A new oxa-Michael reaction and a gold-catalysed cyclisation en route to C-glycosides

Article abstract of DOI:10.1016/j.tetlet.2013.02.022

Two new syntheses of benzyl C-glycosides have been developed. The first one involves an unprecedented oxa-Michael cyclisation and the second one relies on an efficient gold-catalysed ring-closure.

Full text of DOI:10.1016/j.tetlet.2013.02.022

Tetrahedron Letters 54 (2013) 2089–2092
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A new oxa-Michael reaction and a gold-catalysed cyclisation en route to
C-glycosides
Sébastien Redon a,c, Michel Wierzbicki b, Joëlle Prunet a,c,
⇑
a Laboratoire de Synthèse Organique, CNRS UMR 7652, Ecole Polytechnique, DCSO, 91128 Palaiseau, France
b Institut de Recherches Servier, 11 rue des Moulineaux, 92150 Suresnes, France
c WestCHEM, School of Chemistry, University of Glasgow, Joseph Black Building, University Avenue, Glasgow G12 8QQ, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new syntheses of benzyl C-glycosides have been developed. The first one involves an unprecedented
oxa-Michael cyclisation and the second one relies on an efficient gold-catalysed ring-closure.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 4 December 2012
Revised 23 January 2013
Accepted 7 February 2013
Available online 16 February 2013
Keywords:
Benzyl C-glycosides
Oxa-Michael
Metathesis
Gold catalysis
The synthesis of C-glycosides has become an area of intense
study over the last three decades.1 Replacement of the anomeric
oxygen atom with a methylene group allows C-glycosides to have
higher chemical and enzymatic stability. There are few routes to
benzyl C-glycosides reported in the literature. The most common
involve hydroboration of olefinated carbohydrate derivatives and
Suzuki coupling with aryl bromides,2 additions of benzyllithium
to gluconolactones and reduction,3 additions of benzylzinc
reagents to glycals,4 additions of benzylmagnesium reagents to
glucosyl halides,5 ring-closing metathesis to form an endo-
glycal followed by hydroboration,6 iodocyclization7 and Ram-
berg–Bäcklund rearrangement followed by hydrogenation of the
resulting exo-glycal.8
We report herein two new methods for the synthesis of
C-glycosides where the aryl partner can be easily accessed from
a phenol. The first route is based on an unprecedented intramolec-
ular oxa-Michael cyclisation of an electron-deficient styryl deriva-
tive 2 to form the protected C-glycosides 1 directly (Scheme 1).
Only two examples of related intramolecular oxa-Michael reac-
tions are described in the literature.9 The substrates required for
the cyclisation reaction can be prepared by cross-metathesis
(CM) between electron-poor styrenes and the known olefin 3,10
easily obtained by the Wittig reaction between commercially avail-
Olefin 3
was first submitted to cross-metathesis11 with
p-nitrostyrene,12 using Grubbs second-generation catalyst13
(Table 1). The yields were low because of incomplete conversion
of olefin 3, 67% of which was recovered with 10 mol % of the cata-
lyst (entry 1) and 38% with 15 mol % of the catalyst (entry 2). The
best yield for the reaction (62%) was obtained with the Hoveyda–
Grubbs second-generation catalyst (HG2) in refluxing toluene
(entry 3).14 Only the E-isomer of 2a was formed. Performing the
reaction under microwave conditions did not improve the yield.15
The reaction was plagued by isomerisation of the alkene in 3 and
the D2,3 E-isomer of 3 was formed in up to 24% yield.
Various styrenes (EWG = SO2Ph, CHO, COMe, COOMe) were
then submitted to CM with olefin 3 under the previously optimised
conditions and the yields ranged between 50% and 62% (E isomers
only) (Table 2). In all cases, the
D
2,3 isomer was formed, but it was
easily separated from the desired metathesis products.
