Di-pi-methane Rearrangement of 1,4-Dienes

4-Dienes can be converted photochemically to vinyl cyclopropanes. This reaction, also named the Zimmerman rearrangement 322), is common to molecules in which two n moieties are substituents on a methane carbon 323). The skeletal changes follow the mechanistic sequence in (3.14). 

Early applications of this reaction are found in the conversion of barrelene to semibullvalene 324) (3.15) or in the preparation of an azabullvalene (3.16) 325). In a similar reaction azabarbaralene has been prepared from aza-bicyclononatriene 326). 

The di-rc-methane rearrangement is also a convenient way of obtaining polycyclic fused ring systems as illustrated in the synthesis of a tricyclo-undecane (3.17) 327). In the irradiation of dihydrotriquinacene the initial bonding scheme is identical as in (3.14) but ultimate cyclopropane formation is hindered by structural reasons (3.18) 328). 

One of the C—C double bonds can also be part of an enone system as shown in the synthesis of 2-cyclopropyl-2-eyclopentenones. Such a photoproduct was then converted to the sesquiterpene taylorione (3.19) 329). 

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