Three-Component Hetero-[4+2] Cycloaddition/Allylboration

Significance

The ability to selectively effect multiple steps in a single step (one pot) is an extremely attractive method in synthesis. This paper describes a sequential cyclization-allylation sequence that gives interesting products with >95% ee. The use of 3-boronoacrolein esters and alkyl enol ethers for the inverse electron demand hetero-[4+2] cycloaddition, catalyzed by 1, was shown to give allylborate products with high conversion and enantioselectivities. The reaction needs to be carried out at or below room temperature to prevent unwanted allylation of the starting 3-boronoacrolein ester. Addition of various aldehydes after completion of the cyclization step gave the homoallylic alcohol product with very high yield and ee.