Symmetrical tris(4,6-diamino-5-methylene-2-pyrimidones): New building blocks for self-assembly of hollow spherical supramolecules locked by hydrogen bonds

Article abstract of DOI:10.1135/cccc19961464

Concise synthesis of the tris(pyrimidones) 1a,b is described. Molecular modeling study demonstrated that both the prepared models 1a,b are capable of self-assembling under formation of spherical dimers locked by 18 hydrogen bonds. Extreme insolubility in all common solvents precluded investigation of the self-assembly in solution. Circumstantial evidence in favor of the self-assembly has been provided in the solid and gas phase.

Full text of DOI:10.1135/cccc19961464

1464
Alexander, Holy, Fiedler, Havlas, Zavada:
SYMMETRICAL TRIS(4,6-DIAMINO-5-METHYLENE-2-PYRIMIDONES):
NEW BUILDING BLOCKS FOR SELF-ASSEMBLY OF HOLLOW
SPHERICAL SUPRAMOLECULES LOCKED BY HYDROGEN BONDS
,
1
2
Daniel ALEXANDER Petr HOLY , Pavel FIEDLER, Zdenek HAVLAS
and Jiri ZAVADA *
3,
Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic,
166 10 Prague 6, Czech Republic; e-mail: 1 petrholy@uochb.cas.cz, 2 havlas@uochb.cas.cz,
3 zavada@uochb.cas.cz
Received July 27, 1996
Accepted August 28, 1996
Dedicated to the memory of Dr Zdenek Arnold.
Concise synthesis of the tris(pyrimidones) 1a,b is described. Molecular modeling study demonstrated
that both the prepared models 1a,b are capable of self-assembling under formation of spherical
dimers locked by 18 hydrogen bonds. Extreme insolubility in all common solvents precluded inves-
tigation of the self-assembly in solution. Circumstantial evidence in favor of the self-assembly has
been provided in the solid and gas phase.
Key words: Pyrimidones; Hydrogen bonding; Self-assembly.
The spontaneous generation of organized non-covalent structures by self-assembly of
complementary molecular units is a leitmotif of supramolecular chemistry. The self-as-
sembling process is driven by molecular recognition between the individual compo-
nents. Recently, a simple concept has been proposed for the self-assembly which is
based on multiple hydrogen bonding1. Derivatives of 4,6-diamino-2-pyrimidone2,3 have
been found to be very convenient models for self-assembly by intermolecular hydrogen
bonding, each pyrimidone unit providing two self-complementary binding faces. For-
mation of a highly organized two-dimensional polymeric architecture (Scheme 1) has
been demonstrated by X-ray structural analysis2,3.
Now we wish to extend application of the pyrimidone synthon to construction of
closed-surface three-dimensional structures**. Symmetrical tris(pyrimidones) 1 have
* The author to whom correspondence should be addressed.
**Successful synthesis of similar self-assembling dimeric capsules based on glycouril or calixarene
hydrogen-bonding synthon has been recently reported, see ref.4.
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Symmetrical Tris(4,6-diamino-5-methylene-2-pyrimidones)
1465
been designed to serve as the building blocks. According to model examination, their
self-assembly might give rise to spherical dimers with a hollow interior locked by six
triplets of hydrogen bonds.
H
N
R
H
N
H
N
R
H
N
H
H
H
H
O
R
O
R
N
N
N
N
H
H
H
N
N
N
N
O
O
H
H
H
H
N
H
N
H
N
N
H
H
SCHEME 1
EXPERIMENTAL
Melting points were determined on a Kofler block and are uncorrected. 1H NMR spectra were
measured on a spectrometer Varian XL-200 (200 MHz, FT mode) and chemical shifts of protons (δ,
ppm) were referred to TMS as the internal standard. Mass spectra (m/z, rel.% intensity) were ob-
tained with a ZAB-EQ spectrometer (VG Analytical, Manchester, U.K.); EI mass spectra were taken
at 70 eV; matrix and solvent used for FAB spectra are specified in brackets. IR spectra were re-
corded in KBr pellets and fluorolube mull (4 000–1 350 cm–1) on a Bruker IFS 88 Fourier transform
spectrometer. GLC analyses were run on a chromatograph Hewlett–Packard 5890 with a HP-1 column
(methylsilicone, 5 m × 0.53 mm, film thickness 2.65 nm); flame ionization detector. TLC analyses
were performed on silica gel plates Silufol UV (Kavalier, Votice, Czech Republic).
