Novel nitronyl nitroxides: Synthesis and properties

Article abstract of DOI:10.1515/znb-2002-0614

The synthesis of several new nitronyl nitroxides with mesogenic groups is reported (6a, b, 11, 14a, b). Microscopic investigations revealed that in some cases (6b, 14b) highly ordered mesogenic phases are formed.

Full text of DOI:10.1515/znb-2002-0614

Novel Nitronyl Nitroxides: Synthesis and Properties
Stefan Grevea, Volkmar Villb, and Willy Friedrichsena
a
Institute of Organic Chemistry, University of Kiel, Otto-Hahn-Platz 3,
D-24118 Kiel, Germany
b
Institute of Organic Chemistry and Biochemistry, University of Hamburg,
Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany
Reprint requests to Prof. Dr. W. Friedrichsen. E-mail: friedrichsen@oc.uni-kiel.de
Z. Naturforsch. 57b, 677Ð684 (2002); received February 21, 2002
Nitronyl Nitroxides, Mesogenic Behaviour
The synthesis of several new nitronyl nitroxides with mesogenic groups is reported (6a, b,
11, 14a, b). Microscopic investigations revealed that in some cases (6b, 14b) highly ordered
mesogenic phases are formed.
Introduction
gives 1,3-dihydroxyimidazolidine derivative 9 as
colorless crystals, which is already oxidized by air.
Treatment with lead dioxide gives the cyclic nitro-
nyl nitroxide10 in 81% yield as dark blue needles.
Esterification with 5b under Steglich conditions
gives 11 in 63% yield as blue crystals. The synthe-
sis of 14a, b can be accomplished with 12a, b as
starting materials. Esterification with 1 (to form
13a, b) and Jones oxidation [14] yields carboxylic
acids 13c, d. Treatment of these acids with 10
under Steglich conditions gives nitroxides 14a and
14b as blue crystals with m.p. 64 ∞C and 91.5 ∞C,
respectively.
Liquid crystalline radicals with a neutral π-de-
localized spin system seem to be unknown [1, 2].
Among organic radicals, nitronyl nitroxides (NIT)
because of their stability are of considerable inter-
est in this respect. Compounds of this type [3, 4]
have been used extensively as building block(s) for
magnetic materials [5]. Ferromagnetism in com-
pounds with an NIT moiety [6, 7] suggests the pos-
sibility of ferromagnetic liquid crystals with this
structure element [8]. Therefore preliminary in-
vestigations have been carried out to find out
whether mesogenic phases of this type are accessi-
ble [9].
In order to get an insight into the liquid crystal
(LC) behavior of 6a, b, 11, and 14a, b microscopic
measurements with a polarizing microscope were
undertaken. Whereas compounds 6a, 11, and 14a
Preparative Results
A rod-like geometry [11] seems to be a good show no LC behavior, compound 6b has a mono-
prerequisite for a liquid crystalline NIT com- tropic mesophase. Similar observations were made
pound. Therefore compound 4 (Scheme 1) was for compound 14b. The crystals of compound 6b
chosen as starting material.
melt at 90 ∞C to an isotropic liquid. On cooling, a
As has been reported as early as 1968 [3,12] this mesophase is observed at 36 ∞C. At first glance,
compound is readily available from 4-hydroxy- this mesophase looks like a typical nematic meso-
benzaldehyde (1) and bishydroxylamine 2 via 3, phase with Schlieren textures. The transition tem-
although the yield in the dehydrogenation step perature is reproducible on cooling and heating
was quite low (19%) [12]. This could be improved again. The viscosity is high, similar to an ordered
to 26%. Treatment of 4 with 4-octyloxy- and smectic or columnar phase. Very slow cooling
4-(dodecyloxy)benzoic acid (5a, b) under Steglich from 36.5 ∞C to 35 ∞C yields a different texture
conditions [13] (DCC, DMAP) resulted in the for- with lancets typical for ordered smectic phases.
mation of 6a (62%) and 6b (50%) as dark-blue The mesophase is not miscible with the nematic
crystals with mp. 85 ∞C and 90 ∞C, respectively.
phase of PCH7 (4-(trans-4-heptylcyclohexyl-
The syntheses of 11 and 14a, b are also straight- benzonitrile)) [15a]. By annealing the melt around
forward. Etherification of aldehyde 1 with alkyl- 40 ∞C, another crystalline form is obtained melting
bromide 7b yields 8 which after treatment with 2 at 50 ∞C. The crystals of 14b melt at 91.5 ∞C to an
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S. Greve et al. · Novel Nitronyl Nitroxides: Synthesis and Properties
Scheme 1.
