Prototropic Isomerization of Propargyl Compounds

Only a few examples exist describing the products from the allenylic/propargylic carbanion resulting from the deprotonation of 18 and reaction with other electrophiles instead of protons which lead to products analogous to 19 [48]. Thus, treating the propargyl compound 21 with tetrabutylammonium fluoride (TBAF) in the presence of benzaldehyde furnishes the C,C-connected compound 22 [41]. 

In the case of carboxylic acids [33], ketones [47] or sulfones [49], it was possible to prove a further prototropic rearrangement of allenes 19 with R2 = H yielding the alkynes 20. In other examples, the prototropic isomerization of the triple bond leads to conjugated dienes directly thus the allene of type 19 could only be postulated as an intermediate [50]. Braverman et al. showed that the allenes generated by prototropic isomerization of dipropargylic sulfones or sulfoxides, for example 24, are unstable. They are transferred rapidly via diradical intermediates to polycycles such as 27 [51-53], 

The equilibrium 19 20 is not only a succeeding reaction of the acceptor-substituted allenes 19, but can also be used to synthesize the title compounds starting from 20. Whereas the isomerization of the chloro compounds 34a and 34b furnishes the allenes in good yields, the conversion of 34c leads to the unstable azide 35c with low yield [59]. 

Another method using starting materials of type 20 includes ring closure via nucleophilic attack at cyclic ketones [60]. Thus, on treatment of compounds 36 with tetra-w-butyl ammonium fluoride (TBAF), the products 37 were isolated. If cyclohex- 

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