Organocatalytic methods for chemoselective O-tert-butoxycarbonylation of phenols and their regeneration from the O-t-Boc derivatives

Article abstract of DOI:10.1021/jo8013325

(Chemical Equation Presented) Carbon tetrabromide (CBr₄) catalyzes O-tert-butoxycarbonylation of functionalized phenols without any side reactions (bromination, addition of CBr₃ to a double bond, and formation of symmetrical diaryl carbonates, cyclic carbonates, or carbonic-carbonic anhydrides). The parent phenols are regenerated from the O-t-Boc derivatives by the catalyst system CBr₄-PPh₃ without affecting other protecting groups (aryl alkyl ether, alkyl ester, and thioacetal) or competitive side reaction such as bromination, nitrene (from NO₂) and α,α-dibromoolefine (with CHO/COMe) formation, and transesterification (with CO₂Me/Et) taking place.

Full text of DOI:10.1021/jo8013325

TABLE 1. Evaluation of Various Organocatalysts for O-t-Boc
Organocatalytic Methods for Chemoselective
O-tert-Butoxycarbonylation of Phenols and Their
Regeneration from the O-t-Boc Derivatives
Formation of 1a
Sunay V. Chankeshwara, Rajesh Chebolu, and
Asit K. Chakraborti*
entry
catalyst
time (h)
yield (%)b
1
2
3
4
5
6
7
8
none
CBr4
NBS
NCS
I2
2 d
11
24
24
24
24
24
24
24
24
24
0c
93d
0c
Department of Medicinal Chemistry, National Institute of
Pharmaceutical Education and Research (NIPER),
Sector 67, S. A. S. Nagar 160 062, Punjab, India
0c
e f
,
30
15
15
CHBr3
CH2Br2
CHCl3
CH2Cl2
CCl4
akchakraborti@niper.ac.in
0c
ReceiVed June 26, 2008
9
10
11
0c
0c
0c
HBrg
a 1 (2.5 mmol) was treated with Boc2O (2.5 mmol, 1 equiv) under
neat conditions at room temperature (∼35-40 °C) in the presence of the
organocatalyst (10 mol %). b Isolated yield of 2. c Unreacted 1 was
recovered. d 2 was obtained in 96% yield at 65 °C for 0.75 h. e 3 was
the only product formed. f 2 was formed in 10% yield as the only
product at 65 °C after 12 h. g 48% aqueous solution of HBr was used.
butoxycarbonylation is a suitable alternative2 as the O-t-Boc
moiety is compatible with reaction conditions routinely
adopted in organic synthesis. The limited methodologies5
available for the preparation of O-t-Boc phenols have several
drawbacks: the requirement of long reaction times,5d,e low/
high temperatures,5d,e special efforts to prepare the tert-
butoxycarbonylation reagents,5d,e and auxiliary substances
(e.g., solvents, bases, etc.).5 In analogy with the recent trend
of Lewis acid catalyzed N-tert-butoxycarbonylation,6 O-t-
Boc formation does not become feasible as phenols form the
tert-butyl ethers with Boc2O in the presence of a strong Lewis
acid7 and a mild Lewis acid requires heating under reflux in
DCM for 5-19 h.5f Therefore, the need to use/develop
organocatalytic procedures was realized.
Only a few reports are available for O-t-Boc formation using
nucleophilic organocatalyst.5b-e,8 Some of these give side
products such as symmetrical carbonates, cyclic carbonate, and
carbonic-carbonic anhydrides.5c
In our search for an effective organocatalyst, we choose
4-nitrophenol 1 as a model substrate and treated it with Boc2O in
the presence of CBr4, NBS, NCS, and I2 (Table 1). The best results
Carbon tetrabromide (CBr4) catalyzes O-tert-butoxycar-
bonylation of functionalized phenols without any side
reactions (bromination, addition of CBr3 to a double bond,
and formation of symmetrical diaryl carbonates, cyclic
carbonates, or carbonic-carbonic anhydrides). The parent
phenols are regenerated from the O-t-Boc derivatives by
the catalyst system CBr4-PPh3 without affecting other
protecting groups (aryl alkyl ether, alkyl ester, and
thioacetal) or competitive side reaction such as bromina-
tion, nitrene (from NO2) and R,R-dibromoolefine (with
CHO/COMe)formation,andtransesterification(withCO2Me/
Et) taking place.