isotropic liquid. On cooling, a mesophase is level of theory (see Table 1) agree reasonably well
formed at 39 ∞C. This mesophase shows Schlieren with experimental values (taken from [23]) with
and fan-shape textures similar to a smectic C only one exception: the exocyclic NÐO bond
˚
phase. Slow cooling creates more homeotropic like length was obtained as 1.24 A, whereas experi-
˚
textures. Faster cooling allows to supercool the mental values are observed in the 1.28 A region.
melt down to 26 ∞C. Contact preparation with Density functional theoretical calculations [18aÐg,
HOPDOB (4-hexyloxyphenyl 4-decyloxyben- 19] reveal [20] that the B3LYP/6-31G* methodol-
zoate) [15b] shows no miscibility of the meso- ogy gives geometrical data which are close to ex-
phases. A second crystalline form melting at
49.5 ∞C is formed by annealing the supercooled
melt below 45 ∞C. In both cases, the mesophases
are most probably not low ordered phases (N, SC),
but higher ordered phases. The viscosity of the
mesophases are high, the textures obtained by
slow cooling are not of an N or SC type and the
phases are not miscible with phases of reference
compounds.
Computational Results
Quantum chemical calculations of nitronyl nit-
roxides have been reported repeatedly [16aÐd].
Semiempirical PM3 calculations [17aÐc] of 15a
and 15b show that geometry optimizations on this Fig. 1. Calculated structure of 15b (B3LYP/6-31G*).
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S. Greve et al. · Novel Nitronyl Nitroxides: Synthesis and Properties
679
Scheme 2.
Scheme 3.
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S. Greve et al. · Novel Nitronyl Nitroxides: Synthesis and Properties
data are sufficient for a preliminary inspection of
molecules which are not conveniently treated
computationally by ab initio and DFT methods.
The arrangement of the molecules of type 6 (11,
14) in a highly ordered phase is unknown.
Whereas the application of quantum chemical
methods (ab initio, DFT) on large clusters of these
molecules is beyond reach of comtemporary com-
puter methodology, molecular mechanics methods
may give an insight into preferred orderings. Start-
Scheme 4.
perimental values (Table 1). Puckering of the five-
membered ring in 15b is also predicted correctly
(Fig. 1) in contrast to PM3 results.
ing with a rectangular arrangement of about 100
molecules of compound 6b ((C32H45N2O5)100
3200H4500N200O500) in parallel and antiparallel
=
C
X-ray crystal structure determinations of 2-aryl-
substituted nitronyl nitroxides have shown that in
general there is a twist angle θ > 0∞ between the
two rings [22]. PM3 calculations of 15d show that
the conformation with θ = 0∞ is an energy maxi-
mum. ∆H∞f decreases with increasing values of θ
with its minimum at 90∞. For 15c similar results are
obtained. However, DFT calculations (B3LYP/
6-31G*) reveal the energy minimum at θ = 0∞, as
is shown in Fig. 2.
fashion optimizations using the force field MM+
were undertaken [24]. As can be seen from Fig. 3
the parallel arrangement is highly distorted
whereas the antiparallel ordering (Fig. 4) is main-
tained during the optimization procedure. The
same holds for molecules of type 14b
((C45H61N2O8)64 = C2880H3904N128O512; Fig. 5).
Therefore it is highly probable that in line with
The basis set dependence of the rotational bar-
rier at this level of theory is negligible (B3LYP/
6-31G*: 6.80 kcal/mol; 6-31G**: 6.97 kcal/mol). In
conclusion it can be stated that although B3LYP/
6-31G* calculations are in good agreement with
experimental bond lengths (bond angles), PM3
Fig. 3. Parallel ordering of molecules of 6b (MM+ calcu-
lations).
Fig. 2. Calculated rotational barrier around the CÐPh Fig. 4. Antiparallel ordering of molecules of 6b (MM+
bond in 15c (B3LYP/6-31G*, B3LYP/6-31G**). calculations).
