Origins of Regio- and Stereoselectivity in Acid-Promoted Reactions of α-Lactams

α-Lactam Electrophiles for Covalent Chemical Biology

Electrophilic groups are one of the key pillars of contemporary chemical biology and medicinal chemistry. For instance, 3-membered N-heterocyclic compounds-such as aziridines, azirines, and oxaziridines-possess unique electronic and structural properties which underlie their potential and applicability as covalent tools. The α-lactams are also members of this group of compounds, however, their utility within the field remains unexplored. Here, we demonstrate an α-lactam reagent (AM2) that is tolerant to aqueous buffers while being reactive towards biologically relevant nucleophiles. Interestingly, carboxylesterases 1 and 2 (CES1/2), both serine hydrolases with key roles in endo- and xenobiotic metabolism, were found as primary covalent targets for AM2 in HepG2 liver cancer cells. All in all, this study constitutes the starting point for the further development and exploration of α-lactam-based electrophilic probes in covalent chemical biology.

Silver-Promoted Fluorination Reactions of α-Bromoamides

Silver-promoted C-F bond formation in α-bromoamides by using AgF under mild conditions is reported. This simple method enables access to tertiary, secondary, and primary alkyl fluorides involving biomolecular scaffolds. This transformation is applicable to primary and secondary amides and shows broad functional-group tolerance. Kinetics experiments revealed that the reaction rate increased in the order of 3°>2°>1° α-carbon atom. In addition, it was found that the acidic amide proton plays an important role in accelerating the reaction. Mechanistic studies suggested generation of an aziridinone intermediate that undergoes subsequent nucleophilic addition to form the C-F bond with stereospecificity (i.e., retention of configuration). The synthesis of sterically hindered alcohols and ethers by using Ag^I is also demonstrated. Examples of reactions of α-bromoamides with O nucleophiles are presented.

Chemistry of Tertiary Carbon Center in the Formation of Congested C-O Ether Bonds

Nucleophilic substitutions, including S_N 1 and S_N 2, are classical and reliable reactions, but a serious drawback is their intolerance for both bulky nucleophiles and chiral tertiary alkyl electrophiles for the synthesis of a chiral quaternary carbon center. An S_RN 1 reaction via a radical species is another conventional method used to carry out substitution reactions of bulky nucleophiles and alkyl halides, but chiral tertiary alkyl electrophiles cannot be used. Therefore, a stereospecific nucleophilic substitution reaction using chiral tertiary alkyl electrophiles and bulky nucleophiles has not yet been well studied. In this paper, we describe the reaction of tertiary alkyl alcohols and non-chiral or chiral α-bromocarboxamides as a tertiary alkyl source for the formation of congested ether compounds possessing two different tertiary alkyl groups on the oxygen atom with stereoretention.

Synthesis of a Peroxime Proliferator Activated Receptor (PPAR) α/γ Agonist via Stereocontrolled Williamson Ether Synthesis and Stereospecific SN2 Reaction of S-2-Chloro Propionic Acid with Phenoxides

The stereospecific synthesis of the PPAR alpha/gamma agonist 1 was accomplished via ethylation of the optically pure trihydroxy derivative 6, itself derived via an enzymatic resolution. The ethylation can be accomplished without epimerization only under strict control of the reaction conditions and the choice of base (sodium tert-amylate), temperature (-30 degrees C), order of addition, and solvent (DMF). The key diastereospecific SN2 reaction of the phenol 4 with S-2-chloropropionic acid is best achieved via the sodium phenoxide of 4 derived from Na0 as the reagent of choice. The structure elucidation and key purification protocols to achieve pharmaceutical purity will also be described.

Origins of regioselectivity in the reactions of alpha-lactams with nucleophiles. Two distinct acid-catalyzed pathways involving O- and N-protonation

Sterically stabilized alpha-lactams react by two distinct acid-catalyzed pathways. Protonation on oxygen results in nucleophilic attack at the acyl carbon and gives C-2 products. Protonation on nitrogen leads to nucleophilic attack at the C-3 carbon and yields C-3 products. The mechanism thus developed is very useful for understanding the changes in rates and product distributions in the reactions of sterically stabilized alpha-lactams with nucleophiles. It can also be extrapolated to other alpha-lactams so that a more coherent picture of alpha-lactam reactivity can be developed.

The search for stable gas phase b(1) ions derived from aliphatic amino acids: a combined experimental and ab initio study

Several previous studies have shown that b(1) ions (formally acylium ions, H(2)NCHRCO(+)) derived from protonated aliphatic amino acids are unstable in the gas phase, fragmenting via decarbonylation to form a(1) ions (iminium ions, H(2)N = CHR(+)). Herein we examine the fragmentation reactions of ten potential b(1) ion precursors to determine whether stable aliphatic b(1) ions can be formed in the gas phase. Of all the systems studied, only the aziridine b(1) ion and the dehydroalanine b(1) ion were found to be stable. These experimental results are entirely consistent with ab initio calculations (at the MP2(full)/6-311G** level) which indicate that while the loss of CO from the b(1) ion of glycine is barrierless and exoethermic, the related losses from the b(1) ions of aziridine and dehydroalanine have significant barriers (29.5 and 16.2 kcal mol(-1), respectively) and are endothermic overall.