Lewis acid catalysis epoxide ring opening

The limitation of the pinacol is the need for symmetry. This section and the next suggest ways of avoiding this problem. Unsymmetrical epoxides are easily made from alkenes and open with Lewis acid catalysis to give the more substituted of the two possible cations.9 Even such a weak Lewis acid as LiBr opens the epoxide 51 to give the tertiary cation 52 which rearranges by ring contraction to the aldehyde 53. The authors prefer to have the bromocompound 54 as an intermediate.10... 

Nucleophilic substitutions proceeding via Sn2 pathways can be activated by pressure, as has been demonstrated in many examples. In particular, the ring opening of epoxides can be initiated by pressure, and also by Lewis acid catalysis. Consequently, combining these two activation modes might lead to an even more effective way to functionalize epoxides, and indeed, this strategy has been successfully applied. 

When the attacking nucleophile is carbon-centered, intramolecular epoxide ring-opening reactions provide an entry intocarbocyclic systems. For example, epoxy allylsilane 70 cyclizes in an overwhelmingly 5-exo fashion under Lewis acid catalysis to form a putative silyl-stabilized carbocation intermediate (71) which then undergoes... 

The reactions of 2,3-disubstituted epoxides with hydroxylamines and hydroxamates tend to proceed more slowly, giving variable yields of ring-opening products. Catalysis with chiral Lewis acids 20 was reported to allow asymmetric opening of me o-epoxide 19 in a good yield and (equation 13). 

---