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S. Greve et al. · Novel Nitronyl Nitroxides: Synthesis and Properties
681
2-(4-Hydroxyphenyl)-4,5-dihydro-4,4,5,5-
tetramethyl-3-oxido-3-imidazolio-1-oxyl (4)
A solution of 276 mg (1.1 mmol) 3 in benzene
(50 ml) was treated with 1.40 g (5.9 mmol) of lead
dioxide. The suspension was stirred vigorously, fil-
tered after standing for further 2 h, evaporated,
and the residue was purified by RC on silica gel
with cyclohexaneÐethyl acetate (1:1) to provide
71 mg (26%) of 4 as deep blue crystals with m.p.
144Ð146 ∞C (lit. [12] 134Ð135 ∞C).
2-{4-[(4-Octylbenzoyl)oxy]phenyl}-4,5-dihydro-
4,4,5,5-tetramethyl-3-oxido-3-imidazolio-1-oxyl (6a)
To a solution of 4 (26.5 mg, 0.11 mmol), 4-octyl-
oxybenzoic acid 5a (27 mg, 0.11 mmol) and
DMAP (5 mg) in dichloromethane (20 ml) was
added 31 mg (0.15 mmol) of dicyclohexylcarbodi-
imide (DCC). After standing for 24 h, further
24 mg (0.12 mmol) of DCC was added, and the
mixture was stirred for further 24 h. The solution
was filtered, evaporated and the residue was puri-
fied by RC on silica gel with cyclohexaneÐethyl
acetate (1:1) to provide 32 mg (62%) 6a as deep
blue crystals with m.p. 85 ∞C. Ð IR (KBr): ν =2920,
2852, 1720, 1392, 1364, 1256, 1166, 1071 cmÐ1. Ð UV/
vis (CH3CN): λmax (lg ε) =213 (4.164), 274 (4.293),
350 (sh, 3.668), 615 (2.399) nm. Ð MS (EI, 70 eV):
Fig. 5. Antiparallel ordering of molecules of 14b (MM+
calculations).
expectations in an ordered phase molecules of this
type are arranged predominantly or exclusively in
an antiparallel fashion.
Experimental Section
All melting points were determined on a Dr
Tottoli melting point apparatus and are uncor-
rected. IR spectra were recorded on a Perkin-El-
mer FTIR 1600 spectrophotometer. NMR spectra:
Bruker AM 300 (300 MHz: FT 1H NMR; 75 MHz:
+
m/z (%) =481 (29) [M ]. Ð HRMS: C28H37N2O5:
calcd. 481.2702; found 481.2702 (MS).
2-(4-{[4-(Dodecyloxy)benzoyl]oxy}phenyl)-4,5-
dihydro-4,4,5,5-tetramethyl-3-oxido-3-imidazolio-
1-oxyl (6b)
1
13C NMR); Bruker DRX 500 (500 MHz: FT H
NMR; 125 MHz: 13C NMR); Varian EM 360
(60 MHz, 1H NMR), internally referenced on
Me4Si (CDCl3) or DMSO ([2H6]DMSO). J values
are given in Hz. UV spectra: Zeiss DMR 10 spec-
trophotometer; mass spectra: Finnigan MAT 8230
and MAT 8200 mass spectrometer at 70 eV ionisa-
tion potential (EI) or by chemical ionisation (CI)
(isobutane). Radial chromatography (RC) was
carried out with a Harrison-Research Chromato-
tron on silica gel PF254 (Merck, Darmstadt). If as-
signments of spectral data are ambiguous, this is
marked with asterisks (*,**).
As described for the preparation of 6a, from 4
(24.1 mg, 0.10 mmol), 4-dodecyloxybenzoic acid
5b (30.6 mg, 0.10 mmol), DMAP (5 mg) and DCC
(31 mg, 0.15 mmol) in dichloromethane (20 ml)
26.9 mg (50%) of 6b was obtained as deep blue
crystals with m.p. 90 ∞C. Ð IR (KBr): ν =2922,
2851, 1733, 1390, 1365, 1257, 1159, 1050 cmÐ1. Ð
UV/vis (CH3CN): λmax (lg ε) =212 (sh, 4.741), 274
(4.679), 349 (4.023), 364 (4.264), 587 (2.707) nm. Ð
MS (EI, 70 eV): m/z (%) =537 (14) [M +]. Ð
HRMS: C32H45N2O5: calcd. 537.3328; found
537.3326 (MS).
Microscopic (texture) investigations were car-
ried out with a polarizing microscope Olympus
BH2 using a programmable heating device (In-
stec, Inc.).
