Regioselective Preparation of 2,4-, 3,4-, and 2,3,4-Substituted Furan Rings. 1. [1,4] O → C and [1,4] C → O Silyl Migrations of Silyl Ethers and Esters Attached to Furan and Thiophene Rings

Abstract

[1,4] O → C and [1,4] C → O rearrangements are described for a variety of furans and thiophenes. Treatment of 3-((silyloxy)methyl)furans and -thiophenes with n-BuLi in HMPA provided 2-silylated-3-(hydroxymethyl)furans and -thiophenes in good to excellent yields. The reaction was shown by crossover studies to proceed via an intramolecular [1,4] O → C silyl migration. Silyl esters of 3-furoic acids also underwent an intramolecular [1,4] O → C silyl migration to provide 2-silylated-3-furoic acids in moderate to good yield when treated with a mixture of LDA and HMPA. Finally, the above silyl migrations were shown to be reversible. Treatment of 2-silylated-3-(hydroxymethyl)furans and -thiophenes with NaH in DMF provided 3-((silyloxy)methyl)furans and -thiophenes in excellent yields via a [1,4] C → O silyl migration. The [1,4] C → O silyl migration was also shown to be an intramolecular process by a crossover study.