Chiral nonracemic acetylenic acids of general structure
11, prepared using the Schreiber
modification of the Nicholas reaction, have been converted to β-amino
acid derivatives of type 12
by a two-step sequence involving Curtius rearrangement followed by
oxidative cleavage of the
acetylenic bond. Amino acid derivatives 12 are
excellent precursors for β-lactams of the carbapenem
class, including the important antibiotics thienamycin (1)
and PS-5 (4).
Three-membered heterocyclic rings offer an uncommon combination of reactivity, synthetic flexibility, and atom economy. This mini review surveys a selection of the most recent advances amongst epoxides and aziridines. Emphasis is given to novel synthetic methods and new insights into existing methodologies for the selective construction and controlled reaction of the title compounds reported in the past year's literature. While not the exclusive focus of this minireview, well-represented themes in the literature include asymmetric techniques and environmentally compatible reagents.
2,3-Bis(phenylsulfonyl)-l,3-butadiene undergoes conjugate addition in the presence of carbanions giving rise to a variety of unsaturated sulfones. Reaction with lithium enolates proceeds via an allylic anionic intermediate which undergoes a subsequent elimination of phenylsulfinate anion to produce an allene. Generation of enolates from silyl enol ethers results in conjugate addition to the diene without subsequent elimination. Substituted cyclopentenyl sulfones are available via a [4 + 1] annulation reaction of the diene with various distabilized carbanions. The reaction involves a tandem addition-proton exchange-addition sequence. In the special case of 2,4-pentanedione, pyrans are formed, the isomeric identity of which depends upon the reaction conditions. 2-Alkylated 1,3dicarbonyl compounds react with the activated diene to produce substituted allenes in high yield. Phenylsulfonyl alkenyl substituted allenes were conveniently prepared by a similar protocol and were found to serve as substrates for intramolecular [2+2] cycloaddition chemistry.
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