Skeletal Atom Migration

Sesquiterpenes are formed by the head-to-tad arrangement of three isoprene units (15 carbon atoms) there are, however, many exceptions to the rule. Because of the complexity and diversity of the substances produced in nature, it is not surprising that there are many examples of skeletal rearrangements, migrations of methyl groups, and even loss of carbon atoms to produce norsesquiterpenoids. 

There is a special and very important feature of the anticipated open nido twelve-vertex structures in Fig. 12 repetition of single Lipscomb dsd rearrangements (denoted by the two-headed arrows) monotonically allows the six skeletal atoms about the open face to rotate about the second tier of five skeletal atoms (two-tier dsd rotation). Each dsd rearrangement [85, 163) (valence bond tautomerism) recreates the same configuration and involves only the motion of two skeletal atoms (in the ball-and-stick representation) and would allow carbons, if located in different tiers, to migrate apart. Such wholesale valence bond tautomerism is known to accompany the presence of seven-coordinate BH groups, e.g., and CBjoHu 142,155). 

Sesquiterpenoids contain three isoprene units and the precursor for them all, in nature, is famesyl pyrophosphate as shown in Fig. 8.3. Because there are now three double bonds in the molecule, as opposed to the two of monoterpenoids, the variety of possible cyclic structures is much greater, as shown in Fig. 8.6. Skeletal rearrangements, migrations of methyl groups, and even loss of carbon atoms to produce norsesquiterpenoids all contribute further to the variety. There are probably >3000 sesquiterpenes that have been isolated and identified in nature. A large number of sesquiterpenoids possess interesting biological activities, but most are of academic interest only and have no commercial application outside folk medicine. 

The stepwise dehydrocyclization of hydrocarbons with quaternary carbon atoms over chromia was interpreted by Pines 94). Here a skeletal isomerization step prior to cyclization was assumed. This is not of a cationic type reaction, and the results were explained by a free radical mechanism accompanied by vinyl migration (Scheme IXA). Attention is drawn to the fact that... 

Bromine trifluoride is used to selectively substitute fluorine for bromine in brominated alkanes and esters. The reactions are carried out by gradual addition of bromine trifluoride to a solution of the substrate in CFC-113 or CFC-112 at 10-20nC. The bromine-fluorine exchange in mono-bromohaloalkanes is nonstereoselective and accompanied, in some eases, by skeletal rearrangements, hydride shifts, and halogen migrations. All three fluorine atoms in bromine trifluoride are involved in the fluorination reaction. Chlorine atoms in the substrate molecules remain intact.109... 

Very little skeletal rearrangement occurs via pyrolysis, a fact inherent in the failure of free radicals to readily isomerize by hydrogen atom or alkyl group migration. As a result, little branched alkanes are produced. Aromatization through the dehydrogenation of cyclohexanes and condensation to form polynuclear aromatics can take place. Additionally, olefin polymerization also can occur as a secondary process. 

There is evidence that atom A in scheme (76) could be carbon. Phenyl migration has been postulated to account for skeletal rearrangement in phenyl vinyl sulphides (90) initial migration through the four-centre... 

An ene reaction is a concerted intramolecular rearrangement in which a hydrogen atom transfer to a Ti-bond center is accompanied simultaneously by a 7i-bond development, a Ji-bond migration, and a decomposition of the reactant into two unsaturated molecular fragments. The general skeletal transformation is illustrated below... 

---