The synthesis of arylketones via Friedel-Crafts acyldegermylation

Article abstract of DOI:10.1055/s-2005-872258

Electron-rich and electron-neutral aryl trialkylgermanes are shown to be competent precursors to aryl ketones via Friedel-Crafts acyldegermylation. Given the previously demonstrated greater stability towards basic/nucleophilic conditions of arylgermanes as compared to arylsilanes/stannanes, these reactions significantly expand the utility of group 14 acyldemetalation in synthesis. This is exemplified by the cleavage of Ge-based linker models for solid-phase synthesis (SPS). Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2005-872258

LETTER
2167
The Synthesis of Arylketones via Friedel–Crafts Acyldegermylation
Synthesis of
A
rylk
l
etones
a
via Friedel–
n
Crafts Acyldegerm
C
ylation . Spivey,*a Christopher J. G. Gripton,b Catherine Noban,a Nigel J. Parrc
a
Department of Chemistry, Imperial College, South Kensington Campus, London, SW7 2AZ, UK
E-mail: a.c.spivey@imperial.ac.uk
b
c
Department of Chemistry, University of Sheffield, Brook Hill, Sheffield, S3 7HF, UK
Medicinal Chemistry, GlaxoSmithKline, Gunnelswood Road, Stevenage, Hertfordshire, SG1 2NY, UK
Received 17 June 2005
ketone preparation is established, but to the best of our
knowledge there are only two anecdotal reports of acylde-
germylation.13
Abstract: Electron-rich and electron-neutral aryl trialkylgermanes
are shown to be competent precursors to aryl ketones via Friedel–
Crafts acyldegermylation. Given the previously demonstrated
greater stability towards basic/nucleophilic conditions of arylger-
manes as compared to arylsilanes/stannanes, these reactions signif-
icantly expand the utility of group 14 acyldemetalation in synthesis.
This is exemplified by the cleavage of Ge-based linker models for
solid-phase synthesis (SPS).
Initial work on acyldesilylation by Eaborn employed
acetyl chloride and AlCl3 in CS2,10 but more recently
CH2Cl2 has generally been employed as solvent.14
We commenced our study by synthesising a series of
sterically and electronically diverse aryltrimethyl-
germanes via halogen–metal exchange of the correspond-
ing aryl bromides (1a–i) followed by transmetallation
with trimethylgermanium bromide (TMG-Br) to give
arylgermanes (2a–i, Scheme 1 and Table 1).
Key words: arylgermanes, solid-phase synthesis, electrophilic aro-
matic substitution, Friedel–Crafts acylation, oligothiophenes
Solid-phase synthesis (SPS) using parallel techniques
constitutes an attractive method for the rapid preparation
of libraries of discrete molecules for property screening.1
Crucial to this approach is the controlled introduction of
molecular diversity, with the opportunity to achieve this
concurrently with molecule release from the solid support
being particularly attractive.2
i, ii
Ar–Br
Ar-GeMe3
see Table 1
1a–i
2a–i
Scheme 1 Preparation of aryltrimethylgermanes 2a–i. Reagents
and conditions: (i) n-BuLi, THF, –78 °C, 1 h; (ii) TMG-Br, THF, –78
°C to r.t., 3 h.
Studies from our laboratories have recently described the
development and application of organogermanium linker
systems for the immobilisation of aryl based libraries.3–6
Protic ‘traceless’ cleavage of aryl groups from a germani-
um linker has been demonstrated, as has cleavage with ha-
lonium ions.3,4 Arylgermane linker systems have been
applied to the SPS of pyrazole4 and pyrimidine5 libraries,
and also to iterative oligothiophene SPS.6 As an extension
of this work we were interested in exploring cleavage with
concomitant C–C bond formation via Friedel–Crafts
acyldegermylation, with a view to developing a protocol
for terminating oligothiophenes with ketone functionality
suitable for subsequent reductive deoxygenation to elec-
troactive organic semiconductor materials.7
Table 1 Preparation of Aryltrimethylgermanes 2a–i
Entry
Ar-Br
Product yield (%)
2a (91)a
2b (12)
1
2
3
4
5
6
7
8
9
1a (4-MeC6H4-)
1b (3-MeC6H4-)
1c (2-MeC6H4-)
1d (2,6-Me2C6H3-)
1e (3,5-Me2C6H3-)
1f (4-MeOC6H4-)
1g (3-MeOC6H4-)
1h (2-Pyridyl-)
1i (3-CO2EtC6H4-)
2c (72)
2d (80)
2e (69)
2f (89)
2g (20)
Cleavage of an aryl group from germanium via electro-
philic ipso-substitution is facilitated by the b-effect exert-
ed by germanium, whereby the electron rich C–Ge bond
is able to stabilise a cation b to germanium by hypercon-
jugation (sC–Ge → pvac).8 The relative magnitude of the b-
effect for group 14 metals is Sn >> Ge > Si, as demonstrat-
ed by Eaborn’s 1960 study of the cleavage of aryl-metal
bonds in Ph-MMe3 (M = Sn, Ge, Si) with methanolic
HClO4.9 The synthetic utility of Friedel–Crafts acylde-
metallation of aryl/vinyl silanes10,11 and stannanes10b,12 for
2h (81)
2i (75)b
a Prepared by transmetallation of p-TolMgBr with TMG-Br.