1,3,5-Benzenetricarbaldehyde (3a)
Sodium (8.4 g, 0.37 mol) was dissolved in absolute ethanol (600 ml), 2-nitropropane (52 g, 0.58 mol)
was slowly added under intense stirring and the solution was treated immediately with tribromide 2a
(35.7 g, 0.1 mol) dissolved in dimethyl sulfoxide (600 ml). After continuous stirring at room tempera-
ture for 3 h the reaction mixture was poured into ice water (3 l) and extracted with chloroform (4 × 300 ml).
The combined organic extracts were treated with 3% aqueous sodium hydroxide (350 ml) and
washed successively with dilute (1 : 10) hydrochloric acid (270 ml) and saturated aqueous sodium
hydrogen carbonate solution (200 ml). After drying (MgSO4) the solution was taken to dryness, the
residue was washed with ether and recrystallized from water. Yield 8.4 g (52%), m.p. 158–160 °C,
1
ref.5 156.5 °C, ref.6 155 °C, ref.7 155–159 °C. H NMR spectrum was in accord with ref.7.
2,4,6-Trimethyl-1,3,5-benzenetricarbaldehyde (3b)
Prepared analogously from tribromide 2b (40 g, 0.1 mol). Recrystallization from 2-propanol afforded
the pure product (12.5 g, 61%), m.p. 173–260 °C (decomp.). For C12H12O3 (204.2) calculated:
70.57% C, 5.92% H; found 70.63% C, 5.93% H. Mass spectrum (EI): 204 (M+, 100), 189 (20), 147
1
(90), 133 (40), 105 (30), 91 (80), 77 (50). H NMR spectrum (CDCl3): 2.65 s, 9 H (3 × CH3); 10.63 s,
3 H (3 × CH=O). IR spectrum (CHCl3): 2 855 w, 2 754 w (CH in CH=O); 1 696 vs (C=O); 1 382 m,
1 071 m (CH3).
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

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Alexander, Holy, Fiedler, Havlas, Zavada:
1,3,5-Tris(2′,2′-ethenyldicarbonitrile)benzene (4a)
Trialdehyde 3a (4.86 g, 0.03 mol) was suspended under stirring in 1-butanol (120 ml) and malononi-
trile (7.27 g, 0.11 mol), followed by several drops of piperidine catalyst (10% solution of the amine
in 1-butanol), was added. The reaction mixture was stirred at 50 °C for 8 h, the resulting precipitate
was collected by suction, washed successively with water–acetic acid (20 : 1) and ether, and dried in
vacuo. Yield 8.90 g (87%), m.p. 288–290 °C (ethyl acetate). For C18H6N6 (306.3) calculated: 70.59% C,
1.97% H, 27.44% N; found: 70.48% C, 2.01% H, 27.32% N. Mass spectrum (EI): 306 (M+, 100), 279 (60),
257 (42), 252 (32), 243 (28), 124 (28), 100 (48), 87 (42), 75 (70), 51 (48), 43 (70). 1H NMR spectrum ((CD3)2SO):
8.48 s, 3 H (3 × CH); 8.78 s, 3 H (Ar-H). IR spectrum (KBr): 3 084 m (CH arom);
3 037 m (HC=C=); 2 237 vs (CN); 1 603 vs (C=C); 1 581 s, 1446 s (ring); 1 192 s,
960 s, 672 s (arom).
1,3,5-Tris(2′,2′-ethenyldicarbonitrile)-2,4,6-trimethylbenzene (4b)
Prepared analogously from trialdehyde 3b (2.04 g (0.01 mol). Yield 2.95 g (85%), m.p. 214–215 °C
(1-butanol). For C21H12N6 . 0.5 H2O (357.4) calculated: 70.58% C, 3.67% H, 23.52% N; found:
70.68% C, 3.53% H, 23.71% N. Mass spectrum (EI): 348 (M+, 100), 333 (18), 320 (30), 306 (42),
279 (32), 267 (15), 140 (15), 120 (22), 100 (15), 88 (20), 76 (15), 63 (20), 57 (20), 51 (18), 39 (20).
1H NMR spectrum ((CD3)2SO): 2.24 s, 9 H (3 × CH3); 8.87 s, 3 H (3 × CH). IR spectrum (CH2Cl2):
3 024 m (–CH=); 2 962 w, 2 929 w (CH3); 2 239 s (CN); 1 599 vs (C=C); 1 556 m (ring); 1 450 m
(CH3); 1 383 m (CH3).
