Anodic Cyclizations, Densely Functionalized Synthetic Building Blocks, and the Importance of Recent Mechanistic Observations.

Abstract

Anodic cyclization reactions can provide a versatile method for converting newly obtained chiral lactols to densely functionalized cyclic building blocks. The method works by first converting the lactol into an electron-rich olefin and then oxidatively generating a radical cation that is trapped by a nucleophile. Historically, such reactions have benefited from the use of less polar radical cations when the trapping nucleophile is a heteroatom and more polar radical cations when the reaction forms C-C bonds. This forced one to optimize underperforming reactions by resynthesizing the substrate. Here, we show that by taking advantage of methods that serve to drive a reversible initial cyclization reaction toward the product, this dichotomy and need to manipulate the substrate can be avoided. Two such methods were utilized: a faster second oxidation step and a mediated electrolysis. Both led to successful cyclizations using a polar radical cation and heteroatom nucleophiles.