Stereochemical assignment of topsentolide C2 by stereodivergent synthesis of its four diastereomers

Four diastereomers of topsentolide C₂, a cytotoxic nine-membered lactone isolated from the marine sponge Topsentia sp., were synthesized stereodivergently from a common chiral seco acid by the combined use of the Yamaguchi and Mitsunobu lactonizations. Comparison of the NMR spectra of the four diastereomers with those of an authentic sample of topsentolide C₂ led to the stereochemical determination of topsentolide C₂ as 8R, 11S, and 12S.     

Structure elucidation of 21,22-dihydroxyonnamides A1-A4 from the marine sponge Theonella swinhoei: an empirical rule to assign the relative stereochemistry of linear 1,5-diols

A polar cytotoxic fraction of the marine sponge Theonella swinhoei was analyzed to be a mixture of four isomeric compounds, 21,22-dihydroxyonnamides A₁, A₂, A₃, and A₄. They were separated after conversion to the isopropyridene derivatives. The structures of the 21S,22S- and 21R,22R-isomers were determined by comparison of their spectral data with those prepared from onnamide A by asymmetric dihydroxylation. During the analysis of NMR data of these derivatives, an empirical rule was implied to assign the relative stereochemistry of linear 1,5-diol. This rule was applied to assign the stereochemistry of the remaining congeners.     

Total synthesis of the (+)-antimycin A3 family: structure elucidation of (+)-antimycin A3a

(+)-Antimycin A_3a (AA_3a), one component of the natural antibiotic antimycin A₃, was synthesized using an asymmetric aza-Claisen rearrangement. The stereochemistry at the 2′ position on the acyloxy side chain of AA_3a was established as S-configuration by comparison of the synthetic AA_3a and the natural AA_3a.     

Structure elucidation and stereoselective total synthesis of pavettamine, the causal agent of gousiekte

The structure elucidation of a novel natural product pavettamine (1), the causal agent of the plant toxicosis gousiekte, is reported. The structure was defined by analysis of NMR and MS data and the relative configuration followed from the ¹³C NMR data of the acetonide derivative. The absolute stereochemistry was established by total synthesis from (2S)-malic acid using chiral sulfoxide methodology as (2S,4R,8R,10S)-1,11-diamino-6-aza-undecane-2,4,8,10-tetraol.     

Synthesis of topsentolides A2 and C2, and non-enzymatic conversion of the former to the latter

The first total synthesis of the marine-derived cytotoxin topsentolide A₂, which eventually culminated in its stereochemical determination, was accomplished in 17 steps from a known chiral alcohol. An improved synthesis of its congener, topsentolide C₂, from a synthetic intermediate of topsentolide A₂ was also performed by utilizing the Yamaguchi lactonization to construct its nine-membered lactone ring. Treatment of epoxide ring-containing topsentolide A₂ with HCl/MeOH brought about its quantitative conversion into topsentolide C₂.     

Isolation,structural assignment and insecticidal activity of (−)-(1S,2R,3R,4S)-1,2-epoxy-1-methyl-4-(1-methylethyl)-cyclohex-3-yl acetate,a natural product from Minthostachys tomentosa

(−)-(1S,2R,3R,4S)-1,2-Epoxy-1-methyl-4-(1-methylethyl)-cyclohex-3-yl acetate has previously been identified as the active compound of Minthostachys tomentosa responsible for the insecticidal activity against Oncopeltus fasciatus. Its structure was initially assigned on the basis of spectral data. In order to confirm the structure and to define the stereochemistry, stereoselective synthesis of its enantiomer, (+)-(1R,2S,3S,4R)-1,2-epoxy-1-methyl-4-(1-methylethyl)-cyclohex-3-yl acetate, starting from (R)-(−)-piperitone, was carried out using a Sharpless reaction as the key step. The natural product is dextro-rotatory while the synthetic product is levo-rotatory. Measurements of insecticidal activities of the different steroisomers revealed that only the natural product is active.     

