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Photolysis of thiabendazole
Chemosphere No. 2, 77 - 78, 1976.
Pergamon Press°
P r i n t e d i n Great B r i t a i n .
PHOTOLYSIS OF THIABENDAZOLE
David A.M. Watklns Long Ashton Research Station, University of Bristol, Bristol BSI8 gAF (Received i n UK for p u b l i c a t i o n 7 ~4arch 1976)
Following our report I that the Imidazole ring in carbendazlm {methyl benzlmldazol-2ylcarbamate} is cleaved on photolysis to give guanldlno compounds, it was of interest to investigate the effect of u.v. irradiation on thlabendazole (2-thlazol-4'-yl benzlmldazole). Ben Azlz and Aharonson2 had commented that the relatively rapid disappearance of thlabendazole from tomato and pepper leaves was probably due to a degradation occurring outside the living cell. During the time we did this work Jacob et al.3 reported the formation of benzimldazole2-carboxamide and benzlmidazole by cleavage of the thiazole ring, we can confirm this. A solution of thlabendazole {400 mg) in methanol (300 ml:dlstllled from sodium hydroxide) was irradiated with a medium pressure Hanovia lamp {I00 w) in a borosllicate glass apparatus and the reaction followed by the u.v. spectrum of daily samples. had almost disappeared.
After g days the peak at 302 mm
The pale yellow solution was concentrated in a rotary evaporator at 40°
to give a residual oil (640 mg) which in aqueous solution had pH 2.9. As in the previous work I on carbendazlm the methanol distillate had a distinctive smell and contained dlmethyloxalate and other components I. An aqueous solution of the residual oil was rotary evaporated at 40° and the distillate treated with ammonia.
Vacuum evaporation of the
ammonlacal solution left a white solid (45 mg) which by comparison with an authentic specimen showed to be ammonium formate thus suggesting that formic acid was present originally. Irradiating methanol in the same w~y gave no residue or volatile components detected by addition of ammonia to the distillate. The oil remaining after removing the water was dissolved in the minimum of methanol, then adding acetone produced a yellowish precipitate (160 ~).
The filtrates from three experiments
were combined, absorbed on alumina (Camag:neutral, washed with methanol, acetic acid, water and reactivated) and added to an alumina column {60 g) packed in light petroleum (bp 40-60°). containing methanol (½)) eluted a fraction containing thlabendazole {7 mg).
Ether
Increasing the
methanol content to I0% gave an initial fraction which yielded a crystalline solid {40 mg; mp 14g°);
77
78
No, 2
shown to be thiazole-4-carboxamtde by co~arison of infrared and mass spectra and mixed melting point with a synthetic specimen.
The next fraction yielded a solid (71 mg; mp 325° )
which had the same mass spectrum as benzimidazol-Z-carboxamide 3.
Elutton with 25% methanol in
ether yielded, on evaporation, a s t i c ~ solid (76 mg) which was subjected to preparative layer chromatography on Merck s i l i c a gel PF2s~.
Development with chloroform:methanol:ammonia
(200:4:
saturated) gave a well separated band which when crystallised became a solid (9 rag), shown to be benzimidazole by comparing its physical characteristics with an authentic specimen.
Elution
using pure methanol gave an i n i t i a l fraction (42 nKj), the mass spectrum of which indicated the presence of thiazol-4-ylamidine (or a salt thereof), followed by a fraction which yielded methyl thiazole-4-carbox~ylate (48 ~ ) ,
proved by comparison of infrared and mass spectrum with
those of a synthetic specimen. Further elution with increasing proportions of water in the methanol and f i n a l l y l% aqueous acetic acid yielded several fractions so far unidentified. The i d e n t i f i c a t i o n of the thiazole derivatives again demonstrates the cleavage of the imidazole ring but cleavage of the thiazole ring also occurs. I thank Dr D. Woodcock for encouragement, Dr R.L.S. Patterson (Neat Research I n s t i t u t e ) for the mass spectra f a c i l i t i e s and Merck, Sharp and Dohme for a g i f t of thiabendazole.
References l.
D.A.M. Watkins, Chemosphere, 3, 239 (1974).
2.
A. Ben-Aziz and N. Aharonson, Pestic. Biochem. Ph~/stol., 4, 120, 1974.
3.
T.A. Jacob, J.R. Carlin, R.W. Walker, F.J. Wolf and W.J.A. VandenHeuvel, 3. agric. Fd chem. 23, 704, (1975).
