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PAPER

263

Novel and Efficient Synthesis of 2-Aminooxazoles from Pyrimidin-2(1H)-one
Vadim L. Alifanov, Eugene V. Babaev*
Chemistry Department, Moscow State University, Moscow 119992, Russian Federation Fax +7(495)9393020; E-mail: babaev@org.chem.msu.su Received 21 August 2006; revised 18 October 2006

Abstract: Stepwise conversion of pyrimidin-2(1H)-one to 2-amino-5-aryloxazoles via oxazolo[3,2-a]pyrimidinium salts is reported. The sequence involves, (i) regioselective N-alkylation of pyrimidone by phenacyl bromides, (ii) cyclization of obtained 1-(2aryl-2-oxoethyl)pyrimidin-2(1H)-ones into oxazolo[3,2-a]pyrimidinium salts under the action of fuming sulfuric (or triflic) acid, and (iii) reaction of the obtained salts with hydrazine leading to 2-amino-5-aryloxazoles. Key words: fused ring systems, ring closure, ring opening, oxazoles, protecting groups

done 1. The overall sequence shown in Scheme 2 has never been realized, although some related reactions have been briefly discussed in the literature.

N-Phenacylation of Pyrimidin-2(1H)-one (1)
The reaction of 2-pyrimidone 1 and its derivatives with ahalogenocarbonyl compounds is poorly investigated in the literature. There are only two examples of phenacylation in the 2-pyrimidone series, namely for 5-chloro-4phenyl-2-pyrimidone2 and the sterically hindered 4,6dimethyl-2-pyrimidone.3 Regarding the parent pyrimidone 1 only reactions with diethylacetal of bromoacetaldehyde4 and chloroacetic acid derivatives5 have been studied. In all these cases exclusive formation of the N-alkyl isomer was observed. Several 4-aryl-N-phenacyl2-pyrimidones were prepared from a-aminoketones by an alternative strategy, not involving the alkylation step.6 Earlier examined alkylation reactions were carried out in bipolar aprotic solvents with pyrimidone alkali salts. We studied phenacylation of 1 in two different ways: using K2CO3 as the base (Method A) and starting from the initially prepared sodium salt of 1 (Method B). Various phenacyl bromides have been used (Scheme 3, Table 1), and in all cases the N-phenacyl derivatives 2a­h were the only products obtained in high yields. In the IR spectra of 2a­h two types of bands for CO group were observed, one for N­C=O fragment of pyrimidone (~1660 cm­1) and another one for carbonyl group (~1700 cm­1), thus clearly confirming selective N-alkylation. (Evidently, in the case of O-phenacylation one would exO H2RN O

One would expect that similar transformations starting from pyrimidin-2(1H)-one (1, 2-pyrimidone) and involving N-phenacyl derivatives IIb and oxazolo[3,2-a]pyrimidinium salts IIIb may lead to unstable oxazolylsubstituted aza-dienes IVb, which, therefore, could be precursors of 2-aminooxazoles V. In this communication we confirm this idea and report our first successful preparation of 5-aryl-2-aminoxazoles V starting from pyrimiO

Scheme 1
O N

Scheme 2

SYNTHESIS 2007, No. 2, pp 0263­0270 Advanced online publication: 14.12.2006 DOI: 10.1055/s-2006-958941; Art ID: Z16706SS © Georg Thieme Verlag Stuttgart · New York

l This is a copy of the author's personal reprint l

l This is a copy of the author's personal reprint l

In our previous investigation1 we described a simple route to 5-aryloxazoles IVa bearing a w-aminodienyl residue by ring opening of bicyclic oxazolo[3,2-a]pyridinium salts IIIa (Scheme 1), which in turn can easily be obtained from 2-pyridone Ia via N-phenacyl-2-pyridones IIa. The overall simplicity of this methodology to obtain a substituted five-membered azole (like IVa) from a six-membered azine Ia via bridgehead azolo-azine IIIa (rarely used in heterocyclic synthesis) stimulated our interest to expand this strategy to the related family of pyrimidine derivatives. The retrosynthetic sequence of the suggested conversions is shown on Scheme 2.

O

N

1

H N

rA aVI

N
2

rA

RN

O

bII

N

rA

aIII

+N

rA

O

bIII

+N

4OS2H

N

rA

O

aII

rA

N bVI O N
2R

rB2HCOCrA N N O rA aI O V N N2H H N


264

V. L. Alifanov, E. V. Babaev
lC

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rA

Compound 2a 2b 2c 2d 2e 2f 2g 2h

Ar 4-ClC6H
4 4 3

Method A A B B B A
4 4

Yield (%) 90 95 60 65 63 79 81 70

Scheme 4

4-BrC6H

2,4-Cl2C6H 4-NO2C6H 3-NO2C6H Ph 4-MeC6H

4 4

A A

4-MeOC6H

however, were not the desired bicyclic salts. In the 1H NMR spectra of the products the singlet of NCH2-group remained unchanged, and the general downfield shift of all peaks confirmed that the products were protonated starting materials 2 (Scheme 5). Bradsher had reported9 that sulfuric acid could be a suitable dehydration agent for analogous cyclization in the pyridone series (IIa to IIIa). However, our attempts to use H2SO4 or PPA for cyclization of pyrimidones 2 also led to the salts of starting compounds.
rA O
+H

Figure 1

X-ray crystal structure of the compound 2a

Scheme 5

Cyclization of N-Phenacyl-2-pyrimidones 2 to Oxazolo[3,2-a]pyrimidinium Salts 3
Aromatic oxazolo[3,2-a]pyrimidinium cation with bridgehead nitrogen atom could be prepared by two different ways, either from oxazole or from pyrimidine, and both strategies have been realized. The first approach is illustrated by condensation of 4,5-disubstituted 2-aminooxazoles with acetylacetone7 leading to 5,7-dimethyloxazolo[3,2-a]pyrimidinium salts. The alternative way (Scheme 4) involved cyclization of hardly available Nphenacyl-2-pyrimidones VI (by using somewhat unsafe combination of Ac2O and HClO4) or their thione precursors VII, leading to 2,7-diaryl-substututed salts VIII.6,8 We have tried to perform the cyclization of N-phenacyl-2pyrimidones 2 using the above protocol6 in a mixture of Ac2O and HClO4. The isolated crystalline substances,

