Condensations of ethyl 3-ethoxy-4-(triphenylphosphoranylidene)-2-butenoate with α,β-unsaturated carbonyl compounds

Article abstract of DOI:10.1007/s00706-002-0527-2

Wittig condensations of α,β-unsaturated carbonyl compounds with ethyl 3-ethoxy-4-(triphenylphosphoranylidene)-2-butenoate gave good to high yields of (2E,4E,6E)-ethyl 3-ethoxy-2,4,6-alkatrienoates. Some of last mentioned compounds were almost quatitativel

Full text of DOI:10.1007/s00706-002-0527-2

Monatshefte f€ur Chemie 134, 61–67 (2003)
DOI 10.1007/s00706-002-0527-2
Condensations of Ethyl 3-Ethoxy-4-
(triphenylphosphoranylidene)-2-butenoate
with a,b-Unsaturated Carbonyl Compounds
ꢀ
Cornelis M. Moorhoff
Department of Chemistry, University of Tasmania, GPO Box 252-75C, Hobart,
Tasmania, Australia 7001
Received June 17, 2002; accepted June 21, 2002
Published online November 7, 2002 # Springer-Verlag 2003
Summary. Wittig condensations of ꢀ,ꢁ-unsaturated carbonyl compounds with ethyl 3-ethoxy-4-
(triphenylphosphoranylidene)-2-butenoate gave good to high yields of (2E,4E,6E)-ethyl 3-ethoxy-
2,4,6-alkatrienoates. Some of last mentioned compounds were almost quatitatively hydrolysed to
(4E,6E)-ethyl 3-oxo-4,6-alkadienoates. This method can therefore be used as an attractive alternative
for the preparation of unsaturated conjugated ꢁ-keto esters previously prepared in very low yields from
ꢀ,ꢁ-unsaturated carbonyl compounds and ethyl 3-oxo-4-(triphenylphosphoranylidene)butanoate.
Keywords. Ethyl 3-ethoxy-2,4,6-alkatrienoates; Ethyl 3-ethoxy-4-(triphenylphosphoranylidene)-2-
butenoate; Unsaturated-ꢁ-ketoesters; Wittig condensations.
Introduction
For quite some time we have been interested in the synthetic capabilities of esters
of 3-oxo-4-(triphenylphosphoranylidene)-butanoate (1) (Scheme 1) [1, 2]. Thus,
Wittig condensations of the phosphonium ylide 1 with ꢀ,ꢁ-unsaturated carbonyl
compounds 2, gave poor yields of (4E,6E)-ethyl 3-oxo-2,4-alkadienoates 3
(Scheme 1). The low yield of unsaturated conjugated ꢁ-keto esters 3 was due to
further condensation reactions of 3 with carbonyl compounds 2, like aldol con-
densations and subsequent Michael-Wittig condensations of these aldol products
with the ylide 1 [2, 3]. However, under certain conditions this ylide 1 can be manip-
ulated into useful Michael-Wittig condensations with ꢀ,ꢁ-unsaturated carbonyl
compounds 2 to form substituted 3-cyclohexen-2-one-1-carboxylates, for example
with 2-butenal (2b), ethyl 6-methyl-2-oxocyclohex-3-ene-1-carboxylate (9) is
obtained (Scheme 1) [4]. In this paper we describe the condensation of ethyl
ꢀ
E-mail: cmoorhoff@boronmolecular.com
Present address: Boron Molecular Pty. Ltd., PO Box 756, Noble Park, VIC 3174, Australia.