1,3,5-Tris(2′,2′-ethyldicarbonitrile)benzene (5a)
Hexanitrile 4a (2.02 g, 6.6 mmol) was dissolved in dimethylformamide (40 ml) and treated with tri-
ethylammonium formate (5.03 g; prepared by a co-distillation of 2 mole equivalents of trie-
thylamine with 5 mole equivalents of formic acid). The reaction mixture was stirred at 25–30 °C for
3 h and poured into ice water (120 ml). The resulting precipitate was collected by suction and
washed successively with water and ether. Yield 1.98 g (96%), m.p. 190–192 °C. Mass spectrum
(EI): 312 (M+, 10), 289 (100), 247 (100), 182 (20), 155 (10), 115 (20), 91 (12), 77 (10). HRMS (EI):
found 312.1114 (M+), C18H12N6 requires 312.1123. 1H NMR spectrum ((CD3)2SO): 3.36 d, J = 7.02 Hz,
6 H (3 × CH2); 5.07 t, J = 7.02 Hz, 3 H (3 × CH); 7.37 s, 3 H (Ar-H). IR spectrum (KBr): 3 029 w,
3 017 w (CH arom); 2 926 vs, 2 922 vs (CH2); 2 260 m, 1 607 m (CN); 894 m, 886 m, 723 s.
1,3,5-Tris(2,2′-ethyldicarbonitrile)-2,4,6-trimethylbenzene (5b)
Prepared analogously from the hexanitrile 4b (2.30 g, 6.6 mmol). Yield 2.26 g (97%), m.p. 198–200 °C
(acetonitrile). Mass spectrum (EI): 354 (M+, 20), 289 (100), 262 (10), 224 (35), 159 (42), 128 (35),
115 (20), 91 (18), 69 (30), 55 (38), 43 (40). HRMS (EI): found 354.1544 (M+), C21H18N6 requires
1
354.1593. H NMR spectrum ((CD3)2SO): 2.41 s, 9 H (3 × CH3); 3.56 d, J = 8.24 Hz, 6 H (3 × CH2);
4.87 t, J = 8.24 Hz, 3 H (3 × CH). IR spectrum (KBr): 2 921 s (CH2); 2 258 m (CN); 1 568 m (ring);
1 479 m, 1 452 m, 1 421 m, 1 365 s (CH3).
1,3,5-Tris[4′,6′-diamino-5′-methylenepyrimidine-2′(1′H)-thione]benzene (6a)
To a solution of sodium ethoxide (from 0.3 g, 13.2 mmol of sodium) in ethanol (14 ml) was added
thiourea (1.37 g, 18 mmol) and hexanitrile 5a (1.25 g, 4 mmol). The reaction mixture was heated
under reflux for 10 h, and taken to dryness in vacuo. The residue was dissolved in water (15 ml) and
neutralized (to pH 6) with dilute hydrochloric acid (1 : 10). The precipitate was collected by suction,
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Symmetrical Tris(4,6-diamino-5-methylene-2-pyrimidones)
1467
washed successively with water, ethanol and ether, and dried in vacuo. Yield 2.12 g (98%), m.p.
>380 °C. Mass spectrum (FAB, thioglycerine + glycerine, DMSO): 541 ([M + H]+, 48), 512 (100),
488 (33), 465 (48), 400 (13), 257 (51). HRMS (FAB): found 541.1396 ([M + H]+), C21H25N12S3
1
requires 541.1487. H NMR spectrum ((CD3)2SO): 3.68 s, 6 H (3 × CH2); 6.94 s, 3 H (Ar-H); 7.74 s,
12 H (6 × NH2); 12.85 brs, 3 H (3 × NH). IR spectrum (KBr): 3 304 vs (NH2); 3 163 vs (NH); 1 637 vs,
1 567 vs, 1 234 vs, 1 166 s (pyrimidinethione); 1 462 m (benzene); 1 430 m (CH2); 1 374 (C–N).
