Master of Science (MS)
Semester of Degree Completion
T. Howard Black
Our research group has been interested in β-lactones as useful synthetic intermediates. Transformations of β-lactones, generally ring expansion/opening processes initiated by Lewis acids via ionization/cation-rearrangement or elimination mechanisms, provide efficient protocols for the syntheses of biologically important butyrolactones, β,γ-unsaturated carboxylic acids, or butenolides.
α-Trimethylsilyl β-lactones, synthesized via the BF3- catalyzed [2+2] cycloaddition of trimethylsilylketene to carbonyl compounds, spontaneously undergo ionization and trimethylsilyl group migration to form α,β-unsaturated trimethylsilyl esters. The trimethylsilyl moiety facilitates the ionization process because of its β-cation stabilizing ability. The two-carbon homologated α,β-unsaturated acids are formed in yields from moderate to high (41-99%) upon aqueous workup. This one-pot procedure, which occurs under mildly acidic conditions, provides an effective, nonbasic alternative to the Wittig reaction.
The only current drawback to this method is that α,β-unsaturated acids are formed as an approximately 1:1 mixture of E/Z isomers, which reflects the cis-trans ratio of the β-lactone intermediates. The low stereoselectivity, probably a consequence of the stereorandom nature of the trimethylsilylketene cycloaddition reaction, could possibly be improved by employing more sterically demanding alkyl silyl ketenes or Lewis acids.
Zhang, Yong, "Carbonyl Homologation via α-Trimethylsilyl β-Lactone Rearrangement: A Nonbasic Wittig Alternative" (1994). Masters Theses. 2052.