Monensin is a potent antibiotic compound isolated from Streptomyces cinnamonensis. The

Chapter 9, Problem 9.93

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Monensin is a potent antibiotic compound isolated from Streptomyces cinnamonensis. The following reaction was employed during W. C. Stills synthesis of monensin (J. Am. Chem. Soc. 1980, 102, 21182120). Sodium bicarbonate (NaHCO3) functions as a mild base to deprotonate the carboxylic acid group. The transformation is said to be diastereoselective, because only one diastereomeric product (5S,6S) is obtained. The other expected diastereomer (5R,6R) is not observed. O O O O O O O O H H NaHCO3 1 2 4 5 3 6 1 2 I2 I (a) Draw a mechanism for this transformation. (b) The stereochemical outcome can be rationalized with a conformational analysis of compound 1. Draw a Newman projection looking down the C4C5 bond and identify the lowest energy conformation. Use your findings to rationalize the observed diastereoselectivity.