Nevertheless, fuming sulfuric acid (20­30 mass% of SO3) turned out to be the reagent of choice, and its use resulted in the formation of the desired salts 3a­e (Scheme 5) in excellent yields (Table 2, Method A).
Table 2 2-Aryloxazolo[3,2-a]pyrimidinium Perchlorates 3a­g Prepared Compound 3a 3b 3c 3d 3e 3f 3f 3g Ar 4-ClC6H
4 4 3

4-BrC6H

2,4-Cl2C6H 4-NO2C6H 3-NO2C6H Ph Ph 4-MeC6H
4

4 4

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© Thieme Stuttgart · New York

5O2P/H3OS3FC )iv( ;3OS*4OS2H )v( ;APP )vi( ;4OlCH/O2H )iii( ;4OS2H )ii( ;O2cA/4OlCH )i(

Method A A A A A A B B

rA

O

)iv( rof g,f3 )v( rof f­a3

O

+N

N

N

N

)iii(,)vi( ro )iii(,)ii( ro )i(

h­g3 rof )iii( ,)iv( h­a2 rof )iii(,)v(

O

rA

O

N

h­a2

N

pect no amide frequencies.) The structure of compound 2a (and the regioselectivity of alkylation) was confirmed by single crystal X-ray analysis (Figure 1).

IIIV

N

Table 1

N-Phenacyl-2-pyrimidones 2a­h Prepared

hP

O

+

N

rA

yp ,IeM

hPOC2HCN

S

IIV

N

rA

Scheme 3

4O

hPOC2HCN lCH/O2cA IV

O

N

rA HN R OR

hPOC2hC2HN ,NCSK

2 +N

R h­a2

O

O rA

N N

h 2 ,xulfer ,OC2eM ,rB2HCOCrA neht ;HOeM/aNOeM :B dohteM d 2 ,.t.r ,OC2eM/3OC2K :A dohteM

rB2HCOCrA O

N H N 1

Yield (%) 90 85 83 80 87 55 79 75


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Synthesis of 2-Aminooxazoles

265

Scheme 6

As an indirect proof of this hypothesis, one could consider the changes in the color of reaction mixtures: usual acids caused no changes, whereas super acids brought to solutions bright (yellow to red) colors. The same colors appeared when the final solid perchlorates 3 were dissolved in triflic acid.

In order to avoid the sulfonation problem, we have tried to perform cyclization of SO3-sensitive compounds 2f­h in a mixture of triflic acid and P2O5 (Scheme 5). In this case the desired salts 3f,g were obtained in good yields (Table 2, Method B), whereas the compound 2h decomposed completely.

Conversion of Oxazolo[3,2-a]pyrimidinium Salts 3 to 2-Amino-5-aryloxazoles 4
Although the structures of the type 3 are known for decades, there are no examples of their reactions with nucleophiles. We found that in reactions with hydrazine bicyclic salts 3a­g underwent selective cleavage of the pyrimidine fragment leading with excellent yields to 2amino-5-aryloxazoles 4a­g (Scheme 7, Table 3). Although hydrazinolysis of structurally related oxazolopyridinium salt IIIa led to the cleavage and transformation of oxazole part,11 in the case of the salts 3 there is no evidence for ambident properties of the bicycle. Melting points and 1H NMR spectral data of the aminooxazoles 4a,b,d,f,g correspond to literature data, and for previously unknown compound 4c,e we obtained satisfactory data of elementary analysis. Structure of most oxazoles 4 was confirmed by 13C NMR spectra and mass spectra (which are absent in the literature), and the structure 4b was proved by X-ray analysis (Figure 3). Derivatives of 2-aminoxazole possess various types of biological activities; the parent scaffold is present in some

Cyclization Mechanism
The formation of fused oxazoles 3 from 2 resembles closure of N-acyl-a-aminocarbonyl compounds to oxazoles (known as Robinson­Gabriel cyclization in the case of monocycles). The influence of the nature of acid on the results of the cyclization of 2 to 3 may bring some light to the mechanism of this conversion. Clearly, the oxygen in the bicycles 3 originates from the amide group in pyrimidones 2; this mechanism was proved by isotope label for monocyclic case,10 and there is no evidence for its change in the case of bicyclic analogs. Therefore, protonation of the ketone oxygen in phenacyl derivatives 2 is required. However, 2-pyrimidones 2 have the concurrent basic cen-

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© Thieme Stuttgart · New York

HH

bXI

aXI

In the case of N-phenacyl-2-pyrimidones 2g,h with donor groups (4-Me or 4-MeO) in the benzene ring, the obtained products were insoluble in most solvents and had high melting points, so it was difficult to prove their structures. We suppose that in these cases SO3 caused sulfonation of the benzene ring. (Analogous sulfonation was observed during the attempts to prepare oxazolopyridinium salts from electron rich phenacyl pyridones in sulfuric acid,9 that is even a weaker sulfonation media than oleum.) In the case of N-phenacyl derivative 2f (unsubstituted at benzene ring) partial sulfonation was also observed, however, decrease of temperature and using DMSO for purification has allowed us to obtain 2-phenyl derivative 3f pure.

rA

+N

noitazilcyc

HO

+ rA

Figure 2

X-ray crystal structure of the compound 3b

noitatardyhed

H3OS3FC

rA

HO O

cXI

O

+

3

N

O2H

+

+N

N H

H

O

dica -repus

rA

O

O

N

2

noitanotorp elbuod

N

+N

dica "lausu"

The salts 3 could be easily isolated as perchlorates. The singlet of CH2 group at 5.4 ppm (initially observed in parent N-phenacyl-2-pyrimidones) disappeared in the 1H NMR spectra of the salts 3, and new downfield singlet of formed oxazole ring appeared at 9.24­9.59 ppm. Respectively, in the 13C spectra the signal for the methylene group observed for phenacyl derivatives 2 (at ~55 ppm) is changed to aromatic oxazole signal (at 110­115 ppm) for 3. Final confirmation of the structural change followed from X-ray structure analysis of the crystal of compound 3b (Figure 2).

ter ­ the second nitrogen atom in the ring. As we have seen, use of H2SO4, PPA or HClO4­Ac2O systems led only to the salts of 2, whereas the use of stronger acids (super acids) like H2SO4­SO3 or CF3SO3H resulted in successful cyclization. This may indicate that the real cyclization mechanism requires somewhat unusual assumption on the formation of dications IX as the reaction intermediates (Scheme 6).

