Derek Whybrow

Total synthesis of the macrocyclic diterpene (–)-casbene, the putative biogenetic precursor of lathyrane, tigliane, ingenane, and related terpenoid structures

A total synthesis of the [12.1.0]bicyclic diterpene 1S,3R(–)-casbene (19), based on elaboration of 1R,3S(+)-cis-chrystanthemic acid (21) to the bis-allylic bromide (20), followed by closure of the 14-membered ring with tetracarbonylnickel, is described.Synthetic casbene did not separate from natural casbene, isolated from Ricinus communis, in mixed chromatographic analysis, and their 1H n.m.r, and mass spectral data were closely similar. 13C N.m.r. data support the casbene structure (19), having a 1S,3R-cis-cyclopropane ring, and a 4,8,12-all-E-triene system.

Photochemical synthesis of the carbon skeleton found in taylorione from mylia taylorii

Abstract Irradiation of the cyclopentenone(15) resulted in efficient regiospecific di-π-methane rearrangement (> 70%) to the vinylcyclopropane(18), which was then converted into trans -desoxytaylorione(19).

Synthetic photochemistry. A synthesis of the carbon skeleton found in taylorione from Mylia taylorii, using the di-π-methane rearrangement

Synthetic routes to the E- and Z-cyclopentenones (20) and (25), which incorporate photosensitive 1,4-diene systems, are described. Irradiation of the E-cyclopentenone (20) is shown to result in efficient regiospecific di-π-methane rearrangement (>70%) to the vinylcyclopropane (26), which is then converted into trans-deoxytaylorione (28). Irradiation of the corresponding Z-cyclopentenone (25) also leads to (26), presumably by way of the E-cyclopentenone (20), produced by Z—E photoisomerisation.

Thermal rearrangements of 2-cyclopropylcycloalk-2-enones

Thermolysis of 2-(2,2-dimethylcyclopropyl)cyclopent-2-enone (2) and 2-cyclopropylcyclohex-2-enone (6) is shown to produce 2-(3-methylbut-2-enylidene)cyclopentanone (10) and 2-propylphenol (14), respectively, via homo[1,5]sigmatropic H-migration and isomerisation. Heating 2-cyclopropylcyclopent-2-enone (8) under a variety of conditions led only to decomposition; no products [e.g.(11)] resulting from vinylcyclopropane–cyclopentene rearrangement were detected.