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Chapter 19 Suggested solutions for Chapter 19 147 second example already has the electron-rich alkene present in the starting material so this is of an E1 than an reaction. The starting material is an enol ether and the intermediate ion hydrolyses to the ketone by an extension of acetal hydrolysis (see Chapter 14). The has two chiral centres unaffected by the reaction and all intermediates are chiral so this product also is a single enantiomer. + CO2H H MeO + MeO MeO MeO H Problem 9 Explain the stereochemistry of the double bonds in the products of these reactions. OMe 0 acid OMe Ph pyridine H Ph Ho Ph of the problem your skill in a deceptive example of control of alkene geometry by elimination. Suggested solution The first reaction is a stereospecific cis addition of hydrogen to the alkyne to give a cis-alkene. The mermediate is therefore a cis, cis-diene and it seems remarkable that it should turn into a trans, on elimination. However, when we draw the mechanism for the elimination we see that need be no relationship between the stereochemistry of the two compounds as this is an E1 Even after conversion to the and the stable and therefore long-lived intermediate cation can rotate into the most stable dienal, reversible conjugate addition of water would allow cis/trans before conversion to the ketone. interconversion. OMe OMe H Ho H Ho Ph Ph Ph hemiacetal Ph Ph + Problem 10 Why is elimination preferred to hemiacetal formation in the acid-catalysed cyclization of this ketone? 0 0 0 Ph H oH Ph Purpose of the problem Exploring what else might happen on the way to an elimination.