Absolute stereochemistry of TT-1 (rasfonin), an α-pyrone-containing natural product from a fungus, Trichurus terrophilus

The absolute stereochemistry of TT-1 (1=rasfonin), an α-pyrone-containing natural product from a Fungi Imperfecti, Trichurus terrophilus, was determined as 5R,6R,7S,9R, and 6′S on the basis of synthesis of diastereomers of two fragments of 1 in optically active forms, and comparison of their spectral and optical data with those of natural specimens.     

Synthesis of the ABC-ring models of goniodomin A: preference for the unnatural configuration at C11 of the BC-ring in a non-macrocyclic model system

To confirm the natural relative stereochemistry of the ABC-ring of goniodomin A (1), the corresponding three stereoisomeric compounds, (2R,5S,6S,7S,9S,11R,15S)-, (2R,5S,6S,7R,9R,11S,15R)-, and (2R,5S,6S,7R,9R,11R,15S)-isomers (2, 3, and 5, respectively), were stereoselectively synthesized using a Nozaki-Hiyama-Kishi reaction as a key step. It was also found that a (2R,5S,6S,7R,9R,11S,15S)-isomer (4), corresponding to the absolute configuration of 1 recently proposed by Sasaki, was not detected during the formation of 5 from a common ketodiol substrate under acid-catalyzed spiroacetalization conditions. This would be attributable to the absence of a macrocyclic framework.     

Synthesis and structure revision of tyroscherin, a growth inhibitor of IGF-1-dependent tumor cells

Synthesis of the proposed structure of tyroscherin, a growth inhibitor of IGF-1-dependent cancer cells, was succeeded by one-pot Julia coupling. However, spectral data of the synthetic compound were not identical with those of natural tyroscherin. The stereochemistry of tyroscherin was revised to be 2S,3R,8R,10R by syntheses of stereoisomers.     

Synthesis of trimethyl (2S,3R)- and (2R,3R)-[2-H1]-homocitrates and the corresponding dimethyl ester lactones—towards elucidating the stereochemistry of the reaction catalysed by homocitrate synthase and by the Nif-V protein

Trimethyl (2S,3R)- and (2R,3R)-[2-²H₁]-homocitrates, 10b and 10c respectively, and dimethyl (2S,3R)- and (2R,3R)-[2-²H₁]-homocitric lactones, 11b and 11c respectively, have been synthesised from shikimic acid and [2-²H]-shikimic acid by a route which defines the stereochemistry of the two chiral centres in each compound. The NMR spectra of these products will enable the stereochemistry of the biological reaction catalysed by homocitrate synthase and by the protein from the nifV gene to be elucidated.     

Stereoselective synthesis of 4-amino-3-hydroxy-2-methylpentanoic acids: stereochemistry of the amino acid occurring in the marine toxin janolusimide

Four diastereomers of 4-amino-3-hydroxy-2-methylpentanoic acid, an amino acid constituent of the hexapeptide portion of the antitumor antibiotic bleomycin A₂, have been stereoselectively synthesized by crotylboration of N-Boc-l-alaninal. The synthesis allowed the assignment of the stereochemistry as 2R,3S,4S to the 4-amino-3-hydroxy-2-methylpentanoic acid occurring in the tripeptide marine toxin janolusimide.     

Asymmetric synthesis of (−)-(S,S)-homaline

The asymmetric synthesis of (−)-(S,S)-homaline was achieved in 8 steps from commercially available starting materials using the diastereoselective conjugate addition of the novel lithium amide reagent lithium (R)-N-(3-chloropropyl)-N-(α-methyl-p-methoxybenzyl)amide to methyl cinnamate to install the correct stereochemistry. Subsequent functional group manipulation of the resultant β-amino ester and Sb(OEt)₃-mediated macrolactamisation was followed by homodimerisation to give (−)-(S,S)-homaline in 18% overall yield, representing the first asymmetric, and by far the most efficient synthesis of this natural product reported to date.     

