Mendeleev Commun., 2008, 18, 144–146
graphic study (Figure 1).†† Based on these results, we accurately
indicate the stereochemical outcome of this reaction. The stereo-
selective alkylation of isoxazolo[5,4-d]pyrimidine 3e by ethyl
O
N
OR2
O
N
O
R
R
ClCH2CO2R2
4a,b
NH
R1
§
N
N
General procedure for preparation of 4-chloroisoxazolo[5,4-d]pyri-
K2CO3, DMF or acetone
(69–96%)
N
R1
O
O
midines 6a,b,e,f. Phosphorus oxychloride (1 mol) and dimethylaniline
(0.2 mol) were added to a solution of isoxazolo[5,4-d]pyrimidin-4(5H)-
one 3a,b,e,f (0.2 mol) in dry toluene. The resulting suspension was
stirred at reflux for 3–4 h. The reaction was followed by TLC (5% MeOH
in CH2Cl2). The solution was cooled to room temperature, poured onto
crushed ice and neutralised with an aqueous solution of NaOH (10%).
The organic layer was separated; the aqueous layer was extracted with
benzene (3×200 ml). Combined extracts were washed with HCl (6%),
water, NaHCO3 and water again. The extracts were purified by flash
chromatography on a silica gel/aluminium oxide column (eluent, benzene).
The solvent was evaporated in vacuo and the resulting residue was poured
into cold hexane. The formed precipitate was filtered off, washed with
hexane and dried in vacuo over P2O5. Chlorides 6a,b,e,f were obtained
in 85–96% yield.
3a–k
5a–j
4a R2 = Me
4b R2 = Et
5a R = R2 = Me, R1 = H
5b R = Me, R1 = H, R2 = Et
5c R = R1 = R2 = Me
5d R = Ph, R1 = H, R2 = Me
5e R = Ph, R1 = H, R2 = Et
5f R = 4-MeC6H4, R1 = H, R2 = Me
5g R = 4-MeC6H4, R1 = H, R2 = Et
5h R = 4-MeOC6H4, R1 = H, R2 = Me
5i R = 2-FC6H4, R1 = H, R2 = Me
5j R = 2-FC6H4, R1 = H, R2 = Et
Scheme 2
4-Chloro-3-methylisoxazolo[5,4-d]pyrimidine 6a: yield 92%, mp 67–
68 °C. 1H NMR (DMSO + CCl4, 400 MHz) d: 2.71 (s, 3H, Me), 9.01 (s,
1H, 6-H). Found (%): C, 42.61; H, 2.35; Cl, 20.93; N, 24.82. Calc. for
C6H4ClN3O (%): C, 42.50; H, 2.38; Cl, 20.91; N, 24.78.
4-Chloro-3-phenylisoxazolo[5,4-d]pyrimidine 6e: yield 96%, mp 100–
102 °C. 1H NMR (DMSO + CCl4, 400 MHz) d: 7.59–7.67 (m, 3H, HAr),
7.80–7.86 (m, 2H, HAr), 9.10 (s, 1H, 6-H). Found (%): C, 57.11; H, 2.67;
Cl, 15.41; N, 18.22. Calc. for C11H6ClN3O (%): C, 57.04; H, 2.61; Cl,
15.31; N, 18.14.
The reactions of equimolar amounts of 4-chloroisoxazolo[5,4-d]-
pyrimidines 6a,b,e,f and ethyl hydroxyacetate in the presence
of K2CO3 (1.1 equiv.) afforded ethyl [(isoxazolo[5,4-d]pyrimidin-
4-yl)oxy]acetates 7a,b,d–f (Scheme 3).¶ The reaction proceeded
in DMF at 80 °C to give pure products, which were precipitated
from the reaction mixture in good yields (62–68%).
The structures of compounds 5e and 7d were assigned using
NMR spectroscopy and firmly established by an X-ray crystallo-
4-Chloro-6-methyl-3-phenylisoxazolo[5,4-d]pyrimidine 6f: yield 94%,
mp 112–114 °C. 1H NMR (DMSO + CCl4, 400 MHz) d: 2.80 (s, 3H,
Me), 7.56–7.66 (m, 3H, HAr), 7.77–7.82 (m, 2H, HAr). Found (%): C,
58.73; H, 3.33; Cl, 14.45; N, 17.08. Calc. for C12H8ClN3O (%): C, 58.67;
H, 3.28; Cl, 14.43; N, 17.10.
