Abstract
The total synthesis of both enantiomers of copalol ( 6) was accomplished via the optical resolution of a racemic diol [(±)- 4] which is a general synthetic intermediate for drimane sesquiterpenes and labdane diterpenes. Esterification between (±)- 4 and Boc- l-proline gave the diastereomeric pair of monoesters ( 5a and 5b) which could be readily separated by flash column chromatography. PDC-oxidation of the resolved 5a and 5b, and subsequent β-elimination gave optically active enones ( 9 and ent-9 ). Both enones were respectively converted into (+)- 6 in 36% yield and (−)- 6 in 26% yield in five steps: (1) Sakurai reaction (TiCl 4-promoted conjugate addition of allylsilane), (2) Wittig methylenation, (3) Wacker oxidation, (4) Horner–Emmons reaction, and (5) DIBAL-H reduction.
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