62
C. M. Moorhoff
R1
R2
R3
R4
Method A
Method B
7
8
7
8
a
b
c
Me
Me
H
H
H
93%
61%
81%
0%
0%
3%a
0%
0%
0%
75%
27%
0%
8%
Me
H
H
b
b
H
H
Me
Me
H
H
–
–
d
e
Me
H
Me
H
0%
0%
0%
(CH3)2C¼CH(CH2)2
b
Isolated as the hydrolysed product 9; not carried out
Me
53%
48%
a
Scheme 1. Condensations of ethyl 3-ethoxy-4-(triphenylphosphoranylidene)-2-butenoate with
ꢀ,ꢁ-unsaturated carbonyl compounds
3-ethoxy-4-(triphenylphosphoranylidene)-2-butenoate (6a) [5–8] with ꢀ,ꢁ-unsatu-
rated carbonyl compounds 2 (Scheme 1) as an attractive alternative for the syn-
thesis of 3-oxo-4,6-alkadienoates 3. Bestmann et al. introduced the ethoxy
phosphonium ylide 6a some thirty years ago [5–8]. Only in the last fifteen years
was there a renewed interest in this ylide 6a [9–22]. Some classical Wittig con-
densation reactions were carried out [7, 9, 15, 17, 19, 22, 25, 26] and novel
cyclisations to five membered-[10, 12–14, 16, 18, 23], six membered unsaturated

Condensations of Ethyl 3-Ethoxy-4-(triphenylphosphoranylidene)-2-butenoate
63
carbocyclic rings [16] and aromatics [20, 21]. Cyclopropanations of the corre-
sponding arsonium ylide [19] were documented. The phosphonium salt 4 was
prepared by Bestmann from 1,1-diethoxyvinyl-2-(triphenylphosphoranylidene)
[5–8]; However, Hudson prepared the phosphonium salt 4 from triphenylphosphine
and ethyl ꢂ-bromo-3-ethoxycrotonate [9]. We have found that the reaction of ethyl
bromide with ethyl 3-oxo-4-(triphenylphosphoranylidene) butanoate (1) gave apart
from 4 also the vinylic salt 5 [27].
Results and Discussion
Condensation of ethyl 3-ethoxy-4-(triphenylphosphoranylidene)-2-butenoate (6a)
and 3-methylbut-2-enal (2a) in benzene gave the classical Wittig product ethyl
3-ethoxy-7-methyl-2,4,6-octatrienoate (7a) as a 9:1 mixture of (2E,4E) and (2E,4Z)
geometric isomers in a high yield of 93% [28]. This compound 7a easily hydro-
lysed with HCl in CDCl3 with little water to give only (2E,4E)-ethyl 7-methyl-3-
oxo-4,6-octadienoate (3a). The condensation of 6a with 2-butenal (2b) gave the
Wittig product 7b (61%) as a 4:1 mixture of (2E,4E,6E) and (2E,4Z,6E) isomers.
Recently, a similar result with a different ester of 6a was obtained [29]. Ethyl
4-(triphenylphosphoranylidene)-2-butanoate (6b) is well known to undergo
Michael-Wittig condensations with for example 2-butenal 2b [30]. However, the
ylide 6a gave only some of the Michael-Wittig product 8b. Last mentioned com-
pound 8b was very labile and in the presence of HCl in chloroform, hydrolysed to
the corresponding cyclic ꢁ-keto ester: ethyl 6-methyl-2-oxocyclohex-3-ene-1-car-
boxylate (9). Eluation of 9 seems to suggest that partially hydrolyses of 8b to 9
must have taken place on silica gel during chromatography. It is interesting to note
that condensation of 2-butenal (2b) with the in situ generated phosphonium ylide
6a, from a mixture of the allylic-4 and vinylic phosphonium 5 salts and potassium
carbonate, gave a lower overall yield but mainly the (2E,4E,6E) geometric isomer
of ethyl 3-ethoxy-2,4,6-octatrienoate (7b), and none of the (2E,4Z,6E) isomer. This
method gave also a higher yield of the cyclic ester 8b. The higher yields that are
obtained using the isolated phosphonium ylide 6a as compared to the use of the in
situ formed phosphonium ylide 6a, seems to be a general outcome for 3-methyl-2-
butenal (2a) and 2-methylpropenal (2c) condensations as well. In our hands, con-
jugated ketones 4-methyl-3-penten-2-one and 3-methyl-3-penten-2-one (2d) did
not condense with the ylide 6a. A similar result has also been communicated
before, namely that methylvinylketone condensed with 6a only in very low yields
(<10%) [16].
Experimental
1
All reactions were carried out under nitrogen. H-NMR (ꢃ, ppm, with SiMe4 as an internal standard)
and 13C-NMR (ꢃ, ppm) were recorded on a Varian Gemini 200 spectrometer at 200MHz and
50.3MHz, or on a Varian FT-80 at 80MHz and 20MHz respectively as indicated. High resolution
electron ionization (EI) mass spectra were obtained from a Varian MAT 311 A instrument. Infrared
spectra were obtained on a Hitachi 270-30 FTIR spectrophotometer (film, NaCl plates). Ultraviolet
absorbance was measured as solutions in 96% EtOH on a Varian SuperScan 3 spectrophotometer or on
a Shimadzu UV-150 spectrophotometer. Microanalyses were performed by Microanalytisches Labor
Pascher (Bonn, Germany). Column chromatography was performed using Merck Si-60 (40–63 mm)

64
C. M. Moorhoff
silica gel. Bulb-to-bulb distillations (b.p.) were carried out on a B€uchi GKR-51 apparatus. Diethyl ether
(ether) and tetrahydrofuran (THF) were dried and distilled from LiAlH4. Light petroleum is the
fraction between 40–60ꢁC.
