Bicyclo octane skeleton combination

The results obtained in combination with the data on the transformations of cations 9-11, led to the conclusion that 1,2-shifts of methyl groups occur readily in carbocations having a pentalene fragment. On the basis of the data obtained for rearrangements of such carbocations, an alternative mechanism has been proposed (52) for the rearrangement of structurally related carbocation 16 having a bicyclo[3.3.0]octane skeleton described in (33) (Scheme 12). 

Fig. 24 shows as an example the construction of the bicyclo[3.2.1]octane skeleton, which consists of one five-membered and one six-membered ring system (SSSR). In the first step, the two low-energy conformations, the envelope conformation of the five-membered and the chair conformation of the six-membered ring, are combined. Also, the more strained boat conformation of the six-membered ring can be fused to the envelope form of five-membered ring. 

Figure 25 Unallowed combination of two-ring templates to generate a conformation of a bicyclo[1.2.3]octane skeleton.

---