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Typical Procedure
for the Synthesis of (
Z
)-Ethyl 2-(Tetrahydrothiazolo[4,3-
b
][1,3]thiazin-6
(2
H
)-ylidene)acetate
(9f)
The
thioester 7f (50.5 mg, 0.17 mmol) was dissolved
in dry EtOH (5 mL) and a solution of NaOEt (0.2 M in EtOH, 0.85 mL,
0.17 mmol) was added at r.t. with vigorous stirring. After hydrolysis
to thiol, as evidenced by complete consumption of the reactant (ca.
15 min; TLC), the reaction mixture was cooled down to 0 ˚C.
Twofold mass excess of NaBH4 (101 mg, 15-20
mmol equiv) was added, followed by 3 drops of 0.4 M HCl in EtOH.
The addition of the same amount of acid was continued in regular
10 min intervals until the end of the reaction (45 min), when the
reaction mixture was quenched with 1 M HCl in EtOH. The suspension
was stirred for an additional 30 min at 0 ˚C, diluted
with H2O, extracted with CHCl3, the organic
phase separated, dried with Na2SO4, and the
solvent evaporated under reduced pressure. Purification of a crude
product by column chromatography (SiO2; PE-EtOAc
solvent gradient 100:0 → 80:20) afforded the final product 9f as a white solid (27.4 mg, 66%);
mp 108-109 ˚C. ¹H
NMR (200 MHz, CDCl3): δ = 1.27 (3 H,
t, J = 7.4
Hz, CH
3CH2), 1.65-2.05
(2 H, m, C3H2), 2.76-2.83 (1 H, m,
C2H), 2.83 (1 H, dd, J
1 = 11.8
Hz, J
2 = 2.0 Hz, C8H),
3.12 (1 H, ddd, J
1 = 13.6 Hz, J
2 = 12.2
Hz, J
3 = 3.0
Hz, C2H), 3.27 (1 H, ddd, J
1 = 14.4
Hz, J
2 = 12.6
Hz, J
3 = 2.6
Hz, C4H), 3.39 (1 H, dd, J
1 = 11.8
Hz, J
2 = 7.2
Hz, C8H), 3.80-3.87 (1 H, m, C4H), 4.17
(2 H, q, J = 7.4
Hz), 5.05 (1 H, s, C6
′H), 5.18 (1
H, dd, J
1 = 7.2
Hz, J
2 = 2.0, C8aH)
ppm. ¹³C NMR (50.3 MHz, CDCl3): δ = 14.6
(CH3CH2), 22.0
(C3), 29.1 (C2), 33.0 (C8), 46.8
(C4), 59.3 (CH2CH3),
67.5 (C8a), 83.5 (C6
′),
162.3 (C6), 168.9 (CO2) ppm. IR: 2977, 2951,
2925, 2897, 1662, 1531, 1459, 1427, 1335, 1276, 1226, 1205, 1177,
1144, 1102, 1043, 1003, 900, 809 cm-¹.
HR MS (CI/TOF): m/z [M + H]+ calcd:
246.06170; found: 246.06176 ± 0.24 ppm.
<A NAME="RG06909ST-16">16</A>
Analytical Data of (Z)-Ethyl
2-{7-Methyl-2H-thiazolo[4,3-b]thiazol-5 (3H,7H,7aH)-ylidene}acetate
(9c)
Isolated in 37% yield
as a mixture of trans- and cis-isomer in a 75:25 ratio.
Compound trans-9c: ¹H
NMR (500 MHz, CDCl3): δ = 1.26 (3 H,
t, J = 7.0
Hz, CH
3CH2), 1.49
(3 H, d, J = 6.5
Hz, CH
3CHS), 3.09-3.12
(1 H, m, CH2S), 3.15-3.20 (1 H, m, CH2S),
3.26-3.32 (1 H m, CH2N), 3.66 (1 H, dq, J
1 = 6.5
Hz, J
2 = 5.5 Hz, CHCH3S), 3.97 (1 H, ddd, J
1 = 9.0
Hz, J
2 = 6.0 Hz, J
3 = 3.0
Hz, CH2N) 4.16 (2 H, q, J = 7.0
Hz, CH
2CH3), 4.89
(1 H, d, J = 5.5
Hz, CHSN), 5.07 (1 H, s, = CH) ppm. ¹³C
NMR (50.3 MHz, CDCl3): δ = 14.5 (CH3CH2), 20.1 (CH3CHS), 32.0 (CH2S),
45.1 (CHCH3S), 50.7 (CH2N), 59.4
(CH2CH3), 76.8
(CHSN), 84.4 (=CH), 163.76 (C=), 168.6 (CO2)
ppm.
Compound cis-9c: ¹H NMR (500 MHz,
CDCl3): δ = 1.27 (3 H, t, J = 7.0 Hz,
CH
3CH2), 1.46 (3
H, d, J = 7.0
Hz, CH
3CHS), 3.05-3.09
(1 H, m, CH2S), 3.24-3.33 (1 H, m, CH2S),
3.29-3.41 (1 H, m, CH2N), 3.89 (1 H, dq, J
1 = 7.0 Hz, J
2 = 5.5
Hz, CHCH3S), 4.00 (1 H, ddd, J
1 = 9.2
Hz, J
2 = 5.8
Hz, J
3 = 3.2
Hz, CH2N) 4.16 (2 H, q, J = 7.0
Hz, CH2CH3), 5.10
(1 H, s, =CH), 5.28 (1 H, d, J = 5.5
Hz, CHSN) ppm. ¹³C NMR (50.3 MHz, CDCl3): δ = 14.5 (CH3CH2), 17.0 (CH3CHS), 31.0 (CH2S),
41.3 (CHCH3S), 51.0 (CH2N),
59.4 (CH2CH3),
76.8 (CHSN), 84.8 (=CH), 163.8 (C=), 168.6 ppm.
For recent examples of the 5-endo-trig mode ring-closing reaction,
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