2,5-Dimethoxy-2,5-dihydrofuran chemistry: a new approach to 2(5H)-furanone derivatives

Article abstract of DOI:10.1016/j.tet.2008.02.096

2,5-Dimethoxy-2,5-dihydrofuran is a synthetic equivalent of 2(5H)-furanone or 2-trimethylsilyloxyfuran, useful C₄ synthons in the preparation of 5-substituted-2(5H)-furanone derivatives. The reaction conditions adopted allow to obtain different classes of complex and biologically interesting compounds, in only one step, with high yields.

Full text of DOI:10.1016/j.tet.2008.02.096

Available online at www.sciencedirect.com
Tetrahedron 64 (2008) 4183e4186
www.elsevier.com/locate/tet
2,5-Dimethoxy-2,5-dihydrofuran chemistry: a new
approach to 2(5H)-furanone derivatives
*
R. Garzelli, S. Samaritani, C. Malanga
Dipartimento di Chimica e Chimica Industriale, University of Pisa, via Risorgimento 35, 56126 Pisa, Italy
Received 17 December 2007; received in revised form 13 February 2008; accepted 28 February 2008
Available online 4 March 2008
Abstract
2,5-Dimethoxy-2,5-dihydrofuran is a synthetic equivalent of 2(5H)-furanone or 2-trimethylsilyloxyfuran, useful C4 synthons in the prepara-
tion of 5-substituted-2(5H)-furanone derivatives. The reaction conditions adopted allow to obtain different classes of complex and biologically
interesting compounds, in only one step, with high yields.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: 2,5-Dimethoxy-2,5-dihydrofuran; Mukaiyama-aldol reaction; 2(5H)-Furanones
1. Introduction
MeBr
Our recent studies on the reactivity of 2,5-dimethoxy-
2,5-dihydrofuran1 (1) as C4 building block in the syntheses
HBr
O
MeO
O
4
of many furofuran1a and 2-alkylfuran1bed moieties of biologi-
cal interest,2 encouraged us to investigate the reactivity of the
allylic diacetal functional group of 1, towards trimethylsilyl
halides.
Only few examples3 of this approach are described but no
general interpretation has ever been proposed.
O
3
Et3N
TMSOTf
TMSOMe
Br
TMSO
MeO
OMe
MeO
O
O
O
2
5
1
It was noted that the treatment of 1 with an equimolar amount
of trimethylsilyl bromide, in CH2Cl2 at room temperature,
afforded a yellow solution containing the more expensive
2(5H)-furanone (4) in quantitative yield (Scheme 1). Com-
pound 4 could be employed in the following reactions without
further purification. Under particular reaction conditions, the
one-pot reaction with an equimolar amount of trimethylsilyl
triflate (TMSOTf), in the presence of triethylamine, allowed
the conversion of 4 into 2-trimethylsilyloxyfuran 5 (Scheme 1).
Compound 5, which is a key intermediate in the Mukaiyama-
aldol synthetic sequence,4a could be reacted one-pot with
TMSBr
E
O
O
6
Scheme 1. Transformation of 2,5-dimethoxy-2,5-dihydrofuran.
suitable electrophiles to give the corresponding products in
high yields.
It is noteworthy that no Lewis acid catalyst was necessary.4
2. Results and discussion
A rationale of the described transformations is outlined in
Scheme 1.
* Corresponding author.
E-mail address: malanga@dcci.unipi.it (C. Malanga).
0040-4020/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.02.096

4184
R. Garzelli et al. / Tetrahedron 64 (2008) 4183e4186
ZnBr2 cat., CH2Cl2
reflux, 4h
Trimethylsilyl bromide reacts with 1 (Scheme 1) affording
Br
+
O
Br
0.004 mol
O
O
2-methoxy-5-bromo-2,5-dihydrofuran (2) as an elusive yet
detectable intermediate (GLCeMS), which decomposes into
2-methoxylfuran (3) (GLCeMS, NMR) in the same reaction
conditions.
O
O
O
(quantitative yield)
2.5 ml
7
Scheme 2. Furan ring opening.
The transformation of 1 into 3 represents a novel approach
towards the formation of key intermediates 4 and 5.
