Tricyclic diterpene moiety

Photochemical reactions can produce highly reactive intermediates for subsequent groimd state reactions. Photoisomerization of the cycloheptenone moiety in 151 led to a highly strained trans enol (152) that readily underwent a Diels-Alder reaction with isoprene to produce a mixture of 153 and 154, both of which were carried forward to a structure related to a naturally occurring tricyclic diterpene, vibsanin E (equation 12.94). Irradiation of the aldehyde... 

The very good yields of especially the bridgehead alkoxy derivatives 231 and the excellent regioselectivity with which they can be transformed to other skeletons plus the fact that a spirocyclopropane moiety is a mimic of and can in fact be considered as a masked gem-dimethyl substituent [28, 112] makes these products versatile precursors of certain natural products. In fact, one may conceive new approaches to the total syntheses of taxol [113-115] and of mediter-raneol [116]. Both strategies rely on the MIMIRC reaction of lithium cycloalka-dienolates [117] with the a-chloro acrylate 1-Me to produce a tricyclic precursor to the appropriate bicyclo [n.2.1]alkanedione derivative, which are key structural units of several diterpenes and their metabolites. 

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