We then decided to test the Michael cyclisation on substrate 2a,
which possesses the strongest electron-withdrawing group
(EWG = NO2, Table 3). When this olefin was treated with strong
bases such as t-BuOK9b or KHMDS, no cyclisation occurred. Instead,
we observed elimination of a benzyloxy group, even at À78 °C, to
furnish the conjugated diene 4a. With a weaker base such as trieth-
ylamine, no reaction occurred after 12 h and with sodium hydride,
the starting material was recovered at 20 °C and only degradation
products were obtained at 50 °C. With DBU at ambient tempera-
ture and low concentration, the starting material was recovered
(entry 6). The use of 3 equiv of DBU at 0.05 M led to 1a in 66% yield
able 2,3,4,6-tetra-O-benzyl-D-glucopyranose and methylenetri-
phenylphosphorane (93% yield).
⇑
Corresponding author. Tel.: +44 141 330 8774; fax: +44 141 330 4888.
E-mail address: joelle.prunet@glasgow.ac.uk (J. Prunet).
after 12 h (entry 7). Selectivity was in favour of the
a-isomer
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.02.022

2090
S. Redon et al. / Tetrahedron Letters 54 (2013) 2089–2092
Table 3
Oxa-Michael cyclisation of 2a
EWG
O
oxa-Michael
BnO
BnO
EWG
EWG
OBn
OBn
NO2
OBn
NO2
NO2
OBn
OBn
1
OH
EWG = NO2, SO2Ph
BnO
OBn
OBn
OBn
OBn
CHO, COOEt, COMe
OBn
2
OH
O
OH
cross-
Conditions
metathesis
OBn
BnO
2a
BnO
OBn BnO
OH
OBn
OBn
1a
4a
BnO
OBn
OBn
3
Scheme 1. Approach towards benzyl C-glycosides.
Entry
Conditions
Yield
1
2
3
4
5
6
7
8
9
t-BuOK (1.5 equiv), THF, À78 °C, 30 min
KHMDS (1 equiv), THF, À78 °C, 1 h
4a
4a
Table 1
Et3N (10 equiv), CH2Cl2, 20 °C, 12 h
NaH (2 equiv), THF, 20 °C, 12 h
No reaction
No reaction
Degradation
No reaction
66%a
CM between olefin 3 and p-nitrostyrene
NaH (2 equiv), THF, 50 °C, 1 h
DBU (0.2 equiv), CH2Cl2 (0.05 M), 20 °C, 10 h
DBU (3 equiv), CH2Cl2 (0.05 M), 20 °C, 12 h
DBU (0.8 equiv), CH2Cl2 (0.2 M), 20 °C, 24 h
K2CO3 (1 equiv), MeOH (0.1 M), 20 °C, 48 h
NO2
78%a
64%a
OBn
OH
OBn
NO2
Conditions, 24 h
a
a
/b = 75:25.
OH
BnO
OBn
BnO
2a
OBn
E only
OBn
3
Table 4
OBn
Oxa-Michael cyclisations of 2a–e
EWG
EWG
Entry
Conditions
Yield (%)
1
2
3
Grubbs 2 (10 mol %), toluene, reflux
Grubbs 2 (15 mol %), toluene, reflux
Hoveyda–Grubbs 2 (10 mol %), toluene, reflux
20
44
62
OBn
OBn
OH
Conditions
O
BnO
OBn
BnO
2a-e
OBn
OBn
OBn
1a-e
Table 2
CM between olefin 3 and various styrenes
EWG
Entry
EWG
Product
Yield
1
2
3
4
5
6
NO2
SO2Ph
CHO
COMe
COOMe
COOMe
1a
1b
1c
1d
1e
1e
78%a
74%b
OBn
OH
OBn
No reactionc
Traces of 1dc
No reactionc
20%d,e
OH
EWG
10 mol% HG 2
toluene, reflux, 24 h
BnO
OBn
BnO
2a-e
OBn
OBn
OBn
a
b
c
DBU (0.8 equiv), CH2Cl2 (0.2 M), 20 °C, 24 h,
DBU (0.8 equiv), CH2Cl2 (0.2 M), 20 °C, 24 h,
DBU (3 equiv), CH2Cl2 (0.2 M), 20 °C, 5 d.
Sn(OTf)2 or Zn(OTf)2 (3 equiv), DBU (2 equiv), THF (0.2 M), 50 °C, 12 h.
Slightly impure product.
a
a
/b = 75:25.
/b = 70:30.