Condensations of ethyl 3-ethoxy-4-(triphenylphosphoranylidene)but-2-enoate (6a)
with unsaturated carbonyl compounds
With 3-methyl-2-butenal (2a) (Method A)
Ethyl 3-ethoxy-4-(triphenylphosphoranylidene)but-2-enoate (6a) (3.0g, 7.17mmol) in benzene (5cm3)
was treated with 3-methyl-2-butenal 2a (1.0g, 11.9mmol) and heated for 11h at 75ꢁC. The reaction
mixture was diluted with light petroleum (10 cm3) and filtered over silica gel. The filtrate was con-
centrated and chromatographed over silica gel and eluted with diethyl ether:light petroleum 1:9 to give
a (9:1) mixture of (2E,4E)- and (2E,4Z)-ethyl 3-ethoxy-7-methylocta-2,4,6-trienoate (7a) (1.49 g,
93%), bp 110ꢁC=0.05mm Hg. ꢄmax (EtOH)=nm: 308, 218 ("=dm3 dmÀ 1 cmÀ 1 28700, 7300); HRMS
(EI) calcd. for C13H20O3 (Mþ ) m=z ¼ 224.1412, found 224.1428; 1H NMR (CDCl3, 80MHz):
ꢃ ¼ 1.21 (t, J ¼ 7.1Hz, 3H), 1.33 (t, J ¼ 7.0 Hz, 3H), 1.78 (sm, 6H), 3.83 (q, J ¼ 7.0 Hz, 2H), 4.07
(q, J ¼ 7.1 Hz, 2H), 4.95 (s, 1H), 5.97 (dm, J ¼ 11.0Hz, 1H), 7.13 (dd, J ¼ 15.3, 11.0 Hz, 1H), 7.31 (d,
J ¼ 15.3Hz, 1H); 13C NMR (CDCl3, 20MHz): ꢃ ¼ 14.0, 14.1, 18.6, 26.4 (4ꢂCH3), 59.0, 63.3
(2ꢂCH2), 91.1 (C2), 121.1 (C4), 125.2 (C6), 131.6 (C5), 141.2 (C7), 166.4, 167.2 (C1, C3); IR
(cmÀ 1): ꢅ ¼ 3080w, 2984s, 2940s, 2870s, 1701 s, 1621 s, 1566 s, 1480 m, 1446 s, 1378s, 1280s,
1254 s, 1141s, 1064s, 989 m, 809 m.
With 3-methyl-2-butenal (2a) (Method B)
[2-Ethoxy-3-(ethoxycarbonyl)prop-2-enyl]triphenylphosphonium bromide (4) ( þ 5) (0.7g,
1.353 mmol) in chloroform (10 cm3) was treated with 3-methyl-2-butenal (2a) (0.25 g, 2.98 mmol)
and the reaction mixture heated for 6 h at 60ꢁC in the presence of potassium carbonate (2.00 g). The
reaction mixture was diluted with light petroleum (10 cm3) and filtered over silica gel. The filtrate
was concentrated and chromatographed over silica gel and eluted with ethyl acetate:light petroleum 1:9
to give (2E,4E)-ethyl 3-ethoxy-7-methylocta-2,4,6-trienoate (7a) (0.228 g, 75.2%) with only traces of
the (2E,4Z) isomer. Hydrolysis of 7a with little aqueous HCl in CDCl3 occurred instantaneously and
gave ethyl 7-methyl-3-oxoocta-2,4-dienoate (3a). 1H NMR (CDCl3, 200 MHz): ꢃ ¼ 1.28 (t, J ¼ 7.0 Hz,
3H), 1.93 (sm, 6H), 3.59 (s, 2H), 4.21 (q, J ¼ 7.0Hz, 2H), 6.03 (dm, J ¼ 11.5Hz, 1H), 6.12 (d,
J ¼ 15.3Hz, 1H), 7.53 (dd, J ¼ 15.3, 11.5 Hz, 1H); this compound was identical with 3a obtained
before [3].
