Synthesis of New Chiral Derivatives of N , N ′‐Dimethylpropyleneurea (DMPU) and Examination of Their Influence on the Regio‐ and Enantioselectivity of Addition of 2‐(1,3‐Dithianyl)lithium to Cyclohex‐2‐en‐1‐one

The preparation of three new chiral derivatives of DMPU ( N , N ′‐dimethylpropyleneurea) is described ( Schemes 2 – 4 ); one type of derivative carries 1‐phenylethyl or 1‐cyclohexylethyl groups at the N‐atoms of the tetrahydropyrimidin‐2(1 H )‐one ring ( 2 and 4 ), another type of derivative is substituted at C(4) and C(6) of the heterocyclic ring ( 7 ). The potential of these chiral Lewis bases as promoters in the regio‐ and/or enantioselective addition of 2‐(1,3‐dithianyl)lithium to cyclohex‐2‐en‐1‐one was explored; they are all unable to effect enantioselective addition; the derivatives with branched substituents at the N‐atoms do not shift the addition mode from 1,2 to 1,4, while the 3,4,5,6‐tetrahydro‐1,3,4,6‐tetramethylpyrimidin‐2(1 H )‐one does ( Scheme 5 ). The results provide useful information regarding the nature of the nucleophilic organolithium reagent: obviously, the steric hindrance to Li complexation on the CO O‐atom of the tetrahydropyrimidin‐2(1 H )‐one by branched substituents at N‐atoms ( cf. X‐ray crystal structure of 2 in the Fig. ) prevents solvent‐separated‐ion‐pair (SSIP) formation; this was confirmed by PM3 and B3LYP/3‐21‐G(d)//PM3 calculations ( Scheme 6 ).