Silphinene total synthesis

Scheme 13.1 New reactions and step economy a three-step total synthesis of silphinene.

Ito s regio- and stereoselective total synthesis relied on dicyclopentadiene as starting material The derived ketone 755 was cleaved to provide 756 which was crafted into 757 (Scheme LXXXV). FoUowing reduction of the carbomethoxy group, conversion to iodo ketone 758 was realked with trimethylsilyl iodide. Reaction of 758 with DBU resulted chiefly in conversion to 752 which was transformed into silphinene essentially as described above. 

These intramolecular meta-addition processes were utilized in the key steps for the total synthesis of a-cedrene [245], isocomene [246], hirsutene [247], coriolin [248], silphinene [249], rudmollin [250], laurenene [251], and fenestranes [252-254], which were synthesized by Wender s and Keese s groups (Scheme 58). 

Similar methodology was used in a total synthesis of the angularly fused triquinane pentalenene (53) [65]. The known phenol 271 was used as the starting material. In this and the previous instance (viz. with 269) constant-current electrolyses were performed, this time converting 271 to a mixture of the desired adduct 272 (64%) as well as a material epimeric at the CH20Ac-bearing carbon (16%). The major adduct corresponds to one wherein the original olefin stereochemistry has been maintained. Conversion of 272 to the natural product followed chemistry similar to that used in the synthesis of silphinene (56). 

Miesch, M., Miesch-Gross, L., Franck-Neumann, M. Total synthesis of ( )-silphinene non photochemical cyclobutenic route to a crucial intermediate. Tetrahedron 1997, 53, 2103-2110. 

The arene-olefin meta-photocycloaddition approach to triquinane natural products is further illustrated this year with a three-step total synthesis of ( )-silphinene (5) starting from 2-bromotoluene (1). Thus, irradiation of the arene-olefin (2) derived from (1) gave a 1 1 mixture of the photoadducts (3) and (4) in 70% yield. Regioselective reductive cleavage of the cyclopropane bond in (3),... 

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