A Synthetic Approach to the Isobacteriochlorin Macrocycle

✍ Scribed by Dr. Franz-Peter Montforts; dipl.-Chem. Silvio Ofner; dipl.-Naturwiss. Vittorio Rasetti; Prof. Dr. Albert Eschenmoser; Dr. Wolf-Dietrich Woggon; Keith Jones; Prof. Dr. Alan R. Battersby

• Publisher: John Wiley and Sons
• Year: 1979
• Tongue: English
• Weight: 364 KB
• Volume: 18
• Category: Article
• ISSN: 0044-8249
• DOI: 10.1002/anie.197906751

✦ Synopsis

clo[6.1 .O]nonatriene has been interpreted by a participation of an aromatic transition state. The ca. 7 kcal/mol lower Gibbs activation energy of ( l a ) -(24 would accordingly suggest an aromatic transition state for the bicyclopentene rearrangement. We assume, however, that the lower activation barrier of the bicyclopentene rearrangement does not only result from the resonance stabilization of the transition state, but largely from the high ground-state enthalpy of the bicyclopentene system. The heat of hydrogenation of the olefink double bond in the parent compound, bicy-cl0[2.1 .O]pent-2-ene (25 "C: A H = 42.5 kcal/mol) is higher by ca. 9 kcal/mol than that of the double bond in bicyclo[2.2.0]hex-2-ene and bicycl0[3.2.0]hept-6-ene~~~. The additional energy content of 9 kcal/mol is attributed to an antiaromatic destabilization of the bicyclo[2.1.0]pent-2-ene ground state and must be considered for estimating the resonance stabilization of the transition state of the "walk" rearrangement (la) -+ (2a).