H

+

N H


266
N

V. L. Alifanov, E. V. Babaev

PAPER

compounds 4 it would be necessary to start from hardly available 2-halogen derivatives of arylacetic aldehydes. The are only few alternative methodologies to prepare 2amino-5-aryloxazoles: Curtius rearrangement of oxazolyl-2-carboxylic acids hydrazides,17 multistep synthesis starting from N-(tosylmethyl)-N¢-tritylcarbodiimide,18 and condensation of a-bromoketones with N-cyanourea.19 Hence the strategy proposed in this communication, involving simple sequence with high yields and requiring cheap materials, may serve as a competitive complementary route to the class of compounds 4. Closer inspection of the suggested strategy may clarify, that actual use of 2-pyrimidone as the source of NCO fragment of 2-aminoxazole is nothing else but applying the idea of protecting group. Indeed, reaction of urea with abromoketones proceeds in a complex manner without clear regioselectivity (in contrast to thiourea). Pyrimidone may be considered as a sort of `protected' urea that may be selectively N-phenacylated. Here the protecting group is the fragment of malonaldehyde, which is safely removed by hydrazinolysis at the final step. The role of malonaldehyde as protecting group is not only envisaged: in fact, most common way to obtain 2-pyrimidone is the condensation of urea with malonaldehyde derivatives. Hence, the overall sequence of `protection and deprotection' of urea in the synthesis of 2-aminoxazoles can be described as shown in Scheme 8.
RO RO

Scheme 7 Table 3 2-Amino-5-aryloxazoles Prepared Ar 4-ClC6H
4 4 3

Compound 4a 4b 4c 4d 4e 4f 4g

4-BrC6H

2,4-Cl2C6H 4-NO2C6H 3-NO2C6H Ph 4-MeC6H
4

4 4

Figure 3

X-ray crystal structure of the compound 4b

Scheme 8

common drugs: antiseptic sulfamoxole and sulfaguanole, anti-asthmatic isamoxole, hypotensive azepexole, and anti-inflammatory ditazole. Many simple N-acyl- and Nalkyl derivatives of this family display antiinflammatory12 and antiviral13 activities, and very recently N-substituted 2-aminoxazoles have been found as a novel class of VEGFR2 kinase inhibitors.14 Therefore, development of flexible routes to this family remains an actual task. It should be mentioned that common methods leading to 2-aminooxazoles are frequently multistep reactions that require complicated reagents or the processes are accompanied by side reactions. The `handbook' strategy to 2-aminooxazoles ­ reaction of cyanamide with a-hydroxycarbonyl compounds15 ­ is difficult to apply for the synthesis of 5-aryl substituted 2-aminooxazoles 4, since the starting materials would be poorly available a-hydroxy derivatives of phenylacetic aldehyde. Another old strategy is cyclocondensation of urea with a-halogencarbonyl compounds;16 this process is complicated by concurrent formation of imidazolones.15b Again, for the synthesis of
Synthesis 2007, No. 2, 263 ­ 270 © Thieme Stuttgart · New York

Melting points are uncorrected. 1H and 13C NMR spectra were recorded on AM 400 Bruker spectrometer for 1H at 360 MHz and for 13 C at 90 or 100 MHz. Chemical shifts are reported in ppm referenced to residual protons in the deuterated solvent. IR spectra were obtained on UR-20 spectrometer in Nujol. Mass spectra were determined on Kratos MS-30 mass spectrometer (EI 70 eV). TLC was performed with Silufol UV-254 (Merck). The experimental intensities of diffraction reflections for single crystals of compounds 2a,20 3b,21 and 4b22 were measured on a CAD4 automated diffractometer (MoKa radiation, graphite monochromator, w scan mode) at r.t. All subsequent calculations were carried out with the SHELX97 program package. 2-Pyrimidone hydrochloride was prepared by reaction of urea with malonaldehyde bis(dimethyl acetal) (1,1,3,3-tetramethoxypropane) by a method described in the literature20 [yield: 71%; yellow solid; mp 203­207 °C; (Lit.23 mp 210 °C)]. 1-(2-Aryl-2-oxoethyl)pyrimidin-2(1H)-ones 2a,b,f­h; 1-[(2-(4Chlorophenyl)-2-oxoethyl]pyrimidin-2(1H)-one (2a); Typical Procedure; Method A K2CO3 (55.2 g, 0.4 mol) was added under stirring to a suspension of 2-pyrimidone hydrochloride (26.5 g, 0.2 mol) in anhyd acetone

R

noitcetorped

O

N2H

N

4H2N

R

O

N

+

N

elozaxoonima-2 fo noitamrof evitcelesoiger

O

HN

N

O

aeru fo noitcetorp

2HN

N2H

+

OR RO

rA

g­a4

rA O

O
2H

N

N NN N H rA g­a3 O

N2H

rA

O
2H +

N N NHN O2H*4H2N N

Yield (%) 95 96 88 93 88 90 82


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(500 mL). A solution of 4-chlorophenacyl bromide (23.3 g, 0.1 mol) in acetone (100 mL) was added, and the mixture was stirred for 2 days at r.t. Acetone was evaporated in vacuum, and the residue was washed with H2O, then with EtOAc. The product was isolated by suction and recrystallized from MeCN to afford 2a; yield: 22.3 g (90%). 1-(2-Aryl-2-oxoethyl)pyrimidin-2(1H)-ones 2c­e; 1-[(2-(2,4Dichlorophenyl)-2-oxoethyl]pyrimidin-2(1H)-one (2c); Typical Procedure; Method B 2-Pyrimidone hydrochloride (26.5 g, 0.2 mol) was added to a freshly prepared solution of NaOMe obtained by reaction of Na (9.2 g, 0.4 mol) with absolute MeOH (400 mL). The mixture was stirred for 30 min, and MeOH was evaporated under reduced pressure. The residue was suspended in anhyd acetone (400 mL), and a solution of 2,4-dichlorophenacyl bromide (26.8 g, 0.1 mol) in acetone (200 mL) was added with stirring. The mixture was refluxed for 2 h and treated as described in Method A to give 2c; yield: 17.0 g (60%). 1-[(2-(4-Chlorophenyl)-2-oxoethyl]pyrimidin-2(1H)-one (2a) Yield: 90%; white solid; mp 230­231 °C; Rf =0.15 (CHCl3­ MeOH, 10:1). IR (Nujol): 1695, 1615 cm­1. H NMR (360 MHz, DMSO-d6): d = 5.48 (s, 2 H, CH2), 6.51 (m, 1 H, H-5), 7.68 (m, 2 H, Ar-H, BB¢), 8.07 (m, 2 H, Ar-H, AA¢), 8.13 (m, 1 H, H-4), 8.62 (m, 1 H, H-6). C NMR (90 MHz, DMSO-d6): d =56.1 (CH2), 103.8 (C-5), 129.1 (C-3¢, Ar), 129.8 (C-2¢, Ar), 133.0 (C-1¢, Ar), 139.0 (C-4¢, Ar), 150.5 [C-4 (6)], 155.5 (C-2), 166.7 [C-6 (4)], 191.6 (C=O). Anal. Calcd for C12H9ClN2O2: C, 57.96; H, 3.65; N, 11.27. Found: C, 57.67; H, 3.41; N, 11.29. 1-[(2-(4-Bromophenyl)-2-oxoethyl]pyrimidin-2(1H)-one (2b) Yield: 95%; white solid; mp 237­239 °C; Rf =0.15 (CHCl3­ MeOH, 10:1). IR (Nujol): 1700, 1660 cm­1.
1 H NMR (360 MHz, DMSO-d6): d = 5.43 (s, 2 H, CH2), 6.41 (m, 1 H, H-5), 7.72 (m, 2 H, Ar-H, BB¢), 7.99 (m, 2 H, Ar-H, AA¢), 8.08 (m, 1 H, H-4), 8.57 (m, 1 H, H-6). 13 1 13

