Cycloproparenes, synthesis

Alder-Rickert cleavage has not been widely used for cycloproparene synthesis, since the preparation of the precursors is often tedious, except for the simple cases like 7,7-difluorobenzocyclopropene (21). The approach offers, however, decisive advantages in special situations. If the Alder-Rickert cleavage is carried out under flash-vacuum pyrolysis conditions, the products may be isolated under neutral conditions and at low temperature. Thus the synthesis of the highly reactive li/-cyclopropa[a]naphthalene (56) by pyrolysis of 68 has been achieved by this approach. Several other approaches to 56 failed. 

Much more recently, and with the advantage of sophisticated NMR techniques that have become available since the early 1970s, it has been shown that the spiro-3i/-pyrazole structure for 29a is incorrect. It is known that the outcome of diazocyclopentadiene addition to dimethyl acetylenedicarboxylate is dependent upon the five-membered ring substituents In the case at hand, tetraphenyldiazocyclopentadiene adds to the alkyne to give 29a as a labile product that rearranges under the reaction conditions to the 3H-indazole 30a (Scheme 3) 1,3-di-r-butyldiazocyclopentadiene behaves similarly l Thus in the formation of 31a at least, the spiropyrazole 29a is not the substrate and one must question the nature of the educt (29 versus 30) employed in cycloproparene synthesis by the spiro-3i/-pyrazole route. Nonetheless, there can be little doubt that spirocycle 29d is the substrate employed by Mataka and coworkers because, upon thermolysis, the corresponding indazole 30d was isolated. What must be noted here is that the thermal reaction did not provide any of the cyclopropa[/]phenanthrene 31d, but neither did independent photolysis of the isolable indazole 30d in benzene solution a 9,10-disubstituted phenanthrene is formed from diradical interaction with the solvent (equation 7). 

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