = transoid-5a: 21.98 (CH3),
110.13 (C(5)), 110.92 (C(3)), 124.11 (C(2)), 126.11 (C(6)); cisoid-5a:
21.98 (CH3), 116.99 (C(3)), 117.86 (C(5)), 122.17 (C(6)), 124.16
(C(2)); other resonances: 116.24 (C(a)), 116.37 (C(a)), 126.08,
127.80, 128.62, 129.16, 137.72, 137.83, 166.22 (CO).
13C NMR (100.6 MHz, CD3OCD3): d = 41.54 (CH2), 127.19 (Bz-i-
C), 127.47 (Py-3,5-C), 127.64 (Ph-p-C), 129.17, 129.49 (Ph-
o,o¢,m,m¢-C), 129.61 (Bz-m,m¢-C), 132.29 (Bz-o,o¢-C), 136.18 (Bz-p-
C), 136.69 (Ph-i-C), 142.50 (Py-2,6-C), 167.23 (Py-4-C), 168.69
(CO).
a Ph = PhCH2-moiety. b Bz = PhCO-moiety.
4-Benzyl-1-pivaloylpyridinium Tetrafluoroborate (7e):
From dihydropyridine 5c (5.1 g); 54% tetrafluoroboric acid in Et2O
(6a) (3.0 mL); yield: 7.61 g (92%); yellowish oil which does not so-
lidify.
1-Benzoyl-4-benzylidene-1,4-dihydropyridine (5b):
1H NMR (400 MHz, CDCl3): d = 1.46 [s, 9H, C(CH3)3], 4.32 (s, 2H,
From benzoyl chloride (4b) (23.2 mL); recrystallized from Et2O; CH2), 7.20–7.32 (m, 5H, PhH), 7.95 (dd, 2H, Py-3,5-H), 8.94 (dd, 2H,
yield: 47.8 g (88%); orange crystals; mp 90–92°C [ref12 mp 9l–92°C Py-2,6-H).
(acetone)].
13C NMR (100.6 MHz, CDCl3): d = 27.45 [C(CH3)3], 41.96 (CH2),
1H NMR (400 MHz, CDCl3, 295 K): d = 5.92 (s, 1H, H(a)), 5.94 (d, 43.39 [C(CH3)3], 127.82 (Ph-p-C), 127.96 (Py-3,5-C), 129.39,
1H, H(5)), 6.95 (d, 1H, H(3)), 7.10 (br, 2H, H(2,6)), 7.17 (m, 1H, Ph- 129.50 (Ph-o,o¢,m,m¢-C), 135.32 (Ph-i-C), 140.44 (Py-2,6-C), 166.54
p-H), 7.31–7.34 (m, 4H, Ph-o,o¢, m,m¢-H), 7.47–7.69 (m, 5H, BzH).
13C NMR (100.6 MHz, CDCl3, 295 K): d = cisoid-5b: 110.55 (br,
C(3,5)), 126.57 (br, C(2,6)); transoid-5b: 117.43 (br, C(3,5)), 124.45
(br, C(2,6)); other resonances: 116.60 (C(a)), 126.01, 127.82, 128.41,
128.50, 128.73, 129.37, 131.46, 132.96, 137.80, 166.60 (CO).
(Py-4-C), 176.79 (CO).
4-Benzyl-1-pivaloylpyridinium Trifluoromethanesulfonate (7f):
From dihydropyridine 5c (5.1 g) in benzene/Et2O mixture (1:1, 160
mL); trifluoromethanesulfonic acid (6b) (1.95 mL); yield: 6.9 g
(79%); may be oily but solidifies upon standing and cooling, colorless
solid.
4-Benzylidene-1-pivaloyl-1,4-dihydropyridine (5c):
From pivaloyl chloride (4c) (24.6 mL); the reaction was complete af- 1H NMR (400 MHz, CDCl3): d = 1.49 [s, 9H, C(CH3)3], 4.32 (s, 2H,
ter 7 h of heating, giving an orange oil after evaporation of toluene; CH2), 7.16–7.37 (m, 5H, PhH), 7.91 (dd, 2H, Py-3,5-H), 8.87 (dd, 2H,
crystallized from acetone; yield: 36.1 g (71%); yellowish powder; mp Py-2,6-H).
56–58°C.
