A concise synthesis of (+)-polyoxamic acid and (+)-5-O-carbamoyl polyoxamic acid

Article abstract of DOI:10.1055/s-0028-1083629

We report the concise synthesis of (+)-polyoxamic acid and (+)-5-O-carbamoyl polyoxamic acid from L-xylose in seven or six steps, respectively. The key steps include a diastereoselective amination of syn-1,2-polybenzyl ethers using chlorosulfonyl isocyanate (CSI) and the one-pot introduction of carbamates into the allylic benzyl ether and primary hydroxyl moieties. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0028-1083629

LETTER
2985
A Concise Synthesis of (+)-Polyoxamic Acid and (+)-5-O-Carbamoyl
Polyoxamic Acid
S
ynthesi
n
s
of (+)-Polyoxami
S
c
A
cid u Kim, Qing Ri Li, Guang Ri Dong, Seol Hee Woo, Hyun-ju Park, Ok Pyo Zee, Young Hoon Jung*
College of Pharmacy, Sungkyunkwan University, Suwon 440-746, Korea
Fax +82(31)2907773; E-mail: yhjung@skku.ac.kr
Received 12 August 2008
itory activity against chitin synthase, which is an impor-
tant component of the fungal cell wall structure. In
addition, it has powerful therapeutic activity against the
human pathogen, Candida albicans.4
Abstract: We report the concise synthesis of (+)-polyoxamic acid
and (+)-5-O-carbamoyl polyoxamic acid from L-xylose in seven or
six steps, respectively. The key steps include a diastereoselective
amination of syn-1,2-polybenzyl ethers using chlorosulfonyl isocy-
anate (CSI) and the one-pot introduction of carbamates into the al-
lylic benzyl ether and primary hydroxyl moieties.
Due to the potent biological activity and structural rela-
tionship with polyoxins, several synthetic approaches to
produce polyoxamic acid (1) and its O-carbamoyl deriva-
tive 2 have been reported.5 However, these synthetic ap-
proaches involve a large number of steps. Therefore, the
development of efficient and concise synthetic strategies
for compounds 1 and 2 is still an active field.
Key words: polyoxamic acid, polyoxin, chlorosulfonyl isocyanate,
amination, sugar
a-Amino acids comprise a number of important building
blocks of living systems, and are the principal compo-
nents of naturally occurring biologically active com-
pounds. These compounds and their derivatives have
attracted considerable attention as potential medical
agents on account of their interesting physiological and
As part of an ongoing research program directed towards
the stereoselective amination of various allylic ethers us-
OBn
OH
pharmacological activities.1 As a consequence the devel-
opment of methods for synthesizing a-amino acids has
been an area of intense research over the last decade.
Moreover, the increasing demand for these compounds
has prompted the development of new synthetic method-
ologies.2
a
BnO
OBn
OH
b
HO
OH
OH
77%
86%
O
O
L-xylose
4
OBn
OBn
c
BnO
HO
(+)-Polyoxamic acid (1; Figure 1) is an acyclic trihydrox-
ylated a-amino acid that is used as an important synthetic
building block for the formation of peptidiyl nucleoside
antifungal polyoxin J (3; Figure 1), which consists of
thymine polyoxin C and 5-O-carbamoyl polyoxamic acid
(2; Figure 1). In particular, polyoxin J, isolated from
Streptomyces cacaoi var. asoensis,3 exhibits potent inhib-
90%
OBn OBn
OBn OBn
6
5
OBn
d
e
BnO
74%
91%
OBn NHCbz
7
OH
O
OBn
O
OBn
O
f
RO
OH
BnO
OH
BnO
H
OH NH2
94%
OBn NHCbz
OBn NHCbz
1 R = H
2 R = CONH2
9
8
OH
O
CO2H
HO
g
O
OH
O
O
HO
OH
O
N
99%
NH
OH NH2
H2N
O
N
H
OH NH2
O
1
OH
3
Scheme 1 Reagents and conditions: (a) (i) MeOH, SOCl2, reflux, 4
h; (ii) NaH, BnBr, DMF–THF (1:1), r.t., 24 h; (iii) 1 M H2SO4, AcOH,
1,4-dioxane, reflux, 12 h; (b) NaH, BnPPh3Cl, DMSO, THF, 60 °C, 2
h; (c) NaH, BnBr, DMF–THF (1:1), r.t., 8 h; (d) (i) CSI, Na2CO3, to-
luene–hexane (10:1), –20 °C, 36 h; (ii) 25% Na2SO3, r.t., 24 h; (e) (i)
O3, CH2Cl2, –78 °C, 1 h; (ii) PPh3, r.t., 1 h; (f) NaClO2, NaH2PO4, 2-
methyl-2-butene, t-BuOH, H2O, r.t., 3 h; (g) (i) Pd/C, H2, 6 N HCl,
MeOH, r.t., 12 h; (ii) DOWEX-50Wx8.