The rapid and quantitative interconversion of the methyl
ketene acetal (3) into a,b-unsaturated-g-lactone (4) could be
reasonably ascribed to the hydrogen bromide, produced in
the first step of the reaction. Compound 1, a synthetic equiva-
lent of 4, is an important intermediate in the preparation of
3-substituted furan-2-ones5 or 2(5H)-furanones 6, useful build-
ing blocks in the preparation of more complex biological
organic structures (Table 1, entry 5).6
It seems possible to rationalize this last step as a SN2 con-
certed reaction8d with a possible transition state showed in
Figure 1.
Br
ZnBr
Br
O
O
H
O
Figure 1. Proposed adduct for the tetrahydrofuran ring opening reaction.
3. Conclusions
In conclusion, a new one-pot electrophilic derivatization of
2(5H)-furanone, starting from the cheaper 2,5-dimethoxy-
2,5-dihydrofuran (1) has been reported. The products can be
obtained by a simple work-up procedure and promise to give
valuable intermediates in the preparation of natural and
biologically active products.
Table 1
Reactivity of 1 with electrophiles in the presence of TMSBr/Et3N, TMSOTf
Entry
1
Electrophile
Producta
Yieldb,c (%)
O
O
O
O
CH(OMe)3
6a
6b
95
OH
O
Further studies are in progress to apply these new synthetic
approaches to various other electrophiles.
O
2
3
90
85
O
O
4. Experimental section
O
O
O
O
O
6c
4.1. Materials and methods
O
O
Dichloromethane was refluxed and distilled over P2O5. Et3N
was refluxed and distilled over KOH. TMSBr and TMSOTf
(Aldrich) were used as-received. ZnBr2 (Fluka) was used as-
received. GLC analyses were performed on a PerkineElmer
8500 instrument [ZB1 capillary column (15 mꢀ0.25 mm),
film 0.25 mm] equipped with a flame ionization detector and
O
4
5
6
6d
6e
90
75
O
O
H
HO
O
O
O
O
O
O
O
O
N
O
1
a splitesplitless injector, with N2 as carrier gas. H and 13C
O
Br
6f
90
70
NMR (200 and 50 MHz, respectively) spectra were recorded
on a Varian Geminy 200 spectrometer; all NMR data were
obtained using CDCl3 solutions. Chemical shifts (d, ppm) are
referred to tetramethylsilane (TMS) (1H NMR) or CDCl3
(13C NMR) as internal standard. Mass spectra (m/z, I%) were
taken on a 5980 HewlettePackard GC instrument, equipped
with an HP-5MS column (30 mꢀ0.25 mm, film 0.25 mm) in-
terfaced with a HewlettePackard 5995A instrument, with He
as carrier gas. Indicated yields are reported on the isolated
chemically pure products.
O
Br
O
O
O
7
6g
O
O
a
Mixtures of diastereomers in a ca. 1:1 ratio (1H, 13C NMR, GLC, GLCe
MS).
b
Evaluated on the isolated products.
IR, NMR, mass spectroscopies and elemental analyses are in agreement
with the structures proposed for products 6.
c
In particular, entry 6 (Table 1), represents a new approach
to the synthesis of compound 6f,7 where N-bromosuccinimide
is used as electrophilic bromine source.
Compound 6f acts as a vinylogous organic acid halide8 as
shown by its peculiar reactivity. In fact, in the presence of
a catalytic amount of ZnBr2, 6f reacts with THF and causes
its ring opening giving 5-(4-bromobutoxy)-5H-furan-2-one
(7, Scheme 2), useful precursor in the synthesis of furopyrans
by well known free radical cyclization reaction.9
4.2. General procedure
A solution of trimethylsilyl bromide (1.32 ml, 0.013 mol)
in 20 ml of anhydrous CH2Cl2 was slowly added to a solution
of 1 (1.30 g, 0.010 mol) dissolved in 30 ml of anhydrous
CH2Cl2, at room temperature.
The resulting yellow solution of 4 was stirred at room tem-
perature for 24 h and then cooled to ꢁ80 ꢂC. After the addition
of freshly distilled triethylamine (1.80 ml, 0.013 mol, in 20 ml

R. Garzelli et al. / Tetrahedron 64 (2008) 4183e4186
4185
of CH2Cl2), the yellow-brown solution is stirred at room tem-
perature for 10 min and then cooled to ꢁ80 ꢂC. The following
slow addition of a TMSOTf solution (2.35 ml, 0.013 mol, in
20 ml of CH2Cl2) under stirring, afforded a pink solution
containing compound 5 as the sole product.