1,3-Dihydroxy-2-(6-hydroxyhexyloxy)phenyl-
4,4,5,5-tetramethylimidazolidine (9)
Materials
A solution of 2 (222 mg, 1.5 mmol) and 4-(6-hy-
All solvents were dried or purified using stan- droxyhexyloxy)benzaldehyde
dard procedures [25]. 1,3-Dihydroxy-2-(4-hydro- mmol) in 2-propanol (5 ml) was stirred for 12 h at
8
(0.333 g, 1.5
xyphenyl)-4,4,5,5-tetramethylimidazolidine
[12].
(3) 60 ∞C. After standing for further 12 h at 4 ∞C crys-
tals of 8 were separated by filtration. Yield: 0.167 g
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682
S. Greve et al. · Novel Nitronyl Nitroxides: Synthesis and Properties
(32%) colorless crystals with m.p. 127Ð130 ∞C. Ð 2909, 2846, 1734, 1700, 1267, 1169 cmÐ1. Ð 1H
1
IR (KBr): ν =3447, 2935, 1378, 1245 cm Ð1. - H NMR (300 MHz, CDCl3): δ =0.90 (t, 3H, CH ),
NMR (300 MHz, [D6]DMSO): δ =1.03 (s, 6H,
1.25Ð1.42 (m, 8H, CH2), 1.43Ð1.55 (m, 2H, CH32),
CH3), 1.06 (s, 6H, CH3), 1.30Ð1.50 (m, 6H, CH2), 1.82 (m, 2H, CH2), 4.05 (t, 2H, OCH2), 6.98 (m,
1.70 (m, 2H, CH2), 3.39 (m, 2H, CH2), 3.94 (t, 2H, 2H, ar-H), 7.40 (m, 2H, ar-H), 7.97 (m, 2H, ar-H),
OCH2), 4.37 (t, 1H, OH), 4.44 (s, 1H, CH), 6.86 8.14 (m, 2H, ar-H), 10.03 (s, 1H, CHO). Ð 13C
(m, 2H, ar-H), 7.35 (m, 2H, ar-H), 7.68 (s, 2H, NMR (75 MHz, CDCl3): δ =14.10 (q), 22.65 (t),
OH). Ð 13C-NMR (75 MHz, [D6]DMSO): δ = 25.98 (t), 29.08 (t), 29.22 (t), 29.32 (t), 31.80 (t),
17.19 (q), 24.44 (q), 25.33 (t), 25.47 (t), 28.81 (t), 68.40 (t), 114.45 (d), 120.82 (s), 122.60 (d), 131.21
32.52 (t), 60.67 (t), 65.92 (s), 67.38 (t), 89.95 (d), (d), 132.43 (d), 133.90 (s), 155.93 (s), 163.89 (s),
113.55 (d), 129.59 (d), 133.89 (s), 158.11 (s).
164.21 (s), 190.94 (d).
2-[4-(6-Hydroxyhexyloxy)phenyl]-4,5-dihydro-
4,4,5,5-tetramethyl-3-oxido-3-imidazolio-1-oxyl
(10)
4-Formylphenyl 4-(dodecyloxy)benzoate (13b)
Compound 12b (2.50 g, 8.20 mmol) was esteri-
fied with 1 (1.20 g, 10.0 mmol) using the Steglich
procedure [DMAP (92 mg, 0.75 mmol), DCC
(2.25 g, 10.9 mmol), dichloromethane (20 ml), 24 h
at r.t.]. Yield: 1.84 g (55%) fine, colorless crystals
of 13b with m.p. 64 ∞C (n-hexane). Ð IR (KBr):
A solution of 9 (203 mg, 0.57 mmol) in ethanol
(40 ml) was treated with lead dioxide (500 mg).
After a few seconds the suspension showed a deep
blue color. After stirring for 30 min the solution
was filtered, evaporated and purified by RC on
silica gel with cyclohexaneÐethyl acetate (1:1).
Yield: 0.163 g (81%) dark blue needles with m.p.
