A Flexible Stereoselective Synthesis of the Spirosesquiterpenes (±)-β-Acorenol, (±)-β-Acoradiene, (±)-Acorenone-B and (±)-Acorenone via an Intramolecular Ene-Reaction
✍ Scribed by Wolfgang Oppolzer; Kumar K. Mahalanabis; Kurt Bättig
- Publisher
- John Wiley and Sons
- Year
- 1977
- Tongue
- German
- Weight
- 929 KB
- Volume
- 60
- Category
- Article
- ISSN
- 0018-019X
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✦ Synopsis
Abstract
The racemic spirosesquiterpenes β‐acorenol (1), β‐acoradiene (2), acorenone‐B (3) and acorenone (4) (Scheme 2) have been synthesized in a simple, flexible and highly stereoselective manner from the ester 5. The key step (Schemes 3 and 4), an intramolecular thermal ene reaction of the 1,6‐diene 6, proceeded with 100% endo‐selectivity to give the separable and interconvertible epimers 7a and 7b. Transformation of the ‘trans’‐ester 7a to (±)‐1 and (±)‐2 via the enone 9 (Scheme 5) involved either a thermal retro‐ene reaction 10 → 12 or, alternatively, an acid‐catalysed elimination 11 → 13 + 14 followed by conversion to the 2‐propanols 16 and 17 and their reduction with sodium in ammonia into 1 which was then dehydrated to 2. The conversion of the ‘cis’‐ester 7b to either 3 (Scheme 6) or 4 (Scheme 7) was accomplished by transforming firstly the carbethoxy group to an isopropyl group via 7b → 18 → 19 → 20, oxidation of 20 to 21, then alkylative 1,2‐enone transposition 21 → 22 → 23 → 3. By regioselective hydroboration and oxidation, the same precursor 20 gave a single ketone 25 which was subjected to the regioselective sulfenylation‐alkylation‐desulfenylation sequence 25 → 26 → 27 → 4.