Total synthesis of leustroducsin B via a convergent route

The phosphate ester group in secondary metabolites

Covering: 2000 to mid-2021The phosphate ester is a versatile, widespread functional group involved in a plethora of biological activities. Its presence in secondary metabolites, however, is relatively rare compared to other functionalities and thus is part of a rather unexplored chemical space. Herein, the chemistry of secondary metabolites containing the phosphate ester group is discussed. The text emphasizes their structural diversity, biological and pharmacological profiles, and synthetic approaches employed in the phosphorylation step during total synthesis campaigns, covering the literature from 2000 to mid-2021.

Kinetic Resolution of Nearly Symmetric 3-Cyclohexene-1-carboxylate Esters Using a Bacterial Carboxylesterase Identified by Genome Mining

A new bacterial carboxylesterase (CarEst3) was identified by genome mining and found to efficiently hydrolyze racemic methyl 3-cyclohexene-1-carboxylate (rac-CHCM) with a nearly symmetric structure for the synthesis of (S)-CHCM. CarEst3 displayed a high substrate tolerance and a stable catalytic performance. The enantioselective hydrolysis of 4.0 M (560 g·L^-1) rac-CHCM was accomplished, yielding (S)-CHCM with a >99% ee, a substrate to catalyst ratio of 1400 g·g^-1, and a space-time yield of 538 g·L^-1·d^-1.

Synthetic Strategies Employed for the Construction of Fostriecin and Related Natural Products

Fostriecin and related natural products present a significant challenge for synthetic chemists due to their structural complexity and chemical sensitivity. This review will chronicle the successful efforts of synthetic chemists in the construction of these biologically active molecules. Key carbon-carbon bond forming reactions will be highlighted, as well as the methods used to install the numerous stereocenters present in this class of compounds.

A Highly Convergent Total Synthesis of Leustroducsin B

Leustroducsin B exhibits a large variety of biological activities and unique structural features. An efficient and highly convergent total synthesis of Leustroducsin B was achieved in 17 longest linear and 39 total steps by disconnecting the molecule into three fragments having similar levels of complexity. These pieces were connected via a highly efficient chelate-controlled addition of a vinyl zincate to an α-hydroxy ketone and a silicon-mediated cross-coupling. The stereochemistry of the central and western fragments was set catalytically in high yields and excellent de by a zinc-ProPhenol-catalyzed aldol reaction and a palladium-catalyzed asymmetric allylic alkylation.

Construction of the co-expression plasmids of fostriecin polyketide synthases and heterologous expression in Streptomyces

CONTEXT

Polyketides are bioactive natural products with diverse bioactivities, and heterologous production of polyketides in easily engineered microbial hosts is preferred for the production of structurally diverse and the therapeutically active polyketides.

OBJECTIVE

In this study, heterologous expression of the biosynthetic genes encoding type I polyketide synthases (PKS) involved in biosynthesis of fostriecin, a unique phosphate monoester polyketide antibiotic, was attempted.

MATERIALS AND METHODS

Fostriecin PKS (Fos-PKS) biosynthetic gene cluster in a total of 48.4 kb were cloned downstream of the act I promoter in two compatible Streptomyces vectors using Red/ET recombination. The co-expression plasmids were sequentially transferred into Streptomyces lividans and Streptomyces coelicolor. Active transcription of the polyketide genes was confirmed by reverse transcription PCR (RT-PCR) analysis, and the metabolites were detected using high-performance liquid chromatography (HPLC).

RESULTS

The recombinant strains S. lividans TK24/p6-fosAB-p4-fosCDEF and S. coelicolor M512/p6-fosAB-p4-fosCDEF were obtained for heterologous expression in Streptomyces. Pigmentation was observed in the recombinant strains, whereas the control strain with empty vector displayed no change in pigment production. Active transcription of the polyketide genes was confirmed by RT-PCR analysis and subsequent sequencing.

