CHAPTER 5: Question 7

Simple Answers

Under the reaction conditions, the C-H_a bond can break and reform, providing a mechanism for the inversion of the stereochemistry at this stereocentre. However, compound A contains a second stereocentre, so the two stereoisomers formed at the stereocentre adjacent to H_a will be diastereomers rather than enantiomers; ie. this is an epimerization. One of these two diastereomers is insoluble in petrol and so precipitates from the solution. Once it has precipitated, epimerization can no longer occur. Eventually, in an ideal case, all of compound A would be converted into the least soluble diastereomer and precipitate from the reaction mixture. In this particular case, a 73% yield is obtained. Note that the diastereomer which is isolated need not be the most stable diastereomer, just the least soluble in petrol.

This example is taken from: H.T. Stock, and N.J. Turner; Tetrahedron Lett., 1996, 37, 6575-8.

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