Stereochemical relationship, control starting material

The synthesis in Scheme 13.34 also begins with carbohydrate-derived starting material and also uses catalytic hydrogenation to establish the stereochemical relationship between the C-4 and C-6 methyl groups. As was the case in Scheme 13.33, the configuration at C-2 is not controlled in this synthesis, and separation of the diastereomeric products was necessary. 

Most of the compounds we shall be looking at in this chapter will be in racemic form. We are concerned only with the control of relative stereochemistry and not with the control of absolute stereochemistry. However, many of the reactions have been developed into asymmetric versions. It is certainly true that many of the reactions have been employed within asymmetric synthesis - that is, where the asymmetric part has come from elsewhere and this idea will be revisited in Chapter 30. If we are to concern ourselves simply with relative stereochemistry then, for there to be any stereochemical relationship, we must have at least two chiral centres. If there is no chirality in the starting materials this means that two chiral centres must form in one reaction and if there is only one new chiral centre that forms in the reaction then there must have been a chiral centre already in one of the starting materials. 

---