Figure 1 Structures of (+)-polyoxamic acid (1), (+)-5-O-carbamoyl
polyoxamic acid (2), and polyoxin J (3)
SYNLETT 2008, No. 19, pp 2985–2988
Advanced online publication: 12.11.2008
x
x
.x
x
.2
0
0
8
DOI: 10.1055/s-0028-1083629; Art ID: U08608ST
© Georg Thieme Verlag Stuttgart · New York

2986
I. S. Kim et al.
LETTER
OBn
O
OBn
a
HO
H2N
O
63%
OBn OBn
OBn NHCbz
10
5
O
OBn
O
c
b
H2N
O
H
91%
86%
OBn NHCbz
11
OBn
O
O
OH
O
O
O
d
H2N
O
OH
H2N
OH
94%
OH NH2
OBn NHCbz
2
12
Scheme 2 Reagents and conditions: (a) (i) CSI, toluene–hexane (10:1), –20 °C, 36 h; (ii) 25% Na2SO3, r.t., 24 h; (iii) 6 N HCl, EtOAc, r.t.,
2 h; (b) (i) O3, CH2Cl2, –78 °C, 1 h; (ii) PPh3, r.t., 1 h; (c) NaClO2, NaH2PO4, 2-methyl-2-butene, t-BuOH, H2O, r.t., 3 h; (d) (i) Pd/C, H2, 6 N
HCl, MeOH, r.t., 12 h; (ii) DOWEX-50Wx8.
ing chlorosulfonyl isocyanate (CSI),6 and the application the CSI-mediated stereoselective amination of the cin-
of this method to the total synthesis of polyhydroxylated namylic benzyl ether moiety and the urethane elaboration
alkaloids,7 we became interested in developing an effi- of the alcohol moiety (Scheme 2). A reaction of com-
cient synthetic route to (+)-polyoxamic acid (1) and (+)-5- pound 5 with CSI in the presence of Na2CO3 at –20 °C,
O-carbamoyl polyoxamic acid (2). This paper reports the followed by reduction of the N-chlorosulfonyl group, fur-
concise synthesis of compounds 1 and 2 via the diastereo- nished the desired biscarbamate 10 in low yield (12%).
selective amination of syn-1,2-dibenzyl ethers 5 and 6 us- However, in the absence of Na2CO3, treatment of com-
ing CSI and the one-pot introduction of the carbamate pound 5 with eight equivalents of CSI followed by work-
moieties into allylic benzyl ether and primary hydroxyl up using 6 N HCl provided the desired compound 10, in
group.
