Cyclobutenes fused

Cyclohexadiene derivatives are less reactive than butadiene derivatives, thus only a few examples of cycloadditions with these compoimds are known (Figure 4.3) [37 0]. The cyclohexadiene bicychc derivative 32 was synthesized by rhodium-catalyzed reaction of toluene with tert-butyldiazoacetate and cycloadds in about 40% yield to Cjq [39]. The product has anti-cyclopropane orientation relative to the entering dienophile Cjq. Valence isomerization of 33 (Scheme 4.4) leads to the cyclobutene-fused cyclohexene 35 that adds in good yields (50%) at moderate temperatures (110 °C) to Cjq [40]. The reaction of with the electron-deficient cyclohexene 34 is also possible in moderate yields [38]. 

The site selectivity switch, that is the site to which BF3 coordinated carbonyl rearrangement occurs, in the skeletal rearrangements of cyclobutene-fused diarylhomoben-zoquinones (43) is reported to result from higher order complexation. Thermodynamic... 

Cyclobutene-fused tetrahydropyridazines 1 and 2 are prepared by the diastereoselective cycloaddition of acyldiazenes to 1,2-diethylidenecyclobutanes. They can be converted to bicyclic diazenes 3, which are readily isomerized to the hydrazones f Pyrolysis of the bicyclic diazenes 3 affords 2,6-octadienes 4 with >99.9% stereospecificity. 

Protonation of cyclobutene-fused pyrene leads to a mixture of two a-protonated monocations. Only H NMR data are available (Fig. 34).30... 

Fig. 34. H and 13C NMR chemical shifts for cyclobutene-fused pyrenium cations.

A simple example of epoxidation occurs with a cyclobutene fused to a five-membered ring. This is a very rigid system and attack occurs exclusively from the outside to give a single epoxide in good yield. 

Examples have also been reported of the ring-opening of cyclobutenes fused to larger cyclic olefins, which may be followed by further rearrangements of the initially formed cyclic trienes. ... 

Cyclobutene fused with a carbocyclic ring gives isomeric product by more than one electrocyclic processes. For example, cyclobutene 13 gives 14 and cyclobutene 15 gives 16. 

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