Synthesis of novel polycyclic β-lactams from D-glucose via unusual substrate-controlled radical cyclization

Article abstract of DOI:10.1055/s-2004-822920

A highly stereoselective and substrate-controlled synthesis of polycyclic β-lactams from D-glucose derived chiral template via intramolecular free radical cyclization is described. The cyclization is highly substrate dependant, proceeding via 6-exo and 7-

Full text of DOI:10.1055/s-2004-822920

LETTER
1249
Synthesis of Novel Polycyclic b-Lactams from D-Glucose via Unusual
Substrate-Controlled Radical Cyclization
S
a
ynthesisof
N
ove
.
l
Polycyclic
J
-Lactams
a
from
D
-G
l
y
ucose anthi,a Vedavati G. Puranik,b A. R. A. S. Deshmukh*a
Division of Organic Chemistry (Synthesis), National Chemical Laboratory, Pune – 411 008, India
b
Division of Physical Chemistry, National Chemical Laboratory, Pune – 411 008, India
Fax +91(20)25893153; E-mail: arasd@dalton.ncl.res.in
Received 30 January 2004
Dedicated to Dr. S. Rajappa on his 70th birthday
controlled synthesis of polycyclic b-lactams via 6-exo and
7-endo heptynyl type radical cyclization with a radical
acceptor at N-1 of b-lactam and a radical progenitor on a
sugar part, which is anchored to b-lactam ring at C-4. We
Abstract: A highly stereoselective and substrate-controlled synthe-
sis of polycyclic b-lactams from D-glucose derived chiral template
via intramolecular free radical cyclization is described. The cycliza-
tion is highly substrate dependant, proceeding via 6-exo and 7-endo
heptynyl type radical cyclization with the radical acceptor at N-1 selected D-glucose derived chiral template for the synthe-
and the radical progenitor on a sugar moiety, anchored to the b-lac-
sis of b-lactams with suitably sited radical progenitor and
tam ring at C-4.
acceptor appendages (Scheme 1) using Staudinger reac-
Key words: azetidin-2-ones, polycyclic b-lactams, radical cycliza-
tion, tributyltin hydride
tion, as they were known to show high level of diastereo-
selectivity for b-lactam formation.8
The increasing bacterial resistance to the commercially
available b-lactam antibiotics through the cleavage of
strained b-lactam ring by b-lactamase enzymes motivated
chemists to synthesize stable fused polycyclic b-lactams
of non-classical structure.1,2 Presently the construction of
polycyclic ring structures by free radical cyclization has
been accepted as a useful synthetic methodology, espe-
cially in the total synthesis of natural products.3,4 The
large body of research and systematic studies that have
gone into this subject have led to the evolution of certain
principles and guidelines5 regarding the regio and stereo-
chemistry of radical cyclization. Hexenyl and hexynyl
radical cyclizations are more efficient and well studied for
mechanistic as well as synthetic applications.3 The stere-
ochemical outcome of these kind of cyclizations can be
predicted by using Beckwith’s transition state model.6 Ac-
cording to this model and Baldwin’s rules, 5-exo is more
common than 6-endo cyclization in both the cases. How-
ever, in case of heptynyl radical cyclization, the examples
of 6-exo cyclizations are well known6c–g while 7-endo cy-
clizations are very rare.6h,i Considering the high sensitivity
of b-lactam antibiotics to nucleophilic reagents, several
groups have employed radical cyclization methodology
for the construction of fused polycyclic structures by us-
ing b-lactam with radical acceptor and radical progenitor
appendages.7 In a recent publication we have employed
this methodology for diastereospecific synthesis of novel
polycyclic b-lactams7m,n wherein radical progenitor and
radical acceptor are appended at C-3 and C-4 positions,
respectively, on the b-lactam ring skeleton. We report
herein our results on highly stereoselective and substrate-
Scheme 1 Reagents and conditions: (a) Ref.9; (b) PPh3, I2, imidazo-
le, toluene, reflux, 17 h; (c) 0.8% H2SO4 in MeOH, r.t., 24 h; (d) 0.65
M NaIO4 on SiO2, CH2Cl2, r.t., 1 h; (e) HC≡CCH2NH2, MgSO4,
CH2Cl2, r.t., 3–4 h; (f) ROCH2COCl, Et3N, CH2Cl2, 0 ° C to r.t., 12–
15 h.