‡
General procedure for the preparation of (4-oxoisoxazolo[5,4-d]-
pyrimidin-5(4H)-yl)acetates 5a–j. α-Chloroester 4a,b (0.12 mol) and
anhydrous K2CO3 (0.13 mol) were added to a solution of isoxazolo[5,4-d]-
pyrimidin-4(5H)-one 3a,c,e,h,i,k in dry DMF (125 ml). The reaction
mixture was stirred at 70 °C for 3–4 h. The reaction was followed by
TLC (5% MeOH in CH2Cl2). The solution was cooled to room tempera-
ture and then poured into cold water (500 ml). The precipitate was
filtered off and washed with water and hexane. The desired esters 5a–j
were obtained in good yields (76–98%).
Ethyl (3-methyl-4-oxoisoxazolo[5,4-d]pyrimidin-5(4H)-yl)acetate 5b:
yield 83%, mp 143–145 °C. 1H NMR (DMSO + CCl4, 400 MHz) d: 1.27
(t, 3H, Me, J 7.0 Hz), 2.52 (s, 3H, Me), 4.26 (q, 2H, OCH2, J 7.0 Hz),
4.79 (s, 2H, NCH2), 8.58 (s, 1H, 6-H). Found (%): C, 50.69; H, 4.61;
N, 17.68. Calc. for C10H11N3O4 (%): C, 50.63; H, 4.67; N, 17.71.
Methyl (3,6-dimethyl-4-oxoisoxazolo[5,4-d]pyrimidin-5(4H)-yl)acetate
5c: yield 76%, mp 50–52 °C. 1H NMR (DMSO + CCl4, 400 MHz) d:
2.44 (s, 3H, Me), 2.54 (s, 3H, Me), 3.76 (s, 3H, OMe), 4.60 (s, 2H,
NCH2). Found (%): C, 50.67; H, 4.71; N, 17.75. Calc. for C10H11N3O4
(%): C, 50.63; H, 4.67; N, 17.71.
Methyl (4-oxo-3-phenylisoxazolo[5,4-d]pyrimidin-5(4H)-yl)acetate 5d:
yield 90%, mp 144–145 °C. 1H NMR (DMSO + CCl4, 400 MHz) d: 3.83
(s, 3H, OMe), 4.79 (s, 2H, NCH2), 7.65–7.69 (m, 3H, ArH), 8.12–8.16
(m, 2H, ArH), 8.68 (s, 1H, 6-H). 13C NMR (DMSO + CCl4, 400 MHz) d:
47.37 (NCH2), 53.15 (OMe), 100.51 (C-3a), 126.97(CAr-4), 128.67
(CAr-3,5), 128.79 (CAr-2,6), 131.03 (CAr-1), 152.41 (C-3), 156.51 (C-4),
159.95 (C-6), 167.06 (COOMe), 175.43 (C-7a). Found (%): C, 58.99;
H, 3.82; N, 14.68. Calc. for C14H11N3O4 (%): C, 58.95; H, 3.89; N, 14.73.
Ethyl (4-oxo-3-phenylisoxazolo[5,4-d]pyrimidin-5(4H)-yl)acetate 5e:
yield 77%, mp 159–161 °C. 1H NMR (DMSO + CCl4, 400 MHz) d: 1.25
(t, 3H, Me, J 7.0 Hz), 4.18 (q, 2H, OCH2, J 7.0 Hz), 4.87 (s, 2H, NCH2),
7.45–7.49 (m, 3H, HAr), 8.20–8.25 (m, 2H, HAr), 8.69 (s, 1H, 6-H).
13C NMR (DMSO + CCl4, 400 MHz) d: 12.44 (Me), 45.82 (NCH2), 60.03
(OCH2), 98.10 (C-3a), 125.47 (CAr-4), 126.97 (CAr-3,5), 127.11 (CAr-2,6),
129.29 (CAr-1), 152.99 (C-3), 154.81 (C-4), 157.77 (C-6), 165.59 (COOEt),
173.90 (C-7a). Found (%): C, 60.24; H, 4.32; N, 13.98. Calc. for
C15H13N3O4 (%): C, 60.20; H, 4.38; N, 14.04.
¶
General procedure for the preparation of (isoxazolo[5,4-d]-pyri-
midin-4-yl)oxyacetates 7a,b,d,e,f. Ethyl hydroxyacetate (0.11 mol) and
anhydrous K2CO3 (0.11 mol) were added to a solution of chloride
6a,b,e,f (0.1 mol) in dry DMF (115 ml). The reaction mixture was
stirred at 80 °C for 4–5 h, then cooled to rt and poured into cold water.
The precipitate was filtered off, washed with hexane and recrystallised
from ethanol or acetonitrile to give desired esters 7a,b,d,e,f in moderate
yields (62–68%).