3
E only
d
e
Entry
EWG
Product
Yield (%)
1
2
3
4
5
NO2
SO2Ph
CHO
COMe
COOMe
2a
2b
2c
2d
2e
62
60
50
51
55
stereochemistry of the minor diastereomer was determined by
examining the coupling constants of the proton at the newly
formed stereogenic centre (J2,3 = 9.2 Hz, trans relationship).16 Sub-
mitting a mixture of diasteromers of 1a enriched in the b-isomer
(a/b = 25:75) to 28 equiv of DBU for 24 h (0.2 M concentration)
did not change the isomeric ratio, implying that the conjugate
(a/b = 75:25). The two diastereomers could be separated by
addition was under kinetic control.
column chromatography, yielding 50% of the -isomer and 16%
a
Various substrates (EWG = SO2Ph, CHO, COMe, COOMe) were
submitted to the optimised oxa-Michael cyclisation conditions (Ta-
ble 4).17 A similar result was found with EWG = SO2Ph (74% yield,
of the b-isomer. Finally, optimum conditions required a substoi-
chiometric amount of DBU (0.8 equiv) at a higher concentration
(0.2 M) and compound 1a was obtained in 78% yield after 24 h
(entry 8). Another weak base K2CO3 also furnished C-glycoside 1a
in good yield with the same diastereomeric ratio (entry 9). The
a/b = 70:30, entry 2). With weaker electron-withdrawing groups
such as CHO, COMe or COOMe, no reaction occurred, even in
refluxing dichloromethane (entries 3–5). Lewis acids were added

S. Redon et al. / Tetrahedron Letters 54 (2013) 2089–2092
2091
R
OBn
OBn
OBn
O
OBn
H2
O
AuCl3,THF
OH
O
R
Work-up Aa
82%
BnO
OBn
BnO
OBn
OBn
BnO
OBn
BnO
6f
OBn
OBn
OBn
5
1
OBn
OBn
5f
Z only
O-cyclisation
AuCl3,THF
Work-up Bb
H2, Pd/C
EtOAc
82%
OBn
89%
OBn
R
OBn
Sonogashira
coupling
OH
OH
OH
O
O
.
BF3 OEt2
R
BnO
OBn
Et3SiH
BnO
OBn
BnO
8f
BnO
1f
OBn
OBn
OBn
X
CH3CN
90%
OBn
6
7
X = OTf, I
OBn
OBn
β only
Scheme 2. Retrosynthetic approach involving alkyne substrates.
a Work-up A: filtration on silica gel, then concentration under vacuum
b Work-up B: concentration under vacuum, then filtration on silica gel
to the reaction with the ester substrate 2e to some effect, but the
yield never exceeded 20% (entry 6). This conjugate addition seems
to be limited to substrates with strong electron-withdrawing sub-
stituents such as a nitro or a sulfonyl group, but a large range of
functional groups on the phenyl ring should be easily accessible
from the nitro group.
Another approach was then envisaged, that could produce
benzyl C-glycosides with no electron-withdrawing substituents
on the phenyl ring (Scheme 2). These glycosides would be formed
by cyclisation of hydroxy alkynes 6, followed by reduction of the
resulting alkenes 5. Alkynes 6 would be prepared by Sonogashira
coupling of terminal alkyne 7 with the appropriate aryl triflates
or aryl iodides.
The formation of alkyne 7 proved to be more difficult than ex-
pected. Corey–Fuchs18 or Bestmann–Ohira19 reactions did not con-
vert the lactol at ambient temperature, or gave degradation
products in refluxing THF. Fortunately, Wittig reaction with (chlo-
romethyl)triphenylphosphonium iodide afforded the correspond-
ing chloro-alkene in 80% yield as a 55:45 E/Z mixture (Scheme
3),20 and subsequent elimination of HCl led to alkyne 7.21 Sono-
gashira coupling between 7 and iodobenzene furnished alkyne 6f
in 87% yield.