63% yield, with a diastereoselectivity of 7.2:1.15 These re-
sults suggest that a reaction of CSI with the alcohol moi-
ety under basic conditions might accelerate the
polymerization of urethane in the product. Ozonolysis of
compound 10 and the subsequent Pinnick oxidation fur-
nished the acid 12 in 78% overall yield. Fortunately, the
minor anti diastereomer of the desired aldehyde 11 could
be removed completely (cat. 11%) by column chromatog-
raphy.16 Hydrogenation of compound 1217 followed by
resin purification gave the (+)-5-O-carbamoyl polyoxamic
acid (2) in 94% yield.18 The spectral data (1H NMR and
13C NMR) and specific rotation of compound 2 were in
full agreement with the reported literature values.5y,z
The synthesis of compound 1 began with 2,3,4-tri-O-ben-
zyl-L-xylopyranose (4), which could be easily prepared
from commercially available L-xylose according to the
methodology reported in the literature (Scheme 1).7g
A Wittig olefination of compound 4 with DMSO anion in
THF at 60 °C afforded the olefin 5 as a ca. 2.2:1 mixture
of cis/trans isomers in 86% yield.8 The hydroxyl moiety
of compound 5 was then converted into the tetrabenzyl
ether 6 in 90% yield.9 The regioselective and diastereose-
lective CSI reaction of compound 6 was carried out in a
mixture of toluene and hexane (10:1) at –20 °C for 36
hours, followed by desulfonylation with aqueous 25% so-
dium sulfite solution to give the desired syn-1,2-amino al-
cohol 7 with diastereoselectivity (syn/anti = 8.8:1 by
NMR analysis) in 74% yield.10 The diastereoselectivity of
compound 7 can be explained by the neighboring group
effect, whereby the NHCbz group orientation retains its
original configuration through double inversion of the
configuration.7g Ozonolysis of compound 7 afforded a
separable mixture of desirable aldehyde 811 and its diaste-
reomer. Finally, Pinnick oxidation12 provided the acid 9,13
which was then hydrogenated using Pd/C to afford (+)-
polyoxamic acid (1)14 with a specific rotation, [a]D25 +2.2
(c 1.0, H2O) {lit.5y [a]D25 +2.1 (c 1.0, H2O)}, and identical
spectral data (1H and 13C NMR) to that reported in the lit-
erature.5a
In conclusion, we have reported the concise synthesis of
(+)-polyoxamic acid and 5-O-carbamoyl polyoxamic acid
as pivotal components of polyoxin J. The key steps in the
route involve the diastereoselective amination of syn-1,2-
polybenzyl ethers using chlorosulfonyl isocyanate and the
one-pot introduction of the carbamate moiety into the al-
lylic benzyl ether and primary hydroxyl group.
Acknowledgment
This research was supported by the Korean Research Foundation
Grant funded by the Korean Government (MOEHRD) (KRF-2006-
311-E00613), and by the Seoul Research and Business Develop-
ment (Seoul R&BD) Program (#10541).
For the synthesis of 5-O-carbamoyl polyoxamic acid (2)
as a side chain of polyoxin J, we initially focused on a one-
pot preparation of biscarbamate 10 from compound 5 via
References and Notes
(1) (a) Hunt, S. Chemistry and Biochemistry of the Amino
Acids; Barrett, G. C., Ed.; Chapman and Hall: London, 1985,
Synlett 2008, No. 19, 2985–2988 © Thieme Stuttgart · New York

LETTER
55. (b) Barrett, G. C. Amino Acids, Peptides and Proteins,
Vol. 13; The Chemical Society: London, 1980, 1.
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Synthesis of (+)-Polyoxamic Acid
2987
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(8) Characterization data for 5: [a]D25 –25.8 (c 1.0, CHCl3). IR
(CH2Cl2): 3450, 3011, 2866, 1513, 1494, 1055 cm–1. cis-
Isomer: 1H NMR (500 MHz, CDCl3): d = 2.22–2.24 (br,
1 H), 3.45 (dd, J = 12.5, 3.5 Hz, 1 H), 3.68 (dd, J = 11.5, 3.5
Hz, 1 H), 3.78–3.83 (m, 2 H), 4.19 (d, J = 12.0 Hz, 1 H),
4.57–4.82 (m, 6 H), 5.88 (dd, J = 12.0, 10.0 Hz, 1 H), 6.78
(d, J = 12.0 Hz, 1 H), 7.07–7.40 (m, 20 H). 13C NMR (125
MHz, CDCl3): d = 61.63, 70.40, 73.15, 73.21, 75.38, 80.27,
82.47, 126.85, 127.54, 127.88, 127.98, 128.13, 128.31,
128.55, 128.56, 128.61, 128.77, 129.12, 133.99, 136.66,
137.82, 138.48. trans-Isomer: 1H NMR (500 MHz, CDCl3):
d = 2.22–2.24 (br, 1 H), 3.61 (dd, J = 11.5, 3.5 Hz, 1 H),
3.72–3.77 (m, 3 H), 4.30 (dd, J = 7.5, 4.0 Hz, 1 H), 4.45 (d,
J = 11.5 Hz, 1 H), 4.60–4.82 (m, 5 H), 6.22 (d, J = 16.0, 7.5
Hz, 1 H), 6.59 (d, J = 16.0 Hz, 1 H), 7.07–7.40 (m, 20 H).