To study the intramolecular radical cyclization we pre-
pared azetidin-2-ones, with N-propargyl substituent as
radical acceptor and iodo group as radical progenitor
appendage, from commercially available D-glucose
(Scheme 1). Diacetonide 1 was prepared from D-glucose,
following a known procedure,9 by stirring with acetone in
the presence of anhydrous zinc chloride and phosphoric
acid. This was converted into 3-iodo derivative 2 by tri-
phenylphosphine, iodine and imidazole.10 The selective
SYNLETT 2004, No. 7, pp 1249–1253
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3.
0
6.
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Advanced online publication: 10.05.2004
DOI: 10.1055/s-2004-822920; Art ID: G00304ST
© Georg Thieme Verlag Stuttgart · New York

1250
A. Jayanthi et al.
LETTER
deprotection of acetonide by 0.8% H2SO4 in MeOH gave
3, which on oxidative cleavage with NaIO4 adsorbed on
silica gel furnished chiral iodoaldehyde 4 in quantitative
yield.11 The chiral imine 5 prepared from iodoaldehyde 4
and propargyl amine was found to be unstable and used as
such for the next reaction. This imine underwent smooth
cycloaddition reaction (Staudinger reaction) with ketenes
derived from substituted acid chlorides (phenoxy, benzyl-
oxy and methoxy acetyl chlorides) and Et3N, to give 1:1
diastereomeric mixture of only cis b-lactams 6a–c and
7a–c (J = 4–6 Hz for cis b-lactam ring protons). However,
both the diastereomers could be separated by flash column
chromatography or crystallization from MeOH. The
structure and relative stereochemistry was assigned from
spectral data12 and the absolute stereochemistry for b-lac-
tam ring protons of 6a was further established from the
single crystal X-ray structure analysis as 3R, 4S based on
the known absolute stereochemistry of the carbohydrate
moiety (Figure 1).13
Figure 2 ORTEP diagram of 8b
exo-dig cyclization and relative stereochemistry of C3, C4
and C6 was assigned as 3R, 4S and 6R.
Scheme 3 Reagents and conditions: (a) Bu3SnH, AIBN, toluene,
reflux, 6h; (b) H2, Pd/C (10%).
On the other hand, when alpha diastereomer 7a is subject-
ed to radical cyclization under similar reaction conditions,
the radical attacks the terminal carbon regiospecifically to
give endo-dig cyclized product 9a (Scheme 3). IR and
NMR spectral data14 established the structure of 9a. How-
ever, the compound could not be obtained in crystalline
form for the single crystal X-ray analysis. Therefore, it
was subjected to catalytic hydrogenation at room temper-
ature using Pd/C (10%) and 60 psi H2 pressure to get white
crystalline compound. The absence of CH3 signal, which
was expected for the exo-cyclization, and the appearance
of three-methylene carbon peaks in 13C DEPT spectrum of
cyclized product 10 and 2D NMR studies revealed endo-
dig radical cyclization of 9a.16
Figure 1 ORTEP diagram of 6a
Scheme 2 Reagents and conditions: (a) Bu3SnH, AIBN, toluene, re-
flux, 6 h.
The pure diastereomer 6a when treated with tributyltin
hydride in the presence of AIBN in toluene under reflux-
ing condition underwent radical cyclization to give kinet-
ically controlled stable exo-dig cyclized tetracyclic
product 8a (Scheme 2) via 1,6-bond coupling of hept-6-
ynyl radical.14 The presence of olefinic methylene peak
down in the 13C DEPT experiment at 113.5 ppm and sin-
gle crystal X-ray analysis15 of 8b (Figure 2) confirmed the
Figure 3 ORTEP diagram of 10
Synlett 2004, No. 7, 1249–1253 © Thieme Stuttgart · New York

LETTER
Synthesis of Novel Polycyclic b-Lactams from D-Glucose
1251
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This is in contrast with the normally expected exo-radical
cyclization of hept-6-ynyl system.6c–g Single crystal X-ray
analysis17 of 10 further confirmed the structure and the ab-
solute stereochemistry at the newly formed centers was
established as 3S, 4R, 8S based on the known absolute ste-
reochemistry of 6R, 7R, 9S of sugar moiety. The carbon
atoms C7 and C8 are disordered and occupy two positions
at C7A, C7B and C8A, C8B with 0.7 and 0.3 occupancies
respectively, as shown in the ORTEP diagram (Figure 3).