Pergamon Press°
P r i n t e d i n Great B r i t a i n .
PHOTOLYSIS OF THIABENDAZOLE
David A.M. Watklns Long Ashton Research Station, University of Bristol, Bristol BSI8 gAF (Received i n UK for p u b l i c a t i o n 7 ~4arch 1976)
Following our report I that the Imidazole ring in carbendazlm {methyl benzlmldazol-2ylcarbamate} is cleaved on photolysis to give guanldlno compounds, it was of interest to investigate the effect of u.v. irradiation on thlabendazole (2-thlazol-4'-yl benzlmldazole). Ben Azlz and Aharonson2 had commented that the relatively rapid disappearance of thlabendazole from tomato and pepper leaves was probably due to a degradation occurring outside the living cell. During the time we did this work Jacob et al.3 reported the formation of benzimldazole2-carboxamide and benzlmidazole by cleavage of the thiazole ring, we can confirm this. A solution of thlabendazole {400 mg) in methanol (300 ml:dlstllled from sodium hydroxide) was irradiated with a medium pressure Hanovia lamp {I00 w) in a borosllicate glass apparatus and the reaction followed by the u.v. spectrum of daily samples. had almost disappeared.
After g days the peak at 302 mm
The pale yellow solution was concentrated in a rotary evaporator at 40°
to give a residual oil (640 mg) which in aqueous solution had pH 2.9. As in the previous work I on carbendazlm the methanol distillate had a distinctive smell and contained dlmethyloxalate and other components I. An aqueous solution of the residual oil was rotary evaporated at 40° and the distillate treated with ammonia.
Vacuum evaporation of the
ammonlacal solution left a white solid (45 mg) which by comparison with an authentic specimen showed to be ammonium formate thus suggesting that formic acid was present originally. Irradiating methanol in the same w~y gave no residue or volatile components detected by addition of ammonia to the distillate. The oil remaining after removing the water was dissolved in the minimum of methanol, then adding acetone produced a yellowish precipitate (160 ~).
The filtrates from three experiments
were combined, absorbed on alumina (Camag:neutral, washed with methanol, acetic acid, water and reactivated) and added to an alumina column {60 g) packed in light petroleum (bp 40-60°). containing methanol (½)) eluted a fraction containing thlabendazole {7 mg).
Ether
Increasing the
methanol content to I0% gave an initial fraction which yielded a crystalline solid {40 mg; mp 14g°);
77
78
No, 2
shown to be thiazole-4-carboxamtde by co~arison of infrared and mass spectra and mixed melting point with a synthetic specimen.
The next fraction yielded a solid (71 mg; mp 325° )
which had the same mass spectrum as benzimidazol-Z-carboxamide 3.
Elutton with 25% methanol in
ether yielded, on evaporation, a s t i c ~ solid (76 mg) which was subjected to preparative layer chromatography on Merck s i l i c a gel PF2s~.
Development with chloroform:methanol:ammonia
(200:4:
saturated) gave a well separated band which when crystallised became a solid (9 rag), shown to be benzimidazole by comparing its physical characteristics with an authentic specimen.
Elution
using pure methanol gave an i n i t i a l fraction (42 nKj), the mass spectrum of which indicated the presence of thiazol-4-ylamidine (or a salt thereof), followed by a fraction which yielded methyl thiazole-4-carbox~ylate (48 ~ ) ,
proved by comparison of infrared and mass spectrum with
those of a synthetic specimen. Further elution with increasing proportions of water in the methanol and f i n a l l y l% aqueous acetic acid yielded several fractions so far unidentified. The i d e n t i f i c a t i o n of the thiazole derivatives again demonstrates the cleavage of the imidazole ring but cleavage of the thiazole ring also occurs. I thank Dr D. Woodcock for encouragement, Dr R.L.S. Patterson (Neat Research I n s t i t u t e ) for the mass spectra f a c i l i t i e s and Merck, Sharp and Dohme for a g i f t of thiabendazole.
References l.
D.A.M. Watkins, Chemosphere, 3, 239 (1974).
2.
A. Ben-Aziz and N. Aharonson, Pestic. Biochem. Ph~/stol., 4, 120, 1974.
3.
T.A. Jacob, J.R. Carlin, R.W. Walker, F.J. Wolf and W.J.A. VandenHeuvel, 3. agric. Fd chem. 23, 704, (1975).