13C NMR (125 MHz, CDCl3): d = 61.78, 71.03, 72.96, 75.15,
79.70, 80.49, 82.22, 126.84, 127.53, 127.87, 127.94, 127.97,
128.04, 128.17, 128.42, 128.56, 128.60, 128.65, 128.77,
128.83, 129.94, 133.82, 133.99, 138.68. HRMS (FAB): m/z
[M + H+] calcd for C33H35O4: 495.2535; found: 495.2525.
(9) Characterization data for 6: [a]D25 –33.8 (c 0.5, CHCl3). IR
(CH2Cl2): 2864, 2360, 1541, 1496, 1454, 1395, 1352, 1027,
1089, 1028, 914, 806, 736 cm–1. cis-Isomer: 1H NMR (500
MHz, CDCl3): d = 3.23 (d, J = 9.0 Hz, 1 H), 3.57–3.59 (m,
1 H), 3.92 (br s, 2 H), 4.12 (d, J = 12.0 Hz, 1 H), 4.25 (d, J =
12.0 Hz, 1 H), 4.41–4.88 (m, 7 H), 5.89 (t, J = 10.0 Hz, 1 H),
6.77 (d, J = 10.0 Hz, 1 H), 7.01–7.39 (m, 25 H). 13C NMR
(125 MHz, CDCl3): d = 69.59, 70.16, 73.17, 73.47, 75.74,
78.90, 82.51, 126.85, 127.43, 127.72, 127.75, 127.89,
128.03, 128.31, 128.43, 128.52, 128.59, 128.62, 128.78,
129.22, 130.48, 133.55, 136.78, 138.10, 138.95. trans-
Isomer: 1H NMR (500 MHz, CDCl3): d = 3.60–3.66 (m,
2 H), 3.64 (dd, J = 10.0, 4.5 Hz, 1 H), 3.76 (dd, J = 6.0, 4.5
Hz, 1 H), 4.29–4.39 (m, 1 H), 4.41–4.88 (m, 8 H), 6.16 (dd,
J = 12.5, 7.5 Hz, 1 H), 6.52 (d, J = 12.5 Hz, 1 H), 7.01–7.39
(m, 25 H). 13C NMR (125 MHz, CDCl3): d = 70.36, 70.95,
73.34, 73.56, 75.43, 80.14, 81.21, 81.97, 116.53, 127.30,
127.56, 127.76, 127.84, 127.99, 128.20, 128.42, 128.50,
128.55, 128.61, 128.72, 128.96, 130.47, 133.71, 136.84,
138.51, 139.12. HRMS (FAB): m/z [M + H+] calcd for
C40H41O4: 585.3005; found: 585.3010.
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(10) Characterization data for 7: [a]D25 +25.8 (c 1.0, CHCl3). IR
(CH2Cl2): 3030, 2867, 1719, 1491, 1454, 1214, 1090, 1027,
967, 737, 697 cm–1. cis-Isomer: 1H NMR (500 MHz,
CDCl3): d = 3.53–3.73 (m, 3 H), 3.85 (dd, J = 7.0, 2.0 Hz,
1 H), 4.35 (d, J = 12.0 Hz, 1 H), 4.48–6.63 (m, 4 H), 4.82 (d,
J = 11.0 Hz, 1 H), 5.06 (br m, 3 H), 5.37 (d, J = 8.0 Hz, 1 H),
5.72 (dd, J = 12.0, 9.5 Hz, 1 H), 6.52 (d, J = 12.0 Hz, 1 H),
7.18–7.44 (m, 25 H). 13C NMR (125 MHz, CDCl3):
