Pellasoren: Structure Elucidation, Biosynthesis, and Total Synthesis of a Cytotoxic Secondary Metabolite from Sorangium cellulosum

Abstract

Myxobacteria are efficient producers of numerous secondary metabolites, and the genus Sorangium is frequently described as an proficient source for new, biologically active natural products. [1][2][3] We report here the discovery and complete structure elucidation of pellasoren from the myxobacterium Sorangium cellulosum. Identification of the corresponding pel gene cluster from S. cellulosum So ce38 allowed us to establish a model for pellasoren biosynthesis, providing evidence for an unusual route to glycolate extender unit generation. Moreover, we present the first total synthesis of pellasoren and thereby confirm the absolute configuration of this natural product.Pellasoren (1; Scheme 1) was initially isolated from S. cellulosum So ce35 in the course of an activity-guided discovery program. [4] Additionally, we recently identified 1 in relatively high amounts in extracts from the related strain S. cellulosum So ce38 by using LC-MS analysis. Here we determine its cytotoxicity against HCT-116 human colon cancer cells at a concentration of 155 nm (IC50). Full structural elucidation by NMR and ESI-MS analysis was performed and confirmed the identity of pellasoren from both sources (Figures S1-S3 and Table S1 in the Supporting Information).The pellasoren scaffold features an unusual enol ether moiety, also known from a small number of other natural products, which was corroborated by specific HMBC correlations between a methoxy signal and sp 2 -hybridized carbon atoms ( Figure S1 in the Supporting Information). [5][6][7] The lactone moiety was identified through HMBC correlation between C1 and C5 and characteristic shifts for H-5 and C5 of d = 4.02 and 90.3 ppm, respectively. The position of the amide bond was assigned on the basis of indicative fragments in CID spectra. Efforts were made to establish the molecules relative configuration by using NOE spectroscopy: ROESY interactions together with molecular modeling suggest an anti configuration of the substituents at C4 and C5, as well as a syn configuration of the methyl groups at C2 and C4 ( Figure S2 in the Supporting Information). The stereocenter at C14 maintains the configuration derived from the incorporation of an l-alanine building block during biosynthesis. Stereochemical assignments that could initially not be validated by NOE analysis, such as the configuration at C6, were later established following total synthesis. Additionally, two isomeric pellasorens differing in the configuration of the C10-C11 double bond were isolated from S. cellulosum extracts. Pellasoren A (1 a) represents the all-E configuration while pellasoren B (1 b) has a Z-configured double bond at C10-C11 which can be rationalized by photochemical isomerization.Following the structural elucidation of pellasoren, we set out to identify the underlying biosynthetic machinery using fragmentary whole-genome sequence information for strain S. cellulosum So ce38. Assuming that pellasoren is most likely the product of a hybrid PKS/NRPS biosynthetic pathway, [8] the full complement of putative...

The vinylogous Mukaiyama aldol reaction (VMAR) in natural product synthesis

“…84 Pellasoren A (253), a natural product isolated from myxobacteria that exhibits potent cytotoxic activity against HCT- 116 human colon cancer cells, was synthesized by the Kalesse group in 2012. 85 For their synthesis of the hemiketal moiety 257 they used silyl ketene N,O-acetal 33b and a-chiral aldehyde 254. The transformation proceeded with 61% yield and 17 : 1 dr. At a later stage they made use of Stryker's reagent ([(PPh 3 ) CuH] 6 ) for conjugate reduction of an a,b-unsaturated ester and subsequent stereoselective intramolecular protonation 86 that leads to hemiketal 257 in 61% yield and 20 : 1 dr. (Fig.…”

Further insight into the asymmetric vinylogous Mukaiyama aldol reaction (VMAR); application to the synthesis of the C27–C45 segment of lagunamide A

“…1 H NMR (500 MHz, CDCl 3 ) δ 6.04 (ddq, MeC=CHCH 2 , J = 9.8, 6, 154.2, 135.9, 132.8, 73.6, 63.5, 58.2, 40.0, 33.9, 28.4, 25.8, 17.8, 15.1, 14.1, 13.7, 11.7;FTIR (neat, cm -1 ) 3529, 2968FTIR (neat, cm -1 ) 3529, , 2928FTIR (neat, cm -1 ) 3529, , 1772FTIR (neat, cm -1 ) 3529, , 1686FTIR (neat, cm -1 ) 3529, , 1389FTIR (neat, cm -1 ) 3529, , 1366FTIR (neat, cm -1 ) 3529, , 1297FTIR (neat, cm -1 ) 3529, , 1211FTIR (neat, cm -1 ) 3529, , 1054FTIR (neat, cm -1 ) 3529, , 1015 MS (ESI) m/z calcd for C 16 …”

“…A solution of solidKHMDS (3.20 g,16.02 mmol) in anhydrous THF (230 mL) was added dropwise to a solution of imide C19 (1.81 g, 10.68 mmol) in THF (110 mL) at -78 ºC. After the reaction mixture was stirred for 90 min at -78 ºC a solution of TBSCl(4.83 g, 18.16 mmol) in THF (25 mL) was added dropwise over 20 min at -78 ºC.…”

Antimicrobial Activity Screening of Rhizosphere Soil Bacteria from Tomato and Genome-Based Analysis of their Antimicrobial Biosynthetic Potential

“…Moreover, only 33% and 21% of the genes of cluster 13 and cluster 12 exhibit similarity to known pellasoren and xenocoumacin BGCs respectively. Pellasoren [30] was isolated from myxobacterium, which has shown to possess potential anti-cancer activity. The known pellasoren BGC, is a Type I PKs-NRPs hybrid cluster identified from Sorangium cellulosum So ce38 and consists of six genes of Type I PKs and one single gene of NRPs as compared to the trans-AT PKs-NRPs hybrid gene (cluster 13) of P. xylanexedens edo6, which in turn consists of four trans-AT PKs genes and one trans-AT PKs-NRPs hybrid gene.…”