Catalyst-Free Synthesis of Borylated Lactones from Esters via Electrophilic Oxyboration

Abstract

A catalyst-free oxyboration reaction of alkynes is developed. The resulting borylated isocoumarins and 2-pyrones are isolated as boronic acids, pinacolboronate esters, or potassium organotrifluoroborate salts, providing a variety of bench-stable organoboron building blocks for downstream functionalization. This method has functional group compatibility, is scalable, and proceeds with readily available materials: B-chlorocatecholborane and methyl esters. Mechanistic studies indicate that the B-chlorocatecholborane acts as a carbophilic Lewis acid toward the alkyne, providing a mechanistically distinct pathway for oxyboration that avoids B-O σ bond formation and enables this catalyst-free route.

Figure 1

(a) Previously reported electrophilic cyclization/dealkylation methods to generate O-heterocycles from ethers or esters. (b) Previously reported heterocycle-forming B–X σ bond addition. (c) This work demonstrating mechanistically distinct oxyboration.

Figure 2

X-ray crystallographic structure of 3aa, confirming six-membered ring formation, with the thermal ellipsoids shown at 50% probability (B, yellow; C, gray; O, red).

Scheme 1. Two Potential Oxyboration Pathways

Scheme 2. Intramolecular Competition Experiment