Skeletal rearrangement, ring closure

A common feature of any cyclization reaction is that a new intramolecular C—C bond is produced that would not have been formed in the absence of the catalyst. Those reactions in which one ring closure step is sufficient to explain the formation of a given cyclic product will be called simple cyclization processes, although their mechanism is, as a rule, complex. We shall distinguish those cases in which any additional skeletal rearrangement step(s) is (are) required to explain the process. Some specific varieties of hydrocarbon ring closure processes are not included. A recent excellent review deals with the formation of a second ring in an alkyl-substituted aromatic compound (12). Dehydrocyclodimerization reactions have also to be omitted—all the more since it is doubtful whether a metallic function itself is able to catalyze this process (13). 

Ring closure can proceed through dehydrogenation of the paraffin on a platinum site followed by carbon skeletal rearrangement on an acid site. However, there is also evidence (5) that it can proceed directly on a single site composed of Pt, alumina, and chloride. 

In addition to the skeletal rearrangements of the adamantane nucleus described above, several ring closure reactions have also been developed for the... 

In other reactions, like skeletal rearrangements of saturated hydrocarbons, the decomposition of CePd3 into Pd and Ce02 was also found to increase its activity by two orders of magnitude with an enhanced selectivity towards aromatization and 1-5 ring closure (Le Normand et al. 1984). 

Figure 2.10 shows some of the pathways that are possible with the 2,3-bond in the cis form and when the initial closure occurs at the 6,7-bond. This ring closure resembles the cyclisation of the geranyl cation to the /7-menthane skeleton, and the product, the bisabolane skeleton, similarly resembles a p-menthane skeleton, which has been extended by addition of an isoprene unit to its isopropyl tail. Elimination of a proton from this bisabolyl carbocation, or its trapping by a nucleophile, will lead to a sesquiterpenoid of the bisabolane family. However, there is a number of possibilities for further skeletal rearrangement, three of which are shown in Figure 2.10. 

The gold-catalyzed formal cycloaddition reactions of 2-ethynylbenzyl ethers with 8-methylquinoline oxide and ethyl diazoacetate led to a skeletal rearrangement of the benzo[c]furan motif through an attack of the diazo compound on the initial oxonium species in the alkyne activation route, followed by a Roskamp-type rearrangement and ring closure (13AGE7559). 

Gold-catalysed ring closure of 1,5-enyne eontaining a silyl ether at the allylic position has been reported to induce a skeletal rearrangement to form an oxonium intermediate, which undergoes intra- and inter-molecular allylation (Scheme 119). ... 

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