Preliminary studies on the incorporation of sugars into naphthoquinones: Synthesis of (1R,2S,3S,4R,4aS,11bS)-2-(benzyloxy)-1,2,3,4,4a,5-hexahydro-1,3,4- trihydroxy-11bH-benzo[b]carbazole-6,11-dione

Article abstract of DOI:10.1016/j.tetasy.2004.11.045

The first total synthesis of (1R,2S,3S,4R,4aS,11bS)-2-(benzyloxy)-1,2,3,4, 4a,5-hexahydro-1,3,4-trihydroxy-11bH-benzo[b]carbazole-6,11-dione from D-glucose is described. The key steps of this synthesis are the stereoselective Michael addition of 2-lithium-1,4-dimethoxynaphthalene to 3-O-benzyl-5,6-dideoxy-1,2-O- isopropylidene-6-nitro-α-D-xilohex-5-enefuranose followed by the enantioselective intramolecular Henry reaction of 3-O-benzyl-5,6-dideoxy-5-C-(1, 4-dimethoxynaphthalene-2-yl)-6-nitro-β-l-idofuranose to the key (1R,2S,3S,4R,5S,6S)-3-(benzyloxy)-5-(1,4-dimethoxynaphthalene-2-yl)-1,2, 4-trihydroxy-6-nitrocyclohexane.

Full text of DOI:10.1016/j.tetasy.2004.11.045

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 16 (2005) 11–14
Preliminary studies on the incorporation of sugars
into naphthoquinones: synthesis of
(1R,2S,3S,4R,4aS,11bS)-2-(benzyloxy)-1,2,3,4,4a,5-hexahydro-
1,3,4-trihydroxy-11bH-benzo[b]carbazole-6,11-dione
*
´ ´ ´ ´ ´
Jose M. Otero, Jose C. Barcia, Juan C. Estevez and Ramon J. Estevez
´ ´
Departamento de Quımica Organica, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain
Received 2 November 2004; accepted 19 November 2004
Available online 21 December 2004
Abstract—The first total synthesis of (1R,2S,3S,4R,4aS,11bS)-2-(benzyloxy)-1,2,3,4,4a,5-hexahydro-1,3,4-trihydroxy-11bH-
benzo[b]carbazole-6,11-dione from D-glucose is described. The key steps of this synthesis are the stereoselective Michael addition of
2-lithium-1,4-dimethoxynaphthalene to 3-O-benzyl-5,6-dideoxy-1,2-O-isopropylidene-6-nitro-a-D-xilohex-5-enefuranose followed
by the enantioselective intramolecular Henry reaction of 3-O-benzyl-5,6-dideoxy-5-C-(1,4-dimethoxynaphthalene-2-yl)-6-nitro-
b-L-idofuranose to the key (1R,2S,3S,4R,5S,6S)-3-(benzyloxy)-5-(1,4-dimethoxynaphthalene-2-yl)-1,2,4-trihydroxy-6-nitrocyclo-
hexane.
ꢀ 2004 Elsevier Ltd. All rights reserved.
Naphthoquinones are of perennial chemical interest due
to the widespread occurrence of the naphthoquinone
nucleus in natural products, from simple hydroxynaph-
thoquinones such as lawsone, the main component of a
natural dye, to complex structures such as the trimeric
availability and transformation into a wide variety of
functionalities.8 Their chemistry is largely dominated
by the electron withdrawing character of the nitro
group. Thus the nitroaldol condensation (the Henry
reaction) is a classical method for carbon–carbon form-
ation, which couples a carbonyl compound with a nitro-
alkane and can result in the formation of one or two
stereogenic centers.9 Its intramolecular modality has
proven to be a powerful method for the preparation of
nitrocycloalkanes. On the other hand, nitroalkenes are
potent dienophiles in the Diels–Alder reaction and they
give easily addition reactions with a great variety of
nucleophiles. Their chemistry has been the object of sev-
eral reviews.10 Persual of the literature indicates that
reactions of nitroalkenes with 1,4-naphthoquinones
has not yet been described.
hydroxynaphthoquinone conocurvone,
a
potential
anti-HIV agent.1 The chemistry of naphthoquinones is
mainly dependent on the substituents on the quinone
moiety. Thus 1,4-naphthoquinones bearing a phenyl
substituent attached to its C-2 position proved to be
convenient precursors for the synthesis of polycyclic
naphthoquinone derivatives that exhibit anti-neoplasic2
activity, such as indolonaphthoquinones,3 benzofuro-
naphthoquinones,4 and benzopyronaphthoquinones.5
The similar transformation of related, less abundant,
2-cyclohexyl-1,4-naphthoquinones6 into promising
1,2,3,4,5,6-hexahydroindolonaphthoquinones 97 has
not been studied, probably due to the lack of available
methods for the preparation of highly functionalized
cyclohexylnaphthoquinones.
