Organozinc-promoted ring opening of cyclopropanols

Article abstract of DOI:10.3998/ark.5550190.0013.208

Regioselective ring opening of cyclopropanols by the use of organozinc reagents or ZnI2 is presented. Mechanistic studies are discussed with respect to the possible involvement of zinc ketone homoenolates. ARKAT-USA, Inc.

Full text of DOI:10.3998/ark.5550190.0013.208

Issue in Honor of Prof. Anthony J. Arduengo, III
ARKIVOC 2012 (ii) 74-84
Organozinc-promoted ring opening of cyclopropanols
Pragna Pratic Das, Bibhuti Bhusan Parida and Jin Kun Cha*
Department of Chemistry, Wayne State University, 5101 Cass Ave, Detroit, Michigan 48202
E-mail: jcha@chem.wayne.edu
Dedicated to Professor Anthony J. Arduengo, III on the occasion of his 60th birthday
Abstract
Regioselective ring opening of cyclopropanols by the use of organozinc reagents or ZnI2 is
presented. Mechanistic studies are discussed with respect to the possible involvement of zinc
ketone homoenolates.
Keywords: Cyclopropanol, ring opening, organozinc, homoenolate
Introduction
Cyclopropanes are useful building blocks in organic synthesis.1 A number of ring opening and
expansion reactions of cyclopropanes are available by taking advantage of release of ring strain.
Incorporation of electron-donating and/or withdrawing groups not only activates cyclopropanes
toward ring scission, but also provides useful functionalities for subsequent elaboration.2
Cyclopropanols are most frequently utilized among heteroatom-substituted cyclopropanes and
are easily prepared by several reliable methods including the Kulinkovich cyclopropanation of
esters.3,4 Ring opening of cyclopropanols can be induced by electrophiles, such as Brönsted acid,
halogens, oxocarbenium ions, and transition- or non-transition-metal salts, in addition to strong
base.3 Conspicuously absent is the use of organozinc reagents for ring opening of
cyclopropanols,5 except for one unusual rearrangement of hydroxyl-substituted cyclopropanols.6
As part of research programs on synthetic applications of the Kulinkovich reagent,7 we report
herein regioselective ring opening of cyclopropanols promoted by organozinc reagents for the
preparation of α-methyl substituted ketones.
Results and Discussion
We were intrigued by the possibility of preparing functionalized zinc homoenolates from ring
opening of cyclopropanols triggered by sacrificial dialkylzinc reagents (Scheme 1). Treatment of
a cyclopropanol with a dialkylzinc reagent could provide the corresponding zinc alkoxide A and
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the alkane by-product (R–H) by analogy with a widely accepted mechanism of the Simmons-
Smith cyclopropanation of allylic alcohols.8 Initial formation of the zinc alkoxide A could be
accompanied by ring opening to generate a zinc homoenolate B. However, little was known
about zinc homoenolates of ketones in sharp contrast to those of esters and amide derivatives.
We were particularly concerned about the known propensity of metal homoenolates to cyclize to
the corresponding cyclopropoxides, which would become more pronounced for electrophilic
ketones.9 Matsubara’s attractive synthesis of cyclopropanols by the action of CH2(ZnI)2 was
supportive of facile cyclization of B to A.10 Nonetheless, we thought that a suitable method could
be devised to drive the otherwise unfavorable equilibrium. It would also be useful to gain
information about the stability and reactivity of zinc ketone homoenolates.
ZnR
R Zn R
OH
R2
O
R2
R1
R1
A
R H
ring
opening
O
R2
protonolysis
R1
ZnR
O
R2
R3
R2
R1
B
cross-coupling
with R3–X
O
R1
Scheme 1. A working hypothesis on organozinc-mediated ring opening of cyclopropanols.
Toward ring scission of cyclopropanol 1, we surveyed several common organozinc reagents
that were prepared according to known methods (Table 1).11-13 Treatment of 1 with a THF
solution of EtZnI at reflux resulted in clean ring opening to afford 2 in 72% yield (entry 1). Not
surprisingly, methylene chloride was unsuitable as solvent due to the limited solubility of EtZnI
(entry 2). Furukawa’s modification, EtZnCH2I (prepared from CH2I2 + Et2Zn), was also effective
for the preparation of 2 (entries 3 and 4), whereas high temperature was necessary (Cf. entry 5).14
Ring opening proceeded smoothly with gem-dizinc CH2(ZnI)2 (entries 6 and 7). Interestingly, the
use of Zn(CH2I)2 delivered a mixture of both regioisomers 2 and 2’ (entry 8). Diethylzinc also
induced ring opening to provide 2, but the reaction was sluggish and suffered from low
conversion (entry 9). Finally, the formation of 2 was also possible by the action of ZnI2, albeit in
low conversion even after longer reaction times.