The protection and deprotection of hydroxyl group are
encountered with 30% and 14% frequency, respectively, in
the preparation of drug candidates.1 Acylation2,3 is a common
approach to protect hydroxyl groups, but the regeneration
of the parent compounds requires hydrolysis under strong
alkaline conditions2 incompatible with other functional
groups. Although chemoselective deacylation is achievable
by “demand-based thiolate anion generation”4 under neutral
or virtually neutral and nonaqueous conditions, O-tert-
(4) (a) Chakraborti, A. K.; Sharma, L.; Nayak, M. K. J. Org. Chem. 2002,
67, 2541. (b) Chakraborti, A. K.; Nayak, M. K.; Sharma, L. J. Org. Chem. 1999,
64, 8027.
(5) (a) Houlihan, F.; Bouchard, F.; Fre´chet, J. M. J.; Willson, C. G. Can.
J. Chem. 1985, 63, 153. (b) Hansen, M.; Riggs, J. R. Tetrahedron Lett. 1998,
39, 2705. (c) Basel, Y.; Hassner, A. J. Org. Chem. 2000, 65, 6368. (d) Ouchi,
H.; Saito, Y.; Yamamoto, Y.; Takahata, H. Org. Lett. 2002, 4, 585. (e) Saito,
Y.; Ouchi, H.; Takahata, H. Tetrahedron 2006, 62, 11599. (f) Bartoli, G.; Bosco,
M.; Carlone, A.; Dalpozzo, R.; Locatelli, M.; Melchiorre, P.; Palazzi, P.; Sambri,
L. Synlett 2006, 2104.
(1) Carey, J. S.; Laffan, D.; Thomson, C.; Williams, M. T. Org. Biomol.
Chem. 2006, 4, 2337.
(2) Greene, T. W.; Wuts, P. G. M. ProtectiVe Groups Organic Synthesis,
3rd ed.; Wiley: New York, 1999.
(3) (a) Shivani; Gulhane, R.; Chakraborti, A. K. J. Mol. Catal. A: Chem.
2007, 264, 208. (b) Chakraborti, A. K.; Shivani, J. Org. Chem. 2006, 71, 5785.
(c) Chakraborti, A. K.; Gulhane, R. Synlett 2004, 627. (d) Chakraborti, A. K.;
Gulhane, R. Synthesis 2004, 111. (e) Chakraborti, A. K.; Gulhane, R.; Shivani,
Synlett 2003, 1805. (f) Chakraborti, A. K.; Sharma, L.; Gulhane, R.; Shivani,
Tetrahedron 2003, 59, 7661. (g) Chakraborti, A. K.; Gulhane, R. J. Chem. Soc.,
Chem. Commun. 2003, 1896. (h) Chakraborti, A. K.; Gulhane, R. Tetrahedron
Lett. 2003, 44, 3521.
(6) (a) Chankeshwara, S. V.; Chakraborti, A. K. Synthesis 2006, 2784. (b)
Chakraborti, A. K.; Chankeshwara, S. V. Org. Biomol. Chem. 2006, 4, 2769.
(c) Chankeshwara, S. V.; Chakraborti, J. Mol. Catal. A: Chem. 2006, 253, 198.
(d) Chankeshwara, S. V.; Chakraborti, Tetrahedron Lett. 2006, 47, 1087.
(7) Bartoli, G.; Bosco, M.; Carlone, A.; Dalpozzo, R.; Locatelli, M.;
Marcantoni, E.; Melchiorre, P.; Sambri, L. J. Org. Chem. 2006, 71, 9580.
(8) Chebolu, R.; Chankeshwara, S. V.; Chakraborti, A. K. Synthesis 2008,
1448.