CONCLUSION

The present study is the first attempt to overexpress Fos-PKS biosynthetic gene cluster in Streptomyces. More studies on heterologous expression of the fostriecin biosynthetic gene cluster would be beneficial for further understanding the mechanisms of its structural as well as the potential pharmaceutically effect.

Silyl glyoxylates. Conception and realization of flexible conjunctive reagents for multicomponent coupling

This Perspective describes the discovery and development of silyl glyoxylates, a new family of conjunctive reagents for use in multicomponent coupling reactions. The selection of the nucleophilic and electrophilic components determines whether the silyl glyoxylate reagent will function as a synthetic equivalent to the dipolar glycolic acid synthon, the glyoxylate anion synthon, or the α-keto ester homoenolate synthon. The ability to select for any of these reaction modes has translated to excellent structural diversity in the derived three- and four-component coupling adducts. Preliminary findings on the development of catalytic reactions using these reagents are detailed, as are the design and discovery of new reactions directed toward particular functional group arrays embedded within bioactive natural products.

Formal synthesis of leustroducsin B via Reformatsky/Claisen condensation of silyl glyoxylates

A formal synthesis of leustroducsin B has been completed. The synthesis relies upon a recently developed Reformatsky/Claisen condensation of silyl glyoxylates and enantioenriched β-lactones that establishes two of the molecule's three core stereocenters and permits further elaboration to an intermediate in Imanishi's synthesis via reliable chemistry (Prasad reduction, asymmetric pentenylation, Mitsunobu inversion).

Good timing in total synthesis: the case of phoslactomycin A

The importance for the right order of functional group introduction and manipulation (good timing) was demonstrated in the course of a total synthesis of phoslactomycin A. The synthetic strategy comprised a Cu(I)-thiophene carboxylate (CuTC, Liebeskind's reagent)-mediated coupling to introduce the Z,Z-diene at the final stage of the synthesis in the presence of a protected phosphate. Key features for the assembly of the C1-C13 fragment were an asymmetric dihydroxylation, an Evans-aldol reaction and an advanced protective group strategy. The C14-C21 fragment was accessible via an asymmetric 1,2-addition to cyclohexenone and a subsequent diastereoselective ketone reduction. One crucial task was the dihydroxylation of the C8-C9 alkene, the introduction of the C6-C7 double bond and the generation of the C25-nitrogen functionality. A second example consisted of the best sequence for the generation of the functional groups in the core part (first phosphorylation, second iodo-olefination, third azide/carbamate conversion). The synthetic solutions from this approach are compared with the already existing contributions in the phoslactomycin area.

[Overview of 40 years' chemical study]

1,6-Dihydro-3(2H)-pyridinone, designed as a common synthon for synthesis of various natural products, was found to be easily prepared in large scale and successfully used to synthesize a variety of alkaloids such as ibogamine, quinine and tecomanine. A tricyclo[3.3.0.0(2.8)]octane was also served as a common synthon for several sesquiterpenes such as pentalenene and quadrone. Synthetic studies by using sulfinyl chirality via an intramolecular Michael addition gave the novel route to construct spiro-ketal moiety in enantiomerically pure form. By applying this method, many natural spiro-ketal compounds were asymmetrically synthesized effectively. 3-Sulfinylated 1,4-dihydropyridine, a chiral NADH model compound, reduced activated ketones such as methyl benzoylformate to give the corresponding alcohols in excellent optical yields. A kind of 3-O-substituted pyridoxal chiral model compound was useful for preparation of alpha,alpha-dialkylated alpha-amino acids by asymmetric alpha-alkylation of alpha-amino acids. 2'-O,4'-C-Bridged nucleic acid analogs, BNAs, developed as novel type of artificial nucleic acids, showed an extraordinarily high binding affinity toward single stranded RNA and double stranded DNA complements along with excellent nuclease-resistant ability. Oligonucleotides containing BNA monomer units were proved to be very useful for various biotechnologies, such as antisense and antigene methodologies.