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Table 1.Organozinc-mediated ring opening of cyclopropanol 1
Entry
Organozinc
EtZnI (1 equiv)c
EtZnI (1 equiv)c
Conditionsa
reflux, 8 h
Product yields (%)
1
2
2 (72)
2 (30) (72 brsm)h
b
CH2Cl2
reflux, 3 h
reflux, 8 h
reflux, 3 h
rt, 24 h
3
4
EtZnCH2I (1 equiv)d
EtZnCH2I (2 equiv)d
EtZnCH2I (2 equiv)d
CH2(ZnI)2 (1 equiv)e
CH2(ZnI)2 (2 equiv)e
Zn(CH2I)2 (1 equiv)f
Et2Zn (1 equiv)
2 (70)
2 (82)
5
2 (10)
6
reflux, 6 h
reflux, 6 h
reflux, 8 h
reflux, 8 h
reflux, 2 d
reflux, 1d
2 (69)
7
2 (65)
8
2 (30) + 2’(25)
2 (32) (76 brsm)h
2 (42) (72 brsm)h
2 (45) (73 brsm)h
9
10
11
ZnI2 (1 equiv)g
ZnI2 (2 equiv)g
aTHF was employed as solvent, except for entry 2. bCH2Cl2 was employed as solvent for entry 2.
cPrepared from Et2Zn (1 equiv) + I2 (1 equiv). dPrepared from Et2Zn (1 equiv) + CH2I2 (1 equiv).
ePrepared from Zn (2 equiv) + CH2I2 (1 equiv). fPrepared from Et2Zn (1 equiv) + CH2I2 (2
equiv). gPrepared from Et2Zn (1 equiv) + I2 (2 equiv). hbrsm: based on recovered SM.
We next examined the generality of this ring scission procedure by employing EtZnCH2I,
CH2(ZnI)2, or Et2Zn (Table 2): two zinc carbenoid reagents were equally effective for opening of
cyclopropanols bearing a range of common functional groups. Ring opening was found to be
complete in 3 h, and it was thus unnecessary to keep the reaction at reflux overnight. As was the
case for ring opening of 1, low conversion was observed when Et2Zn was employed (entries 3, 6,
13, and 16). Ring opening of 9 and 11 was markedly faster than other more substituted
cyclopropanols. When the reaction was quenched in 15 min, 10 was obtained in 61% yield, free
from double bond isomerization (entry 14).
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Table 2. Additi onal ring opening examples
organozinc
reagent
(1 equiv)
OH
O
R1
H
R1
THF, reflux
3a: R1 = OTBS
3b: R1 = OBn
4a: R1 = OTBS
4b: R1 = OBn
3c: R1 = N(Ts)Boc
4c: R1 = N(Ts)Boc
organozinc
reagent
(1 equiv)
OH
H
O
OEt
THF, reflux
OEt
OEt
6
OEt
5
organozinc
reagent
(1 equiv)
OH
O
H
Ph
THF, reflux
Ph
7
8
organozinc
reagent
O
OH
(1 equiv)
THF, reflux
9
10
organozinc
reagent
OH
O
(1 equiv)
THF, reflux
11
12
organozinc
reagent
(1 equiv)
OH
O
R2
H
R2
R
3
3
OTIPS
THF, reflux
13a: R2 = OTBDPS
13b: R2 = OPiv
14a: R2 = OTBDPS
14b: R2 = OPiv
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Entry SM Organozinc Time (h) Yield (%)
1
2
EtZnCH2I
CH2(ZnI)2
Et2Zn
3
3
4a (71)
4a (75)
4a (42)a
4b (81)
4b (71)
4b (45)a
4c (68)
4c (68)
6 (55)
3a
3a
3a
3b
3b
3b
3c
3c
5
3
12
3
4
EtZnCH2I
CH2(ZnI)2
Et2Zn
5
3
6
12
3
7
EtZnCH2I
CH2(ZnI)2
EtZnCH2I
CH2(ZnI)2
EtZnCH2I
CH2(ZnI)2
Et2Zn
8
3
9
3
10
11
12
13
14
15
16
17
18
19
20
3
6 (58)
5
3
8 (69)
7
3
8 (62)
7
8
8 (40)a
10 (61)
12 (68)
12 (36)a
14a (76)
14a (72)
14a (75)
14a (73)
7
EtZnCH2I 15 minb
9
CH2(ZnI)2
Et2Zn
3
12
3
11
11
13a
13a
13b
13b
EtZnCH2I
CH2(ZnI)2
EtZnCH2I
CH2(ZnI)2
3
3
3
aIn ring opening reactions with Et2Zn, the starting cyclo -propanol was recovered unreacted in
17–21% yields. bA short reaction time was necessary to avoid formation of a complex mixture.
