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Transannular Ring Expansion in the Acid‐Catalyzed Reaction of the Oxirane Derived from Spirocyclopropane‐Substituted Bicyclo[2.2.2]oct‐2‐ene
Author(s) -
Adam Waldemar,
Crämer Elisabeth,
Peters EvaMaria,
Peters Karl,
Schnering Hans Georg Von
Publication year - 1987
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19871200507
Subject(s) - chemistry , bicyclic molecule , substituent , trifluoroacetic acid , cycloaddition , cyclopropane , stereochemistry , ring (chemistry) , epoxide , medicinal chemistry , adduct , ene reaction , catalysis , organic chemistry
Spiro[bicyclo[2.2.2]oct‐5‐ene‐2,1′‐cyclopropane] ( 1 ), obtained by [4+2] cycloaddition of 1,3‐cyclohexadiene to methylenecyclopropane, was epoxidized to give the exo and endo epoxides 2 . The exo epoxide 2 gave with trifluoroacetic acid the 7,9‐disubstituted homobrendane 7b (main product) and the 2,6‐disubstituted isotwistane 7a (minor product) via transannular ring expansion of the spirocyclopropane substituent with and without skeletal rearrangement of the bicyclic ring system, respectively. No 1,2‐ trans adducts were observed, as is the case in the reaction of the spiroalkene 1 with arenesulfenyl chloride, which in turn gave no ring‐expanded products. The fact that an isomeric mixture of all four possible 1,2‐ trans adducts 4 were obtained, proves that the spirocyclopropane substituent promotes neither regio‐ nor stereoselectivity. In contrast, bicyclo[2.2.2]oct‐2‐en‐5‐one proceeded with arenesulfenyl chloride regio‐ and stereoselectively, leading to ( exo ‐5, endo ‐6)‐6‐chloro‐5‐( p ‐tolylthio)bicyclo[2.2.2]octan‐2‐one ( 5a ). The stereochemistry of the latter was established by means of X‐ray analysis of its sulfoxide derivative 6a .