Our continuous interest into indolonaphthoquinones11
together with our recent interest in nitrosugars12 led us
to developthe first total synthesis of a highly function-
alized indolonaphthoquinone (compound 9) by a route
including two key steps: a Michael addition of naphtha-
lene 1 to nitrosugar 2 and an intramolecular Henry reac-
tion of nitrosugar derivative 4 (Scheme 1).
On the other hand, nitrocompounds are compounds of
great versatility in organic synthesis due to their easy
Reaction of bromonaphthalene 113 with t-BuLi in THF at
ꢀ78 ꢁC, followed by the addition of sugarnitroethylene
*
Corresponding author. Tel.: +34 981 563100; fax: +34 981 591014;
e-mail: qorjec@usc.es
0957-4166/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2004.11.045

12
J. M. Otero et al. / Tetrahedron: Asymmetry 16 (2005) 11–14
NO2
NO2
O
OMe
OMe
MeO
MeO
MeO
MeO
Br
a) tBuLi, THF, -78ºC, 15 min
O
O
O
O2N
O
O
O
BnO
BnO
O
b)
-78ºC, 4 h
1
3a
3b
(77 % yield)
O
BnO
2
dioxan, H2O, TFA (3:1:2)
50 ºC, 16 h
OBn
NO2
OBn
NH2
MeO
MeO
HO
OH
OH
HO
OH
OH
MeO
O
MeO
MeO
H2 (1 atm)
Raney-Ni
OH
2% aq. NaHCO3
OH
NO2
BnO
MeOH
rt, 3h
MeOH
rt, 24 h
4
5
6
(95% yield)
MeO
(87% yield, two steps)
OBn
OBn
OH
OBn
OH
HO
HO
HO
OH
OH
OH
OH
O
O
O
air
(10% yield)
CAN
THF
CH3CN-H2O
0º C, 1 h
(92% yield)
60º C, 24 h
OH
OH
N
H
N
H
NH2
O
7
8
9
Scheme 1.
214 to the resulting 2-lithionaphthalene of 1 gave a
4.7:1.0 epimeric mixture 3a and 3b15 of the expected ad-
duct, as a result of the attack of the organolithium deri-
vative of 1 on both faces of the nitroethylene moiety of
2. This moderate stereoselectivity should be due to the
preferential attack on the si-face of the double bond.
This was confirmed when the structure of 3a16 was
firmly established by X-ray diffraction (Fig. 1).
Formation of compound 5 may occur as indicated in
Scheme 2. The intramolecular Henry reaction in the
open form 10 of nitrosugar 4 probably gives a mixture
of nitrocyclohexanes 5 and 12 via the corresponding
chair transition states 11a and 11b, both having an equa-
torial nitro group. Compounds 5 and 12 could easily re-
vert to compound 10 by a retro-Henry reaction. At the
equilibrium, compound 5 should be favored with respect
to compound 12, because 11a is thermodynamically
more stable than 11b, where the carbonyl is axial. This
explains the highly remarkable stereoselectivity of the
cyclization.
Catalytic hydrogenation of naphthylnitrocyclohexane 5
allowed its efficient transformation into the correspond-
ing aminocyclohexane 6. Finally, treatment of this
compound with CAN gave aminocyclohexanenaphtho-
quinone 7, which on heating in THF provided a poor
yield of a dark purple compound tentatively identified
as hexahydroindolonaphthoquinone 9 from its spectro-
scopical data.15 Formation of this compound might
occur by conjugate addition of the amino groupto the
quinone moiety followed by oxidation of the resulting
naphthol 8. The unfavorable attack of the amino to
the quinone 7 would justify the low efficiency of the
intramolecular addition leading the pentacyclic nitrogen
ring of 8.
