Synthesis of adamantanoid dienones: Decarboxylation studies in 2.7-dioxabicyclo[2,2,1] systems.

The synthesis of two adamantanoid dienones (3) and (4) has been achieved from the phenolic iodide (l) via an Ar,-6 participation involving desaromatisation. The phenolic iodide was synthesised as a mixture of stereoisomers and on heating in a sealed tube with potassium t-butoxide in t-butanol two different dienones were formed depending on the concentration of base used: using 2.5 equivalents, the phenolic dienone (2) was isolated, but using 1.0 equivalents the adamantanoid dienone (3) was obtained. The adamantanoid bis-dienone (4) was formed by dehydrogenation of (3).Ultimately the adamantanoid skeleton for (3) was assigned by a consideration of the mechanism involved (scheme 1), but considerable evidence supports this assignment. [diagram included] Part 2 Endo- and exo-6-isobutyl-1, 4-dimethyl-2 .7-dioxabicvclo- [2, 2, l] heptane-6-carboxylic acids (1) (2) have been synthesised and identified from the chemical shifts of the 5-endo and 'W' coupled 5-exo-iorotons. Decarboxylation of the endo-acid (1) was found to be considerably faster than the exo-acid (2), and both decarboxylated faster than a model monocyclic acid (3). Product studies showed that (1) decarboxylated to initially form the dihydrofuran (6), whereas the exo-acid initially formed a product formulated as the dihydropyran (7). Both (6) and (7) isomerise to yield a similar mixture of the bicyclic acetals (8). An explanation for the faster rate of decarboxylation of (1) over (2) is that decarboxylation takes place from the zwitterions (4) and (5) (scheme I). The less favourable geometry for proton transfer and expected lower basicity of the 7-oxygen would result in a lower concentration of (5) relative to (4). Also any incipient carbonium ion character at position 1 would be stabilized better by the 7-oxygen in (4) than by the 2-oxygen in (5), but the exact timing of bond breaking and making in these decarboxylative eliminations is unknown. [diagram included]