The final addition of the electrophile solution (0.010 mol in
25 ml of CH2Cl2) was carried out at ꢁ80 ꢂC, afforded 6 in
a few minutes.
The reaction mixture was allowed to warm up to room tem-
perature and washed with a 5% NaHCO3 solution and extracted
with diethyl ether. The organic layers were dried over Na2SO4
and the solvent removed in vacuo to give a dark oil that was
dissolved in benzene. The solution was filtered to remove the
triethylammonium triflate. The resulting yellow oil residue
was repeatedly extracted with hot pentane, and, after the elim-
ination of the solvent, gave the final product 6.
Elemental analysis: Found: C, 70.63; H, 6.02%; Anal.
Calcd for C12H12O3: C, 70.58; H, 5.92; O, 23.50%.
4.3.4. 5-(3-Oxocyclohexyl)furan-2(5H)-one (6c)
1H NMR: 7.75e7.25 (m, 1H, O]CeCH]CHe), 6.40e
6.00 (m, 1H, O]CeCH]CHe), 5.25e4.85 (m, 1H, CH]
CHeCHo), 3.50e3.00 (m, 4H, CH2eCOeCH2), 3.00e1.50
(m, 5H, COeCH2eCH2eCH2eCHo).
13C NMR: 209.3, 173.8, 154.1, 122.1, 86.3, (41.5, 41.4),
(41.2, 41.1), (27.3, 27.5), 25.5, 24.5.
FTIR liquid film (n, cmꢁ1): 3090, 3035, 2940, 2869, 1760,
1713, 1600, 1479, 1449, 1266, 1223, 1154, 1095, 1031, 967,
924, 897, 817, 758.
EIMS (m/z (%)): 180 (Mþ, 12.98), 152 (9.91), 137 (4.15), 123
(19.69), 111 (47.63), 97 (C6H9O, 31.25), 82 (MþꢁC6H10O,
32.69), 69 (25.99), 55 (100).
Elemental analysis: Found: C, 66.75; H, 6.65%; Anal.
Calcd for C10H12O3: C, 66.65; H, 6.71; O, 26.64%.
4.3. Spectroscopic characterization of the products
4.3.1. 2(5H)-Furanone (4)
4.3.5. 5-(((Z)-4-Hydroxybut-2-enyloxy)(methoxy)methyl)-
furan-2(5H)-one (6d)
1H NMR: 7.65 (dt, 1H, J1¼1.7 Hz, J2¼5.8 Hz, O]Ce
CH]CHe), 6.18 (dt, 1H, J3¼2.2 Hz, J2¼5.8 Hz, O]Ce
CH]CHe), 4.95 (dd, 2H, J1¼1.7 Hz, J3¼2.2 Hz,
CH]CHeCH2eO).
1H NMR: 7.77e7.17 (m, 1H, O]CeCH]CHe), 6.40e
6.00 (m, 1H, O]CeCH]CHe), 5.95e5.45 (m, 2H, Oe
CH2eCH]CHeCH2), 5.25e4.85 (m, 1H, CH]CHeCHo),
4.65e4.45 (m, 1H, pCHeCH(O)eOCH3), 4.35e4.15 (m, 4H,
OeCH2eCH]CHeCH2), 3.50, 3.45 (2s, 3H, pCHeCH(O)e
OCH3), 2.70 (br s, 1H, CH]CHeCH2eOH).
13C NMR: 173.6, 153.0, 121.2, 72.1.
4.3.2. 5-(Dimethoxymethyl)furan-2(5H)-one (6a)
1H NMR: 7.38 (dd, 1H, J1¼1.4 Hz, J2¼5.8 Hz, O]Ce
CH]CHe), 6.05 (dd, 1H, J3¼2.0 Hz, J2¼5.8 Hz, O]Ce
CH]CHe), 4.91 (ddd, 1H, J1¼1.4 Hz, J4¼1.2 Hz, J5¼
5.1 Hz, CH]CHeCHo), 4.23 (d, 1H, J5¼5.1 Hz, pCHe
CH(OMe)2), 3.36, 3.31 (2s, 6H, OCH3).