1,3,5-Tris[4′,6′-diamino-5′-methylenepyrimidine-2′(1′H)-thione]-2,4,6-trimethylbenzene (6b)
Prepared analogously from hexanitrile 5b (1.42 g, 4 mmol). Yield 2.24 g (96%), m.p. >380 °C. Mass
spectrum (FAB, thioglycerine + glycerine, DMSO): 583 ([M + H+], 100), 554 (44), 515 (40), 507
1
(22). HRMS (FAB): found 583.1947 ([M + H+]), C24H31N12S3 requires 583.1957. H NMR spectrum
((CD3)2SO): 2.18 s, 9 H (3 × CH3); 3.71 s, 6 H (3 × CH2); 6.83 brs, 12 H (3 × NH2); 12.70 brs, 3 H (3 ×
NH). IR spectrum (KBr): 3 305 s (NH2); 3 179 s (NH); 1 626 vs, 1 561 vs, 1 507 vs, 1 227 s, 1 169 s,
964 m (pyrimidinethione); 1 378 s, 1 327 s, 1 299 (C–N).
1,3,5-Tris[4′,6′-diamino-5′-methylenepyrimidine-2′(1′H)-one]benzene (1a)
Tris(thiopyrimidone) 6a (540 mg, 1 mmol) was suspended in water (11.2 ml) and heated with chloro-
acetic acid (0.75 g, 7.94 mmol) under reflux for 3 h. The reaction mixture was then treated with
concentrated sulfuric acid (1.35 ml) and heated for another 2 h. The clear solution was neutralized
with 30% aqueous sodium hydroxide (to pH 8). The precipitate was collected by centrifugation and
washed successively with water and ethanol. Yield 360 mg (73%), m.p. >380 °C. Mass spectrum
(FAB, 2-hydroxyethyl disulfide, DMSO): 1 008 ([2 M + Na]+, 2), 985 ([2 M + H]+, 3), 515 ([M + Na]+, 100),
493 ([M + H]+, 95), 401 (80), 369 (25), 279 (80), 237 (100). HRMS (FAB): found 493.2059 ([M + H]+),
1
C21H25N12O3 requires 493.2173. H NMR (CD3COOD + D2O): 3.72 s, 6 H (3 × CH2); 7.03 s, 3 H
(Ar-H).
1,3,5-Tris[4′,6′-diamino-5′-methylenepyrimidine-2′(1′H)-one]-2,4,6-trimethylbenzene (1b)
Prepared analogously from tris(thiopyrimidone) 6b (582 mg, 1 mmol). Yield 450 mg (85%), m.p.
>380 °C. Mass spectrum (FAB, 2-hydroxyethyl disulfide, DMSO): 1 091 ([2 M + Na]+, 1), 1 069 ([2 M + H]+, 1),
558 ([M + Na]+, 48), 536 ([M + H]+, 100), 516 (40), 482 (41), 437 (54), 401 (30), 293 (100), 279
1
(62), 237 (62). HRMS (FAB): found 535.2557 ([M + H]+), C24H31N12O3 requires 535.2642. H NMR
spectrum (CD3COOD + D2O): 2.37 s, 9 H (3 × CH3); 3.86 s, 6 H (3 × CH2).
4,6-Diamino-5-benzylpyrimidine-2(1H)-thione (7)
Prepared from benzylmalononitrile8 (3.12 g, 20 mmol) by an analogous procedure as 6a. Yield 4.18 g
(90%), m.p. 276–279 °C (dimethylformamide–benzene). Mass spectrum (EI): 232 (M+, 100), 214
(20), 174 (18), 155 (15), 142 (50), 115 (85), 91 (28), 55 (50), 43 (60). HRMS (EI): found 232.0726
1
(M+), C11H12N4S requires 232.0783. H NMR spectrum ((CD3)2SO): 3.76 s, 2 H (CH2); 7.25 m, 5 H
(Ar-H); 7.74 brs, 4 H (2 × NH2); 12.93 brs, 1 H (NH). IR spectrum (KBr): 3 485 s, 3 424 s, 3 269
(NH2); 3 093 vs (NH); 1 627 vs, 1 572 vs, 1 520 vs, 1 236 vs, 1 176 s, 967 m (pyrimidinethione);
1 495 s, 1 453 m, 1 072 m, 1 030 m, 697 m (benzene); 1 383 s, 1 324 s (CN); 1 196 m (C=S).
4,6-Diamino-5-benzylpyrimidine-2(1H)-one (8)
Prepared from 7 (4.18 g, 18 mmol) analogously as described for 1a. Yield 3.46 g (89%), m.p. 371–374 °C
after crystallization from dimethyl sulfoxide. Mass spectrum (EI): 216 (M+, 100), 187 (15), 155 (20),
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

1468
Alexander, Holy, Fiedler, Havlas, Zavada:
139 (22), 122 (15), 91 (15), 69 (20), 57 (32), 43 (30). HRMS (EI): found 216.1003 (M+), C11H12N4O
1
requires 216.1011. H NMR spectrum (D2O + CD3COOD): 3.76 s, 2 H (CH2); 7.34 m, 5 H (Ar-H).