We investigated the cyclisation under different conditions:
acidic (PPTS), basic (MeONa, KH) or with PdCl2(CN)2, unsuccess-
fully. We then considered gold catalysis, which has proved to be
efficient for several heterocyclisation reactions.22 Contrary to what
was observed by Pale and co-workers for similar substrates,23 the
reaction proceeded smoothly under Au(III) catalysis to furnish
compound 5f8b in 82% yield as the Z-isomer, exclusively (Scheme
4).24 When the gold catalyst was not filtered from the reaction
Scheme 4. Cyclisations/reductions of 6f.
mixture before evaporation of the solvent, hemiketal 8f was ob-
tained as the major product.25 Olefin 5f was then hydrogenated
following the procedure reported by Belica and Franck8a to furnish
benzyl C-glycoside 1f in 89% yield. Reduction of hemiketal 8f with
Et3SiH/BF3ÁOEt2 was also efficient, giving 1f in 90% yield.26
In conclusion, we have developed two short syntheses of benzyl
C-glycosides featuring an unprecedented oxa-Michael cyclisation
and an efficient gold-catalysed ring-closure. The second approach
also constitutes a new synthesis of benzyl exo-glycals, which could
be a good alternative to the Ramberg–Bäcklund rearrangement.8a,b
Depending on the route, we can obtain either a- or b-C-glycosides.
The preparation of more complex benzyl C-glycosides using these
approaches is underway.
Acknowledgments
Financial support was provided by the CNRS, the Ecole Poly-
technique, the University of Glasgow, and the Institut de Recher-
ches Servier.
References and notes
1. (a) Levy, D. E.; Tang, C. The Chemistry of C-Glycosides; Elsevier Science:
Amsterdam, 1995; (b) Postema, M. H. D. C-Glycoside Synthesis; USA: CRC,
1995; (c) Beau, J.-M.; Gallagher, T. Top. Curr. Chem. 1997, 187, 1–54; (d) Nicotra,
F. Top. Curr. Chem. 1997, 187, 55–83; (e) Sinaÿ, P. Pure Appl. Chem. 1997, 69,
459–463; (f) Du, Y.; Lindhart, R. J.; Vlahov, I. R. Tetrahedron 1998, 54, 9913–
9959.
2. (a) Walker, J. R.; Alshafie, G.; Abou-Issa, H.; Curley, R. W. Bioorg. Med. Chem. Lett.
2002, 12, 2447–2450; (b) Johns, B. A.; Pan, Y. T.; Elbein, A. D.; Jonhson, C. R. J.
Am. Chem. Soc. 1997, 119, 4856–4865; (c) Johnson, C. R.; Johns, B. A. Synlett
1997, 1406–1408.
3. Stricher, H.; Reiner, M.; Schmidt, R. R. J. Carbohydr. Chem. 1997, 16, 277–298.
4. Pearce, A. J.; Ramaya, S.; Thorn, S. T.; Bloomberg, G. B.; Walter, D. S.; Gallagher,
T. J. Org. Chem. 1999, 64, 5453–5462.
5. Wong, M. F.; Weiss, K. L.; Curley, R. W. J. Carbohydr. Chem. 1996, 15, 763–768.
6. (a) Calimente, D.; Postema, M. H. D. J. Org. Chem. 1999, 64, 1770–1771; (b)
Postema, M. H. D.; Piper, J. L.; Betts, R. L. J. Org. Chem. 2005, 70, 829–836.
7. Khan, A. T.; Ahmed, W.; Schmidt, R. R. Carbohydr. Res. 1996, 280, 277–286.
8. (a) Belica, P. S.; Franck, R. W. Tetrahedron Lett. 1998, 39, 8225–8228; For other
syntheses of substituted exo-glycals that could be hydrogenated, see: (b)
Griffin, F. K.; Murphy, P. V.; Paterson, D. E.; Taylor, R. J. K. Tetrahedron Lett. 1998,
39, 8179–8182; (c) Bourdon, B.; Corbet, M.; Fontaine, P.; Goekjian, P. G.;
Gueyrard, D. Tetrahedron Lett. 2008, 49, 747–749.
OBn
OBn
OH
1) BuLi,THF, HMPA
ClCH2PPh3I
O
OH
2) BuLi, THF
45% (2 steps)
BnO
OBn
BnO
OBn
OBn
OBn
7
PdCl2(PPh3)2
PhI, CuI, Et3N
DMF, 5 h, 50 °C
87%
OBn
OH
9. With a phenol as the nucleophile: (a) Masuoka, Y.; Asako, T.; Goto, G.; Noguchi,
S. Chem. Pharm. Bull. 1986, 34, 130–139; Transannular cyclization: (b) Li, M.;
O’Doherty, G. A. Org. Lett. 2006, 8, 6087–6090.