With 2-butenal (2b) (Method A)
Ethyl 3-ethoxy-4-(triphenylphosphoranylidene)but-2-enoate (6a) (2.0g, 4.78mmol) in THF (10 cm3)
was treated with 2-butenal (2a) (0.25 g, 3.57mmol) and heated for 16h at 60ꢁC. The reaction mixture
was diluted with light petroleum (20 cm3) and filtered over silica gel. The filtrate was concentrated and
chromatographed over silica gel and eluted with diethyl ether:light petroleum 1:9 to give a (4:1)
mixture of (2E,4E,6E)- and (2E,4Z,6E)-ethyl 3-ethoxyocta-2,4,6-trienoate (7b) (0.46 g, 61%). ꢄmax
(EtOH)=nm: 296.5, 215 ("=dm3 dmÀ 1 cmÀ 1 26300, 8200); HRMS (EI) calcd. for C12H18O3 (Mþ )
m=z ¼ 210.1256, found 210.1238; 1H NMR (CDCl3, 200 MHz) (2E,4E,6E-isomer): ꢃ ¼ 1.23 (t,
J ¼ 7.1 Hz, 3H), 1.34 (t, J ¼ 7.0 Hz, 3H), 1.78 (d, J ¼ 6.6Hz, 3H), 3.84 (q, J ¼ 7.0 Hz, 2H), 4.11 (q,
J ¼ 7.1 Hz, 2H), 4.99 (s, 1H), 5.92 (dq, J ¼ 15.0, 6.6 Hz, 1H), 6.21 (ddm, J ¼ 15.0, 10.5Hz, 1H), 6.88
(dd, J ¼ 15.3, 10.5 Hz, 1H), 7.34 (d, J ¼ 15.3Hz, 1H); 13C NMR (CDCl3, 20MHz) (2E,4E,6E)-iso-
mer): ꢃ ¼ 14.0, 14.2 (2ꢂ OCH2CH3), 18.3 (CH3), 59.1 (OCH2CH3), 63.4 (CO2CH2CH3), 91.5 (C2),
121.4 (C4), 131.4 (C6), 134.7 (C7), 135.6 (C5), 166.2, 167.2 (2ꢂ –C¼); IR (cmÀ 1): ꢅ ¼ 3070 w,

Condensations of Ethyl 3-Ethoxy-4-(triphenylphosphoranylidene)-2-butenoate
65
2986 s, 2940 s, 1704s, 1621 s, 1568 s, 1445s, 1382 s, 1278s, 1220m, 1142 s, 1059s, 995 m, 810 m.
After further elution, ethyl 6-methyl-2-oxocyclohex-3-ene-1-carboxylate (9) (25 mg, 3%) was isolated.
1H NMR (CDCl3, 200 MHz) ꢃ ¼ 1.03 (d, J ¼ 6.4Hz, 3H), 1.24 (t, J ¼ 7.0 Hz, 3H), 2.65–2.35 and 2.2–
2.0 (2m, 2H), 3.06 (d, J ¼ 11.7 Hz, 1H), 4.19 (q, J ¼ 7.0Hz, 2H), 6.01 (ddd, J ¼ 10.1, 2.6, 1.0Hz, 1H),
6.95 (ddd, J ¼ 10.1, 5.8, 2.8 Hz, 1H), 6.88 (dd, J ¼ 15.3, 10.5Hz, 1H), 7.34 (d, J ¼ 15.3Hz, 1H); 13C
NMR (CDCl3, 20MHz): ꢃ ¼ 14.1 (OCH2CH3), 19.7 (CH3), 32.8, 33.1 (C5 and C6), 60.9 (C1), 61.6
(CO2CH2CH3), 128.6 (C3), 149.9 (C4), 169.9 (CO2Et), 194.6 (C2) [4]. Hydrolysis of 7b with little
aqueous HCl in CDCl3 occurred instantaneously and gave ethyl 3-oxoocta-2,4-dienoate (3b). 1H NMR
(CDCl3, 200 MHz): ꢃ ¼ 1.29 (t, J ¼ 7.0 Hz, 3H), 1.90 (d, J ¼ 6.4 Hz, 3H), 3.60 (s, 2H), 4.22
(q, J ¼ 7.0 Hz, 2H), 6.14 (d, J ¼ 11.5Hz, 1H), 6.1–6.4 (m, 1H), 6.95–7.25 (m, 2H); this compound
was identical with 3b obtained before [3].
