Stereocontrolled Synthesis of Polypropionate Fragments based on a Building Block Assembly Strategy using Lithiation‐Borylation Methodologies

Polypropionates are important structural motifs in nature and are commonly made by iterative aldol or crotylation methodologies. Herein, an alternative strategy is presented in which stereochemically predefined building blocks, bearing appropriate functionality, are coupled together using a lithiation‐borylation methodology with complete stereocontrol. The building blocks comprise lithiated carbamates acting as donors, and boronic esters acting as acceptors. The acceptor building blocks contain β‐hydroxyl groups masked as silyl groups to avoid elimination of the boronate intermediates. Subsequent oxidation of both the boron and silyl moieties can then deliver an array of polypropionate fragments with full stereochemical control, including the synthetically challenging anti‐anti isomers. Any stereochemical outcome for short chain polypropionate fragments has been achieved by assembly of carbamates and β‐silaboronic esters through lithiation‐borylation reactions. Stereotriads, tetrads, and pentads are obtained in good yields and excellent diastereoselectivity under reagent‐control conditions.