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Heterocycles 1988, 27, 875. (g) Kametani, T.; Chu, S.-D.;
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2003, 5, 3795. (m) Joshi, S. N.; Puranik, V. G.; Deshmukh,
A. R. A. S.; Bhawal, B. M. Tetrahedron: Asymmetry 2001,
12, 3073. (n) Joshi, S. N.; Phalgune, U. D.; Bhawal, B. M.;
Deshmukh, A. R. A. S. Tetrahedron Lett. 2003, 44, 1827.
(8) (a) Bose, A. K.; Manhas, M. S.; Hedge, V. R.; Wagle, D. R.;
Bari, S. S. J. Chem. Soc., Chem. Commun. 1986, 181.
(b) Bose, A. K.; Mathur, C.; Wagle, D. R.; Nagui, R.;
Manhas, M. S. Heterocycles 1994, 39, 491. (c) Hanessian,
S.; Desilets, D.; Rancourt, G.; Fosfin, R. Can. J. Chem.
1982, 60, 2292. (d) Koga, K.; Ikata, N.; Toshino, O. Chem.
Pharm. Bull. 1982, 30, 1929. (e) Bernardo, S. D.; Tengi, J.
P.; Sasso, G. J.; Weigele, M. J. Org. Chem. 1985, 50, 3457.
(f) Arun, M.; Joshi, S. N.; Puranik, V. G.; Bhawal, B. M.;
Deshmukh, A. R. A. S. Tetrahedron 2003, 59, 2309.
(9) Furniss, B.; Hann Ford, A. J.; Smith, P. W. G.; Tatchell, A.
R. Vogel’s Textbook of Practical Org. Chem. 1989, 5th ed.
654.
Scheme 4
A drastic change in the mode of radical cyclization is ob-
served with the change in stereochemistry of b-lactam nu-
cleus (Scheme 4). We believe that the steric control of
bulky substituents at C3 and sugar moiety at C4 positions
of azetidin-2-one ring help in tuning the regioselectivity
of cyclization. The electrostatic repulsion between the b-
lactam ring nitrogen and the furanose ring oxygen atoms
may be responsible for endo radical cyclization in 7
(Scheme 4). Radical addition to the triple bond was also
stereospecific and the newly formed C-C bond directed
anti to the acetonide group presumably due to the steric
interactions between the b-lactam ring and the acetonide
group. This unusual behavior in the mode of radical
cyclization was not observed in the case of N-allyl b-lac-
tams. Further study on intramolecular radical cyclization
of various substituted N-allyl as well as N-propargyl sub-
strates is in progress.
In conclusion, tributyltin hydride-AIBN mediated radical
cyclization of N-propargyl b-lactams was studied and the
observed products unambiguously proved the cyclization
to be stereospecific and substrate controlled, either 6-exo
or 7-endo depending on the stereochemistry of the b-lac-
tam ring.
Acknowledgment
The authors thank DST for financial support and CSIR for research
fellowships to AJ.
References
(1) For selected reviews, see: (a) Setti, E. L.; Micetich, R. G.
Curr. Med. Chem. 1998, 5, 101. (b) The Organic Chemistry
of b-Lactams; Georg, G. I., Ed.; VCH: New York, 1993.
(c) Neuhaus, F. C.; Georgeopapadakou, N. H. In Emerging
Targets in Antibacterial and Antifungal Chemotherapy;
Sutcliffe, J.; Georgeopapadakou, N. H., Eds.; Chapman and
Hall: New York, 1992. (d) Alcaide, B.; Almendros, P. Curr.
Org. Chem. 2002, 6, 245.
(10) Garegg, P. J.; Samuelsson, B. J. Chem. Soc., Perkin Trans. 1
1980, 2866.
(11) Daumas, M.; Vo-Quang, Y.; Vo-Quang, L.; Le Goffic, F.
Synthesis 1989, 64.
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A. Jayanthi et al.
LETTER
(12) General Procedure for the Synthesis of N-Propargyl b-
Lactams (6a–c and 7a–c): To a solution of propargylamine
(2 mmol) in CH2Cl2 (20 mL), was added an anhyd MgSO4 (4
equiv) and a CH2Cl2 solution of iodoaldehyde 4 (2 mmol, in
5 mL) under argon atmosphere at r.t. and stirred for 3–4 h.