b Prepared by Hal–Mg exchange on the aryl iodide using i-PrMgCl15
followed by transmetallation with TMG-Br.
These trimethylgermane derivatives were then subjected
to typical conditions for acetyldemetallation. This con-
sisted of premixing AcCl with AlCl3 in CH2Cl2 at 0 °C
to form the requisite acylium ion, cooling this mixture to
–78 °C and then adding the arylgermane as a solution in
CH2Cl2 (Scheme 2 and Table 2).
SYNLETT 2005, No. 14, pp 2167–2170
0
2
.0
9
.2
0
0
5
Advanced online publication: 03.08.2005
DOI: 10.1055/s-2005-872258; Art ID: D17105ST
© Georg Thieme Verlag Stuttgart · New York

2168
A. C. Spivey et al.
LETTER
Scheme 3).16 Dunogues has reported that benzoylation of
compound 11, the TMS analogue of germane 2g, affords
a mixture of four products (12–15) all of which result
from initial benzoyldeprotonation at the activated posi-
tions ortho or para to the OMe group (Scheme 3).17,11d
i
products
(see Table 2)
Ar-GeMe3
2a–i
Scheme 2 Attempted acetyldegermylation of aryltrimethylger-
manes 2a–i. Reagents and conditions: (i) AlCl3–AcCl, CH2Cl2, –78
°C to r.t., 3 h.
11%
OMe
24%
Me3Ge
H
Table 2 Attempted Acetyldegermylation of Aryltrimethylgermanes
2a–i
Me3Ge
OMe
i
16%
10 78%
O
Entry Ar-GeMe3
Product
Yield
(%)
(Table 2
entry 7)
H
2g
(NOE difference enhancements)
Me
H
1
2
3
4
5
2a (4-MeC6H4-)
4-Methylacetophenone (3)
3-Methylacetophenone (4)
2-Methylacetophenone (5)
85
65
82
O
Ph
21%
OMe
2b (3-MeC6H4-)
2c (2-MeC6H4-)
2d (2,6-Me2C6H3-)
2e (3,5-Me2C6H3-)
Me3Si
OMe
O
Me3Si
OMe
ii
12 50%
Ph
14 13%
2,6-Dimethylacetophenone (6) 53
(ref. 17)
Me Si
OMe
Me3Si
O
OMe
3
11
3,5-Dimethyl- and 2,4-dimethyl 76a
acetophenones (7 and 8)
O
13 33%
15 4%
Ph
Ph
6
7
2f (4-MeOC6H4-)
2g (3-MeOC6H4-)
4-Methoxyacetophenone (9)
63
Scheme 3 Contrasting behaviour of germane 2g (entry 7, Table 2)
and silane 11 (Dunogues)17 towards acyldemetallation. Reagents and
conditions: AlCl3–AcCl, CH2Cl2, –78 °C to r.t., 3 h; (ii) AlCl3–BzCl,
CS2, O °C.
2-Trimethylgermyl-4-methoxy- 78
acetophenone (10)
b
8
9
2h (2-Pyridyl-)
–
–
–
–
b
2i (3-CO2EtC6H4-)
A rough estimate of the magnitude of the ipso-directing
effects of TMS and TMG groups vs. the ortho-/para-
directing effects of Me and OMe groups can be obtained
a Combined yield of isomers; ratio by 1H NMR ca. 1:1.
b Only starting material recovered.