With 2-butenal (2b) (Method B)
[2-Ethoxy-3-(ethoxycarbonyl)prop-2-enyl]-triphenylphosphonium bromide, (0.7g, 1.353mmol) in
chloroform (10 cm3) was treated with 2-butenal (2b) (0.25 g, 3.57mmol) and heated for 6 h at 60ꢁC
in the presence of potassium carbonate (2.00 g). The reaction mixture was diluted with light petroleum
(20 cm3) and filtered over silica gel. The filtrate was concentrated and chromatographed over silica gel
and eluted with ethyl acetate:light petroleum 1:9 to give only (2E,4E,6E) ethyl 3-ethoxyocta-2,4,6-
trienoate (7b) (0.077 g, 27.1%). Followed by ethyl 2-ethoxy-6-methyl-1,3-cyclohexadiene-1-carbox-
ylate (8b) (22 mg, 7.8%); 1H NMR (CDCl3, 200 MHz): ꢃ ¼ 1.00 (d, J ¼ 6.9 Hz, 3H), 1.30 (t,
J ¼ 7.0 Hz, 3H), 1.32 (t, J ¼ 7.2Hz, 3H), 2.07 (ddd, J ¼ 17.5, 5.6, 2.1 Hz, 1H), 2.46 (dddd, J ¼ 17.5,
8.4, 2.6, 2.3 Hz, 1H), 2.88 (dqm, J ¼ 6.9, 5.6 Hz, 1H), 4.20 (q, J ¼ 7.0 Hz, 2H), 4.21 (q, J ¼ 7.2 Hz,
2H), 6.07 (dd, J ¼ 9.2, 2.6 Hz, 1H), 6.10 (ddm, J ¼ 9.2, 8.4 Hz, 1H). Followed by ethyl 6-methyl-2-
oxocyclohex-3-ene-1-carboxylate (9) [4].
With 2-methylpropenal (2c)
Ethyl 3-ethoxy-4-(triphenylphosphoranylidene)-but-2-enoate (6a) (0.50 g, 1.195mmol) in THF
(10 cm3) was treated with 2-methylpropenal (2c) (0.25 g, 3.57 mmol) and heated for 16h at 60ꢁC.
The reaction mixture was diluted with light petroleum (20 cm3) and filtered over silica gel. The filtrate
was concentrated and chromatographed over silica gel and eluted with ethyl acetate:light petroleum
1:9 to give (2E,4E)-ethyl 3-ethoxy-6-methylhepta-2,4,6-trienoate (7c) (4:1) (0.203g, 80.7%). ꢄmax
1
(EtOH)=nm¼ 240, H NMR (CDCl3, 200 MHz): ꢃ ¼ 1.29 (t, J ¼ 7.0 Hz, 3H), 1.41 (t, J ¼ 7.0 Hz,
3H), 1.96 (sm, 3H), 3.91 (q, J ¼ 7.0 Hz, 2H), 4.16 (q, J ¼ 7.0 Hz, 2H), 5.09 (s, 1H), 5.22 (sm, 2H),
7.04 (d, J ¼ 15.9 Hz, 1H), 7.51 (d, J ¼ 15.9, 1H); 13C NMR (CDCl3, 50 MHz): ꢃ ¼ 14.3, 14.4
(OCH2CH3), 18.4 (CH3), 59.5 (CO2CH2CH3), 63.7 (OCH2CH3), 92.5 (C2), 120.9 (C4), 121.0 (C7),
137.8 (C6), 142.0 (C6), 166.3, 167.6 (C1, C3); IR (cmÀ 1): ꢅ ¼ 3080w, 2981 s, 2940s, 1706 s, 1620s,
1580 s, 1446s, 1378s, 1253 s, 1141 s, 1060 s, 990 m, 816m.