10.1021/jo8013325 CCC: $40.75
Published on Web 10/10/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 8615–8618 8615

TABLE 2. CBr4-Catalyzed O-t-Boc Formation of Phenolsa
TABLE 3. Reaction of 4 with Boc2O under Various Conditionsa
yield (%)b
Boc2O
entry (equiv)
temp
(°C)
time
(h)
catalyst
solvent
5
6
c
c
c
c
1
2
3
4
5
6
7
8
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
DCM
DCM
neat
reflux 12
reflux 12
50
30
40
20
60
20
70
40
90
40
95
40
95
45
95
65
65
rt
1
1
12
12
1
neat
d
K2CO3
K2CO3
K2CO3
THF
THF
neat
10
5
d
rt
d
65
65
rt
d
K2CO3
neat
1
0.5
9
DMAPe
DMAPe
DMAPe
DMAPe
MeCN
MeCN
neat
neat
neat
neat
neat
neat
10
11
12
13
14
15
16
rt
rt
rt
0.5
0.5
0.5
c
CBr4
CBr4
CBr4
rt
18
78
83
85
c
65
65
65
1
1
5
c
c
CBr4
90
a 4 (2.5 mmol) was treated with Boc2O under various conditions.
b Isolated yield. c 10 mol% of the catalyst was used. d Used in 1.5 equiv
of 4. e Used in 0.5 equiv of 4.
such as bromination11 and carbonic-carbonic anhydride formation5c
were observed.
The reported procedure forms cyclic carbonate5c by the reaction
of Boc2O with 1,2-diols. Therefore, to demonstrate the advantage
of this newly developed methodology over the reported procedures
for selective O-t-Boc formation, the reaction of 1,2-dihydroxyben-
zene 4 with Boc2O was carried in the presence of some of the
reported catalysts/reagent and CBr4 (Table 3).
The CBr4-catalyzed reactions were found to be highly selective.
The mono-O-t-Boc was the only product in using 1 equiv of Boc2O
at room temperature after 18 h and at 65 °C after 1 h. The use of
2 equiv of Boc2O afforded the mono-O-t-Boc after 1 h at 65 °C
and the di-O-t-Boc product as the only product at 65 °C after 5 h.
The reactions performed with the reported catalysts (following
reported experimental conditions or the conditions operative for
the CBr4-catalyzed reactions) were either nonselective (affording
a mixture of the mono- and di-Boc compounds) or produced the
di-Boc compound (entries 4, 6, and 8; Table 3). In case of DMAP,
no significant amount of the mono-Boc compound was formed
(entries 9-12, Table 3). None of the reported catalysts was effective
for selective mono-Boc formation.
As the ultimate utility of a protecting group is judged by its
ease of manipulation in the presence of other functional groups,
we planned to test the compatibility of the reaction conditions
with various functional/protecting groups such as nitrile, formyl,
acetyl, silyl, carboalkoxy, dithiolane, acetoxy, R,ꢀ-unsaturated
carbonyl, N-t-Boc, acetamido and benzamido (Table 4). The
desired O-t-Boc compounds were formed in high yields with
excellent chemoselectivity. No competitive side reactions such
as bromination,11 N-t-Boc formation at the NHCOR moiety
(entries 11-13, Table 4),12 addition of the CBr3 radical to the
R,ꢀ-unsaturated carbonyl moiety,13 and desilylation14 were
a The phenol (2.5 mmol) was treated with (Boc)2O (2.5 mmol, 1
equiv) under neat condition at room temperature (∼35-40 °C; Method
A) and at 65 °C (Method B) in the presence of CBr4 (10 mol%). b Yield
of the purified O-t-Boc derivative.
were obtained with CBr4, affording the 4-(tert-butoxycarbonyl-
oxy)nitrobenzene 2 in 93% yield at room temperature (35-40 °C)
after 11 h and in 96% yield at 65 °C after 0.75 h (entry 2, Table
1). No significant amount of 2 was formed either in the absence
of any catalyst (footnote, entry 1, Table 1) or in the presence of
NBS, NCS, I2 (entries 3-5, Table 1). Surprisingly, the use of I2
resulted in the formation of 4-tert-butoxynitrobenzene 39 as the
only isolable product in 30% yield (entry 5, Table 1). Other
bromocarbons such as CHBr3 and CH2Br2 were less effective,
affording 2 in 15% yield (entries 6 and 7, Table 1) but chlorocar-
bons such as CCl4, CHCl3, and CH2Cl2 were ineffective (entries
8-10, Table 1). To find the effect of solvent, the reaction of 1
with Boc2O (1 equiv) was carried out in DCM, THF, MeCN,
MeNO2, and MeOH in the presence of CBr4 (10 mol%), and 2
was formed in 50%, 0, 10%, 0, and 5% yields, respectively.