Synthesis of 2-Aminooxazoles

267

C NMR (90 MHz, DMSO-d6): d =56.6 (CH2), 103.9 (C-5), 124.1 (C-3¢, Ar), 129.5 (C-2¢, Ar), 138.9 (C-1¢, Ar), 150.4 (C-4¢, Ar), 150.5 [C-4 (6)], 155.5 (C-2), 166.9 [C-6 (4)], 192.0 (C=O). Anal. Calcd for C12H9N3O4: C, 55.60; H, 3.50; N, 16.21. Found: C, 55.69; H, 3.39; N, 16.49. 1-[(2-(3-Nitrophenyl)-2-oxoethyl]pyrimidin-2(1H)-one (2e) Yield: 63%; white solid; mp 162­164 °C; Rf =0.15 (CHCl3­ MeOH, 10:1). IR (Nujol): 1710, 1670 cm­1.
1 H NMR (360 MHz, DMSO-d6): d = 5.55 (s, 2 H, CH2), 6.45 (m, 1 H, H-5), 7.86 (m, 1 H, H-5¢-Ar), 8.12 (m, 1 H, H-4), 8.48 (m, 2 H, H-4¢-Ar, H-6¢-Ar), 8.59 (m, 1 H, H-6), 8.76 (m, 1 H, H-2¢-Ar). 13 C NMR (90 MHz, DMSO-d6): d =56.5 (CH2), 103.9 (C-5), 122.3 (C-2¢, Ar), 128.2 (C-4¢, Ar), 130.8 (C-5¢, Ar), 134.1 (C-6¢, Ar), 135.6 (C-1¢, Ar), 148.1 (C-3¢, Ar), 150.4 [C-4 (6)], 155.5 (C-2), 166.9 [C-6 (4)], 191.4 (C=O).

Anal. Calcd for C12H9N3O4: C, 55.60; H, 3.50; N, 16.21. Found: C, 55.58; H, 3.42; N, 16.19. 1-(2-Oxo-2-phenylethyl)pyrimidin-2(1H)-one (2f) Yield: 79%; white solid; mp 184­186 °C; Rf =0.15 (CHCl3­ MeOH, 10:1). IR (Nujol): 1710, 1650 cm­1.
1 H NMR (360 MHz, DMSO-d6): d = 5.50 (s, 2 H, CH2), 6.51 (m, 1 H, H-5), 7.60 (m, 2 H, H-3¢-Ar), 7.73 (m, 1 H, H-4¢-Ar), 8.06 (m, 2 H, H-2¢-Ar), 8.15 (m, 1 H, H-4), 8.63 (m, 1 H, H-6). 13 C NMR (90 MHz, DMSO-d6): d =56.2 (CH2), 103.7 (C-5), 127.9 (C-2¢, Ar), 128.9 (C-3¢, Ar), 134.1 (C-4¢, Ar), 134.3 (C-1¢, Ar), 150.6 [C-4 (6)], 155.5 (C-2), 166.7 [C-6 (4)], 192.4 (C=O).

Anal. Calcd for C12H10N2O2: C, 67.28; H, 4.71; N, 13.08. Found: C, 66.99; H, 4.86; N, 13.24. 1-[(2-(4-Methylphenyl)-2-oxoethyl]pyrimidin-2(1H)-one (2g) Yield: 81%; white solid; mp 197­199 °C; Rf =0.15 (CHCl3­ MeOH, 10:1). IR (Nujol): 1690, 1660 cm­1.
1

Anal. Calcd for C12H9BrN2O2: C, 49.17; H, 3.09; N, 9.56. Found: C, 48.75; H, 2.89; N, 9.36. 1-[(2-(2,4-Dichlorophenyl)-2-oxoethyl]pyrimidin-2(1H)-one (2c) Yield: 60%; white solid; mp 205­207 °C; Rf =0.15 (CHCl3­ MeOH, 10:1). IR (Nujol): 1710, 1680 cm­1.
1 H NMR (360 MHz, DMSO-d6): d = 5.27 (s, 2 H, CH2), 6.42 (m, 1 H, H-5), 7.51 (d, J5¢,6¢ = 8.2 Hz, 1 H, H-5¢-Ar), 7.57 (s, 1 H, H-3¢Ar), 7.91 (d, J6¢,5¢ = 8.2 Hz, 1 H, H-6¢-Ar), 8.19 (m, 1 H, H-4), 8.57 (m, 1 H, H-6).

H NMR (360 MHz, DMSO-d6): d = 3.29 (s, 3 H, CH3), 5.45 (s, 2 H, CH2), 6.50 (m, 1 H, H-5), 7.40 (m, 2 H, Ar-H, BB¢), 7.95 (m, 2 H, Ar-H, AA¢), 8.14 (m, 1 H, H-4), 8.62 (m, 1 H, H-6). C NMR (90 MHz, DMSO-d6): d =21.2 (CH3), 56.0 (CH2), 103.6 (C-5), 128.0 (C-2¢, Ar), 129.4 (C-3¢, Ar), 131.9 (C-1¢, Ar), 144.6 (C4¢, Ar), 150.6 [C-4 (6)], 155.5 (C-2), 166.6 [C-6 (4)], 191.9 (C=O).