RESULTS AND DISCUSSION
Synthesis
The synthetic protocol used for the preparation of the target tris(4,6-diamino-5-
methylene-2-pyrimidones) 1a,b is outlined in Scheme 2.
CH Br
CHO
2
(CH ) CHNO
3 2
2
R
R
R
R
NaOEt
EtOH/DMSO
BrH C
CH Br
OHC
CHO
2
2
R
R
3a,b
2a,b
CH (CN)
2
2
CH (CN)
2
2
C H N
5 11
CN
CN
CN
R
CN
R
R
R
Et N . HCOOH
3
NC
NC
NC
NC
R
NC
R
NC
CN
CN
5a,b
4a,b
(NH ) CS
2 2
EtOH, NaOEt
O
N
S
N
N
H N
H
H N
2
N
H
2
2
NH
NH
2
1. ClCH COOH
2
R
R
R
R
2. H SO ; 3. NaOH
H N
2
H N
2
4
2
N
N
N
N
O
S
NH
N
NH
N
2
2
R
H N
2
R
H N
2
NH
NH
H
H
2
H
H
2
N
N
O
S
6a,b
1a,b
In formulae 1-6 a: R = H
b: R = CH
3
SCHEME 2
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Symmetrical Tris(4,6-diamino-5-methylene-2-pyrimidones)
1469
The starting tribromides 2a,b have been prepared from mesitylene, the former by
radical bromination9 and the latter by bromomethylation10. Since the direct alkylation
of malononitrile with the tribromides 2a,b failed to afford the corresponding tris(malo-
nonitriles) 5a,b, an alternative procedure had to be developed. Tribromides 2a,b have
been accordingly converted into the trialdehydes* 3a,b upon a treatment with 2-nitro-
propane carbanion13 in EtOH–DMSO solution. Knoevenagel condensation14 of the trial-
dehydes 3a,b with malononitrile afforded smoothly the unsaturated hexanitriles 4a,b.
Triethylammonium formate has been found to be a mild and highly selective reducing
agent15 for conversion of 4a,b into the desired saturated analogues 5a,b. Condensation
of 5a,b with thiourea afforded tris(thiopyrimidones) 6a,b which upon treatment with
chloroacetic acid and subsequent hydrolysis yielded the target building blocks 1a,b.
Analogous protocol, starting from benzaldehyde, was used for the preparation of
4,6-diamino-5-benzyl-2-pyrimidone 8 serving as a reference compound.
Molecular Modeling Studies
The structures of tris(pyrimidones) 1a,b have been optimized at the semiempirical PM3
level16 using program package MOPAC17. The calculation shows that the derivative 1a
(R = H) should exist preferably in a “cone” conformation that is by ≈20 kJ/mol more
stable than the alternative “alternate” arrangement.
Further examination of the optimized conformation of 1a shows that interpenetration
P
P
R
R
P
P
P
R
R
P
R
R
"cone"
"alternate"
P = 2,4-diaminopyrimidine-2-(1H)-thion-5-yl; R = H, CH
3
of two head-to-head oriented “cones”, twisted mutually by 60°, allows formation of
interleaved supramolecular dimer that seals the resulting cavity by intermolecular hy-
drogen bonds. The symmetry of the dimer can be seen from a top view (Fig.1a),
whereas the hydrogen bonding network among the individual pyrimidone units, which
totals 18 hydrogen bonds, is apparent from the side view (Fig. 1b). Based on the dif-
* Several other synthetic approaches have been earlier reported for preparation of 3a, cf. e.g. refs5–7. Two
ingenious syntheses have been designed by the late Dr Z. Arnold (refs11,12).
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

1470
Alexander, Holy, Fiedler, Havlas, Zavada:
ference between energies of the optimized dimer and monomer, the energy gain result-
ing from the non-covalent dimerization corresponds to about 158 kJ/mol.
In a contrast, the corresponding energy gain calculated for the analogous dimeriza-
tion of the tris(pyrimidone) 1b (R = CH3) amounts only to ≈70 kJ/mol. A steric inter-
ference between the individual pyrimidone units and the neighboring methyls is
assumedly responsible for this difference between 1a and 1b. Notably, molecular
modeling studies show that the “cone” conformation of the monomeric form of 1b is
less stable than the “alternate” arrangement, in a contradistinction to the situation found
above for the non-methylated compound 1a.