With regard to limitations, cyclopropanols 15–19 containing a free hydroxyl group in the side
chain did not undergo ring opening, but were recovered presumably due to formation of a
chelate, even with the use of 2–3 equivalents of EtZnCH2I, CH2(ZnI)2, or Et2Zn (Scheme 2). The
ring scission procedure seemed to be sensitive to steric effects: ring opening of 20 and 22 proved
to be conspicuously slower than that of 3a-c.15
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OH
OH OH
R1
OH
H
H
18
15: R1 = H
OH OH
16: R1 = iPr
17: R1 = Ph
Ph
H
19
O
OH
EtZnCH2I
(2 equiv)
OBn
H
OBn
THF, reflux
20
21
24% (72% brsm)
O
OH
EtZnCH2I
(2 equiv)
OBn
23
H
22
OBn
THF, reflux
41% (76% brsm)
Scheme 2. Ring opening examples comparison.
The free cyclopropanol functionality is required for ring opening, as the corresponding TBS
ethers were found to be stable toward alkylzinc reagents under identical reaction conditions.
However, zinc iodide induced ring opening, albeit in low conversion: ketone 8 was isolated in
40% (82% brsm) yield from the TBS ether of 7.5 The observed regiochemistry (i.e., opening at
the less substituted C–C bond), except for formation of 2’ (Table 1, entry 8), rules out the
involvement of alkoxy radical-induced β-fragmentation. At the outset the nature of zinc reagents
was believed to affect the reaction mechanism. Isomerization of a cyclopropanol by ZnI2 to the
corresponding ketone is assumed to occur via intermediate I (Pathway 1, Scheme 3), in part
because no gas evolution was observed upon addition of ZnI2. On the other hand, addition of
diethylzinc results in vigorous evolution of gas from a reaction mixture in accord with formation
of zinc cyclopropoxide A (Pathway 2). Alkoxide A could be in equilibrium with homoenolate B,
where the former is expected to be strongly favored.16 Deuterium-labeling experiments were
carried out to probe the presumed intermediacy of a zinc homoenolate by using cyclopropanol 7.
No deuterium incorporation into the methyl group (e.g., –CH2D in C: R1 = Me, R2 = CH2Ph) was
detected upon addition of D2O, MeOD, or AcOD (Scheme 3). As a plausible fate of B, we
considered proton transfer leading to enolate D, protonolysis of which would afford E. The
regioisomeric zinc enolate (structure not shown) was discounted in view of exclusive formation
of 21 and 23. Regardless of the position of equilibrium between A and B, ring scission would be
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driven to completion by the proton transfer step. However, the proton transfer scenario was ruled
out by negative trapping experiments of enolate D (with D2O, MeOD, and PhCHO). Another
possibility was next considered that B might abstract a hydrogen atom from the solvent in light
of the known use of dialkylzincs as a source of alkyl radicals (albeit typically in the presence of
oxygen).17 This H-transfer pathway was discounted by no deuterium incorporation (e.g., F) from
the use of THF-d8. Similarly, the cognate labeling studies revealed no deuterium incorporation in
the ring opening reactions of 7 by EtZnCH2I, to which either pathway could be applicable.18
Unfortunately, the exact “proton” source remains unclear to date. With respect to the possible
intermediacy of a zinc homoenolate, it is noteworthy that B can be trapped in situ by
transmetalation at low temperatures.19
Pathway 1. ZnI2
I2Zn
ZnI2
OH
O
R2
O
R2
R2
H
R1
THF,
R1
R1
I
Pathway 2. ZnEt2
ZnEt2
THF,
ZnEt
ZnEt
O
OH
O
R2
R2
R2
R1
R1
R1
A
B
where
R1 = Me
THF
(d0 or d8)
D2O
H or D
O
ZnY
CH2D
O
O
R2
R2
R2
R1
R1
F
D
C
+
D2O
d0 or d7
O
O
R2
CH2D
E
Scheme 3. Plausible pathways for organozinc-mediated ring opening of cyclopropanols.
