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4. Reaction Mechanisms Serine protease enzymes catalyze peptide bond cleavage in proteins. Molecules that control protease function (inhibitors) can alleviate disease conditions (e.g. cardiovascular ACE inhibitors) and as antibiotics. The following synthesis was used to prepare an "enol lactone" for investigation of serine protease inhibition. Step 2 Step 3 Br Ph O Br Ph O a) NaNH2 Ph O { 5 H3O+ Br { Br (4 equivalents) OEt OH OH 2 H2O 3 4 b) "Workup" H2O pH 2 HgSO4 Br2 additives Step 1 CCI4 Ph CH2CI2 Step 4 Ph O O O OEt 1 Enol Lactone i) Provide a step-by-step mechanism for Step 1. Use R as molecule 1 R ii) Step 2 of the synthesis hydrolyzes the ester functional group, RCO2Et, to the carboxylic acid functional group, RCO2H. Briefly explain (words or structures) why Step 1 was performed with an ester in molecule 1, not a carboxylic acid iii,iv) Step 3a uses 4 equivalents of NaNH2. All 4 equivalents (moles NaNH2 per mole of structure 3) are needed to convert structure 3 4. Draw the structure formed by reaction of 3 with only 1 NaNH2 in Step 3a. Br Ph o NaNH2 Br OH 3 1 equivalent 6 Draw the structure formed by reaction of 3 with all 4 NaNH2 in Step 3a. Br Ph O NaNH2 Br- OH 3 4 equivalents v) Different additives were tested to maximize the yield of Step 4 and to minimize the amount of Hg2+ required. A 90% yield of "enol lactone" was attained using 0.1 equivalents of HgSO4 and 0.02 equivalents CH3SO3H (pKa = -1). Provide a step-by-step mechanism for the cyclization of 4 "enol lactone" in the presence of Hg2+ SO2² and CH3SO3H. O II H3C-S-OH - Ph Hg²+ SO42 (0.1 equiv) II Ph O O (0.02 equiv) enol lactone OH CH2CI2 4 O O vi) If CH3SO3H is not added, the reaction forms Compound Q (not the intended enol lactone) in 10% yield. Based on your mechanism, draw the structure of Compound Q 7

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