The use of CBr4 circumvented the notable drawbacks of the
reported procedures such as the use of THF,5a MeCN,5c PhH,5e
and DCM5f as solvents that are not preferred by the recent solvent
selection guide of the pharmaceutical industry from an environ-
mental perspective10 and the requirement of stoichiometric quanti-
ties or more of additional reagents, e.g., isoquinoline5e or K2CO3
5a
that add environmental burden. The generality was demonstrated
with various phenols that afforded the desired O-t-Boc derivatives
in excellent yields on treatment with Boc2O at room temperature
(Method A) and at 65 °C (Method B) (Table 2). No side reactions
(11) Hunter, W. H.; Edgar, D. E. J. Am. Chem. Soc. 1932, 54, 2025.
(12) Burk, M. J.; Allen, J. G. J. Org. Chem. 1997, 62, 7054.
(13) Kharasch, M. S.; Jensen, E. V.; Urry, W. H. J. Am. Chem. Soc. 1946,
68, 154.
(14) (a) Lee, A. S.-Y.; Yeh, H.-C.; Shie, J.-J. Tetrahedron Lett. 1998, 39,
5249. (b) Chen, M.-Y.; Lee, A. S.-Y. J. Org. Chem. 2002, 67, 1384.
(9) Woiwode, T. F.; Rose, C.; Wandless, T. J. J. Org. Chem. 1998, 63, 9594.
(10) (a) Alfonsi, K.; Colberg, J.; Dunn, P. J.; Fevig, T.; Jenings, S.; Johnson,
T. A.; Kleine, H. P.; Knight, C.; Nagy, M. A.; Perry, D. A.; Stefaniak, M. Green
Chem. 2008, 10, 31. (b) Capello, C.; Fischer, U.; Hungerbu¨hler, K. Green Chem.
2007, 9, 927.
8616 J. Org. Chem. Vol. 73, No. 21, 2008

TABLE 4. CBr4-Catalyzed O-t-Boc Formation of Phenols
TABLE 5. Reaction of 7 with Boc2O under Various Conditionsa
Containing Other Functional Groupsa
yield (%)b
Boc2O
entry (equiv)
temp
(°C)
time
(h)
catalyst
solvent
8
9
c
c
c
c
1
2
3
4
5
6
7
8
1
2
1
2
1
2
1
2
1
2
1
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
DCM
DCM
neat
neat
THF
THF
neat
neat
neat
neat
neat
reflux 12
reflux 12
60
60
60
50
60
40
40
30
83
96
98
20
40
20
50
20
60
50
70
65
65
rt
1
1
12
12
1
d
K2CO3
K2CO3
K2CO3
d
rt
d
65
65
rt
d
K2CO3
1
10
c
9
10
11
CBr4
CBr4
c
65
65
0.75
0.75
c
CBr4
a 7 (2.5 mmol) was treated with Boc2O under various conditions.
b Isolated yield. c 10 mol % of the catalyst was used. d 1.5 equiv with
respect to 7 was used.
SCHEME 1. Role of CBr4 in O-t-Boc Formation of Phenols
a The phenol (2.5 mmol) was treated with (Boc)2O (2.5 mmol, 1
equiv) under neat condition at room temperature (∼35-40 °C; Method
A) and at 65 °C (Method B) in the presence of CBr4 (10 mol %).
b Yield of the purified O-t-Boc derivative. c 2 equiv of (Boc)2O was
used. d Unreacted starting material was recovered.
observed. The chiral substrates (entries 11-13, Table 4) afforded
optically pure products. The difference of rate of reaction for
4- and 2-hydroxyacetophenones (entries 4 and 5, Table 4) were
in consonance with those observed for 4- and 2-chlorophenols
(entries 7 and 8, Table 2) and for 4- and 2-nitrophenols (entries
11 and 12, Table 2).
showed selective formation of the Boc derivative of the phenolic
OH group and clearly demonstrated its advantage.
As in the case of alcoholic hydroxyl group, no O-t-Boc formation
took place at room temperature (Method A) and the reaction took
longer time following Method B (entry 17, Table 2) we thought
that the difference in the rate of reactions with phenolic and
alcoholic hydroxyl groups may form the rationale for chemose-
lective O-t-Boc formation of the phenolic OH group in the presence
of the alcoholic OH group. Further, the intramolecular competition
between a phenolic OH and alcoholic OH group may serve to
measure the advantage of this new methodology over the reported
procedures.