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Conclusions
We have described the regioselective preparation of α-methyl substituted ketones by organozinc
reagent-promoted ring opening of cyclopropanols. Mechanistic studies and other methods for
trapping the zinc homoenolates are currently underway to achieve C–C bond formation.
Experimental Section
General. All reactions were conducted under an inert atmosphere of argon. Anhydrous
tetrahydrofuran and diethyl ether were purified on alumina columns by Innovative Technology’s
purification system. Other reagents were purchased from commercial sources and used as
received. FT infrared spectra were obtained as neat films. Proton NMR spectra were recorded at
400 MHz, and carbon NMR spectra were recorded at 100 MHz (on Varian spectrometers).
GC/MS analyses were performed on an Agilent Model 6890N. Analytical thin-layer
chromatography (TLC) was performed on 0.2 mm precoated silica gel plates (60 F254). Column
chromatography was carried out with Merck (Kieselgel 60, 63-200 m) or SiliCycle silica gel
.
Representative procedure for ring opening of cyclopropanols with organozinc reagents
To a solution of CH2I2 (103 mg, 0.4 mmol) in anhydrous THF (0.3 mL) was added at rt under an
atmosphere of nitrogen a 1 M hexane solution of Et2Zn (0.4 mL, 0.4 mmol). After the mixture
had been stirred for 5 min, a solution of cyclopropanol 1 (50 mg, 0.18 mmol) in anhydrous THF
(0.5 mL) was added at rt. The reaction mixture was then refluxed for 3 h, and TLC showed
complete disappearance of the starting material. The mixture was cooled to rt and quenched with
sat NH4Cl (2 mL). The organic layer was diluted with Et2O (10 mL), washed with sat NH4Cl (2 x
10 mL) and brine (1 x 10 mL), and evaporated under reduced pressure. Purification of the crude
product by silica gel column chromatography using 2:98 EtOAc/hexane afforded pure 2 (41 mg,
82%) as a colorless liquid.
3-Methyl-5-triisopropylsiloxypentan-2-one (2). IR (neat, cm-1) 2941, 2866, 1714, 1461, 1100.
1H NMR (400MHz, CDCl3) δ 3.69 (t, J = 6.5 Hz, 2H), 2.80–2.71 (m, 1H), 2.16 (s, 3H), 1.98–
13
1.89 (m, 1H), 1.54–1.46 (m, 1H), 1.10 (d, J = 6.5 Hz, 3H), 1.08–1.00 (m, 21H). C NMR (100
MHz, CDCl3) δ 212.9, 61.1, 43.7, 36.0, 28.5, 18.2, 16.4, 12.1; MS (GC/MS) m/z 229 (M+–iPr,
100%), 199, 187, 131, 103.
5-tert-Butyldimethylsiloxy-3-methylpentan-2-one (4a). IR (neat, cm-1) 2857, 1713, 1255,
1
1097, 835. H NMR (400MHz, CDCl3) δ 3.62–3.59 (t, J = 6.5 Hz, 2H), 2.68 (m,1H), 2.10 (s,
3H), 1.94–1.86 (m, 1H), 1.56–1.48 (m, 1H), 1.09 (d, J = 7.3 Hz, 3H), 0.88 (s, 9H), 0.02 (s, 6H);
13C NMR (100 MHz, CDCl3) δ 212.5, 60.9, 43.9, 35.8, 28.5, 26.1, 18.5, 16.4, −5.2; MS
(GC/MS) m/z 230 (M+), 173 (100%), 155, 143, 129, 115, 99, 75.
