Annulation of pyrrole: Application to the synthesis of indolizidine alkaloids

Article abstract of DOI:10.1016/j.tet.2005.06.026

The nucleophilicity of pyrrole has been exploited to rapidly assemble the bicyclic skeleton of the indolizidine alkaloids. The key sequence is the annulation of a second ring onto pyrrole from a γ-lactone and has been exploited in the synthesis of the natural products (±)-monomorine and (±)-indolizidine 209D.

Full text of DOI:10.1016/j.tet.2005.06.026

Tetrahedron 61 (2005) 8226–8230
Annulation of pyrrole: application to the synthesis of
indolizidine alkaloids
*
Ruth I. J. Amos, Brendon S. Gourlay, Peter P. Molesworth, Jason A. Smith and Owen R. Sprod
School of Chemistry, University of Tasmania, Private Bag 75, Hobart 7001, Australia
Received 3 March 2005; revised 31 May 2005; accepted 9 June 2005
Available online 29 June 2005
Abstract—The nucleophilicity of pyrrole has been exploited to rapidly assemble the bicyclic skeleton of the indolizidine alkaloids. The key
sequence is the annulation of a second ring onto pyrrole from a g-lactone and has been exploited in the synthesis of the natural products (G)-
monomorine and (G)-indolizidine 209D.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
synthesis of the appropriately substituted g-pyrrolic esters, a
rapid synthesis of the indolizidine skeleton of targeted
alkaloids could be developed (Scheme 1).
Indolizidine alkaloids such as those derived from
amphibians and ants have proved popular targets for total
synthesis for both conformation of structure and investi-
gation of the potent biological activity that many of these
possess.1 They have also been exploited to highlight new
synthetic methods.1 Many of the alkaloids isolated possess
alkyl substituents around the bicyclic core such as the
5-alkyl substituted derivative assigned as indolizidine
209D2 and the Pharaoh’s ant trail pheromone (C)-
monomorine,3 which is a 3,5-dialkyl indolizidine (Fig. 1).
Scheme 1. Reagents: (i) 160 8C; (ii) H3OC(90%).
Recently, the unusual SN2 type ring opening of g-butyro-
lactone (2) with the potassium salt of pyrrole (1), as reported
by Li and Snyder,7 has been exploited on a multi-gram scale
to give the g-pyrrolic acid 3 as a key intermediate in the
synthesis of the naturally occurring alkaloid rhazinal, which
posseses anti-mitotic properties.8 Therefore, we set about
investigating the extension of the lactone ring opening to
substituted g-pyrrolic acid derivatives, particularly those
bearing a substituent at the g-position of the lactone. This
could be exploited to introduce a substituent at C-5 of an
indolizidine simply by the choice of lactone. The reaction of
substituted lactones has not been reported and it would be
expected that the size of the substituent at the g-position
may inhibit the reaction if the process is truly an SN2 type
substitution. Subsequent conversion of the g-pyrrolic acids
to an ester, and Lewis acid mediated intramolecular
acylation onto the nucleophilic pyrrole would yield the
bicyclic structure. The availability and low cost of
numerous substituted lactones that could be used to target
indolizidine alkaloids warranted investigation into a rapid
synthesis of these natural products via g-pyrrolic esters.
Figure 1.
The synthesis of bicyclic alkaloids from pyrrole derivatives
has been reported to be an effective method for the synthesis
of indolizidine alkaloids.4 The pyrrole unit provides a
template not only to form a second ring but also to reveal the
saturated heterocycle by hydrogenation. One of the most
efficient methods for the formation of the indolizidine
skeleton was reported by Jefford5 and Taylor,6 who have
shown that the activation of the carboxylate group of a
g-pyrrolic ester with boron tribromide promoted cyclisation
onto the nucleophilic pyrrole ring. By developing a short
Keywords: Pyrrole; Indolizidines; Alkaloids; g-Lactones.
