Concerted synthesis of a spirobicyclic type-VI beta-turn mimic of Pro-Pro-Pro-NH2

Synthesis and applications of symmetric amino acid derivatives

Symmetric α-amino acid derivatives can be used for the synthesis of intermolecularly linked peptides such as dimer-type peptides, and modified peptides in which two amino acids are intramolecularly linked. They are also synthetic intermediates for the total synthesis of natural products and functional molecules. These symmetric amino acid derivatives must be prepared based on organic synthesis. It is necessary to develop an optimal synthetic strategy for constructing the target symmetric amino acid derivative. In this review, we will introduce strategies for synthesizing symmetric amino acid derivatives. Additionally, selected applications of these amino acids in the life sciences will be described.

Silver/ThioClickFerrophos-Catalyzed 1,3-Dipolar Cycloaddition and Tandem Addition-Elimination Reaction of Morita-Baylis-Hillman Adducts

The asymmetric 1,3-dipolar cycloaddition of glycine imino esters to Morita-Baylis-Hillman (MBH) adducts or acetylated MBH adducts is described. The reaction was efficiently catalyzed by AgOAc/(R,S_p)-ThioClickFerrophos at room temperature to afford pyrrolidine derivatives bearing a quaternary carbon as a single diastereomer with excellent enantioselectivity. When a cyclic pyrroline ester was used as the nucleophile instead of a glycine imino ester, the enantioselective tandem addition-elimination reaction with an acetylated MBH adduct proceeded with an excellent yield and enantioselectivity, resulting in the formation of an exo-olefin. The wide substrate scope of these reactions and the transformability of the products enable expeditious access to divergent multifunctionalized pyrrolidines in an optically pure fashion.

Smart and concise entry to chiral spiro[cyclopentane-indolizidine]-tetraol diastereomers as a new aza-spirocyclic framework

Synthesis of chiral oxacarba-spiroindolizidine tetraols was achieved by alkene cis-dihydroxylation, diol protection, and lactam carbonyl reduction followed ultimately by acetonide deprotection.

An analysis of the complementary stereoselective alkylations of imidazolidinone derivatives toward α-quaternary proline-based amino amides

Alkylations of proline-based imidazolidinones are described based on the principle of self-regeneration of stereocenters (SRS), affording high levels of either the cis or trans configured products. Stereoselectivity is dictated solely on the nature of the "temporary" group, where isobutyraldehyde-derived imidazolidinones provide the cis configured products and 1-naphthaldehyde-derived imidazolidinones afford the complementary trans configured products. These stereodivergent products can be readily cleaved to afford both α-alkylated proline enantiomers from readily available L-proline. A series of imidazolidinones were alkylated to investigate the origin of the anti-selectivity. Potential contributions toward the observed anti-selectivity are discussed on the basis of these experiments, suggesting a refined hypothesis for selectivity may be in order.

Metal-free [3 + 2 + 1]/[2 + 2 + 1] biscyclization: stereospecific construction with concomitant functionalization of indolizin-5(1H)-one

A metal-free [3 + 2 + 1]/[2 + 2 + 1] biscyclization strategy has been developed for the stereospecific construction with concomitant derivation of biologically significant indolizin-5(1H)-ones from simple and commercial starting materials. The transformations are notable because they can yield five new σ bonds and six stereocenters including a quaternary carbon center in a single operation.

Development of peptidomimetic ligands of Pro-Leu-Gly-NH(2) as allosteric modulators of the dopamine D(2) receptor

A variety of stable, small-molecule peptidomimetic ligands have been developed to elucidate the mechanism by which the neuropeptide Pro-Leu-Gly-NH₂ (PLG) modulates dopaminergic neurotransmission. Photoaffinity labeling ligands based upon PLG peptidomimetics have been used to establish that PLG binds to the D₂ dopamine receptor at a site that is different from the orthosteric site, thus making PLG and its peptidomimetics allosteric modulators of the dopamine receptor. Through the design, synthesis and pharmacological evaluation of conformationally constrained peptidomimetics containing lactam, bicyclic, and spiro-bicyclic scaffolds, support was provided for the hypothesis that the bioactive conformation of PLG is a type II β-turn. In addition, studies with peptidomimetics designed to mimic either a type VI β-turn or polyproline II helix conformation yielded molecules that were able to modulate dopamine receptors because of their ability to place the carboxamide NH₂ pharmacophore in the same topological space as that seen in the type II β-turn. Extensive studies with the spiro-bicyclic PLG peptidomimetics also established that both positive and negative modes of modulation were possible for the same series of peptidomimetics simply as a result of minor differences in the stereochemistry about the bridgehead carbon within the scaffold. This information was used to transform existing positive modulators into negative modulators, which demonstrated that small structural changes in the spiro-bicyclic dopamine receptor modulators are capable of causing major changes in the modulatory activity of PLG peptidomimetics.

Allosteric modulation of the dopamine receptor by conformationally constrained type VI beta-turn peptidomimetics of Pro-Leu-Gly-NH2

A peptidomimetic of Pro-Leu-Pro-NH2, 7, possessing an indolizidinone type VI beta-turn mimic was synthesized via improved high-yielding protocols for the preparation and Cbz protection of alpha-allylproline. Bicyclic peptidomimetic 7 and spirobicylic peptidomimetic 8 enhanced the binding of [3H] N-propylnorapomorphine to dopamine receptors indicating that a type VI beta-turn is a possible bioactive conformation of the homochiral Pro-Leu-Pro-NH2 and Pro-Pro-Pro-NH 2 analogues of Pro-Leu-Gly-NH2 at the dopamine receptor allosteric regulatory site.