13C NMR: 172.3, 152.7, 122.4, 103.7, 82.4, 56.5, 56.2.
FTIR liquid film (n, cmꢁ1): 3103, 2940, 2838, 1786, 1757,
1446, 1356, 1335, 1303, 1264, 1192, 1160, 1103, 1066, 976,
917, 890, 824, 733.
EIMS (m/z (%)): 127 (MþꢁMeO, 2.27), 99 (Mþꢁ
MeOꢁCO, 9.44), 83 (MþꢁHC(OMe)2, 3.63), 75 (100), 71
(9.01), 55 (8.21), 47 (16.2).
13C NMR: 173.5, 152.9, (132.7, 132.6), 128.3, (126.7,
126.4), 122.6, (102.3, 102.0), (83.0, 82.5), (64.6, 64.4),
(58.2, 58.1).
FTIR liquid film (n, cmꢁ1): 3468, 3102, 2939, 2882, 2840,
1773, 1752, 1447, 1330, 1276, 1255, 1218, 1164, 1095, 1057,
1031, 913, 892, 822, 732.
EIMS (m/z (%)): 143 (MþꢁCH2CH]CHCH2OH, 44.51),
137 (25.39), 127 (17.89), 113 (7.94), 101 (6.96), 83
(C4H3Oþ2 , 1.68), 73 (C4H9Oþ, 100), 59 (9.70), 53 (7.43), 45
(7.80).
Elemental analysis: Found: C, 55.95; H, 6.65%; Anal.
Calcd for C10H14O5: C, 56.07; H, 6.59; O, 37.34%.
Elemental analysis: Found: C, 53.26; H, 6.31%; Anal.
Calcd for C7H10O4: C, 53.16; H, 6.37; O, 40.47%.
4.3.6. 5-(5-Methoxy-tetrahydrofuran-2-yl)furan-2(5H)-one
(6e)
4.3.3. 5-(1-Hydroxy-2-phenylethyl)furan-2(5H)-one (6b)
1H NMR: 7.60e7.64 (m, 1H, O]CeCH]CHe), 7.22e
7.27 (m, 5H, ePh), 6.12e6.17 (m, 1H, O]CeCH]CHe),
5.00e4.80 (m, 2H, CH]CHeCHo and pCHeCH(OH)e),
4.00e4.08 (m, 1H, pCHeCH(OH)e), 3.12e3.17 (m, 2H,
pCHeCH(OH)eCH2ePh).
1H NMR: 7.80e7.40 (m, 1H, O]CeCH]CHe), 6.40e
6.00 (m, 1H, O]CeCH]CHe), 5.20e4.80 (m, 1H,
CH]CHeCHo), 4.20e3.80 (m, 1H, CH]CHe(O)CHe
CH(O)eCH2eCH2), 3.33 (br s, 3H, CH2e(O)CHeOCH3),
2.20e1.80 (m, 4H, CHeCH2eCH2eCH(O)eOCH3).
13C NMR: 174.0, (153.2, 153.0, 152.8), (122.6, 122.3,
122.0), (105.6, 105.4), (85.5, 85.1), (84.3, 84.2), 55.7, (32.5,
32.4), (25.0, 24.9, 24.7).
FTIR liquid film (n, cmꢁ1): 3096, 2951, 2833, 1752, 1600,
1444, 1349, 1311, 1205, 1161, 1097, 1039, 952, 891, 820, 700.
EIMS (m/z (%)): 152 (MþꢁMeOH, 32.51), 123 (Mþꢁ
MeOHꢁCHO, 100), 109 (15.93), 96 (45.60), 82 (78.34), 68
(32.93), 55 (44.99).
13C NMR: 173.4, (154.7, 154.3), 136.6, 129.4, 129.3, 128.6,
128.4, 126.7, (122.5, 122.3), (85.0, 84.6), (72.4, 71.9), (39.8,
39.6).
FTIR liquid film (n, cmꢁ1): 3449, 3086, 3064, 2925, 1939,
1741, 1597, 1496, 1448, 1399, 1341, 1287, 1245, 1223, 1164,
1090, 1031, 945, 881, 812, 752.
