Preparation and C-Alkylation of Enantiomerically Pure S-Phenyl Aziridinecarbothioates. On the Structure of Small-Ring Ester Lithium Enolates

Racemic and enantiomerically pure methyl N-(tert-butyl)-N-benzyl- and N-1-(phenylethyl)aziridinecarboxylates are prepared by known methods and converted to phenyl thioesters ( 1 , 2 , 15 , 16 ; Schemes 2 and 3). These are deprotonated with lithium diisopropylamide (LDA) and BuLi (for removal of diisopropylamine) in THF at dry-ice temperature. The resulting lithiated species are surprisingly stable and are deuterated, alkylated (CH₃, C₂H₅, allyl, benzyl), and added to aldehydes and nitroolefins in good yields (50–80 %, 18 examples; Schemes 1 and 4–6). The configurational stability of the lithiated species is studied, and conclusions about their structures are drawn. Thus, a C(α)-lithiated ester (see L , Scheme 9) or an O-lithiated ‘enolate’ (see M ) with pyramidalized C(β)-atom is proposed for the species from levorotatory S-phenyl N-benzylaziridinecarbothioate which does not undergo racemization after 1 h at ?60° (THF solution).