Convergent Total Synthesis of Yaku'amide A

Advances in the self-organized total synthesis of natural products

Natural product total synthesis has trailblazed in the era of multistep synthesis. The strategic application of existing synthetic methodologies and the stepwise construction that revolves around newly developed, tailored key steps, are two basic tactics in the principle of classic retrosynthetic analysis. However, a new synthetic model, termed self-organized total synthesis, has emerged in recent years, enabling the rapid creation of specific natural products by a one-pot reaction. Distinct from conventional analysis associated with certain bond disconnections, the design of self-organized total synthesis focuses on seeking a series of self-organized reaction sequences which can be integrated compatibly under a uniform condition, therefore allowing the entire sequence to proceed in one pot, and most importantly, starting from commercially available feedstocks or biomass materials. Whilst dauntingly challenging, this synthetic strategy is more consistent with the biogenetic pathway of natural products compared with conventional counterparts, and will hopefully provide the shortest synthesis for such natural products. Through this rational analysis, one-pot total synthesis is no longer in the way of serendipity but can be precisely designed and manipulated. In this review, we account for the definition, delimitation, and categorization of self-organized total synthesis and then elucidate a comprehensive understanding of this synthetic strategy based on our intensive explorations. We also highlight the contributions of other research groups in this growing field and anticipate that it will give rise to advancing new methodologies, as well as new concepts within organic synthesis.

Synthesis and Studies of Bulky Cycloalkyl α,β-Dehydroamino Acids that Enhance Proteolytic Stability

Three new bulky cycloalkyl α,β-dehydroamino acids (ΔAAs) have been designed and synthesized. Each residue enhances the rigidity of model peptides and their stability to proteolysis, with larger ring sizes exhibiting greater effects. Peptides containing bulky cycloalkyl ΔAAs are inert to conjugate addition by a nucleophilic thiol. The results suggest that these residues will be effective tools for improving the proteolytic stability of bioactive peptides.

[Total Synthesis and Functional Analysis of Complex Peptidic Natural Products and Their Artificial Analogues]

This review focuses on a new solid-phase synthetic strategy for an anticancer natural product yaku'amide B (1) and its target identification and structure-function relationship study using synthetic analogues and probes. To realize the Fmoc-based solid-phase synthesis of 1, we developed new synthetic methods for enamide formation. Namely, modified traceless Staudinger ligation using alkenyl azides and newly designed phosphinophenol esters enabled stereoselective construction of the (E)- and (Z)-ΔIle moieties. Furthermore, resin-cleavage and C-terminus modification were simultaneously achieved with an ester-amide exchange reaction using C-terminal amine and AlMe₃, which successfully afforded 1 via a full solid-phase route. The developed strategy was applied to the construction of seven E/Z isomers of 1. In the target identification of 1, fluorescent imaging study and affinity pull-down assay using the synthetic probes revealed that 1 exerts potent cytostatic activity by binding to subunits α and β of mitochondrial F_oF₁-ATP synthase. On the basis of the mode of action of 1, we conducted biological evaluation of the seven E/Z-isomers of 1. Assessment of growth inhibition activity and the effect on F_oF₁-ATP synthase indicates that the E/Z-stereochemistry of the three ΔIle residues controls the magnitude of biological functions of 1.

Synthesis and evaluation of potent yaku'amide A analogs

Two full-length analogs of the anticancer peptide yaku'amide A (1a) and four partial structures have been synthesized. These analogs were identified by computational studies in which the three E- and Z-ΔIle residues of the natural product were replaced by the more accessible dehydroamino acids ΔVal and ΔEnv. Of the eight possible analogs, modeling showed that the targeted structures 2a and 2b most closely resembled the three-dimensional structure of 1a. Synthesis of 2a and 2b followed a convergent route that was streamlined by the absence of ΔIle in the targets. Screening of the compounds against various cancer cell lines revealed that 2a and 2b mimic the potent anticancer activity of 1a, thereby validating the computational studies.

Simplified full-length analogs of yaku'amide A were designed with the aid of computations and then synthesized. The analog that was predicted to most closely resemble the structure of the natural product mimicked its anticancer activity.

Generation and Biological Evaluation of Degraded Derivatives of the Three E/Z-Isomers of Yaku'amide B

Potently cytostatic yaku'amide B (1) is a highly unsaturated linear tridecapeptide. During our synthetic studies of the E/Z-isomers of the α,β-dehydroisoleucines of 1, an unexpected retro-aldol reaction proceeded to transform E/Z-isomers 2, 3, and 4 into 2a, 3a, and 4a/4b, respectively. Compounds 2a, 3a, and 4a have a glycine at residue-1 instead of β-hydroxyisoleucine, and the β-hydroxyvaline at residue-8 in 4a is further replaced by glycine in 4b. Evaluation of the growth inhibition activities against MCF-7 cells revealed that 4b was approximately 10-fold weaker than the equipotent 2-4 and 2a-4a, demonstrating the biological importance of a bulky side chain at residue-8.

Towards a streamlined synthesis of peptides containing α,β-dehydroamino acids

Investigation of a strategy to streamline the synthesis of peptides containing α,β-dehydroamino acids (ΔAAs) is reported. The key step involves generating the alkene moiety via elimination of a suitable precursor after it has been inserted into a peptide chain. This process obviates the need to prepare ΔAA-containing azlactone dipeptides to facilitate coupling of these residues. Z-dehydroaminobutyric acid (Z-ΔAbu) could be constructed most efficiently via EDC/CuCl-mediated dehydration of Thr. Formation of Z-ΔPhe by this or other dehydration methods was unsuccessful. Production of the bulky ΔVal residue could be accomplished by DAST-promoted dehydrations of β-OHVal or by DBU-triggered eliminations of sulfonium ions derived from penicillamine derivatives. However, competitive formation of an oxazoline byproduct remains problematic.