Diastereotopic, definition

These results clearly indicate that barriers to all isomerisation processes are at least less than about 8kcalmol 1. In /V-benzyloxy-7V-chlorobenzamide 44 the amide isomerisation was not observable but the anomeric overlap resulted in diastereotopic benzylic hydrogens, which at coalescence afforded a barrier for rotation about the N-OBn bond of around 10.3 kcalmol-1.32 Like its /V-chloro analogue, the amide isomerisation barrier in 43 is too low to be observed by 3H NMR and even though there is definitive X-ray and theoretical evidence for anomeric effects in /V-acyloxy-/Y-alkoxyamidcs, the barrier to isomerisation about the N-OBn bond must be lower than 10.3 kcalmol-1. The n0-CN ci anomeric interaction in 44 is predicted to be stronger than the n0-CN OAc interaction in 43 on purturbation arguments.32... 

The general subject of asymmetric synthesis has been reviewed extensively (1-5). The term asymmetric synthesis has been defined in more than one way (1,4) however, a useful definition is the one given by Morrison and Mosher (1) a process which converts a prochiral unit [refs. 6 and 7] into a chiral unit so that unequal amounts of stereoisomeric products result. The stereoisomeric products may be enantiomeric or they may be diastereomeric. The substrate molecule must contain either enantiotopic or diastereotopic groups or faces (8,9), since the attack of a reagent at equivalent groups or faces cannot lead to isomeric products. 

In retrospect, it seems unfortunate that in 1971 Morrison and Mosher8 generalized the definition, while keeping the term, an asymmetric synthesis is a reaction in which an achiral unit in an ensemble of substrate molecules is converted by a reactant into a chiral unit in such a manner that the stereoisomeric products arc produced in unequal amounts ( Footnote The substrate molecule must have either enantiotopic or diastereotopic groups or faces) . Obviously the phrase "an achiral unit in an ensemble of substrate molecules is too inexact and requires a great deal of additional explanation, which was partially given by the footnote (note that molecule, i.e., singular, was used ). Currently, the Morrison-Mosher term appears to be equivalent to stereoselective reaction. Unfortunately, this term was only defined in the modem sense by Izumi in 1971, i.e., in the same year the Morrison-Mosher definition was published. 

Now, does this ring any bells It should remind you very much of the definitions in Chapter 32 of H enantiotopic and diastereotopic in connection with NMR spectra. Replacing one of two enantiotopic Enantiotopic and diastereotopic groups with another group leads to one of two enantiomers replacing one of two diastereotopic chapter 32 pr<837 dl9CU9sed in... 

Compounds 6a, 7a, and 8a were isolated as viscous, colorless liquids. These compounds can be identified with NMR and GC-MS spectroscopy. In some cases unwanted and yield reducing side reactions like oligomerization or monoalkylation of the dichlorosilanes occur. Side products are not detected by NMR specroscopy, but GC-MS studies definitely show peaks at higher masses. In the H NMR spectra of 6a and 7a well-separated signals can be observed for the diastereotopic methylene protons at room temperature. Obviously the ring inversion process is slowed down if bulky substituents are used. 

For definitions of homotopicity, enantiotopicity and diastereotopicity see Mislow and Raban (1967) and Jennings (1975). 

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