13

Anal. Calcd for C13H12N2O2: C, 68.41; H, 5.30; N, 12.27. Found: C, 68.65; H, 5.49; N, 12.38. 1-[(2-(4-Methoxyphenyl)-2-oxoethyl]pyrimidin-2(1H)-one (2h) Yield: 70%; white solid; mp 173­175 °C; Rf =0.15 (CHCl3­ MeOH, 10:1). IR (Nujol): 1700, 1640 cm­1.
1

Anal. Calcd for C12H8Cl2N2O2: C, 50.91; H, 2.85; N, 9.89. Found: C, 50.74; H, 2.83; N, 9.75. 1-[(2-(4-Nitrophenyl)-2-oxoethyl]pyrimidin-2(1H)-one (2d) Yield: 65%; pale yellow solid; mp 215­217 °C; Rf =0.15 (CHCl3­ MeOH, 10:1). IR (Nujol): 1695, 1655 cm­1.
1 H NMR (360 MHz, DMSO-d6): d = 5.55 (s, 2 H, CH2), 6.54 (m, 1 H, H-5), 8.16 (m, 1 H, H-4), 8.28 (m, 2 H, Ar-H, BB¢), 8.41 (m, 2 H, Ar-H, AA¢), 8.65 (m, 1 H, H-6).

H NMR (360 MHz, DMSO-d6): d = 3.90 (s, 3 H, OCH3), 5.40 (s, 2 H, CH2), 6.41 (m, 1 H, H-5), 7.05 (m, 2 H, Ar-H, BB¢), 8.01 (m, 2 H, Ar-H, AA¢), 8.08 (m, 1 H, H-4), 8.56 (m, 1 H, H-6).

Anal. Calcd for C13H12N2O3: C, 63.93; H, 4.95; N, 11.47. Found: C, 63.88; H, 4.88; N, 11.56. Attempted Cyclization of 2a HClO4 (0.3 mL) was carefully added under good cooling to Ac2O (10 mL). Pyrimidone 2a (0.746 g, 0.003 mol) was added, the colorless mixture was stirred for 30 min, and then kept for 24 h at r.t. The obtained precipitate was filtered, dried and recrystallized from

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V. L. Alifanov, E. V. Babaev

PAPER
Anal. Calcd for C12H7Cl3N2O5: C, 39.43; H, 1.93; N, 7.66. Found: C, 39.14; H, 1.91; N, 7.75. 2-(4-Nitrophenyl)oxazolo[3,2-a]pyrimidinium Perchlorate (3d) Yield: 80%; pale yellow solid; mp 271­273 °C. H NMR (360 MHz, DMSO-d6): d = 8.17 (dd, J6,7 =4.5 Hz, J6,5 = 6.3 Hz, 1 H, H-6), 7.36 (m, 2 H, Ar-H, BB¢), 8.46 (m, 2 H, ArH, AA¢), 9.41 (dd, J7,6 =4.5 Hz, J7,5 = 1.8 Hz, 1 H, H-7), 9.50 (s, 1 H, H-3), 9.73 (dd, J5,6 = 6.3 Hz, J5,7 = 1.8 Hz, 1 H, H-5). Anal. Calcd for C12H8ClN3O7: C, 42.18; H, 2.36; N, 12.30. Found: C, 42.36; H, 2.48; N, 12.43. 2-(3-Nitrophenyl)oxazolo[3,2-a]pyrimidinium Perchlorate (3e) Yield: 87%; white solid; mp 248­250 °C. H NMR (360 MHz, CF3CO2D): d = 8.66 (m, 1 H, H-3¢-Ar), 8.83 (m, 1 H, H-6), 9.13 (m, 1 H, H-2¢-Ar), 9.32 (m, 1 H, H-4¢-Ar), 9.67 (m, 2 H, H-3-Ar + H-5¢-Ar), 10.13 (m, 1 H, H-7), 10.19 (m, 1 H, H5). C NMR (90 MHz, DMSO-d6): d = 113.3 (C-3), 118.6 (C-6), 120.7 (C-2¢, Ar), 125.2 (C-1¢, Ar), 126.5 (C-6¢, Ar), 131.6 (C-4¢, Ar), 131.8 (C-5¢, Ar), 142.7 [C-7 (5)], 148.6 (C-3¢, Ar), 149.6 (C-2), 153.3 (C-9), 163.3 [C-5 (7)]. Anal. Calcd for C12H8ClN3O7: C, 42.18; H, 2.36; N, 12.30. Found: C, 42.25; H, 2.22; N, 12.36. 2-Phenyloxazolo[3,2-a]pyrimidinium Perchlorate (3f) Method A: To the precipitate, obtained after cyclization and addition of H2O and HClO4, was added DMSO (50 mL), the mixture was thoroughly suspended, and the insoluble part was filtered off. The filtrate was partially evaporated in vacuum and diluted with Et2O (50 mL). The precipitate was filtered, washed with Et2O and EtOH, and dried in vacuum over P2O5 to afford the 3f; yield: 55%; white solid; mp 261­263 °C.
1 13 1 1

EtOH to afford the perchlorate of 2a; yield: 1.00 g (96%); white solid; mp 325­327 °C. H NMR (360 MHz, DMSO-d6): d = 5.73 (s, 2 H, CH2), 6.98 (m, 1 H, H-5), 7.59 (m, 2 H, Ar-H, BB¢), 8.09 (m, 2 H, Ar-H, AA¢), 8.86 (m, 1 H, H-4), 8.91 (m, 1 H, H-6). Anal. Calcd for C12H10Cl2N2O6: C, 45.80; H, 3.52; N, 8.90. Found: C, 45.69; H, 3.43; N, 8.80. Treatment of 2a with concd H2SO4 and PPA led to the same perchlorate. In the case of TiCl4 or POCl3 no identifiable products were isolated. 2-Aryloxazolo[3,2-a]pyrimidinium Perchlorates 3a­f; 2-(4Chlorophenyl)oxazolo[3,2-a]pyrimidinium Perchlorate (3a); Typical Procedure; Method A Dried 4-chlorohenacylpyrimidone (2a; 4.28 g, 0.02 mol) was carefully added to a stirred mixture of fuming H2SO4 [32 mL; prepared by mixing of H2SO4 (25 mL, d = 1.84) and commercial (60 mass% of SO3) fuming H2SO4 (17 mL, 34 g)]. [For 2c­e fuming sulfuric acid (40 mL) with 30 mass% of SO3 was used for mixing]. The temperature was kept in the range ­5 to 0 °C. The mixture (orange to red) was stirred below 0 °C until the compound 2a completely dissolved and then kept for 5 h under r.t. (For 2c the mixture was kept for 24 h, and for 2d,e, 72 h at r.t.). After that the mixture was carefully poured into crushed ice (500 g). Then HClO4 (10 mL, 65%) was added. The product was isolated by suction filtration, washed with H2O, then with EtOH and Et2O, and dried in vacuum over P2O5 to afford 3a; yield: 5.23 g (79%); white solid; mp 268­270 °C.
1 H NMR (360 MHz, DMSO-d6): d = 7.64 (m, (m, 2 H, Ar-H, AA¢), 8.14 (dd, J6,7 =4.5 Hz, 6), 9.31 (s, 1 H, H-3), 9.34 (dd, J7,6 = 4.5 Hz, 7), 9.66 (dd, J5,6 = 6.3 Hz, J5,7 = 1.7 Hz, 1 H, 1