The mixture was filtered through a pad of celite and the
filtrate was concentrated to get the imines 5, which was
found to be unstable and used immediately without further
purification.
A solution of the acid chloride (phenoxy or benzyloxy or
methoxyacetyl chloride, 1.5 mmol) in anhyd CH2Cl2 (10
mL) was added to a solution of the imine (5, 1.0 mmol) and
Et3N (4.5 mmol) in CH2Cl2 (20 mL) at 0 °C under argon
atmosphere. It was then allowed to warm to r.t. and stirred
for 15 h. The reaction mixture was then washed with water,
sat. NaHCO3 solution, and sat. brine solution. The organic
layer was then dried over anhyd Na2SO4, and concentrated
under reduced pressure to give diastereomeric mixture of cis
b-lactams 6 and 7 in 1:1 ratio (40–50%). The diastereomers
were separated by flash column chromatography using silica
gel (230–400 mesh).
6a (Figure 4): White crystalline solid (mp 109–110 °C);
[a]D25 +133.0 (c = 0.93, CHCl3). IR (CHCl3): 1770 cm–1. 1H
NMR (200 MHz, CDCl3): d = 1.41 (s, 3 H, CH3), 1.58 (s, 3
H, CH3), 2.37 (t, 1 H, H14), 3.97–4.06 (dd, J = 2.5, 17.9 Hz,
1 H, H12), 4.10–4.15 (dd, J = 3.9, 3.9 Hz, 1 H, H6), 4.30 (t, J
= 3.9, 4.9 Hz, 1 H, H4), 4.41–4.50 (dd, J = 2.5, 17.9 Hz, 1 H,
H12), 4.65 (t, J = 3.5, 3.9 Hz, 1 H, H7), 4.70–4.78 (dd, J = 3.9,
3.9 Hz, 1 H, H5), 5.34 (d, J = 4.9 Hz, 1 H, H3), 5.88 (d, J =
3.5 Hz, 1 H, H8), 7.02–7.09 (m, 3 H, aromatic), 7.29–7.37
(m, 2 H, aromatic). 13C NMR (50.32 MHz, CDCl3): d =
165.6 (C2), 157.5 (C15), 129.7 (C19, C17), 122.7 (C18), 115.9
(C16, C20), 112.4 (C9), 103.4 (C8), 81.7 (C7), 80.8 (C5), 80.1
(C3), 75.8 (C14), 74.0 (C13), 56.3 (C4), 30.9 (C12), 26.9 (C10),
26.8 (C11), 19.4 (C6). MS (70 eV): m/z = 470 [M + 1]. Anal.
Calcd for C19H20NO5I: C, 48.63; H, 4.30; N, 2.98. Found: C,
48.49; H, 4.27; N, 2.80.
Figure 5
(13) Crystal structure data for 6a: C19H20INO5, colorless crystals
grown from i-PrOH; M = 469.26; crystal dimensions 0.43 ×
0.21 × 0.14 mm; crystal system orthorhombic, space group
P212121; a = 8.104 (2), b = 9.166 (2), c = 26.351 (6) Å; V =
1957.4 (8) Å3; Z = 4; Dc = 1.592 g/cm3; m (MoKa) (l =
0.7107 Å) = 1.664 mm–1; F(000) = 936; q = 1.55–23.27°; T
= 293 (2) K; Max. and min. transmission = 0.8041 and
0.5320; Reflections collected/unique = 8515/2801 [R(int) =
0.0181]; Completeness to q = 23.27 99.6%; Refinement
method = Full-matrix least-squares on F2 Data/restraints/
;
parameters = 2801/0/240; Goodness-of-fit on F2 = 1.156;
Final R indices [I>2s(I)]: R1 = 0.0196, wR2 = 0.0497; R
indices (all data): R1 = 0.0203, wR2 = 0.0501.
(14) General Procedure for Intramolecular Radical
Cyclization of N-Propargyl b-Lactams (6a–b, and 7a–c):
A solution of Bu3SnH (0.40 mL, 1.5 mmol) and AIBN (15
mg, 0.09 mmol) in toluene (10 mL) was slowly added to a
refluxing solution of b-lactam 6a–b or 7a–c (1 mmol) in
toluene (20 mL) over a period of 5 h. The reaction mixture
was further refluxed for 2–5 h. After completion of the
reaction (TLC), the solvent was concentrated and the crude
reaction mixture was purified by flash column
chromatography (silica gel, petroleum ether–EtOAC) to get
pure cyclized product 8a–b or 9a–c.
