Synthese einer Calicen-Vorstufe für Retro-Diels-Alder-Reaktionen

Synthesis of a Calicene Precursor for Retro-Diels-Alder Reactions In view of retro-Diels-Alder reactions (RDA reactions), the calicene precursor 9 has been synthesized in a comparably simple four-step synthesis by dibromocarbene addition at dibenzobarrelene ( 10 → 11 , 44%), halogenlithium exchange followed by reaction with cyclopentenone ( 11→12 , 91%) and H₂O as well as HBr elimination ( 12→14→9 , 43%) (Scheme 5). First experiments with respect to the thermal behavior of 9 show that, although RDA reaction seems to be relatively easily occurring according to the results of ‘Curie-Point’ pyrolysis, only anthracene and no calicene 2 has been detected so far.     

Synthese von Triafulven-Vorstufen aus trisubstituierten Cyclopropanen

Synthesis of Triafulvene-Precursors from Trisubstituted Cyclopropanes Trisubstituted cyclopropanes 5a–f are prepared by carbene addition to the appropriate olefins. While 5a (Y = OAc) and 5c (Y = Cl) rearrange in the presence of BuLi, 5d (Y = SPh) is stable enough to allow the envisaged sequence for triafulvene (Scheme 2) : halogen-Li exchange, followed by methylation of 6d , gives 7d in a 93% yield; after base-induced elimination of HB r from 7d , the key precursor 1-methylene-2-(phenylthio)cyclopropane ( 9 , 70% yield) is isolated. Compound 9 is transformed into the corresponding sulfoxide 10 (83%), sulfone 11 (80%), and sulfonium fluoroborate 12 (95% yield) by subsequent oxidation and methylation, respectively. Some ¹H-NMR results of cyclopropanes 5a–f and 7d as well as of methylidene-cyclopropanes 9–11 are discussed.