Oxiranes Lewis acid activation

Oxetanes are generally much more stable to nucleophilic attack than the more strained three-membered ring oxiranes. However, activation of the oxygen atom by a Lewis acid increases the electronegativity of the adjacent carbon atom and renders oxetanes susceptible to attack from a nucleophile accompanied by subsequent ring opening. 

There has been only a few reports on reactions of small rings with metal ynolates. Oxiranes are much less electrophilic than carbonyls and sometimes need activation by Lewis acids or Lewis-acidic organometals. The lithium-trimethylaluminum ate complex of i/ZyZ-substituted ynolate 105 reacts with the oxirane 106 to give the y-lactone 107 (equation 44), while lithium silyl-substituted ynolates are inert to oxiranes. There have been no reports using carbon-substituted metal ynolates. 

Other thia-Payne-type rearrangements involve activation of 2,3-epoxypropyl thioethers with trimethylaluminum as the Lewis acid f Scheme IQ.ScL This activates the oxirane C—O bonds and leads to ready acceptance of intramolecular nucleophilic attack at the 2-position by the proximate sulfur atom Transposition of a methyl group from Me3Al to the episulfonium intermediate Int-3 furnished the product 23 with net retention of stereochemistry at C-2 due to double inversion at this site. 

Hegedus and coworkers have shown that the reaction of optically active a-oxazolidinonylallenylstannanes (50) with oxiranes, in the presence of BF3 OEt2, produces the p-hydroxypropargylamines (51) with high syn diastereoselectivity (Equation 32) [33]. Here, the Lewis acid is responsible for the rearrangement of the oxiranes to the corresponding aldehydes via alkyl, aryl or a hydride shift. 

---