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A Flexible Stereoselective Synthesis of the Spirosesquiterpenes (±)‐β‐Acorenol, (±)‐β‐Acoradiene, (±)‐Acorenone‐B and (±)‐Acorenone via an Intramolecular Ene‐Reaction
Author(s) -
Oppolzer Wolfgang,
Mahalanabis Kumar K.,
Bättig Kurt
Publication year - 1977
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19770600727
Subject(s) - chemistry , regioselectivity , stereoselectivity , ene reaction , enone , intramolecular force , ketone , stereochemistry , epimer , organic chemistry , catalysis
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 .