2 H, Ar-H, BB¢), 8.06 J6,5 = 6.3 Hz, 1 H, HJ7,5 = 1.7 Hz, 1 H, HH-5).

13 C NMR (100 MHz, DMSO-d6): d = 111.8 (C-3), 118.6 (C-6), 122.6 (C-1¢, Ar), 127.7 (C-2¢, Ar), 130.1 (C-3¢, Ar), 137.1 (C-4¢, Ar), 142.4 [C-7 (5)], 150.8 (C-2), 153.2 (C-9), 162.6 [C-5 (7)].

Anal. Calcd for C12H8Cl2N2O5: C, 43.53; H, 2.44; N, 8.46. Found: C, 43.33; H, 2.40; N, 8.53. 2-(4-Bromophenyl)oxazolo[3,2-a]pyrimidinium Perchlorate (3b) Yield: 85%; white solid; mp 279­281 °C. H NMR (360 MHz, DMSO-d6): d = 7.81 (m, (m, 2 H, Ar-H, AA¢), 8.14 (dd, J6,7 =4.6 Hz, 6), 9.31 (s, 1 H, H-3), 9.36 (dd, J7,6 =4.6 Hz, 7), 9.68 (dd, J5,6 = 6.3 Hz, J5,7 = 1.8 Hz, 1 H,
1

H NMR (360 MHz, CF3CO2D): d = 8.84 (m, 3 H, H-6-Ar + H-3¢Ar), 9.19 (m, 3 H, H-2¢,4¢-Ar), 9.81 (s, 1 H, H-3), 10.49 (m, 1 H, H7), 10.56 (m, 1 H, H-5).

13 C NMR (90 MHz, DMSO-d6): d = 111.2 (C-3), 118.4 (C-6), 123.6 (C-1¢, Ar), 125.9 (C-2¢, Ar), 129.8 (C-3¢, Ar), 132.3 (C-4¢, Ar), 142.2 [C-7 (5)], 151.8 (C-2), 153.2 (C-9), 162.4 [C-5 (7)].

2 H, Ar-H, BB¢), 8.02 J6,5 = 6.3 Hz, 1 H, HJ7,5 = 1.8 Hz, 1 H, HH-5).

Anal. Calcd for C12H9ClN2O5: C, 48.58; H, 3.06; N, 9.44. Found: C, 48.65; H, 2.91; N, 9.38. Method B: N-Phenacyl-2-pyrimidone 2f (0.37 g, 0.0016 mol) was mixed with P2O5 (0.65 g, 0.0046 mol) and freshly distilled triflic acid (5 mL) was added to the mixture. The dark-red mixture obtained was refluxed for 3 h at 85­90 °C. The mixture was cooled and carefully poured into crushed ice (100 g). HClO4 (3 mL) was added, and the precipitate was filtered, washed with H2O, EtOH and Et2O, and dried in vacuum over P2O5 to afford the 3f; yield: 0.375 g (79%); white solid; mp 261­263 °C, identical to the sample obtained by Method A.
1

13 C NMR (90 MHz, DMSO-d6): d = 111.8 (C-3), 118.5 (C-6), 122.9 (C-1¢, Ar), 125.9 (C-4¢, Ar), 127.8 (C-2¢, Ar), 132.8 (C-3¢, Ar), 142.3 [C-7 (5)], 150.8 (C-2), 153.2 (C-9), 162.6 [C-5 (7)].

Anal. Calcd for C12H8Br2N2O5: C, 38.38; H, 2.15; N, 7.46. Found: C, 38.24; H, 1.99; N, 7.46. 2-(2,4-Dichlorophenyl)oxazolo[3,2-a]pyrimidinium Perchlorate (3c) Yield: 83%; white solid; mp 315­317 °C. H NMR (360 MHz, DMSO-d6): d =7.69 (dd, J5¢,6 =8.9 Hz, J5¢,3¢ = 1.8 Hz, 1 H, H-5¢-Ar), 7.82 (d, J3¢,5¢ = 1.8 Hz, 1 H, H-3¢-Ar), 8.16 (m, 2 H, H-5-Ar + H-6¢-Ar), 9.41 (dd, J7,6 =4.4 Hz, J7,5 =1.8 Hz, 1 H, H-7), 9.45 (s, 1 H, H-3), 9.61 (dd, J5,6 = 6.2 Hz, J5,7 =1.8 Hz, 1 H, H-5). C NMR (90 MHz, DMSO-d6): d = 115.2 (C-3), 118.8 (C-6), 121.4 (C-1¢, Ar), 128.9 (C-3¢, Ar), 130.7 (C-6¢, Ar), 131.0 (C-5¢, Ar), 132.2 (C-2¢, Ar), 137.4 (C-4¢, Ar), 142.4 [C-7 (5)], 147.2 (C-2), 152.8 (C-9), 163.6 [C-5 (7)].
13 1

H NMR (360 MHz, DMSO-d6): d = 7.69 (m, 3 H, H-6-Ar + H-3¢Ar), 8.11 (m, 3 H, H-2¢,4¢-Ar), 9.28 (s, 1 H, H-3), 9.40 (m, 1 H, H7), 9.67 (m, 1 H, H-5). 2-(4-Methylphenyl)oxazolo[3,2-a]pyrimidinium Perchlorate (3g) Method B; yield 75%; white solid; mp 239­241 °C.