13C NMR (100.6 MHz, CDCl3): d = 27.62 [C(CH3)3], 42.10 (CH2),
43.33 [C(CH3)3], 127.53 (Py-3,5-C), 128.02 (Ph-p-C), 129.44,
129.56 (Ph-o,o¢,m,m¢-C), 135.13 (Ph-i-C), 140.76 (Py-2,6-C), 167.87
(Py-4-C), 175.87 (CO).
C17H19NO calcd
C
80.60
81.09
H
7.56
7.60
N
5.52
5.61
(253.3)
found
IR (KBr pellet): n(C=O) = 1657 cm–1.
1H NMR (400 MHz, CDCl3, 295 K): d = 1.36 [s, 9H, C(CH3)], 5.86
(s, 1H, H(a)), 5.90 (d, 1H, H(5)), 6.47 (m, 1H, H(3)), 7.22 (m, 1H,
H(6)), 7.27 (m, 1H, H(2)), 7.12–7.33 (m, 5H, PhH).
Esterification of Secondary Alcohols 8a–d; General Procedure:
Method A: The acylpyridinium salts 7a–f (10.0 mmol) were weighed
into a 500-mL flask and CH2Cl2 (300 mL) was added. In some cases
a suspension resulted, but immediately became clear after addition of
the alcohol (10.0 mmol). The mixture was stirred for 16 h at r.t. and
the solvent was removed using a rotary evaporator. The residue was
dissolved in Et2O (20 mL) and the insoluble oily components were
separated with a separating funnel. The solvent was evaporated and
the crude product was distilled in vacuo.
13C NMR (100.6 MHz, CDCl3, 295 K): d = 28.17 [C(CH3)3], 39.69
[C(CH3)3], 109.65 (C(5)), 115.50 (C(a)), 116.45 (C(3)), 124.62
(C(6)), 125.76 (Ph-p-C), 126.60 (C(2)), 127.24, 128.35 (Ph-
o,o¢,m,m¢-C), 129.22 (C(4)), 138.0 (Ph-i-C), 173.42 (CO).
1-Acyl-4-alkylpyridinium Salts 7a–f; General Procedure:
The dihydropyridines 5a–c (20.0 mmol) were dissolved in Et2O (300
mL) and an 1.1 fold excess of acid 6a,b (22.0 mmol) was subsequent-
ly added. The resulting suspension was stirred for 20 min at r.t. and
the precipitated pyridinium salts 7a–f were filtered off, washed with
Et2O and dried in vacuo.
Method B: In a 250-mL flask the dihydropyridines 5a–c (10.0 mmol)
were dissolved in CH2Cl2 (200 mL). To the solution an equimolar
amount of acid (10.0 mmol; 6a: 1.4 mL, 6b: 0.9 mL) was added. After
10 min, the alcohol (10.0 mmol) was added and the mixture was al-
lowed to stand overnight (16 h). The workup procedure as described
above afforded the product.
Yields and analytical data of compounds 7a,c are in accordance with
those given in the literature.12

888
Papers
SYNTHESIS
(22) Freudenberg, K.; Rhino, F. Chem. Ber. 1924, 57, 1547.
Regeneration of 3a:
The insoluble residue (10 g) (from Method A or B), which consisted
of nearly pure 4-benzylpyridinium salt 10a,b, was dissolved in
CH2Cl2 (200 mL), treated with sat. aq NaHCO3 (200 mL), washed
with water (2 ´ 100 mL) and dried (Na2SO4). The solvent was re-
moved with a rotary evaporator and the residue was distilled in vacuo.
Average yield: 85%.
(23) Freudenberg, K.; Markert, L. Chem. Ber. 1925, 58, 1753.
(24) Kenyon, J.; Phillips, H.; Turley, H. G. J. Chem. Soc. 1925, 127,
399.
(25) Kenyon, J.; Lipscomb, A. G.; Phillips, H. J. Chem. Soc. 1931,
133(II), 2275.
(26) Fryzuk, M. D.; Bosnich, B. J. Am. Chem. Soc. 1978, 100, 5491.
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Neises, B.; Steglich, W. Angew. Chem. 1978, 90, 556; Angew.
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Höfle, G.; Steglich, W.; Vorbrüggen, H. Angew. Chem. 1978,
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(33) Wakselman, M.; Guibe-Jampel, E. Tetrahedron Lett. 1970,
1521.
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