*
Corresponding author. Tel.: C61 3 6226 2182: fax: C61 3 6226 2858;
e-mail: jason.smith@utas.edu.au
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.06.026

R. I. J. Amos et al. / Tetrahedron 61 (2005) 8226–8230
8227
2. Results and discussion
the extra bulk at the g-position of the lactone hindered the
reaction and reduced the yield dramatically giving support
for the SN2 mechanism. To overcome this, the reaction was
repeated under the conditions of Li and Snyder7 to give the
intermediate acid, which was then converted directly to the
ester 9 in 48% over two steps. Clearly the substituted
lactones require more forcing conditions but the g-pyrrolic
esters can be formed in reasonable yields and this has been
carried out on a multi-gram scale. Cyclisation of the ester 9
gave the 5-substituted indolizidine 10 as expected in high
yield (Scheme 4).
Initial studies were carried out on the ring opening of
g-butyrolactone to form the unsubstituted indolizidine 6.
While the original reaction was carried out under solventless
conditions, we investigated the use of a solvent to facilitate
the formation of the desired ester directly from the crude
reaction mixture. We found that reaction of the potassium
salt of pyrrole with g-butyrolactone at 160 8C in DMF
affected the desired ring opening and addition of methyl
iodide to the cooled mixture alkylated the intermediate
carboxylate salt (Scheme 2).
Scheme 2. Reagents: (i) KH, DMF, 0 8C then g-butyrolactone, 160 8C, 4 h;
(ii) CH3I (excess) 18 8C (61%); (iii) BBr3 (1.1 equiv) CH2Cl2, 0 8C (91%).
It was interesting to note that no acyl pyrrole 7 was observed
on the treatment of the reaction mixture with methyl iodide
as might have been expected by trapping the, presumably
kinetically favoured, acyl nucleophilic ring opened product
(8) of the lactone. (Scheme 3) This indicates that the
equilibrium favours the starting materials and results in the
thermodynamically favoured g-pyrrolic ester as the only
observed product. This gave the methyl ester 5 in 61% yield
in one step, making it a viable method for the synthesis of
this type of compound. The g-pyrrolic ester was then
cyclised by treatment with boron tribromide5 to yield the
indolizidine 6 as indicated by desymmetrisation of the
pyrrole ring in the 1H NMR spectrum and a shift of
Scheme 4. Reagents: (i) KH, 0 8C; and then g-valerolactone, 160 8C, 4 h
(60%); (ii) K2CO3, DMF, CH3I (excess) 18 8C (80%); (iii) BBr3 (1.1 equiv)
CH2Cl2, 0 8C (90%); (iv) NaBH3CN, ZnI2, CH2Cl2, 45 8C (73%);
(v) butyryl chloride, AgOTf, CH2Cl2, 0 8C (20%); (vi) Ref. 9: Pd/C, H2
(55 psi), CH3OH, catalytic H2SO4.
To exploit this method for the synthesis of natural products,
we targeted the Pharaoh’s ant pheromone monomorine that
bears a methyl substituent at C-5. To achieve this synthesis,
the carbonyl group adjacent to the pyrrole was removed by
reduction with ZnI2/NaBH3CN10 to activate C-3 of the
indolizidine (11) to introduce a butyl substituent. Initially,
we investigated Vilsmeier–Haack formylation and Wittig
olefination to introduce the butyl chain required, however,
formylation was low yielding and not regioselective
occurring equally at C-2 and C-3. This was surprising as
we expected formylation to occur exclusively at C-3 based
upon a similar reaction for the synthesis of rhazinal,8
consequently the methyl group at C-5 must hinder attack at
this position. Surprisingly, the activation of an acid chloride,
butyryl chloride, by typical Lewis acids such as AlCl3 and
ZnCl2 proved unsuccessful. However, the use of silver
triflate promoted the reaction to give the desired acylated
product 12 in 20% yield (40% based upon starting material
consumed). The selective acylation at C-3 of the indolizi-
dine was indicated by the coupling constant of 4.2 Hz for the
two protons of the pyrrole moiety. The ketone 12 has
previously been hydrogenated stereoselectively by
Muchowski9 to yield monomorine. Thus, in five short
steps we have accomplished the formal synthesis of this 3,5-
dialkyl indolizidine (Scheme 5).
the carbonyl stretch in the IR spectrum to 1651 cmK1
.
Therefore, in two short steps we have achieved the
annulation of pyrrole to form the indolizidine skeleton in
56% overall yield. As a result, we then investigated the
application of this reaction to substituted lactones.
Scheme 3. Reagents: (i) DMF, 160 8C, 4 h; (ii) CH3I (excess) 18 8C.