With 3-methylpent-3-en-2-one (2d)
Ethyl 3-ethoxy-4-(triphenylphosphoranylidene)but-2-enoate (6a) (0.50 g, 1.195mmol) in THF
(10 cm3) was treated with 3-methylpent-3-en-2-one (2d) (0.25 g, 3.57mmol) and heated for 16 h at
60ꢁC. The reaction mixture was diluted with light petroleum (20 cm3) and filtered over silica gel. The
filtrate was concentrated and chromatographed over silica gel and eluted with ethyl acetate:light
petroleum 1:9 to give only traces of (2E,4Z,6E)- and mostly the (2E,4E,6E)-isomer of ethyl 3-
ethoxy-2,6-dimethylnona-2,4,6-trienoate (20 mg). 1H NMR (CDCl3, 200 MHz): ꢃ ¼ 1.04 (t, J ¼ 7.5 Hz,
3H), 1.27 (t, J ¼ 7.1 Hz, 3H), 1.40 (t, J ¼ 7.0 Hz, 3H), 1.84 (s, 3H), 2.12 (q, J ¼ 7.5 Hz, 2H), 3.90 (q,
J ¼ 7.0 Hz, 2H), 4.14 (q, J ¼ 7.1 Hz, 2H), 5.02 (s, 1H), 6.04 (dm, J ¼ 10.9Hz, 1H), 7.21 (dd, J ¼ 15.3,
10.9Hz, 1H), 7.39 (d, J ¼ 15.3Hz, 1H); 13C NMR (CDCl3, 50 MHz): ꢃ ¼ 12.3 (CH3CH2C), 14.3, 14.4

66
C. M. Moorhoff
(OCH2CH3), 25.7 (CH3), 32.9 (CCH2CH3), 59.4 (CO2CH2CH3), 63.6 (OCH2CH3), 91.3 (C2), 121.6
(C6), 123.7 (C4), 132.1 (C5), 147.1 (C7), 166.8, 167.8 (C1, C3). This unexpected compound is
probably due to some small amounts of 2-methylpent-2-enal being present in 3-methylpent-3-en-2-
one.
With 3,7-dimethylocta-2,6-dienal (2e)
[2-Ethoxy-3-(ethoxycarbonyl)prop-2-enyl]triphenylphosphonium bromide (4) ( þ 5) (0.7g,
1.353 mmol) in CHCl3 (10 cm3) was treated with 3,7-dimethylocta-2,6-dienal (2e) (0.40 g, 2.63 mmol)
and heated for 6 h at 60ꢁC in the presence of potassium carbonate (2.00 g). The reaction mixture was
diluted with light petroleum (20 cm3) and filtered over silica gel. The filtrate was concentrated and
chromatographed over silica gel and eluted with ethyl acetate:light petroleum 1:9 to give only the
(2E,4E,6E)-isomer of ethyl 3-ethoxydodeca-2,4,6,10-tetraenoate (7e) (0.19 g, 48.1%). ꢄmax
(EtOH)=nm¼ 304, 239.5; 1H NMR (CDCl3, 200 MHz): ꢃ ¼ 1.26 (t, J ¼ 7.1Hz, 3H), 1.38 (t,
J ¼ 7.0 Hz, 3H), 1.59 (sm, 3H), 1.66 (sm, 3H), 1.83 (sm, 3H), 2.1–2.3 (m, 4H), 3.89 (q, J ¼ 7.0 Hz,
2H), 4.13 (q, J ¼ 7.1 Hz, 2H), 5.01 (s, 1H), 5.07 (sm, 1H), 6.04 (d, J ¼ 11.0Hz, 1H), 7.20 (dd, J ¼ 15.2,
11.0Hz, 1H), 7.39 (d, J ¼ 15.2Hz, 1H); 13C NMR (CDCl3, 50 MHz): ꢃ ¼ 14.3, 14.4 (OCH2CH3), 17.1,
25.7, 25.4 (3ꢂCH3), 40.2 (2ꢂCH2), 59.3 (CO2CH2CH3), 63.6 (OCH2CH3), 91.4 (C2), 121.7 (C6),
123.6 (C4), 125.1 (C10), 131.8 (C11), 131.9 (C5), 145.1 (C7), 166.7, 167.5 (C1, C3); IR (cmÀ 1):
ꢅ ¼ 3080 w, 2980 s, 2934s, 2880s, 1710 s, 1617s, 1563 s, 1446m, 1377 s, 1282 s, 1142s, 1066 s,
980 m, 811 m (Method A: 53%).
Acknowledgements
The author thanks David Schneider for helpful discussions during the preliminary stage of this project,
Hendrik Spies for NMR-analyses of compounds 7a and 7b, and Verona Truter for mass spectral
analyses. Financial assistance from the Foundation for Research Development (South Africa) and
Australian Research Council is gratefully acknowledged.
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