Biosynthesis of porphyrins and related macrocycles. Part 45. Determination by a novel X-ray method of the absolute configuration of the spiro lactam which inhibits uroporphyrinogen III synthase (cosynthetase)

A novel approach, involving X-ray analysis of a specifically designed racemate, in combination with correlations by circular dichroism, allows the absolute configuration of the spiro lactam 4 to be determined. The outcome is that the enantiomer of this lactam which strongly inhibits uroporphyrinogen III synthase (cosynthetase) has the R-configuration and the implications of this finding are briefly discussed.