Thermal activation of the normal Claisen products, the enones 4, 7 and 14 in the presence of a catalytic amount of propionic acid generated the isomeric enones 6, 9 and 15 via the sequential intramolecular ene-reaction of the enol tautomer followed by 1,5-hydrogen transfer (or retro ene-reaction) of the resultant acetyl cyclopropane intermediate. Conversion of the enones 9 and 15 into the corresponding
Thermal activation of γ,δ-unsaturated ketones (1, 9 and 12) in the presence of a catalytic amount of propionic acid causes a rearrangement to give new γ,δ-unsaturated ketones (2, 10 and 14)via an intramolecular ene reaction followed by a retro-ene reaction.
Stereospecific construction of multiple contiguous quaternary carbons. Total synthesis of (±)-cis,anti,cis-1,8,12,12-tetramethyl-4-oxatricyclo[6.4.0.02,6]dodecan-3-ol, a thapsane isolated from Thapsia villosa var minor
作者:A. Srikrishna、K. Krishnan
DOI:10.1021/jo00079a020
日期:1993.12
The details of the first total synthesis of a natural thapsane 1g containing three contiguous quaternary carbon atoms, starting from cyclogeraniol (9) is described. The Claisen rearrangement of 9 with methoxypropene in the presence of acatalytic amount of propionic acid produced ketone 10. Rhodium acetate-catalyzed intramolecular cyclopropanation of alpha-diazo-beta-keto ester 12, obtained from 10 via beta-keto ester 8, furnished cyclopropyl keto ester 7. Lithium in liquid ammonia reductive cleavage of cyclopropyl compound 7 gave a 1:1 mixture of hydrindanone 6 and ketol 13. Wittig methylenation of 6 furnished ester 21. Epoxidation of 21, followed by BF3-OEt2-catalyzed rearrangement of epoxide 23 afforded hemiacetal 25. Treatment of hemiacetal 25 with triethylsilane in trifluoroacetic acid furnished lactone 22, a degradation product of various thapsanes. Finally, DIBAH reduction of lactone 22 generated the thapsane 1g.