Migrations 1,2 ring-expansion

In most cases, the cleavage of a carbon-carbon bond causes rearrangements of the carbon skeleton Ring contraction, ring expansion, and alkyl group migration are observed under different conditions These transformations proceed in most cases in the presence of catalysts at elevated temperatures Examples where only temperature causes rearrangements will be discussed m the next section... 

Migration from nitrogen to carbon is observed also in aza-Cope rearrangement [76] Ring expansion occurs in thermal rearrangement of azindine denvauves [77] (equation 17)... 

These reactions involve addition of the diazo ester to an adduct of the carbonyl compound and the Lewis acid. Elimination of nitrogen then triggers migration. Triethyloxonium tetrafluoroborate also effects ring expansion of cyclic ketones by ethyl diazoacetate.83... 

The reaction of allenylsilanes with a,/8-unsaturated acylsilanes presents a new [3 + 3]-cycloaddition approach to a six-membered carbocycle [189]. Lewis acid-promoted ring expansion of the [3 + 2]-annulation product 260 is followed by a second cationic 1,2-silyl migration to produce the cyclohexenone 261 after desilylation. 

A flexible approach to the synthesis of substituted benzazepines 403 was devised (equation 166). As before, the seven-membered ring was made by the use of a ring-expansion reaction and exclusive migration of the aromatic ring was observed. 

Bhawal and colleagnes promoted a ring expansion to prodnce diazocines 417 from the oximes of dioxomorphanthridines 416 (eqnation 173). In all examples, migration of the aromatic ring was completely selective. 

---