Figure 1. ORTEP diagram corresponding to the X-ray molecular
structure of compound 3a.
Removal of acetonide protecting group of 3a with TFA
followed by treatment of the resulting furanose 4 with
2% aqueous sodium bicarbonate allowed us to obtain
a 87% yield of naphthalenecyclohexane 5. The alterna-
tive cyclization product was not detected by TLC nor
by 1H NMR. The configuration of compound 5 was eas-
In conclusion, we have developed the first total synthesis
of a polyhydroxylated 2-nitrocyclohexane-1,4-naphtho-
quinones (compound 5) together with the first total
synthesis of a polyhydroxylated hexahydroindolonaph
thoquinone (compound 9), the later being a new class
of compound that should exhibit antibiotic properties
on the basis of their structural similarities to kinamycin
antibiotics.17
1
ily established from its H NMR spectrum, which in-
cludes at 5.15 ppm a pair of doublets (J1,6 = 10.3 Hz,
J5,6 = 10.7 Hz) corresponding to the highly deshielded
hydrogen at position C6 bearing the NO2. This signal
can be explained in terms of di-axial couplings between
H5 and H6 and between H1 and H6.
Work is now in progress to extend this new route for
nitrocyclohexanenaphthoquinones 5 in order to prepare

J. M. Otero et al. / Tetrahedron: Asymmetry 16 (2005) 11–14
13
NO2
MeO
O
OH
BnO
OH
MeO
4
OBn
NO2
OBn
OBn
HO
HO
HO
OH
OH
OH
OH
OH
BnO
BnO
HO
O2N
HO
O2N
MeO
MeO
MeO
MeO
MeO
O
H
O
R
R
OH
OH
H
O
H
NO2
NO2
H
H
MeO
12
11b
11a
5
10
Scheme 2.
Org. Chem. 1994, 59, 6075; (f) Tamayo, N.; Echavarren,
A. M.; Paredes, M. C. J. Org. Chem. 1991, 56, 6488; (g)
McKenzie, T. C.; Choi, W. B. Synth. Commun. 1989, 19,
1523; (h) Watanabe, M.; Date, M.; Furukawa, S. Chem.
Pharm. Bull. 1989, 37, 292.
a wide range of indolonaphthoquinones 9 for chemical
and biological studies.
Acknowledgements
6. Arai, S.; Tsuge, H.; Oku, M.; Miura, M.; Shioiri, T.
Tetrahedron 2002, 58, 1623; Coppa, F.; Fontana, F.;
Lazzarini, E.; Minisci, F. Chem. Lett. 1992, 7, 1299; Arai,
S.; Oku, M.; Miura, M.; Shioiri, T. Synlett 1998, 11, 1201;
Colonna, S.; Gaggero, N.; Manfredi, A.; Spadoni, M.;
Casella, L.; Carrea, G.; Pasta, P. Tetrahedron 1988, 44,
5169; Colonna, S.; Manfredi, A.; Annunziata, R.;
Spadoni, M. Tetrahedron 1987, 43, 2157; Pluim, H.;
Wynberg, H. J. Org. Chem. 1980, 45, 2498; Krohn, K.;
Ostermeyer, H. H.; Tolkiehn, K. Chem. Ber. 1979, 112,
2640; Fieldgate, D. M.; Woodcock, D. Pestic. Sci. 1973, 4,
193; Fieser, L. F.; Berliner, E.; Bondhus, F. J.; Chang, F.
C.; Dauben, W. G.; Ettlinger, M. G.; Fawaz, G.; Fileds,
M.; Heidelberger, C.; Heymann, H.; Vaughan, W. R.;
Wilson, A. G.; Wilson, E.; Wu, M.; Leffler, M. T.;
Hamlin, K. E.; Matson, E. J.; Moore, E. E.; Moore, M.
B.; Zaugg, H. E. J. Am. Chem. Soc. 1948, 70, 3186.
7. OÕSullivan, P. J.; Moreno, R.; Murphy, W. S. Tetrahedron
Lett. 1992, 33, 535.
