Passage
Chloramphenicol (Compound 1) is an antibiotic that acts against gram-positive and gram-negative bacteria by blocking peptidyl transfer, which inhibits protein synthesis by bacterial ribosomes. However, it tastes bitter, so most patients cannot tolerate it. As a result, synthesis of hydroxyl-substituted chloramphenicol analogues has become of interest because ester chloramphenicol analogues taste better than chloramphenicol. Substituents on the primary hydroxyl group of chloramphenicol analogues are typically hydrolyzed quickly in vivo to regenerate Compound 1. Compound 3, a chloramphenicol analogue, was synthesized by the transesterification of Compound 1 and vinyl propionate (Compound 2) , catalyzed by the enzyme Lip_BA lipase (Figure 1) .
Figure 1 Transesterification of chloramphenicol catalyzed by Lip_BAAlthough enzyme catalysis allows for efficient regiospecific and stereospecific bond formation, the reaction conditions for the conversion efficiency of Compound 1 to Compound 3, as well as the purity of Compound 3, must also be optimized. The reaction solvent, temperature, and time were studied to find the conditions that gave the highest conversion rate and purity. Each parameter was altered individually while all others were kept constant, and the enzyme concentration was 4.5 g/L in each trial. The results of this study are shown in Table 1. The purity of Compound 3 was found to be over 90% when 1,4-dioxane was used and over 70% when dichloromethane was used.Table 1 Transesterification Conditions Optimization
The transesterification reaction was monitored by thin-layer chromatography (Figure 2) , and the product was confirmed by ¹H NMR. The ¹H NMR spectra of Compounds 1 and 3 can be distinguished by the shift of H_e and H_f due to the addition of the electronegative ester functional group.
Figure 2 Thin-layer chromatography of reaction mixture
Adapted from F. Dong et al., "Transesterification Synthesis of Chloramphenicol." Molecules. ©2017 MDPI.
-Which of the following experimental conditions does NOT contribute to the highest conversion efficiency?

Question

Passage
Chloramphenicol (Compound 1) is an antibiotic that acts against gram-positive and gram-negative bacteria by blocking peptidyl transfer, which inhibits protein synthesis by bacterial ribosomes. However, it tastes bitter, so most patients cannot tolerate it. As a result, synthesis of hydroxyl-substituted chloramphenicol analogues has become of interest because ester chloramphenicol analogues taste better than chloramphenicol. Substituents on the primary hydroxyl group of chloramphenicol analogues are typically hydrolyzed quickly in vivo to regenerate Compound 1. Compound 3, a chloramphenicol analogue, was synthesized by the transesterification of Compound 1 and vinyl propionate (Compound 2) , catalyzed by the enzyme Lip_BA lipase (Figure 1) .

Passage Chloramphenicol (Compound <strong>1</strong>) is an antibiotic that acts against gram-positive and gram-negative bacteria by blocking peptidyl transfer, which inhibits protein synthesis by bacterial ribosomes. However, it tastes bitter, so most patients cannot tolerate it. As a result, synthesis of hydroxyl-substituted chloramphenicol analogues has become of interest because ester chloramphenicol analogues taste better than chloramphenicol. Substituents on the primary hydroxyl group of chloramphenicol analogues are typically hydrolyzed quickly in vivo to regenerate Compound <strong>1</strong>. Compound <strong>3</strong>, a chloramphenicol analogue, was synthesized by the transesterification of Compound <strong>1</strong> and vinyl propionate (Compound <strong>2</strong>) , catalyzed by the enzyme Lip<sub>BA</sub> lipase (Figure 1) . <strong>Figure 1</strong> Transesterification of chloramphenicol catalyzed by Lip<sub>BA</sub>Although enzyme catalysis allows for efficient regiospecific and stereospecific bond formation, the reaction conditions for the conversion efficiency of Compound <strong>1</strong> to Compound <strong>3</strong>, as well as the purity of Compound <strong>3</strong>, must also be optimized. The reaction solvent, temperature, and time were studied to find the conditions that gave the highest conversion rate and purity. Each parameter was altered individually while all others were kept constant, and the enzyme concentration was 4.5 g/L in each trial. The results of this study are shown in Table 1. The purity of Compound <strong>3</strong> was found to be over 90% when 1,4-dioxane was used and over 70% when dichloromethane was used.<strong>Table 1</strong> Transesterification Conditions Optimization The transesterification reaction was monitored by thin-layer chromatography (Figure 2) , and the product was confirmed by <sup>1</sup>H NMR. The <sup>1</sup>H NMR spectra of Compounds <strong>1</strong> and <strong>3</strong> can be distinguished by the shift of H<sub>e</sub> and H<sub>f</sub> due to the addition of the electronegative ester functional group. <strong>Figure 2</strong> Thin-layer chromatography of reaction mixture Adapted from F. Dong et al., Transesterification Synthesis of Chloramphenicol. Molecules. ©2017 MDPI. -Which of the following experimental conditions does NOT contribute to the highest conversion efficiency? A) Use of 1,4-dioxane as the solvent B) Reaction temperature of 55 °C C) Reaction time of 20 hours D) Enzyme concentration of 4.5 g/L

Figure 1 Transesterification of chloramphenicol catalyzed by Lip_BAAlthough enzyme catalysis allows for efficient regiospecific and stereospecific bond formation, the reaction conditions for the conversion efficiency of Compound 1 to Compound 3, as well as the purity of Compound 3, must also be optimized. The reaction solvent, temperature, and time were studied to find the conditions that gave the highest conversion rate and purity. Each parameter was altered individually while all others were kept constant, and the enzyme concentration was 4.5 g/L in each trial. The results of this study are shown in Table 1. The purity of Compound 3 was found to be over 90% when 1,4-dioxane was used and over 70% when dichloromethane was used.Table 1 Transesterification Conditions Optimization

The transesterification reaction was monitored by thin-layer chromatography (Figure 2) , and the product was confirmed by ¹H NMR. The ¹H NMR spectra of Compounds 1 and 3 can be distinguished by the shift of H_e and H_f due to the addition of the electronegative ester functional group.

Figure 2 Thin-layer chromatography of reaction mixture
Adapted from F. Dong et al., "Transesterification Synthesis of Chloramphenicol." Molecules. ©2017 MDPI.
-Which of the following experimental conditions does NOT contribute to the highest conversion efficiency?

Quizplus · Accepted Answer

11ecba2d_129e_e306_a3bf_499d89f72421_MD0008_00 Reaction condition optimization is important for achieving efficient synthesis of a molecule. One variable should be evaluated at a time, and all others held constant so that only one variable is considered in each trial. The reaction conditions that give the highest conversion efficiency and purity are optimal. Table 1 shows the results of the reaction condition optimization for the transesterification of chloramphenicol. The reaction was catalyzed by the enzyme Lip_BA, and its concentration was 4.5 g/L in each trial (Choice D). The solvent 1,4-dioxane (Choice A) gave a considerably higher conversion efficiency (91% or higher) and purity than dichloromethane (72% conversion). Of the temperatures tested, 55 °C yielded the highest conversion efficiency (Choice B), and a reaction time of 10 hours, rather than 20, gave the highest conversion efficiency at 55 °C (99%). A longer reaction time likely led to product degradation or side reactions. Therefore, a 20-hour reaction time is not part of the optimal set of reaction conditions. Educational objective: When experimental conditions are optimized, one variable should be evaluated in each trial while other variables are kept constant. The conditions that give the highest yield are optimal.

Passage Chloramphenicol (Compound 1) Is an Antibiotic That Acts Against Gram-Positive