10. Pougny, J.-R.; Nassr, M. A. M.; Sinaÿ, P. J. Chem. Soc., Chem. Commun. 1981, 375–
376.
BnO
6f
OBn
OBn
11. (a) Prunet, J. Curr. Top. Med. Chem. 2005, 5, 1559–1577; (b) Prunet, J. Eur. J. Org.
Chem. 2011, 3634–3647.
Scheme 3. Preparation of alkyne 7 and Sonogashira coupling.

2092
S. Redon et al. / Tetrahedron Letters 54 (2013) 2089–2092
12. For examples of CM with p-nitrostyrene, see: (a) Hodgson, D. M.; Angrish, D.;
Labande, A. H. Chem. Commun. 2006, 627–628; (b) Robertson, J.; Green, S. P.;
Hall, M. J.; Tyrrell, A. J.; Unsworth, W. P. Org. Biomol. Chem. 2008, 6, 2628–2635.
13. Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1, 953–956.
14. Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H. J. Am. Chem. Soc. 2000,
122, 8168–8179.
21. Elimination only occurred from the E-olefin, which explains the modest overall
yield (45% for 2 steps).
22. Hashmi, A. S. K. Angew. Chem., Int. Ed. 2005, 44, 6990–6994.
23. Harkat, H.; Weibel, J.-M.; Pale, P. Tetrahedron Lett. 2007, 48, 1439–1442.
24. We did not perform the reaction with AuCl.
25. General procedures for gold-catalysed cyclisations: A sample of AuCl3 catalyst
(5 mol %) was added to a solution of the alkyne in THF (2 mL) under argon. The
mixture was stirred at ambient temperature for 2 h. Work-up A: the solution
was filtered on silica gel and concentrated under vacuum to furnish the desired
compound. Work-up B: the solution was concentrated under vacuum then
filtered on silica gel (30% EtOAc/petroleum ether) to furnish the desired
compound. The general procedure with work-up A was used for the conversion
of alkyne 6f (100 mg, 0.16 mmol) to afford compound 5f as a yellow oil (82 mg,
82%). 1H NMR (CDCl3, 400 MHz) d 7.71 (d, J = 7.6 Hz, 2H), 7.40–7.10 (m, 23H),
5.75 (s, 1H), 4.83–4.57 (m, 8H), 4.20–4.10 (m, 1H), 4.05 (m, 1H), 3.89–3.80 (m,
4H); 13C NMR (CDCl3, 100 MHz) d 149.1, 138.4, 138.3, 138.0, 135.3, 128.9,
128.6, 128.56, 128.53, 128.50, 128.46, 128.3, 128.06, 128.01, 127.97, 127.92,
127.84, 127.82, 127.7, 126.5, 109.6, 84.7, 79.4, 78.1, 77.0, 74.1, 73.6, 73.5, 71.8,
69.7, 69.4; IR (thin film, cmÀ1) 3450, 2922, 2867, 1723, 1602, 1496, 1453, 1362,
1264, 1086, 1070, 1026, 1051; HRMS (EI) m/z calcd for C41H40O5 612.2876;
found: 612.2859. The general procedure with work-up B was used for the
conversion of alkyne 6f (200 mg, 0.33 mmol) to afford compound 8f as a yellow
oil (165 mg, 82%). 1H NMR (CDCl3, 400 MHz) d 7.45–7.10 (m, 25H), 4.90 (dd,
J = 12, 10.8 Hz, 2H), 4.80–4.74 (m, 2H), 4.63 (d, J = 11.6 Hz, 2H), 4.55 (d,
J = 11.6 Hz, 2H), 4.40 (d, J = 12.0 Hz, 1H), 4.14 (d, J = 9.2 Hz, 1H), 3.85–3.80 (m,
3H), 3.54 (t, J = 9.6 Hz, 1H), 3.32 (dd, J = 14.0, 9.6 Hz, 2H), 3.11 (dd, J = 14.0 Hz,
1H); 13C NMR (CDCl3, 100 MHz) d 131.2, 128.6, 128.51, 128.45, 128.38, 128.33,
128.0, 127.9, 127.84, 127.75, 127.69, 127.65, 127.59, 127.05, 127.01, 84.0, 81.4,
78.5, 75.7, 75.4, 75.0, 73.4, 71.4, 68.9, 65.3, 43.8; IR (thin film, cmÀ1) 3445,
3020, 2865, 1720, 1485, 1430, 1362, 1137, 1103, 1094, 1034; HRMS (EI) m/z
calcd for C41H42O6 630.2981; found: 630.2963.