+
from comparison of the Hammett sp values for CH2TMS
and CH2TMG vs. Me and OMe (–0.61 and –0.61 vs. –0.31
and –0.78, respectively).18,19 The dominant directing
influence of the OMe group (but not Me groups) in these
reactions is therefore not surprising.11b What is not so
clear is why ipso-protodegermylation of germane 10 does
not ensue in situ (cf. for formation of product 8, Table 2,
entry 5); the AlCl3·HCl is conceivably too strongly asso-
ciated with the OMe group in this case.
The ipso-acetyldegermylation proceeded smoothly for 4-,
3-and 2-tolylgermanes 2a, 2b and 2c to give acetophe-
nones 3 (85%), 4 (65%) and 5 (82%), respectively (entries
1–3), the result with compound 2b indicating that, as ex-
pected (vide infra), the ipso-directing influence of the
TMG-group prevails over the ortho/para-directing influ-
ence of the Me group. The more sterically hindered 2,6-
dimethylphenylgermane (2d) was also successfully ipso-
acetyldegermylated to give acetophenone 6 (53%, entry
4). 3,5-Dimethylphenylgermane (2e) gave a ca. 1:1 insep-
arable mixture of 3,5-dimethyl and 2,4-dimethyl ace-
tophenones (7 and 8, 76%, entry 5). The cine-product
presumably arises from acetyldeprotonation ortho to the
TMG group, followed by ipso-protodegermylation by
AlCl3·HCl. This position is activated towards electro-
philic substitution by both of the methyl groups, in com-
petition with the ipso -directing effect of germanium.10b,11c
4-Anisylgermane (2f), for which the directing influences
of the OMe and TMG groups are matched, underwent re-
gioselective ipso-acetyldegermylation to give 4-methoxy-
acetophenone (9) as expected (63%, entry 6). In contrast,
3-anisylgermane (2g), for which the directing influences
are mismatched, gave 2-trimethylgermyl-4-methoxyace-
tophenone (10) as the exclusive product (78%, entry 7).
That acetyldeprotonation para to the OMe group prevails
over both ipso-acetyldegermylation and acetyl deprotona-
tion at either position ortho to the OMe group was con-
firmed by NOE experiments on product 10 (see
Both 2-pyridylgermane (2h) and 3-germyl ethylbenzoate
(2i) failed to undergo degermylation under the standard
conditions or even in refluxing CH2Cl2, presumably due
to their electron-deficient nature and the potential for co-
ordination to AlCl3 (entries 8 and 9).
In the light of these findings it is expected that the use of
ipso-acyldegermylation as a method of linker cleavage
with concomitant diversification via C–C bond formation
for SPS will be restricted to relatively electron-rich and
electron-neutral aryls lacking competitive reactive sites
for protodegermylation. Accordingly, 4-anisyldimethyl-
germane linker model 16 and 2-thiophenylditolylgermane
linker model 17 gave the expected products 9 and 18 (67%
and 91%, respectively) when subjected to the standard
conditions (Scheme 4).
We are currently evaluating ipso-acyldegermylation for
parallel SPS of oligo(3-hexylthiophene)-containing
molecules as potential organic semiconducting materials
using a germanium-based linker. Cleavage of the oligo-
Synlett 2005, No. 14, 2167–2170 © Thieme Stuttgart · New York

LETTER
Synthesis of Arylketones via Friedel–Crafts Acyldegermylation
2169
References
O
OEt
O
Ge
i
(1) Thompson, L. A.; Ellman, J. A. Chem. Rev. 1996, 96, 555.
(2) Gil, C.; Bräse, S. Curr. Opin. Chem. Biol. 2004, 8, 230.
(3) Spivey, A. C.; Diaper, C. M.; Rudge, A. J. Chem. Commun.
1999, 835.
OMe
OMe
16
9 67%
(4) Spivey, A. C.; Diaper, C. M.; Rudge, A. J. J. Org. Chem.
2000, 65, 5253.
(5) Spivey, A. C.; Srikaran, R.; Diaper, C. M.; Turner, D. J. Org.
Biomol. Chem. 2003, 1, 1638.
(6) (a) Spivey, A. C.; Turner, D. J.; Turner, M. L.; Yeates, S.
Org. Lett. 2002, 4, 1899. (b) Spivey, A. C.; Turner, D. J.;
Turner, M. L.; Yeates, S. Synlett 2004, 111.
(7) Katz, H. E.; Bau, Z.; Gilat, S. L. Acc. Chem. Res. 2001, 34,
359.