Observed Self-Assembling Properties
Examination of self-assembling properties of the tris(pyrimidones) 1a,b is rendered dif-
ficult by extreme insolubility of these compounds, the sole exception being strongly
acidic solvents (e.g. CF3COOH) destroying the hydrogen bonding networks. Self-as-
sembling studies in solution could not be accordingly accomplished and acquiring the
physico-chemical evidence has been confined to the solid and gas phase.
IR spectra of the pyrimidones 1a,b and 8 have been taken in KBr pellets as well as
in fluorolube mull and the main data are summarized in Table I. The assignment of the
individual bands is based on the available spectral analysis of the structurally akin
cytosine18.
a
b
FIG. 1
A top (a) and side (b) view on the optimized structure of the dimer 1a.1a
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Symmetrical Tris(4,6-diamino-5-methylene-2-pyrimidones)
1471
All the three investigated pyrimidones exhibit two intense and very broad bands in
the region 3 300–3 000 cm–1, the upper corresponding to the antisymmetric valence
vibration of the –NH2 groups bound strongly by hydrogen bonds (cytosine 3 380 cm–1),
whereas the lower originates assumedly from valence vibration of the N–H bonds, also
strongly hydrogen bound (cytosine 3 169 cm–1). At higher wavenumbers the band
corresponding to the bound –NH2 groups is complemented by the corresponding band
of the “free” (unbound) form. Thus, in accord with expectation, both tris(pyrimidones)
1a,b as well as the mono(pyrimidone) 8 exhibit qualitatively the same hydrogen bond-
ing pattern in the solid phase.
Relative strength of the hydrogen bonds in the individual models was assessed on the
basis of the observed shifts of ν(NH2), ν(NH) and β(NH). It is seen from the cursory
inspection of the data in Table I that the intermolecular hydrogen bonding is somewhat
stronger in the mono(pyrimidone) 8 than in both the tris(pyrimidones) 1a,b, in a quali-
tative accord with the non-ideal arrangement of hydrogen bonds anticipated in the
spherical dimers of 1a,b.
TABLE I
Infrared spectra of diaminopyrimidone derivatives 1a,b and 8 in the region 3 500–950 cm–1 (KBr
pellets)a
Assignmentb
1a
1b
8
νas(NH2) free
3 462 m
3 439 mc
3 329 mc
3 124 m
3 483 s; 3 469 s
3 321 s
νas(NH2) bonded 3 350 m
ν(NH)
3 198 m
3 129 vs; 3 026 vs
1 680 s, sh
βs(NH2)
1 678 s, sh
1 627 vs
1 678 s, sh
1 621 vs
ν(C=O), ν(ring)
1 638 vs
β(NH) + ν(ring) 1 559 s
ν(ring) + β(NH) 1 506 s
1 552 s
1 569 s
1 498 s
1 511 vs
νas, νa(C–N)
ν(ring)
1 420 s; 1 374 m
1 234 w; 1 141 m
1 412 s; 1 376 md
1 424 vs; 1 396 vs
1 242 w, sh; 1 1140 w 1 237 m; 1 156 m; 1 141 m
ν(ring) + βas(NH2) 1 070 w, sh
ν, β(ring) 997 w
1 070 w, sh
995 w
1 076 me
1 001 m
a
Benzene, CH2 and CH3 vibrations and many shoulders on broad, complex bands are omitted; vs
b
very strong, s strong, m medium, w weak, sh shoulder. νas, νs antisymmetric and symmmetric
stretching, β in plane bending, βs scissoring, βas rocking, δ bending. Value from fluorolube mull
spectrum. Overlapping with δs(CH3). Overlapping with benzene vibration.
c
d
e
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

1472
Alexander, Holy, Fiedler, Havlas, Zavada:
There is some indication that the self-assembled dimers 1a.1a and 1b.1b may occur
also in the gas phase. The FAB spectra of both tris(pyrimidones) 1a and 1b showed the
presence of small but significant peaks [2 M + H]+ and [2 M + Na]+.
Summing up, several pieces of circumstantial but not yet conclusive evidence have
been provided supporting feasibility of the projected spherical dimers. Studies aiming
at preparation of more soluble (lipophilic) analogues are in progress.
We are grateful to the Grant Agency of the Czech Republic for the financial support (grant No.
203/93/0059). We are indebted to Dr R. Rericha for discussion of the IR spectra.
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