Special thanks are due to Professor G. W. J. Fleet for
helpful discussions on this chemistry. We also thank
the Xunta de Galicia and the Spanish Ministry of Sci-
ence and Technology for financial support, and the lat-
´ ´
ter for grants to Jose M. Otero and to Jose C. Barcia.
References
1. (a) Thomson, R. H. Naturally Occurring Quinones, 4th ed.;
Chapman and Hall: London/New York, 1997; (b) Spy-
roudis, S. Molecules 2000, 5, 1291.
2. Cheng, C. C. In Progress in Medicinal Chemistry, Struc-
tural Aspects of Antineoplastic Agents—A New Approach;
Ellis, G. P., West, G. B., Eds.; Elsevier: Amsterdam, 1988;
Vol. 25.
3. (a) Luo, Y.-L.; Chou, T.-C.; Cheng, C. C. J. Heterocycl.
Chem. 1996, 33, 113; (b) Kno¨lker, H.-J.; Reddy, K. R.
Chem. Rev. 2002, 102, 4303–4427.
8. Noboru, O. In The Nitro Group in Organic Synthesis;
Feuer, H., Ed.; Organic Nitro Chemistry Series; Wiley-
VCH, 2001.
´
4. (a) Martinez, E.; Martinez, L.; Treus, M.; Estevez, J. C.;
Estevez, R. J.; Castedo, L. Tetrahedron 2000, 56, 6023; (b)
9. (a) Henry, L. C. R. Acad. Sci. Paris 1895, 120, 1265; (b)
Luzzio, F. A. Tetrahedron 2001, 57, 915; (c) Noboru, O..
In The Nitro Group in Organic Synthesis; Feuer, H., Ed.;
Organic Nitro Chemistry Series; Wiley-VCH, 2001, Chap-
ter 2.
10. (a) Varma, R. S.; Kabalka, G. W. Heterocycles 1986, 24,
2645; (b) Barret, A. G. M.; Graboski, G. G. Chem. Rev.
1986, 86, 751.
´
´
´
Martinez, A.; Estevez, J. C.; Estevez, R. J.; Castedo, L.
Tetrahedron Lett. 2000, 41, 2365; (c) Chang, H.-X.; Chou,
T.-C.; Savaraj, N.; Liu, L. F.; Yu, C.; Cheng, C. C. J.
Med. Chem. 1999, 42, 405; (d) Martinez, E.; Martinez, L.;
´
´
Estevez, J. C.; Estevez, R. J.; Castedo, L. Tetrahedron
Lett. 1998, 39, 2175; (e) Echavarren, A. M.; Tamayo, N.;
De Frutos, O.; Garcia, A. Tetrahedron 1997, 53, 16835; (f)
Yoshida, K.; Yamanaka, Y.; Euno, Y. Chem. Lett. 1994,
2051; (g) Cheng, C. C.; Dong, Q.; Liu, D. F.; Luo, Y. L.;
Liu, L. F.; Chen, A. Y.; Yu, C.; Savaraj, N.; Chou, T. C.
J. Med. Chem. 1993, 36, 4108; (h) Forrester, A. R.;
Ingram, A. S.; John, I. L.; Thomson, R. H. J. Chem. Soc.,
Perkin Trans. 1 1975, 1115.
´
11. (a) Fernandez, M.; Barcia, J. C.; Estevez, J. C.; Estevez, R.
J.; Castedo, L. Synlett 2004, 267; (b) Barcia, J. C.; Cruces,
´
´
´
J.; Estevez, J. C.; Estevez, R. J.; Castedo, L. Tetrahedron
Lett. 2002, 43, 5141; (c) Cruces, J.; Martinez, E.; Treus,
´
´
M.; Martinez, L. A.; Estevez, J. C.; Estevez, R. J.;
Castedo, L. Tetrahedron 2002, 58, 3015; (d) Estevez, J. C.;
´
´
´
Estevez, R. J.; Castedo, L. Tetrahedron Lett. 1993, 34,
6479.