15. For
a review on microwave-assisted olefin metathesis, see: Coquerel, Y.;
Rodriguez, J. Eur. J. Org. Chem. 2008, 1125–1132.
16. Pasetto, P.; Chen, X.; Drain, C. M.; Franck, R. W. Chem. Commun. 2001, 81–82.
17. Procedure for the oxa-Michael addition: To a solution of 1,3,4,5-tetra-O-benzyl-
7-(4-nitrophenyl)-hept-6-en-2-ol (2a) (80 mg, 0.12 mmol) in CH2Cl2 (0.4 mL)
was added 10 lL of DBU (0.06 mmol, 0.5 equiv). The solution was stirred for
24 h under a nitrogen atmosphere, and then was concentrated under vacuum
and chromatographed (SiO2, 30% EtOAc/petroleum ether) to furnish the
desired product 1a as a yellow oil (63 mg, 78%) as a 75:25 mixture of
isomers, which could be partially separated for the purpose of characterisation.
-isomer: 1H NMR (CDCl3, 400 MHz) d 8.05 (d, J = 8.8 Hz, 2H), 7.37–7.27 (m,
a/b
a
20H), 7.16–7.20 (m, 2H), 5.02 (d, J = 10.8 Hz, 1H), 4.93–4.86 (m, 3H), 4.64–4.64
(m, 3H), 4.26 (d, J = 11.4 Hz, 1H), 4.22–4.17 (m, 1H), 3.89–3.79 (m, 3H), 3.67–
3.58 (m, 3H), 3.10–3.07 (m, 2H); 13C NMR (CDCl3, 100 MHz) d 147.0, 138.6,
138.2, 138.0, 137.9, 130.0, 128.7, 128.6, 128.5, 128.1, 128.02, 127.96, 123.7,
82.2, 80.2, 78.1, 75.6, 75.4, 75.3, 73.8, 73.6, 71.9, 69.2, 31.2; HRMS (EI) m/z calcd
for
C
41H41NO7 659.2883; found: 659.2877. b-Isomer: 1H NMR (CDCl3,
400 MHz) d 8.11 (d, J = 8.8 Hz, 2H), 7.44–7.30 (m, 20H), 7.26–7.22 (m, 2H),
4.96–4.67 (m, 4H), 4.65–4.48 (m, 4H), 3.78 (t, J = 8.8 Hz, 1H), 3.67–3.62 (m, 4H),
3.50–3.45 (dt, J = 2.0, 9.2 Hz, 1H), 3.64–3.32 (m, 1H), 3.18 (dd, J = 2.0, 14.0 Hz,
1H), 2.84 (dd, J = 8.8, 14.0 Hz, 1H); 13C NMR (CDCl3, 100 MHz) d 146.8, 138.1,
130.5, 128.7, 128.65, 128.61, 128.54, 128.2, 128.1, 128.0, 127.9, 127.8, 123.4,
87.4, 81.6, 79.4, 79.0, 78.6, 77.5, 76.8, 75.8, 75.3, 75.2, 73.5, 69.0, 37.8; HRMS
(EI) m/z calcd for C41H41NO7 659.2883; found: 659.2877.
18. Corey, E. J.; Fuchs, P. L. Tetrahedron Lett. 1972, 13, 3769–3772.
19. (a) Ohira, S. Synth. Commun. 1989, 19, 561–564; (b) Müller, S.; Liepold, B.; Roth,
G.; Bestmann, J. H. Synlett 1996, 521–522.
26. Wellner, E.; Gustafsson, T.; Bäcklund, J.; Holmdhal, R.; Kihlberg, J.
ChemBioChem 2000, 1, 272–280.
20. Mella, M.; Panza, L.; Ronchetti, F.; Toma, L. Tetrahedron 1988, 44, 1673–1678.