1
5-Benzyloxy-3-methylpentan-2-one (4b). IR (neat, cm-1) 3030, 2880, 1706, 1453, 1027. H
NMR (400MHz, CDCl3) δ 7.40–7.22 (m, 5H), 4.41 (s, 2H), 3.50–3.40 (t, J = 6.5 Hz, 2H), 2.70
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(m, 1H), 2.10 (s, 3H), 2.06–1.98 (m, 1H), 1.66–1.58 (m, 1H), 0.95 (d, J = 6.5 Hz, 3H). 13C NMR
(100 MHz, CDCl3) δ 212.7, 138.5, 128.6, 127.9, 119.2, 73.2, 68.1, 44.2, 32.9, 28.6, 16.6; MS
(GC/MS) m/z 206 (M+), 178, 119, 99, 91 (100%).
tert-Butyl N-3-methyl-4-oxopentyl-N-tosylcarbamate (4c). IR (neat, cm-1) 3070, 2985, 1709,
1
1729, 1356, 1157. H NMR (400MHz, CDCl3) δ 7.50 (d, J = 8.1Hz, 2H), 7.29 (d, J = 8.1 Hz,
2H), 3.79 (t, J = 6.5 Hz, 2H), 2.69–2.62 (m, 1H), 2.42 (s, 3H), 2.20 (s, 3H), 2.19–1.99 (m, 1H),
13
1.81–1.72 (m, 1H), 1.33 (s, 9H), 1.1 (d, J = 7.3 Hz, 3H). C NMR (100 MHz, CDCl3) δ 211.8,
151.2, 144.4, 137.5, 129.5, 128.0, 84.6, 45.6, 44.6, 32.8, 28.2, 28.1, 21.8, 16.8; MS (GC/MS) m/z
369 (M+), 251, 186, 155, 96 (100%); ESI-HRMS m/z calcd. for C18H27NO5SNa [M+Na]+
392.1508, found 392.1506.
1
5,5-Diethoxy-3-methylpentan-2-one (6). IR (neat, cm-1) 2978, 1711, 1461, 1114. H NMR
(400MHz, CDCl3) δ 4.51 (t, J = 6.5 Hz, 1H), 3.63–3.60 (m, 2H), 3.50–3.45 (m, 2H), 2.67 (m,
1H), 2.10 (s, 3H), 2.07–1.96 (m, 1H), 1.83–1.75 (m, 1H), 1.20 (t, J = 6.9 Hz, 6H), 1.09 (d, J =
7.3 Hz, 3H). 13C NMR (100 MHz, CDCl3) δ 211.7, 102.5, 61.3, 35.7, 28.4, 26.1, 16.3, 15.5.
1
3-Methyl-4-phenylbutan-2-one (8). IR (neat, cm-1) 3040, 2971, 2931, 1710, 1454, 1359. H
NMR (400MHz, CDCl3) δ 7.78–7.15 (m, 5H), 2.99 (dd, J = 6.4, 13.0 Hz, 1H), 2.83 (m, 1H),
2.56 (dd, J = 7.3, 13.0 Hz, 1H), 2.08 (s, 3H), 1.09 (d, J = 7.3 Hz, 3H). 13C NMR (100 MHz,
CDCl3) δ 212.5, 139.9, 129.2, 129.1, 128.7, 126.5, 49.1, 39.1, 29.1, 16.5; MS (GC/MS) m/z 162
(M+), 147, 119, 91 (100%).
1
(E)-Oct-5-en-3-one (10). IR (neat, cm-1) 2971, 2931, 1710, 1455, 1267. H NMR (400MHz,
CDCl3) δ 5.60–5.48 (m, 2H), 3.10 (d, J = 6.5 Hz, 2H), 2.45 (q, J = 7.3 Hz, 2H), 2.13–2.08 (m,
13
2H), 1.41 (t, J = 7.3 Hz, 3H), 0.96 (t, J = 7.3 Hz, 3H). C NMR (100 MHz, CDCl3) δ 211.1,
136.8, 121.2, 46.6, 35.5, 25.4, 13.8, 7.9; MS (GC/MS) m/z 126 (M+), 111, 69 (100%).
6-Ethyloctan-3-one (12). IR (neat, cm-1) 2985, 1717, 1459, 1102. 1H NMR (400MHz, CDCl3) δ
13
2.40–2.25 (m, 4H), 1.45–1.22 (m, 7H), 1.01 (t, J = 7.3 Hz, 6H), 0.95 (t, J = 7.3 Hz, 3H). C
NMR (100 MHz, CDCl3) δ 211.0, 43.6, 40.1, 35.8, 28.7, 25.4, 13.8, 7.9; MS (GC/MS) m/z 156
(M+), 125, 109, 85 (100%).