O
4-Tol 4-Tol
Ge
i
S
S
OEt
O
I
I
nHex
nHex
17
18 91%
Scheme 4 Acetyldegermylation of linker models 16 and 17.
Reagents and conditions: AlCl3–AcCl, CH2Cl2, –78 °C to r.t., 3 h.
(8) (a) Dallaire, C.; Brook, M. A. Organometallics 1990, 9,
2873. (b) Dallaire, C.; Brook, M. A. Organometallics 1993,
12, 2332.
thiophenes from the resin with concomitant ‘end-capping’
by various ketone functions is expected to provide an
array of compounds for evaluation of the influence of dif-
ferent capping groups on mobility and switching rates in
Field Effect Transistor (FET) devices. These studies will
be reported in due course.
(9) Eaborn, C.; Pande, K. C. J. Chem. Soc. 1960, 1566.
(10) (a) Austin, J. D.; Eabrn, C.; Smith, J. D. J. Chem. Soc. 1963,
4744. (b) Dey, K.; Eaborn, C.; Walton, D. R. M. Organomet.
Chem. Synth. 1971, 1, 151; the ipso-acyldesilylation
reactions of 2- and 3-MeOC6H4SiMe3 reported here have
subsequently been shown to be in error: see ref. 11c, 11d, 17.
(11) For reviews of acyldesilylation see: (a) Eaborn, C. J.
Organomet. Chem. 1975, 100, 43. (b) Chan, T. H.; Fleming,
I. Synthesis 1979, 761. (c) Häbich, D.; Effenberger, F.
Synthesis 1979, 841. (d) Bennetau, B.; Donogues, J. Synlett
1993, 171.
(12) Neumann, W. P.; Hillgärtner, H.; Baines, K. M.; Dicke, R.;
Vorspohl, K.; Kobs, U.; Nussbeutel, U. Tetrahedron 1989,
45, 951.
(13) For mention of acetyldegermylation of PhGeMe3 see:
(a) Maire, J.; Marrot, J.; Nabet, R. Bull. Soc. Chim. Fr. 1981,
429. (b) For acetyldegermylation of trans-PhCH=CHGeEt3
see: Marsubara, S.; Yoshino, H.; Utimoto, K.; Oshima, K.
Synlett 2000, 495.
Representative Procedure for Arylgermane Synthesis:
4-OMeC6H4GeMe3 (2f; Table 1, Entry 6)
4-Bromoanisole (0.380 mL, 0.568 g, 3.04 mmol) was dissolved in
anhyd THF (15 mL) under N2 and cooled to –78 °C. s-BuLi (1.3 M,
3.5 mL, 4.55 mmol) was added slowly and dropwise, and the result-
ing solution was stirred for 5 min. Trimethylgermanium bromide
(0.485 mL, 0.749 g, 3.79 mmol) was then added dropwise and the
reaction mixture stirred for 15 min. The CO2/acetone bath was re-
moved, and the reaction mixture was stirred for a further 15 min be-
fore being quenched with H2O (2 mL) and partitioned between Et2O
(2 ꢀ 20 mL) and H2O (20 mL). The organic extracts were dried
(MgSO4) and concentrated in vacuo. Purification by column chro-
matography (4 ꢀ 2 cm silica gel, eluting with PE–EtOAc, 9:1) gave
trimethylgermane (2f) as a clear colourless liquid (0.605 g, 2.69
mmol, 89%). Spectroscopic data see ref.20
(14) Félix, G.; Laguerre, M.; Dunogues, J.; Calas, R. J. Chem.
Res., Synop. 1980, 236.
(15) Boymond, L.; Rottlander, M.; Cahiez, G.; Knochel, P.
Angew. Chem. Int. Ed. 1998, 37, 1701.
(16) Subjection of 2-TMG-4-methoxyacetophenone 12 to
aluminium(III) chloride/AcCl to see if a second acyl group
could be installed by ipso-degermylation resulted in the
formation of no identifiable products.
(17) Bennetau, B.; Krempp, M.; Dunogues, J. J. Organomet.
Chem. 1987, 334, 263.
(18) Bassindale, A. R.; Eaborn, C.; Walton, D. R. M.; Young, D.
J. J. Organomet. Chem. 1969, 20, 49.
(19) (a) Egorochkin, A. N.; Razuvaev, G. A. Russ. Chem. Rev.