8a (Figure 6): White crystalline solid (mp 109–110 °C);
[a]D25 +42.07 (c = 2.1, CHCl3). IR (CHCl3): 1765 cm–1. 1H
NMR (200 MHz, CDCl3): d = 1.31–1.32 (d, 6 H, CH3), 2.89
(d, J = 4.4 Hz, 1 H, H6), 3.72 (d, J = 14.7 Hz, 1 H, H12), 3.89
(t, J = 3.4, 3.9 Hz, 1 H, H4), 4.34 (d, J = 14.7 Hz, 1 H, H12),
4.87–4.98 (m, 3 H, H14, H7), 5.24 (s, 1 H, H5), 5.37 (d, J =
3.9 Hz, 1 H, H3), 5.87 (d, J = 3.9 Hz, 1 H, H8), 7.00–7.07 (m,
3 H, aromatic), 7.29–7.36 (m, 2 H, aromatic). 13C NMR
(50.32 MHz, CDCl3): d = 166.3 (C2), 156.9 (C15); 137.1
(C13), 129.6 (C19, C17), 122.6 (C18), 115.5 (C16, C20), 113.5
(C14), 111.6 (C9), 104.6 (C8), 81.8 (C7), 80.5 (C3), 75.1 (C5),
54.9 (C4), 47.7 (C6), 45.4 (C12), 26.4 (C10, C11). MS (70 eV):
m/z = 344 [M + 1]. Anal. Calcd for C19H21NO5: C, 66.50; H,
6.16; N, 4.10. Found: C, 66.33; H, 5.97; N, 4.24.
Figure 4
7a (Figure 5): Gummy material; [a]D25 –13.4 (c = 1.08,
CHCl3). IR (CHCl3): 1769 cm–1. 1H NMR (200 MHz,
CDCl3): d = 1.37 (s, 3 H, CH3), 1.43 (s, 3 H, CH3), 2.40 (t, 1
H, H14), 3.87–4.02 (m, 2 H, H12, H6), 4.36 (t, J = 3.4, 4.9 Hz,
1 H, H4), 4.43–4.55 (m, 2 H, H12, H7), 4.64 (t, J = 3.4, 3.4 Hz,
1 H, H5), 5.37 (d, J = 4.9 Hz, 1 H, H3), 5.81 (d, J = 3.4 Hz, 1
H, H8), 7.03–7.12 (m, 2 H, aromatic), 7.31–7.37 (m, 3 H,
aromatic). 13C NMR (50.32 MHz, CDCl3): d = 164.1 (C2),
156.3 (C15), 128.8 (C19, C17), 121.5 (C18), 114.8 (C16), 113.9
(C20), 111.2 (C9), 107.5 (C8), 79.9 (C7), 79.1 (C5), 75.7 (C3),
75.4 (C14), 73.3 (C13), 55.2 (C4), 30.6 (C12), 25.9 (C10), 25.6
(C11), 21.7 (C6). MS (70 eV): m/z = 470 [M + 1]. Anal. Calcd
for C19H20NO5I: C, 48.63; H, 4.30; N, 2.98. Found: C, 48.82;
H, 4.11; N, 2.93.
Figure 6
9a (Figure 7): Gummy material; [a]D25 +25.38 (c = 1.16,
CHCl3). IR (CHCl3): 1762 cm–1. 1H NMR (200 MHz,
CDCl3) d = 1.14 (s, 3 H, CH3), 1.27 (s, 3 H, CH3), 2.99 (br s,
1 H, H6), 3.86 (d, J = 19.0 Hz, 1 H, H12), 4.27–4.44 (m, 4 H,
Synlett 2004, No. 7, 1249–1253 © Thieme Stuttgart · New York

LETTER
Synthesis of Novel Polycyclic b-Lactams from D-Glucose
1253
Figure 7
Figure 8
H5, H7, H4, H12), 5.32–5.57 (m, 3 H, H13, H14, H3), 5.82 (d,
J = 3.0 Hz, 1 H, H8), 6.95–7.05 (m, 3 H, aromatic), 7.28–7.32
(m, 2 H, aromatic). 13C NMR (75.2 MHz, CDCl3): d = 162.0
(C2), 157.4 (C15), 129.5 (C14), 125.8 (C19, C17), 124.8 (C13),
122.0 (C18), 115.5 (C16, C20), 111.8 (C8), 104.9 (C9), 84.5
(C7), 79.6 (C3), 74.2 (C5), 57.5 (C4), 50.5 (C6), 41.7 (C12),
26.9 (C10), 26.7 (C11); MS (70 eV): m/z = 344 [M + 1]. Anal.
