Die Geometrie des aktivierten Komplexes der thermischen und ladungsinduzierten aromatischen para → ortho - Claisen-Umlagerung

Pure 10β-(trans-2′-butenyl)-17β-hydroxy-estra-1,4-dien-3-one ( 6 ), 10-(trans-2′-butenyl)-2-oxo-Δ^1(9),3(4)-hexahydronaphthalene ( 13 ), trans-2′-butenyl 17β-hydroxy-3-estra-1,3,5-(10)-trienyl ether ( 12 ) and trans-2′-butenyl 5,6,7,8-tetrahydro-2-naphthyl ether ( 14 ) were prepared by direct C- and O-alkylation, respectively, of the corresponding phenols (cf. 3] 10]), namely estra-3, 17β-diol and 5,6,7,8-tetrahydro-2-naphthol. The Claisen rearrangement of the ether 14 (200°, 12 h) yielded 53% 1-(1′-methylallyl)- and 34% 3-(1′-methylallyl)-5,6,7,8-tetrahydro-2-naphthol ( 15 and 16 , respectively), whereas in the thermal (120°, 14 h) and in the acid-catalysed (boron trifluoride in ether, 20°, 20 min) reaction of the corresponding dienone 13 nearly equal amounts of 15 (53–54%) and 16 (46%) were formed by thermal and charge-induced aromatic 3s, 3s]-sigmatropic rearrangements 2]. The steroid dienone 6 , on heating at 120°, was rearranged by 3s, 3s]-sigmatropic processes to form 52% of 2-(1′-methylallyl)- and 48% of 4′-(1′-methylallyl)-3, 17β-dihydroxy-estra-1,3,5,(10)-triene ( 7 and 8 , respectively). The steroid phenols 7 and 8 were carefully separated; subsequent hydrogenation (Raney-Ni in alcohol) and ozonolysis yielded 2-methylbutyric acid ( 9 ): from 7 , S-(+)- 9 , and from 8 , R-(?)- 9 , obtained in 88,5 and 88,0% optical purity (cf. 4a]). This means (cf. scheme 2 and Table 2) that both phenols are formed to the extent of at least 94% via a chair-like activated complex, and of at most 6% via a both-like activated complex (ΔΔG = 2.15 kcal/mol). Similarly, the boron trifluoride-induced rearrangement of 6 (born trifluoride in ether, 0°, 45 min) yielded 7 and 8 , from which S-(+)- 9 and R-(?)- 9 were respectively obtained in 89% and 98% optical purity. For these induced rearrangements this corresponds to at least 94,5 and 99%, respectively, of the chair-like, and to only 5.5 and 1% of the boat-like activated complex (ΔΔG = 1.5–2.5 kcal/mol). These results demonstrate that the activated complexes of both 3s,3s]-sigmatropic processes, i.e. the pure thermal reaction at 120° and the charge-induced reaction occurring at 0°, must be very similar. Thus, the boron trifluoride accelerates the Cope-like reactions 6 → 7 + 8 , but does not influence the geometries of their transition states. The Claisen rearrangement of the steroid ether 12 (200°, 15 h), yielding 7 and 8 , was not influenced by the chiral steroid skeleton, because no optical induction was observed (both phenols, 7 and 8 , yielded on degradation racemic 2-methylbutyric acid (9)).