1987, 56, 846. (b) Hansch, C.; Leo, A.; Taft, R. W. Chem.
Rev. 1991, 91, 165.
Representative Procedure for Acyldegermylation:
4-MeOC6H4GeMe3 → 4-MeOC6H4COMe (2f → 9; Table 2,
Entry 6)
To a suspension of AlCl3 (0.0833 g, 0.624 mmol) in anhyd CH2Cl2
(1 mL) under nitrogen and cooled to 0 °C with stirring, was added
acetyl chloride (41 mL, 0.0453 g, 0.577 mmol) dissolved in anhyd
CH2Cl2 (0.5 mL) dropwise and slowly. The solution was then
cooled further to –78 °C. Trimethylgermane (2f, 0.105 g, 0.467
mmol) was dissolved in anhyd CH2Cl2 (0.5 mL) and added to the
acetyl tetrachloroaluminate solution slowly and dropwise, before
being stirred for 1 h at –78 °C, warmed to r.t., and stirred for a
further 2 h. The reaction was quenched with sat. aq NH4Cl and ex-
tracted with CH2Cl2 (3 ꢀ 5 mL). The organic layers were combined,
dried (MgSO4), filtered and concentrated in vacuo. Purification by
column chromatography (4 ꢀ 6 cm silica gel, eluted with PE–
EtOAc, 8:2) gave 4-methoxyacetophenone (9, 0.0442 g, 0.294
mmol, 63%). Spectroscopic data see ref.21
(20) Data for Arylgermanes. (a) For compounds 2a, 2f, see:
Moerlein, S. M. J. Organomet. Chem. 1987, 319, 29.
(b) For compounds 2b, 2c, see: Bennett, S. W.; Eaborn, C.;
Jackson, R. A.; Pearce, R. J. Organomet. Chem. 1971, 28,
59. (c) Compound 2d: a clear colourless oil, Rf = 0.71 (PE).
1H NMR (250 MHz, CDCl3): d = 0.57 [9 H, s, (CH3)3Ge],
2.48 (6 H, s, CH3C), 7.01–7.04 (2 H, m,
CH3CCHCHCHCCH3), 7.14–7.20 (1 H, m, CHCHCH). 13
NMR (63 MHz, CDCl3): d = 3.6 (q), 24.6 (q), 127.8 (d),
C
Acknowledgment
128.4 (d), 143.5 (s), one quaternary carbon not observed. IR
(neat): 3052, 2968, 2909, 1566, 1448, 1236, 833, 768 cm–1.
MS (EI): m/z (%) = 224 (14)[M+], 209(100), 179 (7), 119
(20), 105 (39). HRMS: m/z calcd for C11H1874Ge: 224.0620;
found: 224.0622.
This work was supported by the EPSRC and GSK, Stevenage
(CASE award, CJG) and the ORS award scheme (CN). Merck
Sharp & Dohme Ltd. (MSD) and Pfizer Ltd. are also thanked for
unrestricted research funds. Sue Bradshaw, University of Sheffield
is thanked for running the NOE difference experiments.
Compound 2e, a clear colourless oil, Rf = 0.74 (PE). 1H NMR
(250 MHz, CDCl3): d = 0.52 [9 H, s, (CH3)3Ge], 2.46 (6 H,
Synlett 2005, No. 14, 2167–2170 © Thieme Stuttgart · New York

2170
A. C. Spivey et al.
LETTER
s, CH3C), 7.11 (1 H, s, CH3CCHCCH3), 7.24 {2 H, s,
CH3CCHC[Ge(CH3)3]CHCCH3}. 13C NMR (63 MHz,
CDCl3): d = –1.7 (q), 21.5 (q), 130.1 (d), 130.7 (d), 137.3 (s),
142.4 (s). IR (neat): 2972, 2909, 1597, 1236, 1137, 828, 600
cm–1. MS (EI): m/z (%) = 224 (11) [M+], 209 (100), 179 (11),
119 (8), 105 (16). HRMS: m/z calcd for C11H1874Ge:
224.0620; found: 224.0623. (d) For compound 2g, see:
Moerlein, S. M. J. Org. Chem. 1987, 52, 664. (e) For
compound 2h, see: Riedmiller, F.; Jockisch, A.;
Schmidbaur, H. Z. Naturforsch., B: Chem. Sci. 1999, 54, 13.
