I. β-Functionalization of Enones via Phosphoniosilylation
β-Alkoxycarbonylation, β-acylation, and β-sulfenylation of α,β-enones are achieved by the reaction of ylides, derived from enones via phosphoniosilylation, with benzyl chloroformate, benzoyl fluoride, and diphenyl disulfide, respectively, followed by the desilylation.
β-Conjugate addition and β-hydroxyalkylation of α,β-enones have been accomplished by the reaction of ylides with activated olefins and aldehydes in the presence of trimethylsilyl triflate in tetrahydrofuran at -78℃ and subsequent elimination of phosphonium salts by one-pot procedure.
Also, direct displacement of triphenylphosphonium salt, derived from enones via phosphoniosilylation, with several nucleophiles provides conjugate addition products in which ketone groups are protected as silylenol ethers.
II. Synthesis of β-Lactams from β-Amino Acids Using Organophosphorus Reagents.
Diphenylphosphinic chloride is found to be very effective condensing reagent for the formation of β-lactams from N-substituted β-amino acids.
Bis(5-nitro-2-pyridyl) phenyl phosphate (NPPP) and bis(5-nitro-2-pyridyl) phenylphosphonate (BNPP) are prepared by the reaction of phenyl dichlorophosphate and phenylphosphonic dichloride, respectively, with 2-hydroxy-5-nitropyridine. NPPP and BNPP can be effectively utilized as a condensing reagent for the formation of β-lactams from N-substituted β-amino acids.
Using triphenylphosphine/carbon tetrachloride system as a condensing reagent, N-substituted as well as N-unsubstituted β-amino acids are cleanly cyclized into the corresponding β-lactams in high yields under a concentrated solution.