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Chapter 17 Suggested solutions for Chapter 17 133 eochemistry right when twisting the molecule round to draw it as a ring. The product is a single but is not chiral. OTs OTs OTs SH OTs S SN2 KoH inversion HS rotate SN2 redraw inversion S S S OTs H H Problem 9 What are the mechanisms of these reactions, and what is the role of the ZnCl₂ in the first step and the Nal in the second? 0 ZnCl₂ CI 0 MeCO₂K 0 0 o CI 0 cat. Nal 0 0 Purpose of the problem Exploration of the roles of two different kinds of catalysts in substitution reactions. Suggested solution The ZnCl₂ acts as a Lewis acid and can be used either to remove chloride from MeCOCl or to mplex with its carbonyl oxygen atom, in either case making it a better electrophile so that it can tack the unreactive oxygen atom of the cyclic ether. Ring cleavage by chloride follows. + :0 0 CI 0 CI ZnCl₂ :0 CI 0 You can read more about this in The second reaction is an SN2 displacement of a reasonable leaving group (chloride) by a rather B. S. Furniss et al., Vogel's textbook eak nucleophile (acetate). The reaction is very slow unless catalysed by iodide - a better of organic chemistry (5th edn), cleophile than acetate and a better leaving group than chloride. It is a nucleophilic catalyst. Longmans, Harlow, 1989, p. 492. CI 0 0 0 0 MeCO₂ 0 0 0 Problem 10 Describe the stereochemistry of the products of these reactions. OH CI 0 0 base S S HS S enantiomerically pure