Synthesis of β‑Ketosulfonamides Derived from Amino Acids and Their Conversion to β‑Keto-α,α-difluorosulfonamides via Electrophilic Fluorination

β-Ketosulfonamides derived from Boc or Cbz-protected amino acids bearing hydrophobic side chains were prepared in good to excellent yield by treating N-allyl, N-alkyl methanesulfonamides with n-BuLi, followed by reaction of the resulting carbanion with methyl esters of N-protected l-amino acids. The analogous reaction using the dianion derived from an N-alkyl methanesulfonamide proceeded in much lower yield. Electrophilic fluorination of the β-ketosulfonamides using Selectfluor in the presence of CsF in DMF at room temperature for 15–60 min provided β-keto-α,α-difluorosulfonamides in good to excellent yields. The allyl protecting group could be removed in good yield using cat. Pd­(PPh)3)4 and dimethyl barbituric acid. When the fluorination reaction was performed with Cs2CO3 as base, β-ketosulfonamides derived from Val, Leu or Ile gave the expected β-keto-α,α-difluorosulfonamides, while β-ketosulfonamides derived from Ala, Phe, or hPhe gave the hydrates of the imino β-keto-α,α-difluorosulfonamides.