Methyl [(3-methyl-4,5-dihydroisoxazolo[5,4-d]pyrimidin-4-yl)oxy]-
1
acetate 7a: mp 96–98 °C. H NMR (DMSO + CCl4, 400 MHz) d: 2.64
(s, 3H, Me), 3.74 (s, 3H, OMe), 5.21 (s, 2H, OCH2), 8.70 (s, 1H, 6-H).
Found (%): C, 48.04; H, 4.88; N, 18.69. Calc. for C9H11N3O4 (%): C,
48.00; H, 4.92; N, 18.66.
Methyl [(3-ethyl-4,5-dihydroisoxazolo[5,4-d]pyrimidin-4-yl)oxy]acetate
7b: mp 150–153 °C. 1H NMR (DMSO + CCl4, 400 MHz) d: 1.36 (t, 3H,
Me, J 7.4 Hz), 2.99 (q, 2H, CH2, J 7.4 Hz), 3.72 (s, 3H, OMe), 5.28 (s,
2H, OCH2), 8.79 (s, H, 6-H). Found (%): C, 50.68; H, 4.75; N, 18.82.
Calc. for C10H11N3O4 (%): C, 50.63; H, 4.67; N, 17.71.
Methyl [(3-phenyl-4,5-dihydroisoxazolo[5,4-d]pyrimidin-4-yl)oxy]acetate
7d: mp 122–123 °C. 1H NMR (DMSO + CCl4, 400 MHz) d: 3.73 (s, 3H,
OMe), 5.29 (s, 2H, OCH2), 7.62 (m, 3H, HAr), 8.09 (m, 2H, HAr), 8.87
(s, 1H, 6-H). 13C NMR (DMSO + CCl4, 400 MHz) d: 52.34 (OMe),
64.17 (OCH2), 104.19 (C-3a), 129.54 (CAr-4), 131.45 (CAr-2,6), 134.01
(CAr-3,5), 143.41 (CAr-1), 156.51(CAr-6), 162.28 (C-3), 166.23 (C-7a),
168.08 (C-4), 168.43 (COOMe). Found (%): C, 58.57; H, 4.51; N, 14.66.
Calc. for C14H13N3O4 (%): C, 58.53; H, 4.56; N, 14.63.
Ethyl [(3-phenyl-4,5-dihydroisoxazolo[5,4-d]pyrimidin-4-yl)oxy]acetate
1
7e: mp 96–98 °C. H NMR (DMSO + CCl4, 400 MHz) d: 1.22 (t, 3H,
Me, J 7.1 Hz), 4.19 (q, 2H, OCH2, J 7.1 Hz), 5.27 (s, 2H, OCH2),
7.59–7.63 (m, 3H, Ar), 8.07–8.11 (m, 2H, Ar), 8.88 (s, 1H, 6-H).
13C NMR (DMSO + CCl4, 400 MHz) d: 13.84 (Me), 61.05 (OCH2),
64.43 (OCH2), 105.51 (C-3a), 130.17 (CAr-4), 131.92 (CAr-2,6), 134.99
(CAr-3,5), 143.19 (CAr-1), 157.38 (C-6), 162.18 (C-3), 166.13 (C-7a),
167.98 (C-4), 168.88 (COOEt). Found (%): C, 59.83; H, 4.97; N, 14.01.
Calc. for C15H15N3O4 (%): C, 59.79; H, 5.02; N, 13.95.
Methyl [(6-methyl-3-phenyl-4,5-dihydroisoxazolo[5,4-d]pyrimidin-4-yl)-
oxy]acetate 7f: mp 131–133 °C. 1H NMR (DMSO + CCl4, 400 MHz) d:
2.65 (s, 3H, Me), 3.73 (s, 3H, OMe), 5.25 (s, 2H, OCH2), 7.61 (m, 3H, -
Methyl [3-(4-methylphenyl)-4-oxoisoxazolo[5,4-d]pyrimidin-5(4H)-yl]-
acetate 5f: yield 94%, mp 190–192 °C. 1H NMR (DMSO + CCl4, 400 MHz)
d: 2.39 (s, 3H, Me), 3.72 (s, 3H, OMe), 4.90 (s, 2H, NCH2), 7.37 (d,
2H, HAr, J 8.0 Hz), 8.13 (d, 2H, HAr, J 8.0 Hz), 8.78 (s, 1H, 6-H).
Found (%): C, 60.16; H, 4.31; N, 14.11. Calc. for C15H13N3O4 (%):
C, 60.20; H, 4.38; N, 14.04.
For characteristics of compounds 5a,g,h,j see Online Supplementary
Materials.
H
Ar), 8.09 (m, 2H, HAr). Found (%): C, 59.83; H, 4.98; N, 13.99. Calc.
for C15H15N3O4 (%): C, 59.79; H, 5.02; N, 13.95.
– 145 –