The lack of O-t-Boc formation of 1 in the presence of I2, an
organocatalyst with Lewis acidic character, and generation of the
undesired product 3 (entry 5, Table 1) tempted us to speculate the
plausible role of CBr4 (Scheme 1).15 The lack of formation of any
significant amount of 2 by the reaction of 1 with Boc2O in the
presence of HBr (entry 11, Table 1) ruled out the possibility of
any in situ formed HBr (by the reaction of 1 with CBr4) as the
actual catalytic agent. We invoke a radical-cation-radical-anion pair
(RCRAP) I, through n-σ* transition16 by interaction of the C-Br
bond dipole and the dipole of (Boc)2O, and a cation-neutral-anion
pair (CNAP) II that is involved in complex formation with the
phenol in the six-membered cyclic transition state III. The charge
transfer complex formation between CBr4 and electron-rich sub-
strates (e.g., aromatic hydrocarbon)17 and halide anion18 is known
and forms the basis of the proposition/involvement of the charge
The reaction of 3-hydroxybenzyl alcohol with Boc2O has been
reported5a to be nonselective giving a mixture of the mono- and
di-Boc derivatives. The Zn(OAc)2 catalyzed reaction5f of 3-(4-
hydroxyphenyl)-1-propanol afforded a mixture of primary mono-
protected and diprotected diol. We chose 4-hydroxybenzyl alcohol
7 as the model substrate for adjudging the selectivity of Boc
formation under various methodologies (Table 5). The use of
DMAP has been reported to be nonselective and resulted in the
formation of the di-Boc derivative.8 The other reported catalyst/
reagent such as Zn(OAc)2 and K2CO3 were also nonselective
forming 60:20 to 30:70 mixtures of the mono- and di-Boc
derivatives. Compared to these observations, the present method
(15) CBr4 has been used as organocatalyst, but its role has not been defined
except for the proposition of its mild Lewis acid property. Zhang, L.; Luo, Y.;
Fan, R.; Wu, J. Green Chem. 2007, 9, 1022, and references therein.
(16) Stevenson, D. P.; Coppinger, G. M. J. Am. Chem. Soc. 1962, 84, 149.
(17) Janini, G. M.; King, J. W.; Martire, D. E. J. Am. Chem. Soc. 1974, 96,
5268.
J. Org. Chem. Vol. 73, No. 21, 2008 8617

TABLE 6. Regeneration of Phenols from O-t-Boc Phenols
SCHEME 2. Role of CBr4-PPh3 in O-t-Boc Deprotection
a
Catalyzed by CBr4-PPh3
oxygen atom of the Boc25 followed by the formation of a six-
membered intermediate IV.
In summary, organocatalytic procedures have been developed
for chemo- and regioselective O-tert-butoxycarbonylation of phe-
nols and their regeneration from the O-t-Boc derivatives. No side
reactions such as symmetrical diaryl carbonate, cyclic carbonate,
carbonic-carbonic anhydride, or R,R-dibromoolefin formation,
bromination, nitrene formation, or transesterification were observed.
A mechanistic rationale for the catalytic action of CBr4 in O-t-
Boc formation has been formulated by invoking radical-cation-
radical-anion pair (RCRAP) and cation-neutral-anion pair (CNAP)
intermediates.
a The O-t-Boc phenol (2.5 mmol) in MeOH (5 mL) was treated with
CBr4 (10 mol %) and Ph3P (20 mol %) under reflux (bath temp ∼80
°C). b Yield of the purified phenol. c Figures in parentheses are the
corresponding values when the reaction was carried out in the presence
of CBr4 (2.5 mmol, 1 equiv) and Ph3P (5 mmol, 2 equiv).
Experimental Section
Typical Experimental Procedures. Protection. To a mixture of
4-nitrophenol (1) (0.35 g, 2.5 mmol) and CBr4 (0.083 g, 10 mol
%) was added Boc2O (0.54 g, 2.5 mmol, 1 equiv), and the reaction
mixture was stirred magnetically at room temperature (∼40 °C,
Method A). After complete consumption of 1 (TLC, 12 h), the
reaction mixture was extracted with EtOAc (3 × 5 mL), and the
combined EtOAc extracts were concentrated under vacuum rotary
evaporation. The residue was passed through a bed of silica gel
(10 g; no. 60-120) and eluted with 5% EtOAc in hexane (100
mL) to afford the 4-(tert-butoxycarbonyloxy)nitrobenzene (entry
11, Table 2) as a pale yellow solid (0.57 g, 93%). Mp: 73-74 °C.
