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Chapter 35 Suggested solutions for Chapter 35 307 Alternative syntheses would include a direct Diels-Alder reaction with an alkyne, but that would This is a general synthesis of give the wrong regioisomer, or the Birch reduction of a m-methoxybenzoate ester but that metasubstituted aromatic compounds by S. Danishefsky and too would have given the wrong regioselectivity. group, Am. Chem. Soc.. 1978, 2918. OMe OMe OMe CO₂Me CO₂Me CO₂Me OMe OMe OMe Na, t-BuOH Na. t-BuOH CO₂Me CO₂Me CO₂Me Problem 11 Give mechanisms for these reactions and explain the regio- and stereochemical control (or the lack of it!). H H Ph 0 Ph 1. Zn, HOAc N N 0 H 2. MnO₂ N 0 mixture of (oxidizes to ketones) Ph Purpose of the problem Selectivity and application of 1,3 dipolar cycloadditions. Suggested solution The first thing to do is to sort out the mechanism for the cycloaddition. The nitrone uses its LUMO The dipolar cycloaddition was the of the C=N bond) to react with the HOMO of the diene whose largest coefficient is at the developed by J. J. and R. G. Gatrone, Tetrahedron Lett.. and away from the phenyl group. There is no stereoselectivity as there is no conjugating group and 2753. no selection. H Ph Ph N N 0 0 H Reduction with zinc cleaves the N-O bond and MnO₂ oxidizes the allylic alcohol to the enone. At this point there is only one chiral centre SO the mixture of diastereoisomers has become one compound. Conjugate addition of the amine gives the new ring. H Ph Ph 1. Zn, HOAc 2. N NH OH 0 H 0 0 0 NH N N H Ph Ph Ph The stereochemistry is more difficult to explain. The product has a trans ring junction from choice but has the Ph group axial - obviously not from choice. This must be the kinetic product. It