Cantharidin from Cantharis vesicatoria

Cantharidin is obtained from cantharides, the dried beetle Cantharis vesicatoria ( = Lytta vesicatoria) (Meloidae) or other spp., containing not less than 0.6%, or from mylabris, the dried beetles Mylabris sidae = M. phalerata), M. cichorii and M. pustulata (Meloidae), containing not less than 1%,... 

Cantharidin is a powerfiil vesicant that can be isolated from dried beetles Cantharis vesicatoria, or Spanish fly ). Outlined here is the stereospecific synthesis of cantharidin reported by Gilbert Stork (Columbia University). Supply the missing reagents (a)-(n). 

Q 15. Explain why the following [4 + 2] cycloaddition reaction does not occur to produce cantharidin isolated from dried beetles Cantharis vesicatoria). 

Cantharidin a toxic agent, M, 196.21, m.p. 218°C, produced by the beetle Cantharis vesicatoria (Spanish fly, blistering beetle), native to southern and central Europe. C. is biosynthesized from mevalonic acid. Its use as an aphrodisiac has caused fatalities. 

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