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

[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.