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Isomerisierungen und Umlagerungen in Bicyclischen Systemen via cyclopropan‐carbaldehyd‐enamine

1974; Wiley; Volume: 57; Issue: 3 Linguagem: Inglês

10.1002/hlca.19740570327

1522-2675

Max Rey, Aǹdré S. Dreiding,

Inorganic and Organometallic Chemistry

Abstract The pyrrolidino‐aminal ( 4 ) of bicyclo[3.1.0]hex‐2‐ene‐6‐ endo ‐carbaldehyde ( 3 ) underwent a facile (80°), mildly acid catalyzed isomerization to the corresponding exo ‐aminal ( 6 ), which was characterized by hydrolysis to bicyclo[3.1.0]hex‐2‐ene‐6‐ exo ‐carbaldehyde ( 7 ). At higher temperatures (140°), the two aminals 4 and 6 were converted smoothly to a 1:1 mixture of syn ‐ and anti ‐4‐(pyrrolidino‐methylidene)‐bicyclo[3.1.0]hex‐2‐ene ( syn ‐ and anti ‐7‐pyrrolidino‐homofulvene 1 ) 8 and 9 . The structures of 8 and 9 were derived from spectral data. This corrects a previous interpretation by Cook et al. The endo → exo ‐aminal isomerization ( 4 → 6 ) is considered to occur via the enamine ( 5 ) of bicyclo[3.1.0]hex‐2‐ene‐6‐carbaldehyde ( 3 or 7 ), with which the aminals ( 4 and 6 ) are in equilibrium. The same enamine ( 5 ), a methylidene cyclopropane derivative, is thought to be the intermediate in the aminal ( 4 or 6 ) → amino‐homofulvene ( 8 and 9 ) conversion, which, therefore, belongs to the vinyl‐methylidene‐cyclopropane rearrangement type. A cationic mechanism for the endo → exo ‐aminal isomerization is excluded by the discrepancy in this reaction of the pyrrolidino‐aminals ( 13 and 15 ) of 6‐ exo ‐methyl‐bicyclo[3.1.0]hex‐2‐ene‐6‐ endo ‐carbaldehyde ( 12 ) and of bicyclo[3.1.0]hexane‐6‐ endo ‐carbaldehyde ( 14 ). While the former aminal ( 13 ) is stable even under acid catalysis and at higher temperatures, the latter ( 15 ) isomerizes readily to the exo ‐aminal 16 . The three endo ‐aldehydes 3, 12 and 14 were prepared by the alkali catalyzed rearrangement of the three chlorohydrins 7‐ endo ‐chloro‐bicyclo[3.2.0]hept‐2‐en‐6‐ endo ‐ol ( 20ac ), 7‐ endo ‐chloro‐7‐ exo ‐methyl‐bicyclo[3.2.0]hept‐2‐en‐6‐ endo ‐ol ( 20ad ) and 7‐ endo ‐chloro‐bicyclo[3.2.0]heptan‐6‐ endo ‐ol ( 20bc ) according to the method of Brook . The configurations of the unsaturated endo ‐aldehydes 3 and 12 followed from their equilibria with the corresponding 2‐oxa‐bicyclo[3.2.1]octa‐3,6‐diene systems ( 23 ) and those of the saturated aldehydes 14 and 17 from oxidations to the corresponding carboxylic acids. The endo ‐ and exo ‐ isomers of the aldehydes and aminals treated in this work were readily distinguished by certain highly characteristic NMR.‐signals.