13,13-Diisopropyl-2,2,9,14-tetramethyl-3,3-diphenyl-4,12-dioxa-3,13-disilapentadecan-8-
1
one (14a). IR (neat, cm-1) 2937, 2870, 1711, 1267, 1101. H NMR (400MHz, CDCl3) δ 7.71–
7.61 (m, 4H), 7.40–7.31 (m, 6H), 3.69–3.60 (m, 4H), 2.79–2.71 (m, 1H), 2.60–2.50 (m, 2H),
1.98–1.90 (m, 1H), 1.81–1.75 (pentet, J = 6.4 Hz, 2H), 1.50–1.42 (m, 1H), 1.10 (d, J = 7.3 Hz,
3H), 1.10–1.00 (m, 30H). 13C NMR (100 MHz, CDCl3) δ 214.4, 135.7, 134.0, 129.8, 127.9, 63.2,
61.2, 42.9, 37.5, 36.1, 30.5, 27.1, 26.8, 19.4, 18.2, 16.6, 12.2; ESI-HRMS m/z calcd. for
C31H50O3Si2Na [M+Na]+ 577.3507, found 577.3493.
5-Methyl-4-oxo-7-((triisopropylsilyl)oxy)heptyl pivalate (14b). IR (neat, cm-1) 2938, 2866,
1713, 1750, 1459, 1157, 1099. 1H NMR (400MHz, CDCl3) δ 4.20 (t, J = 6.5 Hz, 2H), 3.68 (t, J =
7.3 Hz, 2H), 2.81–2.78 (m, 1H), 2.59–2.52 (m, 1H), 1.94–1.60 (m, 3H), 1.54–1.46 (m, 1H), 1.19
(s, 9H), 1.10–1.02 (m, 25 H). 13C NMR (100 MHz, CDCl3) δ 213.7, 178.7, 63.8, 61.1, 42.8, 38.9,
37.5, 36.7, 36.1, 36.0, 32.8, 27.4, 26.6, 23.3, 23.0, 18.2, 17.3, 16.6, 12.1; MS (GC/MS) m/z 357,
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255 (100%), 215, 185, 125; ESI-HRMS m/z calcd. for C22H44O4SiNa [M+Na]+ 423.2901, found
423.2897.
5-Benzyloxy-3,6-dimethylheptan-2-one (21). IR (neat, cm-1) 2932, 2858, 1709, 1266, 1100. H
1
NMR (400MHz, CDCl3) δ 7.40–7.25 (m, 5H), 4.66 (d, J = 11.3 Hz, 1H), 4.54 (d, J = 11.3 Hz,
1H), 3.41–3.36 (m, 1H), 2.72 (m, 1H), 2.12 (s, 3H), 1.90–1.80 (m, 1H), 1.70–1.63 (m, 1H), 1.35–
1.22 (m, 1H), 1.10 (d, J = 6.4 z, 3H), 0.96 (d, J = 7.3 Hz, 3H), 0.92 (d, J = 7.3 Hz, 3H). 13C NMR
(100 MHz, CDCl3) δ 212.7, 138.5, 128.6, 127.9, 127.8, 73.2, 68.1, 44.2, 32.9, 28.5, 23.6, 20.2,
19.0, 16.6; MS (GC/MS) m/z 248, 206, 91 (100%).
1
5-Benzyloxy-3-methyloct-7-en-2-one (23). IR(neat, cm-1) 3027, 2881, 1709, 1494, 1452. H
NMR (400MHz, CDCl3) δ 7.41–7.22 (m, 5H), 5.95–5.80 (m, 1H), 5.15–5.01 (m, 2H), 4.58 (s,
3H), 3.55 (m,1H), 2.50–2.30 (m, 1H), 2.01 (s, 3H), 1.80–1.71 (m, 1H), 1.63–1.52 (m, 2H), 1.11
13
(d, J = 7.3 Hz, 3H). C NMR (100 MHz, CDCl3) δ 212.4,135.76, 134.1, 129.8, 127.9, 119.2,
74.9, 63.3, 37.7, 36.1, 30.5, 16.6. MS (GC/MS) m/z 246, 206, 203, 91 (100%).
Acknowledgements
We thank the National Science Foundation (CHE0615604) for generous financial support.
References and Notes
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Das, P. P.; Cha, J. K. Unpublished results.
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