H NMR (360 MHz, DMSO-d6): d = 7.50 (m, 2 H, Ar-H, BB¢), 7.98 (m, 2 H, Ar-H, AA¢), 8.13 (m, 1 H, H-6), 9.20 (s, 1 H, H-3), 9.37 (m, 1 H, H-7), 9.64 (m, 1 H, H-5).
13 C NMR (90 MHz, DMSO-d6): d =21.2 (CH3), 110.5 (C-3), 118.4 (C-6), 120.9 (C-1¢, Ar), 125.8 (C-2¢, Ar), 130.4 (C-3¢, Ar), 142.1 [C7 (5)], 142.8 (C-4¢, Ar), 152.0 (C-2), 153.1 (C-9), 162.0 [C-5 (7)].

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PAPER
Anal. Calcd for C13H11ClN2O5: C, 50.26; H, 3.57; N, 11.41. Found: C, 50.40; H, 3.41; N, 11.32. 2-Amino-5-aryloxazoles 4a­g; 5-(4-Chlorophenyl)-1,3-oxazol2-amine (4a); Typical Procedure Perchlorate 3a (3.3 g, 0.01 mol)) was suspended in anhyd MeCN (50 mL), and hydrazine hydrate (5 mL) was added with stirring. The mixture turned orange, and was refluxed for 20 min (until decolorization). The solution was cooled to r.t. and poured into cold H2O (100 mL). The product was isolated by suction filtration and recrystallized from EtOH to afford 4a; yield: 1.65 g (95%); white solid; mp 220­221 °C (Lit.18a mp 220­222 °C); Rf =0.4 (CHCl3­MeOH, 10:1).
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Synthesis of 2-Aminooxazoles

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Anal. Calcd for C9H7N3O3: C, 52.69; H, 3.44; N, 20.48. Found: C, 52.72; H, 3.32; N, 20.37. 5-Phenyl-1,3-oxazol-2-amine (4f) Yield: 90%; white solid; mp 215­216 °C (Lit.19b mp 215­216 °C); Rf = 0.4 (CHCl3­MeOH, 10:1).
1

H NMR (360 MHz, DMSO-d6): d = 6.48 (br s, 2 H, NH2), 6.96 (s, 1 H, H-4), 7.12 (m, 1 H, H-4¢-Ar), 7.30 (m, 2 H, H-3¢-Ar), 7.42 (m, 2 H, H-2¢-Ar). C NMR (100 MHz, DMSO-d6): d = 121.9 (C-2¢, Ar), 123.0 (C-4), 126.2 (C-1¢, Ar), 128.7 (C-4¢, Ar), 128.8 (C-3¢, Ar), 143.1 (C-5), 161.4 (C-2). MS (EI, 70 eV): m/z (%) = 105 (70), 160 (100).

13

H NMR (360 MHz, DMSO-d6): d = 6.57 (br s, 2 H, NH2), 7.01 (s, 1 H, H-4), 7.26 (m, 2 H, Ar-H, BB¢), 7.41 (m, 2 H, Ar-H, AA¢).

C NMR (90 MHz, DMSO-d6): d = 123.4 (C-2¢, Ar), 123.9 (C-4), 127.5 (C-1¢, Ar), 128.8 (C-3¢, Ar), 130.2 (C-4¢, Ar), 142.0 (C-5), 161,5 (C-2). MS (EI, 70 eV): m/z (%) = 138 (100), 194 (76). 5-(4-Bromophenyl)-1,3-oxazol-2-amine (4b) Yield: 96%; white solid; mp 220­221 °C (Lit Rf = 0.4 (CHCl3­MeOH, 10:1).
1 19b

13

5-(4-Methylphenyl)-1,3-oxazol-2-amine (4g) Yield: 82%; white solid; mp 218­220 (Lit.18a mp 218­220 °C); Rf =0.4 (CHCl3­MeOH, 10:1).
1

H NMR (360 MHz, DMSO-d6): d = 6.74 (br s, 2 H, NH2), 7.09 (s, 1 H, H-4), 7.17 (m, 2 H, Ar-H, BB¢), 7.32 (m, 2 H, Ar-H, AA¢). C NMR (90 MHz, DMSO-d6): d =21.1 (CH3), 121.9 (C-2¢, Ar), 122.0 (C-4), 126.0 (C-1¢, Ar), 129.3 (C-3¢, Ar), 135.5 (C-4¢, Ar), 143.2 (C-5), 161.0 (C-2).

13

mp 220­222 °C);

H NMR (360 MHz, DMSO-d6): d = 6.61 (br s, 2 H, NH2), 7.04 (s, 1 H, H-4), 7.35 (m, 2 H, Ar-H, BB¢), 7.44 (m, 2 H, Ar-H, AA¢). MS (EI, 70 eV): m/z (%) = 183 (100), 238 (100). 5-(2,4-Dichlorophenyl)-1,3-oxazol-2-amine (4c) Yield: 88%; white solid; mp 202­204 °C; Rf = 0.4 (CHCl3­MeOH, 10:1).

Acknowledgment
This work was supported by the Russian Foundation of Basic Research (RFBR Grant no. 05-03-39022-GFEN_a).

