DABCO-catalyzed 1,4-bromolactonization of conjugated enynes: highly stereoselective formation of a stereogenic center and an axially chiral allene.

A DABCO-catalyzed highly regio- and stereoselective syn-1,4-bromolactonization of conjugated enynes was discovered. The 1,4-addition occurred selectively over 1,2-addition for most enynes. In contrast to anti 1,2-addition to alkenes, where two adjacent stereogenic centers are created, 1,4-addition across conjugated enynes can produce a stereogenic center and an axially chiral allene simultaneously. Enynes with Z and E configurations yielded products with complementary stereochemistries at the newly generated stereogenic center and axially chiral allene. Di-, tri-, or even tetrasubstituted allenes together with lactones can be efficiently and selectively prepared from this enyne bromolactonization reaction.