Umwandlung von 6‐Methyliden‐tricyclo[3.2.1.02,7]oct‐3‐en‐8‐onen in Norcaradien/Cycloheptatrien‐Derivate

Abstract

Transformation of 6‐Methyliden‐tricyclo[3.2.1.02,7]oct‐3‐en‐8‐ones into Norcaradiene/Cycloheptatriene‐Derivatives.Tricyclic ketones of type 1,5‐dimethyl‐6‐methyliden‐tricyclo[3.2.1.02,7]oct‐3‐en‐8‐one (1) [1] rearrange on heating with sodium methylate mainly to methyl‐7‐methyl‐bicyclo[4.1.0]hepta‐2,4‐dien‐7‐carboxylates = methyl‐7‐methyl‐norcaradien‐7‐carboxylates (4). Moreover, the formation of methyl 2‐aryl‐propionate (5) is observed. Thus, tricyclic 1 gives a mixture of norcaradiene derivatives 2 and 4 in 50% yield together with 13% of methyl 2‐(2′, 3′‐dimethylphenyl)‐propionate (5) (Scheme 1). Similarly the tricyclic ketone 7 rearranges to norcaradienes 8 and 9 (31,5%) and methyl 2‐(2′, 3′, 5′‐trimethylphenyl)‐propionate (10, 4%). In this case, the reduction products of 7, i.e. the alcohols 11 (24%) and 12 (8%) as well as other products derived from 11 are observed; heating of the endo‐alcohol 11 with sodium methylate leads to the 2‐arylpropan‐1‐ols 13 and 14 (Scheme 2). Under the same conditions the ketone 18 affords the norcaradiene ester 20 (Scheme 3). Scheme 4 shows the rearrangement of the pentacyclic ketone 21 to the cycloheptatriene derivative 22 and the base catalysed isomerisation of 22 to 23.The structure elucidations were achieved with the help of UV.‐, IR.‐ and mainly NMR. spectra. The carboxy or methoxycarbonyl group assumes the exo‐position in all the norcaradiene derivatives (NMR., also in the presence of NMR. shift reagents). The cycloheptatriene‐noncaradiene‐equilibrium is shifted to the norcaradiene side to > 95% in compounds 2, 4, 8 and 9 and to > 90% in 19 and 20. This is due to the 7‐exo‐carboxy or 7‐exo‐methoxycarbonyl group and the methyl groups in positions 2 and 3 (cf. Klärner [9], chapter 4). On the other hand, in the case of 22 the cycloheptatriene structure is almost exclusively predominant.The most probable mechanism for the rearrangement of tricyclic ketones of type 1 to methylnorcaradien‐7‐exo‐carboxylates or to the corresponding carboxylic acids is depicted in Scheme 5. Thus, the reaction path leads from 1 through a, c, f, g and h to 4. The aromatization reaction of 1 to 5 and 3 proceeds preferentially through a, c (Scheme 5) and k (Scheme 6). The conversation of the tricyclic endo‐alcohol 11 to 2‐aryl‐propan‐1‐ols 13 and 14 using sodium methylate occurs to ca. 90% via m → n and ca. 10% via m → p (Scheme 7); the primarily formed 2‐aryl‐propanals are reduced under the reaction conditions. The exo‐isomer 12 shows no similar reaction.