Barbiturates stereochemistry

There is an admirable summary of the stereochemistry of barbiturates and di- to hexahydropyrimidines Further information on reduced pyrimidines is collected <70HC 16-81)322) and some examples of the use of proton NMR spectra in elucidating the conformations of hydropyrimidines is given elsewhere (Section 2.13.1.3.1), based on the general principles of such work <65QR426). 

Some interesting facets of stereochemistry on barbiturate effects have come to light. Depressant activity generally resides more in the S(-) form, while the R(+) isomers have a predominantly excitatory effect. In one example the R(+) as well as the RS (racemic) forms actually were convulsant. 

Figure 6-16. In this match between two polymorphic barbiturates, a small discrepancy in stereochemistry in the iso-butyl chain is minor but, apparently, significant. That AMYTAL 11 crystalhzed with two molecules in the asymmetric unit is significant for crystalline behavior. This is molecule 1. AMYTALIO Ref. same for both [25]...

---