EIMS (m/z (%)): 203 (MþꢁH, 0.32), 191 (0.61), 175 (1.19),
161 (1.41), 149 (3.57), 136 (6.45), 121 (PheCH2eCHOH,
10.31), 107 (8.70), 95 (21.70), 81 (80.37), 69 (100), 55 (5.49).
Elemental analysis: Found: C, 58.73; H, 6.67%; Anal.
Calcd for C9H12O4: C, 58.69; H, 6.57; O, 34.74%.

4186
R. Garzelli et al. / Tetrahedron 64 (2008) 4183e4186
4.3.7. 5-Bromofuran-2(5H)-one (6f)
EIMS (m/z (%)): 151 (MþꢁHBr, 0.90), 137
(MþꢁHBrꢁCH2, 21.14), 83 (C4H3Oþ2 , 100), 71 (13.88), 55
(32.43).
Elemental analysis: Found: C, 40.73; H, 4.81; Br, 34.05%;
Anal. Calcd for C8H11BrO3: C, 40.88; H, 4.72; Br, 33.99; O,
20.42%.
1H NMR: 7.65 (dd, 1H, J1¼1.5 Hz, J2¼5.5 Hz, O]Ce
CH]CHe), 7.20e6.80 (m, 1H, O]CeCH]CHe), 6.24
(dd, 1H, J2¼5.5 Hz, J3¼1.0 Hz CH]CHeCHo).
13C NMR: 168.3, 154.9, 120.7, 75.5.
FTIR liquid film (n, cmꢁ1): 3096, 3000, 1784, 1747, 1715,
1597, 1437, 1325, 1314, 1186, 1143, 1090, 1036, 967, 865,
828, 737.
EIMS (m/z (%)): 83 (Mþꢁ Br, 100), 55 (MþꢁBrꢁCO, 9.00).
Elemental analysis: Found: C, 29.38; H, 1.85; Br, 49.13%;
Anal. Calcd for C4H3BrO2: C, 29.48; H, 1.86; Br, 49.03; O,
19.63%.
References and notes
1. (a) Malanga, C.; Mannucci, S.; Lardicci, L. J. Chem. Res., Synop. 2001,
97e99; (b) Malanga, C.; Mannucci, S. Tetrahedron Lett. 2001, 42,
2023e2025; (c) Malanga, C.; Mannucci, S.; Lardicci, L. Tetrahedron
Lett. 1998, 39, 5615e5616; (d) Malanga, C.; Mannucci, S.; Lardicci, L.
Synth. Commun. 1996, 26, 2317e2327.
4.3.8. 5-(4,4-Dimethoxybutan-2-yl)furan-2(5H)-one (6g)
1H NMR: 7.70e7.30 (m, 1H, O]CeCH]CHe), 6.18 (dd,
1H, J1¼2.0 Hz, J2¼5.7 Hz, O]CeCH]CHe), 5.20e4.80
(m, 1H, O]CeCH]CHeCHo), 4.48 (dd, 1H, J3¼5 Hz,
J4¼4.5 Hz, CH2eCH(OCH3)2), 3.45, 3.31 (2s, 6H, CH2e
CH(OCH3)2), 2.70e2.30 (m, 2H, CHeCH2eCH(OCH3)2),
2.30e1.90 (m, 1H, (O)CHeCH(CH3)eCH2), 1.10, 1.00 (2s,
3H, (O)CHeCH(CH3)eCH2).
2. (a) Rao, S. Y. Chem. Rev. 1964, 64, 353e388; (b) Rao, S. Y. Chem. Rev.
1975, 75, 625e693; (c) Chiacchio, U.; Pipern, A.; Rescifina, A.; Romeo,
A.; Uccella, N. Tetrahedron 1988, 54, 5695e5708.
3. Feringa, B. L. Synth. Commun. 1985, 15, 87e89; (b) Feringa, B. L.;
Dannnberg, W. Synth. Commun. 1983, 13, 509e514; (c) Chan, T. H.;
Lee, S. D. Tetrahedron Lett. 1983, 24, 1225e1228; (d) Vankar, Y. D.;
Shah, K. Tetrahedron Lett. 1991, 32, 1081e1084; (e) Guindon, Y.;
Bernstein, M. A.; Anderson, P. C. Tetrahedron Lett. 1987, 28,
2225e2228; (f) Jung, M. E.; Andrus, M. A.; Ornstein, P. L. Tetrahedron
Lett. 1977, 48, 4175e4178; (g) Kricheldorf, H. R.; Morber, G.; Regel,
W. Synthesis 1981, 81, 383e384; (h) Gil, G. Tetrahedron Lett. 1984, 25,
3805e3808.