d = 49.83, 66.97, 69.39, 73.27, 73.59, 75.64, 80.25, 82.21,
127.38, 127.74, 127.83, 128.01, 128.08, 128.29, 128.51,
128.54, 128.59, 128.72, 128.97, 130.85, 131.02, 136.61,
138.48, 138.71, 155.86. trans-Isomer: 1H NMR (500 MHz,
CDCl3): d = 3.68 (br, 1 H), 3.78 (br m, 2 H), 3.86 (br, 1 H),
4.54–4.66 (m, 5 H), 4.72 (d, J = 12.0 Hz, 1 H), 4.79 (d, J =
12.0 Hz, 1 H), 5.11 (s, 2 H), 5.44 (br, 1 H), 6.15 (dd, J = 16.0,
5.5 Hz, 1 H), 6.50 (d, J = 16.0 Hz, 1 H), 7.25–7.36 (m, 25 H).
13C NMR (125 MHz, CDCl3 ): d = 53.79, 67.07, 69.79,
73.40, 73.75, 75.22, 80.24, 81.30, 126.73, 127.78, 127.83,
127.90, 128.00, 128.04, 128.11, 128.36, 128.43, 128.58,
128.61, 128.63, 128.71, 128.75, 131.00, 136.75, 136.95,
Synlett 2008, No. 19, 2985–2988 © Thieme Stuttgart · New York

2988
I. S. Kim et al.
LETTER
138.38, 138.75, 156.04. HRMS (FAB): m/z [M + H+] calcd
for C41H42NO5: 628.3063; found: 628.3062.
NMR (125 MHz, CDCl3): d = 49.43, 62.86, 67.22, 73.43,
75.95, 78.70, 83.19, 126.78, 127.48, 127.91, 128.09, 128.21,
128.36, 128.53, 128.77, 130.84, 131.64, 136.74, 136.86,
138.07, 138.13, 138.25, 156.15, 156.52. trans-Isomer: 1H
NMR (300 MHz, CDCl3): d = 3.80–3.90 (m, 2 H), 4.32–4.37
(m, 2 H), 4.61–4.85 (m, 5 H), 5.13 (s, 2 H), 5.48–5.50 (m,
1 H), 6.15 (dd, J = 15.6, 6.0 Hz, 1 H), 6.58 (d, J = 15.6 Hz,
1 H), 7.23–7.42 (m, 20 H). 13C NMR (125 MHz, CDCl3):
d = 53.43, 63.13, 66.84, 73.07, 75.47, 77.71, 78.59, 82.01,
126.08, 126.85, 127.90, 128.03, 128.17, 128.29, 128.30,
128.67, 128.81, 131.24, 136.76, 136.85, 137.61, 138.07,
156.26, 156.68. HRMS (FAB): m/z [M + H+] calcd for
C35H37N2O6: 581.2652; found: 581.2655.
(11) Characterization data for 8: [a]D25 –7.5 (c 1.0, CHCl3). IR
(CH2Cl2): 3060, 1717, 1541, 1508, 1456, 1091, 697 cm–1. 1H
NMR (300 MHz, CDCl3): d = 3.58–3.66 (m, 3 H), 3.72–3.78
(m, 1 H), 4.34 (br, 1 H), 4.44 (d, J = 11.7 Hz, 1 H), 4.46 (s,
2 H), 4.59 (d, J = 11.7 Hz, 1 H), 4.73 (s, 2 H), 5.12 (d, J =
15.6 Hz, 1 H), 5.16 (d, J = 15.6 Hz, 1 H), 5.62 (br d, J = 5.7
Hz, 1 H), 7.27–7.41 (m, 20 H), 9.73 (s, 1 H). 13C NMR (125
MHz, CDCl3): d = 60.19, 67.41, 69.67, 73.05, 73.71, 74.23,
76.69, 78.59, 128.01, 128.14, 128.36, 128.52, 128.59,
128.67, 128.72, 128.75, 128.81, 136.29, 137.62, 137.70,
138.10, 156.38, 197.629. HRMS (FAB): m/z [M + H+] calcd
for C34H36NO6: 554.2543; found: 554.2543.
(16) Characterization data for 11: [a]D25 –7.5 (c 1.0, CHCl3). IR
(CH2Cl2): 3444, 2972, 2360, 1716, 1508, 1456, 1418, 1339,
1067, 739, 698 cm–1. 1H NMR (300 MHz, CDCl3): d = 3.72–
3.73 (m 1 H), 4.17–4.74 (m, 9 H), 5.10–5.15 (m, 2 H), 5.64
(d, J = 6.5 Hz, 1 H), 7.27–7.42 (m, 15 H), 9.67 (s, 1 H).
13C NMR (125 MHz, CDCl3): d = 60.03, 62.83, 67.53,
73.10, 74.33, 75.79, 77.91, 128.30, 128.34, 128.47, 128.55,
128.65, 128.69, 128.77, 128.84, 136.24, 137.32, 137.39,
156.39, 156.61, 198.01. HRMS (FAB): m/z [M + H+] calcd
for C28H31N2O7: 507.2131; found: 507.2142.