The reaction of the potassium salt of pyrrole with (G)-g-
valerolactone, which bears a methyl substituent at C-5 of the
lactone, under the reaction conditions reported above
yielded the g-pyrrolic ester 99 in low yield. Not surprisingly,
We then investigated the formation of the 5-hexyl

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3. Conclusions
In conclusion, we have performed a rapid annulation of
lactones onto pyrrole to yield 5-substituted indolizidines via
g-pyrrolic esters in only a few sequential synthetic steps.
While the yields are not consistently high for the proposed
SN2 ring opening of the substituted lactones, the low cost
and availability of the starting substrates makes this a viable
entry into g-pyrrolic esters as the reactions can be readily
carried out on a multi-gram scale. These derivatives have
been manipulated to complete a formal synthesis of (G)-
monomorine and the total synthesis of (G)-indolizidine
209D.
Scheme 5. Reagents: (i) KH, 0 8C; and then g-decanolactone, 160 8C, 4 h;
(ii) K2CO3, DMF, CH3I (excess) 18 8C (30% over two steps); (iii) BBr3
(1.1 equiv) CH2Cl2, 0 8C (90%); (iv) Pd/C, H2 (40 psi) CH3CO2H (90%).
4. Experimental
substituted derivative 209D from the commercially avail-
able (G)-g-decanolactone. The yield for the formation of
the ester 13 in this instance was reduced further to 30% over
two steps. The cause is most likely attributed to the
increasing bulk of the C-4 substituent on the lactone
therefore suggesting a limit to the size of the substituent at
this position. Cyclisation of the ester 13 proceeded in high
yield to give the 5-hexyl indolizidine 14, which was then
hydrogenated with palladium on carbon,11 with high
diastereoselectively, to yield (G)-indolizidine 209D. The
assignment was confirmed by comparison of the 13C NMR
spectrum with that of both possible diastereomers.12
(Table 1). In particular, the resonances of the three carbon
atoms attached to nitrogen, (C3, C5 and C8a) resonating at
50.7, 63.9 and 65.4 ppm, respectively, are diagnostic for the
formation of the desired diastereomer. The chemical shifts
of these carbon atoms in the other possible isomer are 48.8,
55.1 and 55.5 ppm, respectively.12b Therefore, the racemic
alkaloid was formed in 24% yield over four steps from
commercial starting materials. The low yield of the lactone
ring opening is offset by low cost of the reagents and the fact
that the reaction can be carried out on a significant scale to
enable the total synthesis of the targeted alkaloids in a few
short steps. One downfall is that enantio-pure lactones are
extremely expensive therefore limiting the method to the
formation of racemic products at the present time. We are
currently engaged in overcoming this problem to develop an
asymmetric synthesis of g-pyrrolic esters.
4.1. General experimental
1H and 13C NMR spectra were recorded in CDCl3 on a
Varian Mercury Plus spectrometer operating at 300 and
75 MHz, respectively. Infrared spectroscopy was recorded
on a Perkin–Elmer, paragon 1000 FT-IR as neat films on
sodium chloride plates unless otherwise stated. High
resolution and Low resolution mass spectroscopy was
performed on a Kratos Concept ISQ mass instrument.
Melting points were carried out on a Yanagimoto micro
melting point apparatus and are uncorrected. Chemicals and
reagents were purchased from Aldrich and used as received
unless otherwise stated. Solvents were purified by standard
literature methods before use.13 Organic extracts were dried
with anhydrous magnesium sulfate unless otherwise stated.
Column chromatography was carried out using Merck Silica
gel (40–63 mm).
4.2. General procedure for the N-alkylation of pyrrole
Method A. Pyrrole (0.200 g, 2.99 mmol) was added
dropwise to a suspension of potassium hydride (0.120 g,
2.99 mmol) in anhydrous DMF (2 mL) under an atmosphere
of nitrogen at 0 8C. The solution was stirred at room
temperature for 10 min, g-butyrolactone (0.256 g,
2.99 mmol) added and the mixture heated on an oil bath at
160 8C for 4 h. The solution was cooled to room
temperature, methyl iodide (0.053 mL, excess) added and
the mixture stirred overnight at room temperature. Water
(10 mL) was added and the product extracted with 1:1 ethyl
acetate/hexanes (3!10 mL). The organic extract was dried
and evaporated to give the crude product, which was
purified by flash chromatography on silica (eluent: ethyl
acetate/hexanes, 1:9) to give 5 as a colourless oil in 61%
yield. nmax(neat)/cmK1 1737; 1H NMR spectrum (300 MHz,
CDCl3): 2.05–2.15 (m, 2H), 2.30 (t, JZ6.9 Hz, 2H), 3.70 (s,
3H), 3.96 (t, JZ6.9 Hz, 2H), 6.17 (apparent t, 2H), 6.66
(apparent t, 2H); 13C NMR spectrum (75 MHz, CDCl3):
27.0, 31.0, 48.7, 51.9, 108.5, 120.7, 173.5.