13C NMR: 174.2, 154.6, (122.3, 122.2), 102.7, (86.6, 86.0),
(52.9, 52.8), 34.7, (32.4, 31.0), (16.3, 15.2).
FTIR liquid film (n, cmꢁ1): 3090, 2939, 2833, 2733, 1753,
1676, 1602, 1458, 1386, 1273, 1164, 1097, 1063, 1030, 992,
896, 821.
EIMS (m/z (%)): 126 (MþꢁCH2CH(OCH3)2, 0.37), 111
(MþꢁCH2CH(OCH3)2ꢁCH3, 8.93), 97 (17.44), 84
(C4H4Oþ2 , 100), 71 (8.89), 55 (14.87).
4. (a) Casiraghi, G.; Zanardi, F.; Appendino, G.; Rassu, G. Chem. Rev. 2000,
100, 1929e1972; (b) Rassu, G.; Zanardi, F.; Battistini, L.; Casiraghi, G.
Synlett 1999, 1333e1350.
5. Busque, F.; De March, P.; Figueredo, M.; Font, J.; Gonzalez, L. Eur. J. Org.
Chem. 2004, 1492e1499 and references therein.
6. (a) Zanardi, F.; Battistini, L.; Rassu, G.; Auzzas, L.; Pinna, L.; Marzocchi,
L.; Acquotti, D.; Casiraghi, G. J. Org. Chem. 2000, 65, 2048e2064; (b) Li,
P.; Yang, J.; Zhao, K. J. Org. Chem. 1999, 64, 2259e2263; (c) Hoyle, T. R.;
Hanson, P. R. Tetrahedron Lett. 1993, 34, 5043e5046.
Elemental analysis: Found: C, 60.01; H, 7.98%; Anal.
Calcd for C10H16O4: C, 59.98; H, 8.05; O, 31.96%.
7. Chapleo, C. B.; Svanholt, K. L.; Martin, R.; Dreiding, A. S. Helv. Chim.
Acta 1976, 59, 100e107.
4.3.9. 5-(4-Bromobutoxy)furan-2(5H)-one (7)
1H NMR: 7.24 (dd, 1H, J1¼1.0 Hz, J2¼5.8 Hz, O]Ce
CH]CHe), 6.26 (dd, 1H, J1¼1.0 Hz, J2¼5.8 Hz, O]Ce
CH]CHe), 6.15e5.75 (m, 1H, O]CeCH]CHeCHo),
3.70e3.40 (m, 4H, OeCH2eCH2eCH2eCH2eBr), 2.00e
1.50 (m, 4H, OeCH2eCH2eCH2eCH2eBr).
8. (a) Mimero, P.; Saluzzo, C.; Amouroux, R. Synth. Commun. 1995, 25, 613e
627; (b) Node, M.; Kajimoto, T.; Nishide, K.; Fujita, E.; Fuji, K. Tetrahe-
dron Lett. 1984, 25, 219e222; (c) Pri-Bar, I.; Stille, J. K. J. Org. Chem.
1982, 47, 1215e1220; (d) Goldshmith, D.; Kennedy, E.; Campbell, R. G.
J. Org. Chem. 1975, 40, 3571e3574; (e) Yadav, V. K.; Fallis, A. G.
J. Org. Chem. 1986, 51, 3372e3374.
13C NMR: 170.6, 150.3, 124.9, 103.3, 69.3, 33.2, 29.1, 27.9.
FTIR liquid film (n, cmꢁ1): 3105, 2947, 2875, 1787, 1760,
1432, 1353, 1255, 1157, 1130, 1091, 1039, 1006, 960, 927,
901, 822, 724, 685.
9. (a) Beckwith, A. L. J. Tetrahedron 1981, 37, 3073e3100; (b) Hart,
D. J. Science 1984, 223, 883e887; (c) Giese, B. Angew. Chem., Int.
Ed. Engl. 1985, 24, 553e565; Giese, B. Radicals in Organic
Synthesis: Formation of CarboneCarbon Bonds; Pergamon: Oxford,
1986.