5. (a) Qabaja, G.; Jones, G. B. J. Org. Chem. 2000, 65, 7187;
(b) Qabaja, G.; Perchellet, E. M.; Perchellet, J.-P.; Jones,
G. B. Tetrahedron Lett. 2000, 41, 3007; (c) Martinez, E.;
´
12. (a) Soengas, R. G.; Estevez, J. C.; Estevez, R. J. Tetra-
hedron: Asymmetry 2003, 14, 3955; (b) Soengas, R. G.;
´
´
´
Martinez, L.; Treus, M.; Estevez, J. C.; Estevez, R. J.;
Castedo, L. Tetrahedron 2000, 56, 6023; (d) Mohri, S.-I.;
Stefinovic, M.; Snieckus, V. Can. J. Chem. 1997, 62, 7072;
(e) Echavarren, A. M.; Tamayo, N.; Cardenas, D. J. J.
´
Estevez, J. C.; Estevez, R. J. Org. Lett. 2003, 5, 4459; (c)
Soengas, R. G.; Estevez, J. C.; Estevez, R. J. Org. Lett.
´
´
´
´
2003, 5, 1423; (d) Soengas, R. G.; Estevez, J. C.; Estevez,
R. J. Tetrahedron: Asymmetry 2003, 14, 1653.
´

14
J. M. Otero et al. / Tetrahedron: Asymmetry 16 (2005) 11–14
13. (a) Evans, P. A.; Brandt, T. A. J. Org. Chem. 1997, 62,
5321; (b) Bloomer, J.; Zheng, W. Synth. Commun. 1998,
28, 2087.
14. (a) Fleet, G. W. J.; Smith, P. W. Tetrahedron 1987, 43,
971; (b) Funabashi, M.; Kobayashi, K.; Yoshimura, J. J.
Org. Chem. 1979, 44, 1618.
92.7, 103.7, 123.8, 124.0, 126.7, 127.2, 127.8, 128.3, 128.9,
129.4, 129.7, 130.2, 130.1, 141.4, 150.8, 153.8. MS (FAB):
m/z (%) = 469 (5, M+); 133 (100); 91 (25, Bn+); 29 (100).
20
D
Compound 9: ½aŠ ¼ þ26:0 (c 0.10, acetone). 1H NMR
(CD3COCD3): d = 2.64 (s, 1H, OH); 2.80 (s, 1H, OH);
3.57 (dd, 1H, J4a,11b = 9.9 Hz, J1,11b = 7.3 Hz, H-11b);
15. All new compounds gave satisfactory spectroscopical and
analytical data. Selected physical and spectroscopic data
3.73
(ddd,
1H,
J4a,11b = 9.9 Hz,
J4,4a = 7.5 Hz,
J4a,5 = 3.1 Hz, H-4a); 4.36 (d, 1H, JH,OH = 3.4 Hz, OH);
4.63–4.69 (m, 1H, H-2); 4.87 (d, JH,H = 11.7 Hz, CH2Ph);
4.87–4.91 (m, 2H, H-1 + H-4); 5.11 (d, 1H, JH,H = 11.7 Hz,
CH2Ph); 5.17 (d, 1H, J2,3 = 1.3 Hz, H-3); 7.26–7.51 (m,
6H, 6 · Ar–H); 7.68 (td, 1H, JH,H = 7.5 Hz, JH,H = 1.3 Hz,
Ar–H); 7.94–8.03 (m, 2H, 2 · Ar–H); 11.58 (br s, 1H,
NH). 13C NMR (CD3COCD3): d = 42.0, 67.2, 70.6, 71.3,
76.3, 78.4, 85.8, 124.1, 127.2, 129.1, 129.7, 129.9, 130.7,
131.4, 131.9, 134.1, 137.1, 141.4, 157.6, 177.3, 182.9. MS
(FAB): m/z (%) = 407 (1, M+); 390 (5, M+ꢀOH).
20
are as follows. Compound 3a: ½aŠ ¼ ꢀ86:4 (c 1.5, CHCl3).
