Intramolekulare Inversionssubstitution am Dreiring von 77 exo ‐Brombicyclo[4.1.0]heptan‐3 endo ‐ol unter Bildung eines Tetrahydrofuranringes

Intramolecular Substitution under Inversion at the Threemembered Ring of 7 exo ‐Bromobicyclo[4.1.0]heptan‐3 endo ‐ol yielding a Tetrahydrofuran Ring The reaction 1a → 2a involving substitution at a cyclopropane carbon atom can be observed only with the bromophilic alkyllithium reagents but not with the bases lithium diisopropyl amide (LDA) (Table 1) or potassium t ‐butoxide (KTB). The mechanism must be an insertion as outlined in Scheme 1 . ‐ The monobromides 1b , 1c and 1d are prepared stereoselectively from the acetal 3a . Again, cyclization of 1b takes only place with LDA in the presence of alkyllithium (Table 2 , entries 1‐‐4) suggesting an insertion mechanism (route (a) or (b) in Scheme 2 ). In contrast, KTB effects the substitution in high yield with no loss (from 1c ) or incorporation of deuterium at the cyclopropane substitution center ( Table 2 , entries 5‐‐7); the possibility is discussed that this process is an S N 2‐type reaction.