Abstract:

Flavoring compounds such as 2,4,5-trimethyloxazole and 2,5-dimethyl-4-ethyloxazole were synthesized by Dakin- West and Robinson-Gabriel reactions using alanine and 2-aminobutyric acid as the starting materials. The optimal synthesis processing of 2,4,5-trimethyloxazole was explored through evaluating the effects of catalysts, reaction temperature, reaction time and reactant mole ratio on the yield of 2,4,5-trimethyloxazole. During Dakin-West reaction for synthesizing product Ⅰ (α-acylaminoketone), the optimal reaction conditions were alanine-acetic anhydride ratio of 1:11 (mole/mole), dimethylamino pyridine and anhydrous sodium acetate as the catalysts, dimethylamino pyridine-alanine ratio of 1:10, anhydrous sodium acetate-alanine ratio of 2.5:1, reaction temperature of 130 ℃ and reaction time of 5 h. During Robinson-Gabriel reaction for synthesizing product Ⅱ (2,4,5-trimethyloxazole), the optimal reaction conditions were polyphosphoric acid as cyclic dehydration agent, product Ⅰ -PPA ratio of 1:4, reaction temperature of 150 ℃ and reaction time of 2 h. The product Ⅱ(2,4,5- trimethyloxazole) was purified by vacuum distillation. The total yield of 2,4,5-trimethyloxazole was 30.5%. Similarly, according to above synthesis conditions, the total yield of 2,5-dimethyl-4-ethyloxazole was 28.8%. The 2,4,5-trimethyloxazole and 2,5-dimethyl-4-ethyloxazole had the characteristics of roasted and fragrant aroma. Both product structures were elucidated by IR, MS and 1H NMR. This synthesis processing has the advantages of easily available raw materials, simple operation and purely aromatic products, which will have widely prospects for industrial development.

Key words: Dakin-West, Robinson-Gabriel, 2,4,5-trimethyloxazole, 2,5-dimethyl-4-ethyloxazole, edible flavor