134 ∞C. Ð IR (KBr): ν =3250, 2940, 2864, 1386,
1360, 1258, 1186 cmÐ1. Ð UV/vis (CH3CN): λmax
(lg ε) =236 (3.820), 283 (4.221), 295 (sh, 4.110),
347 (sh, 3.764), 365 (3.985), 621 (2.715) nm. Ð MS
ν =2912, 2848, 1736, 1699, 1268, 1169 cm Ð1. Ð H
1
NMR (300 MHz, CDCl3): δ =0.88 (t, 3H, CH ),
1.25Ð1.40 (m, 16H, CH2), 1.40Ð1.55 (m, 2H, CH32),
1.82 (m, 2H, CH2), 4.05 (t, 2H, OCH2), 6.98 (m,
2H, ar-H), 7.40 (m, 2H, ar-H), 7.97 (m, 2H, ar-H),
8.14 (m, 2H, ar-H), 10.03 (s, 1H, CHO). Ð 13C
NMR (75 MHz, CDCl3): δ =14.14 (q), 22.99 (t),
25.97 (t), 29.07 (t), 29.36 (t), 29.56 (t), 29.60 (t),
29.64 (t), 29.66 (t), 31.92 (t), 68.39 (t), 114.42 (d),
120.42 (s), 122.61 (d), 131.23 (d), 132.43 (d), 133.87
(s), 155.91 (s), 163.88 (s), 164.23 (s), 191.01 (d).
+
(EI, 70 eV): m/z (%) =349 (29) [M ]. Ð HRMS:
C19H29N2O4: calcd. 349.2127; found 349.2125.
2-{4-[(6-{[4-(Dodecyloxy)benzoyl]oxy}hexyl)-
oxy]phenyl}-4,5-dihydro-4,4,5,5-tetramethyl-
3-oxido-3-imidazolio-1-oxyl (11)
4-{[4-(Octyloxy)benzoyl]oxy}benzoic acid (13c)
To a solution of 13a (1.50 g, 4.2 mmol) in ace-
As described for 6a, from 10 (28.0 mg,
0.08 mmol), 4-dodecyloxybenzoic acid (24.0 mg,
0.08 mmol), DMAP (5 mg) and DCC (27 mg, tone (40 ml) Jones reagent (4 ml, prepared accord-
0.13 mmol) 32 mg (63%) of 11 was obtained as ing to [27]) was added. The mixture was stirred for
dark blue crystals with m.p. 76Ð77 ∞C. Ð IR (KBr): 2 h at r.t., added to crushed ice (200 g), stirred for
ν =2922, 2852, 1718, 1387, 1362, 1250, 1166, 1097
further 30 min, the crystals were separated by
filtration, washed with water and recrystallized
cmÐ1. Ð UV/vis (CH3CN): λmax (lg ε) =257
(4.570), 282 (4.361), 296 (sh, 4.200), 348 (3.896), from ethanol (50 ml). Yield: 1.31 g (84%) colorless
365 (4.081), 614 (2.894) nm. Ð MS (EI, 70 eV): crystals with m.p. 135 ∞C (lit. [26] 139 ∞C); clearing
+
m/z (%) =637 (36) [M ]. Ð HRMS: C38H57N2O6: point 222 ∞C (lit. [26] 221 ∞C). Ð IR (KBr): ν =
calcd. 637.4217; found 637.4216.
3437, 2922, 1734, 1686, 1254, 1158 cmÐ1. Ð 1H
NMR (300 MHz, [D6]DMSO): δ =0.85 (t, 3H,
CH3), 1.20Ð1.34 (m, 8H, CH2), 1.35Ð1.45 (m, 2H,
CH2), 1.73 (m, 2H, CH2), 4.07 (t, 2H, OCH2), 7.11
4-Formylphenyl 4-(octyloxy)benzoate (13a)
Compound 12a (5.00 g, 20.0 mmol) was esteri- (m, 2H, ar-H), 7.39 (m, 2H, ar-H), 8.03 (m, 2H,
fied with 1 (2.44 g, 20.0 mmol) using the Steglich ar-H), 8.07 (m, 2H, ar-H), 13.05 (s, br, 1H, OH). Ð
procedure [DMAP (184 mg, 1.51 mmol), DCC 13C NMR (75 MHz, [D6]DMSO): δ =13.99 (q),
(4.50 g, 22.0 mmol), dichloromethane (35 ml), 24 h 22.16 (t), 25.50 (t), 28.56 (t), 28.75 (t), 28.82 (t),
at r.t.]. Yield: 4.37 g (62%) colorless crystals of 13a 31.32 (t), 68.04 (t), 114.69 (d), 120.43 (s), 122.20
with m.p. 57 ∞C (lit. [26] 59 ∞C, clearing point (d), 128.38 (s), 130.93 (d), 132.15 (d), 154.23 (s),
69 ∞C, lit. [26] 68 ∞C, n-hexane). Ð IR (KBr): ν = 163.40 (s), 163.83 (s), 166.70 (d).