(12) Bal, B. S.; Childers, W. E. Jr.; Pinnick, H. W. Tetrahedron
1981, 37, 2091.
(13) Characterization data for 9: [a]D25 –12.5 (c 0.5, CHCl3). IR
(CH2Cl2): 3421, 3031, 2870, 2360, 1716, 1507, 1455, 1397,
1214, 1092, 737, 698 cm–1. 1H NMR (300 MHz, CDCl3):
d = 3.60–3.86 (m, 4 H), 4.40–4.78 (m, 7 H), 5.14 (s, 2 H),
5.61 (br, 1 H), 7.30–7.36 (m, 20 H). 13C NMR (125 MHz,
CDCl3): d = 54.21, 67.49, 69.18, 73.32, 73.70, 75.13, 76.68,
78.31, 78.69, 127.95, 128.30, 128.43, 128.59, 128.63,
128.70, 128.76, 132.36, 136.29, 137.34, 138.14, 156.65,
174.37. HRMS (FAB): m/z [M + H+] calcd for C34H36NO7:
570.2498; found: 570.2492.
(17) Characterization data for 12: [a]D25 –10.4 (c 1.0, CHCl3).
IR (CH2Cl2): 3365, 3032, 2965, 2360, 1716, 1508, 1456,
1339, 1220, 1066, 740, 698 cm–1. 1H NMR (500 MHz,
CD3OD): d = 3.79 (br, 1 H), 4.08 (dd, J = 12.0, 5.0 Hz, 1 H),
4.30 (d, J = 7.0 Hz, 1 H), 4.41 (br d, J = 12.0 Hz, 1 H), 4.51
(s, 1 H), 4.57–4.73 (m, 4 H), 5.08–5.14 (m, 2 H), 7.24–7.37
(m, 15 H). 13C NMR (125 MHz, CD3OD): d = 63.26, 66.68,
72.88, 74.66, 78.79, 79.33, 127.50, 127.64, 127.80, 127.96,
128.06, 128.13, 128.29, 136.98, 138.29, 138.47, 157.75,
158.39, 176.12. HRMS (FAB): m/z [M + Na+] calcd for
C28H30N2O8Na: 545.1900; found: 545.1903.
(14) Characterization data for 1: [a]D25 +2.2 (c 1.0, H2O); mp
170–174 °C (dec.). 1H NMR (300 MHz, D2O): d = 4.21 (dd,
J = 3.3, 2.7 Hz, 1 H), 3.93–3.87 (m, 2 H), 3.72–3.60 (m,
2 H). 13C NMR (125 MHz, D2O): d = 172.83, 73.33, 68.26,
62.63, 58.16. HRMS (CI): m/z [M + H+] calcd for C5H12NO5:
166.0715; found: 166.0716.
(15) Characterization data for 10: [a]D25 +35.6 (c 0.5, CHCl3). IR
(CH2Cl2): 3425, 2939, 2865, 2360, 1716, 1507, 1456, 1339,
1067, 697 cm–1. cis-Isomer: 1H NMR (300 MHz, CDCl3):
d = 3.64–3.72 (m, 2 H), 4.17 (br s, 2 H), 4.55 (d, J = 11.1 Hz,
1 H), 4.61 (d, J = 11.1 Hz, 1 H), 4.65 (d, J = 11.1 Hz, 1 H),
4.85 (d, J = 11.1 Hz, 1 H), 5.10 (s, 2 H), 5.18 (t, J = 8.4 Hz,
1 H), 5.47 (d, J = 7.2 Hz, 1 H), 5.75 (dd, J = 11.4, 9.3 Hz,
(18) Characterization data for 2: [a]D25 +1.2 (c 1.0, H2O); mp
215–220 °C (dec.). 1H NMR (500 MHz, D2O): d = 4.17 (br
m, 1 H), 4.09–4.04 (m, 3 H), 3.82 (br m, 1 H). 13C NMR (125
MHz, D2O): d = 172.95, 159.35, 71.07, 68.28, 65.76, 58.13.
HRMS (FAB): m/z [M + H+] calcd for C6H13N2O6:
1 H), 6.54 (d, J = 11.4 Hz, 1 H), 7.23–7.42 (m, 20 H). 13
C
209.0774; found: 209.0776.
Synlett 2008, No. 19, 2985–2988 © Thieme Stuttgart · New York