Mechanistic investigations on the reaction between 1,2-dioxines and bulky stabilized phosphorus ylides: An efficient route to closely related cyclopropane stereoisomers

The bulky stabilized ylides (2a-d) react with a range of 1,2-dioxines (1a-d ) to afford the diversely functionalized cyclopropanes 7 in excellent yield and diastereomeric excess. This is in direct contrast to the situation whe n nonbulky ester ylides are utilized which results in a completely differen t cyclopropyl series. Through a combination of isolation, spectroscopic, te mperature, and deuterium and additive effects studies, the mechanism of cyc lopropane formation from this second pathway can be proposed. Importantly, enolate quenching of the intermediate 1-2 gimel (5)-oxaphospholanes 4 prior to collapse results in an equilibrium mixture of intermediates 10 and 11 w hich have been fully characterized, and their formation is primarily a resu lt of the steric bulk of the stabilized ester ylide. These intermediates (1 0/11) then collapse further and result in formation of the observed closely related cyclopropyl stereoisomers 7 and 8. Moreover, the addition of LiBr to these reactions allows for the control of which of the two possible cycl opropanation pathways will be dominant. Finally, optimal protocols that dem onstrate the potential of this new cyclopropanation methodology for the rea dy construction of closely related cyclopropyl stereoisomers are presented.