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S. Greve et al. · Novel Nitronyl Nitroxides: Synthesis and Properties
683
4-{[4-(Dodecyloxy)benzoyl]oxy}benzoic acid (13d) was purified by RC on silica gel with cyclohex-
aneÐethyl acetate. Yield 13.7 mg [22%; 24 mg
Compound 13d was prepared as described for
(70%) of 13c was recovered] blue crystals, m.p.
13c from 13b (7.00 g, 17.1 mmol) and Jones rea-
64 ∞C. Ð IR (KBr, CH2Cl2): ν =2929, 1735, 1390,
gent (11.9 ml, prepared according to [27]) in ace-
1362, 1257, 1161 cmÐ1. Ð UV/vis (CH3CN): λmax
tone (120 ml). Yield: 4.3 g (61%) colorless needles,
(lg ε) =217 (sh, 4.362), 237 (4.243), 267 (4.569),
m.p. 111Ð113 ∞C (clearing point: 206 ∞C, from eth-
296 (4.062), 349 (3.751), 365 (3.951), 617 (2.577)
anol). Ð IR (KBr): ν =2918, 2851, 1740, 1690,
nm. Ð MS (EI, 70 eV): m/z (%) =701 (17) [M +]. Ð
HRMS: C41H53N2O8: calcd. 701.3802; found
701.3796.
1256, 1159 cmÐ1
. Ð
1H NMR (300 MHz,
[D6]DMSO): δ =0.88 (t, 3H, CH ), 1.19Ð1.42 (m,
18H, CH2), 1.42Ð1.52 (m, 2H, C3H2), 4.04 (t, 2H,
OCH2), 6.98 (m, 2H, ar-H), 7.33 (m, 2H, ar-H),
8.12Ð8.22 (m, 4H, ar-H). Ð 13C NMR (75 MHz,
CDCl3): δ =14.14 (q), 22.70 (t), 25.98 (t), 29.08
(t), 29.36 (t), 29.56 (t), 29.59 (t), 29.64 (t), 29.67
(t), 31.93 (t), 68.39 (t), 114.41 (d), 120.92 (s),
122.02 (d), 126.60 (s), 131.89 (d), 132.44 (d), 155.52
(s), 163.82 (s), 164.34 (s), 170.78 (s).
2-[4-({6-[(4-{[4-(Dodecyloxy)benzoyl]oxy}-
benzoyl)oxy]hexyl}oxy)phenyl]-4,5-dihydro-
4,4,5,5-tetramethyl-3-oxido-3-imidazolio-1-oxyl
(14b)
Compound 13d (31 mg, 0.07 mmol) was esteri-
fied with 10 (25 mg, 0.07 mmol) according to the
Steglich procedure [DMAP (5 mg), DCC (30 mg,
0.15 mmol), 24 h at r.t.]. The compound was puri-
fied by RC on silica gel with cyclohexaneÐethyl
acetate (3:2). Yield: 39 mg (72%), blue crystals,
[7 mg (28%) of 13d was recovered], m.p.
91.5 ∞C. Ð IR (KBr): ν =2922, 2851, 1727, 1362,
2-[4-({6-[(4-{[4-(Octyloxy)benzoyl]oxy}benzoyl)-
oxy]hexyl}oxy)phenyl]-4,5-dihydro-4,4,5,5-
tetramethyl-3-oxido-3-imidazolio-1-oxyl (14a)
Compound 13c (31 mg, 0.09 mmol) was esteri- 1250, 1161 cmÐ1. Ð UV/vis (CH3CN): λmax (lg ε) =
fied with 10 (33 mg, 0.09 mmol) according to the 217 (3.768), 234 (3.881), 293 (3.951), 262 (3.604),
Steglich procedure [DMAP (5 mg), DCC (19 mg) 377 (3.828), 594 (2.036) nm. Ð MS (EI, 70 eV): m/z
in dichloromethane (20 ml), 24 h at r.t., further (%) =757 (5) [M +]. Ð HRMS: C45H61N2O8: calcd.
DCC (19 mg) was added, 24 h at r.t.)]. The product 757.4428; found 757.4426.
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