Table 2: Effect of additives in asymmetric aldol reactions. 1a O HO CO Me (20 mo
ID: 553770 • Letter: T
Question
Table 2: Effect of additives in asymmetric aldol reactions. 1a O HO CO Me (20 mol%) OMe solvent C&FsCO2H; (0-10 equiv) 0°C, 16 h Ph Ph 2a 3a Syn-4a Entry Solvent Equivalents of additive Yieldpl [96] syn/antici eddi [%) MeOH 2 MeOH 3 MeOH MeOH MeCN 63 71 37 32 69 65 47 68 74 8.1:1 94 9.2:1 -42 13:1 -58 13:1 -60 9:1 92 9:1 -43 6:1 -58 10 0 6 MeChN 7 H20 8:19 8:1 -88 lal Unless otherwise specified, the asymmetric aldol reaction of cyclo hexanone and -keto ester 3a was conducted in the presence of 20 mol% of catalyst 1a for 16 h under the given conditions. [b] Yield of isolated product. [c] Diastereoselectivity was determined by 'H NMR spectroscopy. [d] Enantiopurity of aldols was determined by HPLC analysis on a chiral stationary phase with hexane/2-propanol as solvent (see the Supporting Information). (e] Volume ratio 1:1. i) In Table 2, entries 1-4, addition of excess acids reduces the product yield significantly. Explain why. (4 pts)Explanation / Answer
Effect of additives in assymetric aldol reactions
i) In table-2, entries 1-4, addition of excess acids reduces the product yield significantly. This is due to the fact that, once the product is formed it can be hydrolyzed easily with excess acid present in the reaction medium. The product is an ester. Ester hydrolyses to carboxylic acid thus reduces the yield of the desired product considerably.
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