Table 1. 13C NMR comparison of synthetic (G)-indolizidine 209Da
Smith et al.
Polniaszek et al.12b
65.4
63.9
50.7
33.8
31.8
30.0
29.8
29.7
29.5
25.9
24.3
22.6
20.2
14.1
65.1
63.9
51.6
34.7
31.9
31.1
30.9
30.6
29.8
25.9
24.8
22.7
20.5
14.2
Method B. Pyrrole (3.30 g, 49 mmol) was added dropwise to
solid potassium hydride (1.96 g, 49 mmol) under a nitrogen
atmosphere and the mixture stirred at room temperature for
1 h. g-Valerolactone (4.90 g, 62 mmol) was added and the
mixture heated on an oil bath at 160 8C for 4 h. NaHCO3
(5%) solution (50 mL) was added carefully and extracted
a 13C NMR d (75 MHz, CDCl3).

R. I. J. Amos et al. / Tetrahedron 61 (2005) 8226–8230
8229
with dichloromethane (3!20 mL). The aqueous layer was
retained, acidified with 2 M HCl and extracted with
dichloromethane (3!20 mL). The organic extract was
dried and evaporated to give the crude acid that was
dissolved in DMF (20 mL), potassium carbonate (excess)
and methyl iodide (4 equiv) added and stirred overnight at
room temperature. The reaction mixture was worked up the
same as for method A to yield the methyl ester as a
CDCl3): 19.9, 22.3, 23.6, 32.0, 50.4, 103.7, 107.4, 116.7,
129.5.
4.2.4. 5-Methyl-3-butyryl-5,6,7,8-dehydroindolizidine
(12). Butyryl chloride (0.019 mL, 0.17 mmol) was added
dropwise to a solution of indolizidine 11 (16 mg,
0.12 mmol) and silver triflate (52 mg, 0.20 mmol) in
dichloromethane (1 mL) under an atmosphere of nitrogen
and the mixture stirred at room temperature overnight. The
reaction was quenched with water (10 mL) and extracted
with dichloromethane (3!10 mL). The organic extract was
dried and evaporated to give the crude product, which was
purified by flash chromatography (eluent: ethyl acetate/
hexanes, 1:9) to give unreacted starting material (50%) and
12 as a colourless oil in 20% yield. [Found M%C, 205.1471
1
colourless oil 99 in 48% yield. nmax(neat) 1733 cmK1; H
NMR spectrum (300 MHz, CDCl3): 1.45 (d, JZ6.9 Hz,
3H), 1.94–2.08 (m, 2H), 2.10–2.18 (m, 2H), 3.64 (s, 3H),
4.10 (m, 1H), 6.14 (apparent t, 2H), 6.67 (apparent t, 2H);
13C NMR spectrum (75 MHz, CDCl3): 22.2, 30.5, 33.2,
51.6, 54.5, 107.9, 118.4, 173.5.
1
4.2.1. 8-Oxo-5,6,7,8-dehydroindolizidine (6). Boron tri-
bromide (0.718 mL, 7.60 mmol) was added dropwise to a
solution of 5 (1.16 g, 6.91 mmol) in dichloromethane
(20 mL) under an atmosphere of nitrogen at 0 8C. The
mixture was stirred at this temperature for 10 min and the
reaction quenched by the careful addition of water (10 mL)
followed by 2 M Na2CO3 (10 mL). The product was
extracted with dichloromethane (2!20 mL), the organic
extract dried and evaporated to give the crude product,
which was purified by passing through a plug of silica gel
(eluent: ethyl acetate/hexanes, 1:1) to give the indolizidine 6
in 91% yield as a colourless oil that solidified on storage in
C13H19NO requires 205.1467]; nmax(neat)/cmK1 1643; H
NMR spectrum (300 MHz, CDCl3): 0.96 (t, JZ7.5 Hz, 3H),
1.33 (d, JZ6.6 Hz, 3H), 1.60–2.03 (m, 6H), 2.61–2.80 (m,
1H), 2.70 (t, JZ7.5 Hz, 2H), 2.86–2.97 (m, 1H), 5.26–5.32
(m, 1H), 5.86 (d, JZ4.2 Hz, 1H), 6.98 (d, JZ4.2 Hz, 1H);
13C NMR spectrum (75 MHz, CDCl3): 14.0, 15.0, 19.3,
22.0, 23.8, 29.3, 41.1, 49.6, 106.5, 120.5, 189.7, C3 and C8a
not observed.
