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 (CH 3 , C 2 H 5 , 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).