(f) Compound 2i: a clear colourless oil, Rf = 0.79 (PE–
EtOAc, 8:2). 1H NMR (250 MHz, CDCl3): d = 0.46 [9 H, s,
(CH3)3Ge], 1.44 (3 H, t, J = 7.0 Hz, CH2CH3), 4.43 (2 H, q,
J = 7.0 Hz, CH2CH3), 7.42–7.48 (1 H, m,
(21) Data for Acetyl Derivatives. (a) For compounds 3, 5, 7, 9,
see: Cacchi, S.; Fabrizi, G.; Gavazza, F.; Goggiamani, A.
Org. Lett. 2003, 5, 289. (b) For compound 4, see:
Buchwald, S. L.; Watson, B. T.; Lum, R. T.; Nugent, W. A.
J. Am. Chem. Soc. 1987, 109, 7137. (c) For compound 6,
see: Brook, M. A.; Henry, C. Tetrahedron 1996, 52, 861.
(d) For compound 8, see: Friedman, L.; Koca, R. M. J. Org.
Chem. 1968, 33, 1255. (e) Compound 10: a clear colourless
oil, Rf = 0.38 (PE–EtOAc, 9:1). 1H NMR (400 MHz, CDCl3):
d = 0.41 [9 H, s, Ge(CH3)3], 2.58 (3 H, s, COCH3), 3.88 (3
H, s, OCH3), 6.89 (1 H, dd, J = 8.5, 3.0 Hz,
CH3OCCHCHCCOCH3), 7.21 [1 H, d, J = 3.0 Hz,
(CH3)3GeCCH], 7.91 (1 H, d, J = 8.5 Hz, CH3COCCH). 13
C
NMR (63 MHz, CDCl3): d = 0.0 (q), 26.8 (q), 55.1 (q), 111.9
(d), 121.2 (d), 132.5 (d), 143.3 (s), 167.2 (s), two quaternary
carbons not observed. IR (neat): 2964, 1677, 1587, 1563,
1269, 1227, 1045, 827 cm–1. MS (EI): m/z (%) = 253 (100)
[M – Me]+, 238 (11), 223 (27), 119 (23), 89 (18). HRMS
(EI): m/z calcd for C12H1874GeO2: 268.0519; found:
268.0521. (f) Compound 18: as colourless blocks, Rf = 0.54
(PE–EtOAc, 9:1); mp 29–31 °C. 1H NMR (270 MHz,
CDCl3): d = 0.086 (3 H, t, J = 7.0 Hz, CH3CH2), 1.29–1.35
(6 H, m, CH2CH2CH2CH3), 1.53–1.58 (2 H, m, CCH2CH2),
2.45 (3 H, s, COCH3), 2.92 (2 H, t, J = 8.0 Hz, CCH2CH3),
7.13 (1 H, s, CH). 13C NMR (125 MHz, CDCl3): d = 14.1 (q),
22.6 (t), 29.2 (t), 29.8 (t), 30.2 (2 ꢀ t), 31.6 (t), 81.1 (s), 141.5
(d), 141.7 (s), 151.6 (s), 189.4 (s). IR (neat): 2955, 2855,
1653, 1401, 1237, 954 cm–1. MS (CI+): m/z (%) = 337 (100)
[M + H], 354 (51), 279 (9), 228 (17), 211 (23). HRMS (CI+):
m/z calcd for C12H18OSI: 337.0123; found: 337.0120.
GeCCHCHCHCCO2Et), 7.67–7.71 (1 H, m,
GeCCHCHCHCCO2Et), 8.02–8.07 (1 H, m,
GeCCHCHCHCCO2Et), 8.21 (1 H, m, GeCCHCCO2Et). 13C
NMR (63 MHz, CDCl3): d = –1.8 (q), 14.4 (q), 60.8 (t),
127.8 (d), 129.4 (d), 129.9 (s), 133.9 (d), 137.4 (d), 143.0 (s),
166.9 (s). IR (neat): 2977, 2908, 1715, 1590, 1367, 1261,
1117, 826, 742 cm–1. MS (EI): m/z (%) = 268 (1) [M+], 253
(100), 225 (48), 149 (17), 119 (31), 104 (28), 91 (34), 89
(32). HRMS (ESI+): m/z calcd for C12H1974GeO2: 269.0597;
found: 269.0588.
For compounds 16, 17, see ref. 6b.
Synlett 2005, No. 14, 2167–2170 © Thieme Stuttgart · New York