A convergent synthesis of (+)-asteltoxin (1) has been achieved by the Horner-Emmons olefination of bis(tetrahydrofuran) aldehyde 53 and alpha-pyrone phosphonate 5. A key step features the stereoselective construction of a sterically congested quaternary center embedded in the densely functionalized bis(tetrahydrofuran) subunit by a Lewis acid-catalyzed, pinacol-type rearrangement of an epoxy silyl ether. This pivotal rearrangement methodology parallels the proposed biosynthetic pathway of 1 and is ripe for applications to the stereocontrolled synthesis of structurally complex natural products.