D
1H NMR (CDCl3): d = 1.29 (s, 3H, CH3); 1.47 (s, 3H,
CH3); 3.79 (s, 3H, OCH3); 3.83 (d, 1H, J3,4 = 2.8 Hz, H-3);
3.93 (s, 3H, OCH3); 4.50 (d, 1H, JH,H = 12.0 Hz, CH2Ph);
4.60 (dd, 1H, J4,5 = 8.0 Hz, J3,4 = 2.8 Hz, H-4); 4.63 (d,
1H, J1,2 = 3.7 Hz, H-2); 4.70–4.74 (m, 1H, H-5); 4.75 (d,
1H, JH,H = 12.0 Hz, CH2Ph); 4.80 (dd, 1H, J6,6 = 12.9 Hz,
J5,6 = 4.3 Hz, H-6); 5.01 (dd, 1H, J6,6 = 12.9 Hz,
J5,6 = 9.6 Hz, H-6); 5.92 (d, 1H, J1,2 = 3.7 Hz, H-1); 6.62
(s, 1H, H-8); 7.33–7.42 (m, 5H, 5 · Ar–H); 7.46 (dd, 1H,
J10,11 = 8.3 Hz, J11,12 = 7.1 Hz, H-11); 7.52 (dd, 1H,
J12,13 = 8.3 Hz, J11,12 = 7.1 Hz, H-12); 8.01 (d, 1H,
J12,13 = 8.3 Hz, H-13); 8.19 (d, 1H, J10,11 = 8.3 Hz, H-
10). 13C NMR (CDCl3): d = 26.1, 26.6, 37.7, 55.5, 62.5,
71.4, 75.7, 79.7, 81.2, 82.0, 102.0, 104.5, 111.7, 122.2,
122.3, 124.1, 125.6, 126.3, 126.7, 127.2, 128.2, 128.4, 128.7,
16. Crystallographic data (excluding structure factors) for the
structure reported in this paper have been deposited with
the Cambridge Crystallographic Data Centre as supple-
mentary publication no. CCDC 254541 3a. Copies of the
data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK fax:
(+44)1223-336-033; email: deposit@ccdc.cam.ac.uk. Crys-
tallographic data for 3a. C28H31NO8, M = 509.54,
136.9, 148.2, 152.2. MS (CI): m/z (%) = 509 (4, MH+); 420
20
D
(2); 91 (25, Bn+); 29 (100). Compound 5: ½aŠ ¼ þ15:7 (c
T = 293(2) K. Monoclinic, space group
a = 9.7351(10), b = 12.9049(13), c = 11.950(11) A,
a = 90ꢁ, b = 108.266(2)ꢁ, c = 90ꢁ, V = 1323.6(2) A , Dc
(Z = 4) = Ônot measuredÕ. F(000) = 540, absorption coef-
ficient = 0.094 cmꢀ1. Data were obtained on an Enraf-
Nonius CAD4-Mach3 diffractometer (graphite crystal
P 21 with
˚
1
2.00, CHCl3). H NMR (CD3COCD3): d = 3.61 (dd, 1H,
J3,4 = 8.8 Hz, J3,2 = 8.5 Hz, H-3); 3.75 (ddd, 1H,
J1,2 = 9.1 Hz, J2,3 = 9.1 Hz, J2,OH = 4.3 Hz, H-2); 3.91 (s,
3H, OCH3); 3.96–4.06 (m, 1H, H-5); 4.02 (s, 3H, OCH3);
4.09–4.15 (m, 1H, H-4); 4.21 (ddd, 1H, J1,6 = 10.3 Hz,
J1,2 = 9.1 Hz, J1,OH = 4.6 Hz, H-1); 4.45 (d, 1H,
J2,OH = 4.3 Hz, OH-2); 4.75 (d, 1H, J1,OH = 4.6 Hz, OH-
1); 4.95–5.01 (m, 3H, CH2–Ph + OH-4); 5.15 (dd,
1H,J5,6 = 10.7 Hz, J1,6 = 10.3 Hz, H-6); 7.21 (s, 1H, H-8);
7.24–7.35 (m, 3H, 3 · Ar–H); 7.40–7.58 (m, 4H, 4 · Ar–
H); 7.99–8.19 (m, 2H, 2 · Ar–H). 13C NMR (CD3
COCD3): d = 45.4, 57.1, 64.0, 75.0, 76.0, 76.3, 76.4, 86.9,
3
˚
˚
monochromator, k = 1.5418 A) using the x = 2h scan
method; absorption corrections were applied. Refinement,
with anisotropic displacement parameters applied to each
of the non-hydrogen atoms, was by full-matrix least
squares on F2 (SHELXL-93) using all data; w2R =
[(Rw(Fo ꢀFc ) /Rw (Fo2)2]1/2
.
17. Gould, S. J. Chem. Rev. 1997, 97, 2499–2509.
2
2 2