Calcd for C19H21NO5: C, 66.46; H, 6.16; N, 4.08. Found: C,
66.62; H, 6.28; N, 4.20.
H14), 2.3–2.4 (m, 1 H, H6), 3.1–3.2 (m, 1 H, H12), 3.7–3.8 (m,
1 H, H12), 4.17 (d, J = 4.3 Hz, 1 H, H4), 4.31 (d, J = 3.6 Hz,
1 H, H7), 4.33 (d, J = 4.3 Hz, 1 H, H5), 5.36 (d, J = 4.3 Hz, 1
H, H3), 5.85 (d, J = 3.6 Hz, 1 H, H8), 6.9–7.4 (m, 5 H,
aromatic). 13C NMR (50.32 MHz, CDCl3): d = 164.7 (C2),
157.2 (C15), 129.3 (C19, C17), 121.7 (C18), 115.4 (C20, C16),
110.9 (C8), 104.6 (C9), 85.3 (C7), 79.3 (C3), 75.1 (C5), 57.6
(C4), 48.3 (C12), 42.8 (C6), 27.7 (C14), 26.3 (C10, C11), 23.5
(C13). MS (70 eV): m/z = 345 [M+]. Anal. Calcd for
C19H23NO5: C, 66.07; H, 6.71; N, 4.06. Found: C, 66.17; H,
6.87; N, 4.27.
(15) Crystal structure data for 8b: C20H23NO5, colorless crystal
from i-PrOH; M = 357.39; crystal dimensions 0.21 × 0.19 ×
0.06 mm; crystal system monoclinic, space group P21; a =
9.839 (15), b = 8.148 (12), c = 11.946 (18) Å; V = 945.6 (2)
Å3; Z = 2; Dc = 1.255 g/cm3; m (MoKa) (l = 0.7107 Å) =
0.090 mm–1; F(000) = 380; q = 1.73–24.99°; T = 293 (2) K;
Max. and min. transmission = 0.9943 and 0.9813;
(17) Crystal structure data for 10: C19H23NO5, colorless needles
grown from i-PrOH; M = 345.38; crystal dimensions 0.47 ×
0.24 × 0.19 mm; crystal system orthorhombic, space group
P212121; a = 5.498 (2), b = 14.423 (5), c = 21.601 (8) Å; V =
1712.9 (11) Å3; Z = 4; Dc = 1.339 g/cm3; m (MoKa) (l =
0.7107 Å) = 0.097 mm–1; F(000) = 736; q = 1.70–28.22°; T
= 293 (2) K; Max. and min. transmission = 0.9814 and
0.9554; Reflections collected/unique = 8380/3863 [R(int) =
0.0227]; Completeness to q = 28.22°, 94.3%; Refinement
Reflections collected/unique = 9089/3324 [R(int) = 0.0320];
Completeness to q = 24.99 99.8%; Refinement method =
Full-matrix least-squares on F2; Data/restraints/parameters =
3324/1/237; Goodness-of-fit on F2 = 1.071; Final R indices
[I>2s(I)]: R1 = 0.0497, wR2 = 0.1024; R indices (all data):
R1 = 0.0584, wR2 = 0.1065.
method = Full-matrix least-squares on F2 Data/restraints/
;
(16) Spectral data for hydrogenated product 10 (Figure 8): White
crystalline solid (mp 174 °C); [a]D25 +1.88 (c = 0.8, CHCl3).
IR (CHCl3,): 1755 cm–1. 1H NMR (200 MHz, CDCl3) d =
1.15 (s, 3 H, CH3), 1.25 (s, 3 H, CH3), 1.5–1.9 (m, 4 H, H13,
parameters = 3863/0/247; Goodness-of-fit on F2 = 0.837;
Final R indices [I>2s(I)]: R1 = 0.0374, wR2 = 0.0745; R
indices (all data): R1 = 0.0584, wR2 = 0.0794.
Synlett 2004, No. 7, 1249–1253 © Thieme Stuttgart · New York