IR (KBr) ν: 3119, 2985, 1757, 1616, 1594, 1525, 1491, 1369, 1347,
1274, 1220, 1143, 850 cm-1. 1H (CDCl3, 300 MHz, TMS) δ: 8.25
transfer complexes I and II. The more polar O-H bond in phenol
compared to that of alcohol forms the hydrogen-bonded structure
corresponding to III with the phenolic OH moiety of 7 and provides
rationale for selective O-t-Boc formation with the phenolic OH
group (entries 9-11, Table 5).
The lack of formation of 2 during the CBr4-catalyzed reaction of 1
with Boc2O in the presence of solvents may be rationalized due to
inhibition of formation of the hydrogen-bonded structure III.
We next focused our attention to O-t-Boc deprotection. The
regeneration of phenols from the corresponding O-t-Boc deriva-
tives is carried out with TFA5b that is often associated with
side reaction and excess of base.20 We observed that a
combination of CBr4 (10 mol %)-Ph3P (20 mol %) constitutes
an excellent catalyst system21 to regenerate the parent phenols
from the corresponding O-t-Boc derivatives in MeOH under
reflux (Table 6). The use of CBr4 or PPh3 alone was ineffective.
The deprotection is achievable in shorter times (1-2 h) with
higher amounts of the catalyst system (footnote, entries 4 and
10, Table 6). The reaction conditions were compatible with other
groups such as OMe/OBn, CO2Me, and thioactal (entries 1, 2,
8, and 11, Table 6). No side reactions such as bromination,11
R,R-dibromoolefine formation22 with the CHO/COMe groups
(entries 6 and 7, Table 6), nitrene formation of the NO2 group,23
and transesterification24 for substrate bearing CO2Me group
(entry 8, Table 6) took place.
(d, J ) 8.9 Hz, 2 H), 7.36 (d, J ) 8.9 Hz, 2 H), 1.57 (s, 9 H). 13
C
(CDCl3, 75 MHz, TMS) δ: 156.2, 151.0, 145.6, 125.7, 122.4, 85.3,
28.1. MS (EI) m/z: 239 [M+]; identical with an authentic
compound.5e
Deprotection. To the mixture of 4-(tert-butoxycarbonyloxy)-
benzonitrile (0.57 g, 2.5 mmol) were added CBr4 (0.083 g, 10 mol
%) and PPh3 (0.12 g, 20 mol %), and the reaction mixture was
stirred magnetically under reflux (bath temp ∼80 °C). After
complete consumption of the starting material (TLC, 6 h), the
reaction mixture was extracted with EtOAc (3 × 5 mL) and the
combined EtOAc extracts were concentrated under vacuum rotary
evaporation. The residue was passed through a bed of silica gel
(10 g; no. 60-120) and eluted with 5% EtOAc in hexane (100
mL) to afford the 4-hydroxybenzonitrile as an off-white solid (0.27
g, 90%) (entry 4, Table 6). Mp: 111-112 °C. IR (KBr) ν: 3180,
2925, 2850, 2223, 1590, 1505 cm-1. 1H NMR (CDCl3, 300 MHz,
TMS) δ: 7.56 (d, J ) 8.6 Hz, 2 H), 6.94 (d, J ) 8.6 Hz, 2 H), 6.79
(bs, 1 H). MS (EI) m/z: 119 [M+1].
The deprotection may be rationalized by the proposed
mechanism (Scheme 2) involving coordination of the electro-
philic phosphonium adduct of CBr4 and PPh3 with the carbonyl
Acknowledgment. R.C. thanks UGC, New Delhi for the
award of JRF. The authors also thank OPCW, Hague, The
Netherlands for financial support.
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J. Chem. Soc. 1965, 4831. (b) Freeman, A. W.; Urvoy, M.; Criswell, M. E. J.
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Supporting Information Available: Spectral data of all
compounds and scanned spectra of a few representative known
and all unknown compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
JO8013325
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