4.2.5. Methyl 4-(pyrrol-1-yl) decanoate (13). The
g-pyrrolic ester 13 was isolated as a colourless oil in 30%
overall yield from pyrrole by Method B: [Found M%C
,
1
the refrigerator. nmax(neat) 1651 cmK1; H NMR spectrum
251.1887 C15H25NO2 requires 251.1885]; nmax(neat)/cmK1
1740; 1H NMR spectrum (300 MHz, CDCl3) 0.85 (m, 3H),
1.14–1.36 (m, 8H), 1.70–1.78 (m, 2H), 1.89–2.14 (m, 4H),
3.64 (s, 3H), 3.79–3.88 (m, 1H), 6.14 (apparent t, 2H), 6.62
(apparent t, 2H); 13C NMR spectrum (75 MHz, CDCl3):
14.0, 22.6, 26.2, 29.0, 30.5, 31.6, 31.7, 36.7, 51.6, 59.6,
107.8, 118.8, 173.7; m/z (EI) 251(50%), 220(25), 164(100),
134(15), 106(35), 94(70), 81(50), 67(40).
(300 MHz, CDCl3): 2.20–2.29 (m, 2H), 2.56 (t, JZ6.9 Hz,
2H), 4.09 (t, JZ6.0 Hz, 2H), 6.22 (dd, JZ3.9, 2.4 Hz, 2H),
6.83–6.84 (m, 1H), 6.98 (dd, JZ3.9, 1.2 Hz, 1H); 13C NMR
spectrum (75 MHz, CDCl3): 23.4, 36.1, 45.0, 110.2, 113.8,
125.9, 130.2, 187.2.
4.2.2. 5-Methyl-8-oxo-5,6,7,8-dehydroindolizidine (10).
The g-pyrrolic ester 9 was reacted under the same
conditions as for 6 to give the product 10, which was
recrystallised from ether/hexanes (10:3) to yield the titled
product as a colourless solid in 90% yield: mpZ98–100 8C,
[Found M%C, 149.0837 C9H11NO requires 149.0840];
4.2.6. 5-Hexyl-8-oxo-5,6,7,8-dehydroindolizidine (14).
The product was formed by the same conditions as reported
for compound 6. Indolizidine 14 was isolated as a colourless
oil in 90% yield: [Found M%C, 219.1617 C14H21NO
requires 219.1622]; nmax(neat)/cmK1 1662; 1H NMR
spectrum (300 MHz, CDCl3): 1.51 (d, JZ6.3 Hz, 3H),
1.69–1.83 (m, 8H), 1.84–1.97 (m, 1H), 2.06–2.16 (m, 1H),
2.30–2.42 (m, 1H), 2.51 (ddd, JZ17.7, 9.3, 4.3 Hz, 1H),
2.67 (ddd, JZ17.7, 10.2, 4.3 Hz, 1H), 4.15 (m, 1H), 6.24
(dd, JZ3.9, 2.4 Hz, 1H), 6.91 (dd, JZ2.4, 1.5 Hz, 1H), 7.01
(dd, JZ3.9, 1.5 Hz, 1H); 13C NMR spectrum (75 MHz,
CDCl3): 14.0, 22.5, 26.0, 27.7, 29.1, 31.6, 33.3, 34.3, 54.8,
110.1, 114.3, 125.0, 130.2, 187.2 m/z (EI) 219(30%),
191(20), 148(65), 134(95), 106(100), 93(60), 67(30).