References
(1) (a) Review: Babaev, E. V. J. Heterocycl. Chem. (Lectures in Heterocyclic Chemistry) 2000, 37, 519. (b) Babaev, E. V.; Efimov, A. V.; Maiboroda, D. A.; Jug, K. Eur. J. Org. Chem. 1998, 193. (c) Babaev, E. V.; Tsisevich, A. A. J. Chem. Soc., Perkin Trans. 1 1999, 399. (d) Maiboroda, D. A.; Babaev, E. V.; Goncharenko, L. V. Khim.-Pharm. Zhurn. 1998, 32 (6), 24; Pharm. Chem. J. (Engl. Transl.) 1998, 32, 310. Chem. Abstr. 1998, 129, 275864. (e) Rybakov, V. B.; Babaev, E. V.; Tsisevich, A. A.; Arakcheeva, A. V.; Schoenleber, A. Crystallogr. Rep. (Engl. Transl.) 2002, 47, 973. (2) Benneche, T.; Gandersen, L.-L.; Undheim, K. Acta Chem. Scand. 1988, B42, 384. (3) Buchan, R.; Frazer, M.; Shand, C. J. Org. Chem. 1978, 43, 3544. (4) Holy, A.; Ludzisa, A.; Votruba, I.; Sediva, K.; Pischel, H. Collect. Czech. Chem. Comm. 1984, 50, 393. (5) Gefenas, V. I.; Vainilavichus, V. I. Chem. Heterocycl. Comp. (Engl. Transl.) 1984, 20, 1185. (6) Reimer, B.; Patzel, M.; Hassoun, A.; Liebscher, J.; Friedrichsen, W.; Jones, P. G. Tetrahedron 1993, 49, 3767. (7) Chuiguk, V. A.; Leshenko, E. A. Ukr. Chim. Zh. 1974, 633; Chem. Abstr. 1974, 81, 105438. (8) Liebscher, J.; Hassoun, A. Synthesis 1988, 816. (9) Bradsher, C. K.; Zinn, M. F. J. Heterocycl. Chem. 1967, 4, 66. (10) Wasserman, H. H.; Vinick, F. G. J. Org. Chem. 1973, 38, 2407. (11) Babaev, E. V.; Efimov, A. V.; Rybakov, V. B.; Zhukov, S. G. Chem. Heterocycl. Comp. (Engl. Transl.) 1999, 35, 486. (12) Crank, G.; Foulis, M. J. J. Med. Chem. 1971, 14, 1075. (13) Ulbricht, H. Pharmazie 1987, 42, 598. (14) Harris, P. A.; Cheung, M.; Hunter, R. N. III.; Brown, M. L.; Veal, J. M.; Nolte, R. T.; Wang, L.; Liu, W.; Crosby, R. M.;

H NMR (360 MHz, DMSO-d6): d = 6.24 (br s, 2 H, NH2), 7.23 (dd, J5¢,6¢ =8.6 Hz, J5¢,3¢ = 2 Hz, 1 H, H-5¢-Ar), 7.35 (s, 1 H, H-4), 7.38 (d, J3¢,5¢ = 2 Hz, 1 H, H-3¢-Ar), 7.58 (d, J6¢,5¢ = 8.6 Hz, 1 H, H-6¢-Ar).
13 C NMR (100 MHz, DMSO-d6): d = 125.9 (C-1¢, Ar), 126.3 (C-6¢, Ar), 127.7 (C-4), 128.2 (C-2¢, Ar), 128.7 (C-5¢, Ar), 129.8 (C-3¢, Ar), 130.4 (C-4¢, Ar), 138.8 (C-5), 161.5 (C-2).

1

MS (EI, 70 eV): m/z (%) = 173 (64), 228 (100). Anal. Calcd for C9H6Cl2N2O: C, 47.19; H, 2.64; N, 12.23. Found: C, 47.22; H, 2.68; N, 12.37. 5-(4-Nitrophenyl)-1,3-oxazol-2-amine (4d) Yield: 93%; orange solid; mp 235­237 (Lit.18a 235­237 °C); Rf = 0.4 (CHCl3­MeOH, 10:1).
1

H NMR (360 MHz, DMSO-d6): d = 6.99 (br s, 2 H, NH2), 7.39 (s, 1 H, H-4), 7.60 (m, 2 H, Ar-H, BB¢), 8.16 (m, 2 H, Ar-H, AA¢).

13 C NMR (100 MHz, DMSO-d6): d = 121.9 (C-2¢, Ar), 124.6 (C-3¢, Ar), 128.9 (C-4), 134.6 (C-1¢, Ar), 141.5 (C-5), 144.5 (C-4¢, Ar), 162.9 (C-2).

MS (EI, 70 eV): m/z (%) = 175 (47), 205 (100). 5-(3-Nitrophenyl)-1,3-oxazol-2-amine (4e) Yield: 88%; orange solid; mp 201­203 °C; Rf = 0.4 (CHCl3­ MeOH, 10:1).
1

H NMR (360 MHz, DMSO-d6): d = 6.81 (br s, 2 H, NH2), 7.27 (s, 1 H, H-4), 7.53 (m, 1 H, H-5¢-Ar), 7.83 (m, 1 H, H-6¢-Ar), 7.95 (m, 1 H, H-4¢-Ar), 8.21 (m, 1 H, H-2¢-Ar).

13 C NMR (100 MHz, DMSO-d6): d = 115.7 (C-2¢, Ar), 120.3 (C-4¢, Ar), 126.1 (C-4), 127.8 (C-6¢, Ar), 130.2 (C-1¢, Ar), 130.5 (C-5¢, Ar), 141.0 (C-5), 148.4 (C-3¢, Ar), 162.1 (C-2).

MS (EI, 70 eV): m/z (%) = 175 (17), 205 (100).

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V. L. Alifanov, E. V. Babaev
Johnson, J. H.; Epperley, A. H.; Kumar, R.; Luttrell, D. K.; Stafford, J. A. J. Med. Chem. 2005, 48, 1610. (a) Cockerill, A.; Deacon, A.; Harrison, R.; Osbornne, D.; Prime, D. M.; Ross, W. J.; Todd, A.; Verge, J. P. Synthesis 1976, 591. (b) Crank, G.; Khan, H. Aust. J. Chem. 1985, 38, 447. Gompper, R.; Christmann, O. Chem. Ber. 1959, 92, 1945. Tanaka, C. Yakugaku Zasshi 1967, 87, 10; Chem. Abstr. 1967, 66, 94930. (a) van Leusen, A. M.; Jeuring, H. J.; Widelmann, J.; van Nispen, S. P. J. M. J. Org. Chem. 1981, 46, 2069. (b) Tandon, V. K.; Singh, K. A.; Rai, S.; van Leusen, A. M. Heterocycles 2004, 84, 247.

PAPER
(19) (a) Beiling, H.; Barth, P.; Beyer, H. Z. Chem. 1965, 5, 182. (b) Beyer, H.; Schilling, H. Chem. Ber. 1966, 99, 2110. (20) Rybakov, V. B.; Tsisevich, A. A.; Nikitin, K. V.; Alifanov, V. L.; Babaev, E. V. Acta Crystallogr., Sect. E: Struct. Rep. Online 2006, 62, o2546. (21) Rybakov, V. B.; Alifanov, V. L.; Babaev, E. V. Acta Crystallogr., Sect. E: Struct. Rep. Online 2006, 62, o4578. (22) Rybakov, V. B.; Alifanov, V. L.; Babaev, E. V. Acta Crystallogr., Sect. E: Struct. Rep. Online 2006, 62, o4746. (23) Hunt, R. R.; McOmie, J. F. W.; Sayer, E. R. J. Chem. Soc. 1959, 525.

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