Here you will find some interesting examples of how the stereochemistry of a product may be maintained during a reaction, without racemization. To activate the "hyperactive molecules", you will have to specify a helper for the chemical/x-xyz MIME type (e.g. XMol or RasMol).
Above is the molecule trans-1,3-dithiane-1,3-dioxide. Due to the unusual nature of the symmetry of this molecule it can be reacted with an aromatic aldehyde, in this case PhCHO, giving, in a ratio of 95:5, one of the possible diastereoisomers-
The above can be easily converted to the thiolester without racemization -
Since thiolesters are readily converted into many other fundamental organic molecules, such as amines, acids, and esters, the above technique has implications for synthesizing these with high diastereoselectivity.(V.K.Aggarwal, A.Thomas, R.J.Franklin, J. Org. Chem. Chem. Commun., 1994, 14, 1653.)