nmax(neat) 1644 cmK1 1H NMR spectrum (300 MHz,
;
CDCl3): 1.55 (d, JZ6.3 Hz, 3H), 1.97–2.09 (m, 1H),
2.24–2.34 (m, 1H), 2.46–2.70 (m, 2H), 4.20–4.31 (m, 1H),
6.24 (dd, JZ2.4, 4.2 Hz, 1H), 6.92–6.93 (m, 1H), 7.00 (dd,
JZ2.2, 4.2 Hz, 1H); 13C NMR spectrum (75 MHz, CDCl3:
20.6, 30.8, 34.5, 50.5, 110.4, 114.1, 124.1, 130.4, 187.2; m/z
(EI) 149(100%), 134(30), 106(50), 93(40), 80(25), 67(30).
4.2.3. 5-Methyl-5,6,7,8-dehydroindolizidine (11). Sodium
cyanoborohydride (0.10 g, 1.59 mmol) was added to a
solution of the indolizidine 10 (0.20 g, 1.34 mmol) and zinc
iodide (0.41 g, 1.28 mmol) in dichloromethane under an
atmosphere of nitrogen (20 mL) and the mixture refluxed
for 6 h. Water (10 mL) was added and the product extracted
with dichloromethane (2!10 mL), the organic extract dried
and evaporated to give the crude product. The indolizidine
11 was purified by flash chromatography on silica gel
(eluent: ethyl acetate/hexanes, 1:4) to give the product as a
colourless, unstable oil that was used immediately. 1H NMR
spectrum (300 MHz, CDCl3): 1.52 (d, JZ6.3 Hz, 3H),
1.60–1.80 (m, 2H), 1.91–2.10 (m, 1H), 2.04–2.13 (m, 1H),
2.69–2.80 (m, 2H), 4.04–4.17 (m, 1H), 5.92 (br s, 1H), 6.17
(s, 1H), 6.67 (br s, 1H); 13C NMR spectrum (75 MHz,
4.2.7. Indolizidine 209D. The indolizidine 14 (27 mg,
0.12 mmol) was dissolved in acetic acid (10 mL), Pd/carbon
(20 mg of 5%) added and hydrogenated at 40 psi on a Parr
shaker hydrogenator for 10 h. The catalyst was removed by
filtration through Celite and the solvent removed. The
residue was dissolved in 2 M Na2CO3 (10 mL) and
extracted with dichloromethane (3!10 mL). The organic
extracts were dried (Na2SO4), evaporated and the product
purified by flash chromatography (eluent: dichloromethane/
methanol/ammonia, 95:4.75:0.25) to give indolizidine 209D
as a colourless oil in 90% yield; 1H NMR spectrum
(300 MHz, CDCl3): 0.86 (t, JZ6.9 Hz, 3H), 1.16–1.46 (m,

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R. I. J. Amos et al. / Tetrahedron 61 (2005) 8226–8230
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13H), 1.50–1.96 (m, 8H), 2.02–2.20 (m, 2H), 3.37 (td, JZ
9.3, 2.1 Hz, 1H); 13C NMR spectrum (75 MHz, CDCl3): d
14.1, 20.2, 22.6, 24.3, 25.9, 29.5, 29.7, 29.8, 30.0, 31.8,
33.8, 50.7, 63.9, 65.4.
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Acknowledgements
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The authors thank the University of Tasmania for funding.
P.P.M. is thankful to The University of Warwick for a travel
award and O.R.S. to the School of Chemistry for a Summer
Scholarship.
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References and notes
11. The diastereoselective catalytic hydrogenation of a 5-propyl
substituted indolizidine derivative to yield indolizidine 167B
was recently reported: Corvo, M. C.; Pereira, M. M. A.
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1. Michael, J. P. Nat. Prod. Rep. 2004, 21, 625 and references
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2. The original structure proposed for indolizidine 209D has
recently been suggested to be incorrect: Daly, J. W.; Spande,
T. F. In Alkaloids: Chemical and Biological Perspectives;
Pelletier, S. W., Ed.; Wiley: New York, 1986; Vol. 4, pp 112–
124.Daly, J. W.; Garraffo, H. M.; Spande, T. F. In Pelletier, S.
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12. (a) Jefford, C. W.; Sienkiewicz, K.; Thornton, S. R. Helv.
Chim. Acta 1995, 78, 1511. (b) Polniaszek, R. P.; Belmont, S.
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