The enantioselective synthesis of poison-frog alkaloids (−)-203A, (−)-209B, (−)-231C, (−)-233D, and (−)-235B″

The enantioselective synthesis of indolizidines (−)-203A, (−)-209B, (−)-231C, (−)-233D, and (−)-235B″ has been achieved and the absolute stereochemistry of both indolizidines 203A and 233D was established as 5S,8R,9S. The relative stereochemistry of natural 231C was established by the present asymmetric synthesis.     

Synthesis and structure revision of tyroscherin, and bioactivities of its stereoisomers against IGF-1-dependent tumor cells

Synthesis of the proposed structure of tyroscherin, a growth inhibitor of IGF-1-dependent cancer cells, was succeeded by one-pot Julia coupling. However, spectral data of the synthetic compound were not identical with those of natural tyroscherin. The stereochemistry of tyroscherin is revised to be 2S,3R,8R,10R by syntheses of stereoisomers. Synthetic tyroscherin showed more potent activity than its stereoisomers against IGF-1-dependent cancer cells.     

Efficient synthesis of 3,4- and 4,5-dihydroxy-2-amino-cyclohexanecarboxylic acid enantiomers

An efficient method for the synthesis of (1S,2R,4R,5S)- and (1R,2R,4R,5S)-2-amino-4,5-dihydroxycyclohexanecarboxylic acids (?)-6 and (?)-9 and (1R,2R,3S,4R)- and (1S,2R,3S,4R)-2-amino-3,4-dihydroxycyclohexanecarboxylic acids (?)-15 and (?)-18 was developed by using the OsO₄-catalyzed oxidation of Boc-protected (1S,2R)-2-aminocyclohex-4-enecarboxylic acid (+)-2 and (1R,2S)-2-aminocyclohex-3-enecarboxylic acid (+)-11. Good yields were obtained. The stereochemistry of the synthesized compounds was proven by NMR spectroscopy.     

Synthetic studies directed toward amphidinol 2: elucidation of the relative configuration of the C1-C10 fragment

Model compounds (11 and 12) for the C1-C10 tetrahydropyran fragment of amphidinol 2 were prepared from (2S)-benzyloxypropanal in 9 steps. The synthetic route relied on diastereoselective diene-aldehyde cycloaddition, stereoselective C-allylation, and reagent based enantioselective aldehyde allylation. Comparison of the NMR spectra for models 11 and 12 with that for amphidinol 2 indicated that the C1-C10 segment of the natural product possesses the 2R^∗,4R^∗,6R^∗,7S^∗,8R^∗,10S^∗relative configuration.     

First total synthesis of (+)-Vedelianin, a potent antiproliferative agent

The total synthesis of (+)-vedelianin has been accomplished in 18 steps from vanillin. Preparation of a key intermediate in nonracemic form through a Shi epoxidation has allowed determination of the absolute stereochemistry of the natural product as the (2S,3R,4aR,9aR)-isomer.     

Two natural products from the algae Laurencia scoparia

The structures and absolute stereochemistries of two chamigrene-type metabolites (spiro­[5.5]­un­decane derivatives) isolated from the red algae Laurencia scoparia are described. One, a non-sesquiterpene named ma?lione (8-bromo-9-hydroxy-7,7-di­methyl-11-methyl­ene­spiro­[5.5]­undec-1-en-3-one), C₁₄H₁₉BrO₂, was detected previously in Laurencia cartilaginea, while the other, the sesquiterpene isorigidol (8-bromo-3,7,7-tri­methyl-11-methyl­ene­spiro­[5.5]-undec-1-ene-3,9-diol), C₁₅H₂₃BrO₂, is a new isomer of rigidol, first isolated from Laurencia rigida. The A rings of these spiro­cyclic compounds show the same carbon skeleton. However, the relative stereochemistry of the 8-Br and 9-OH substituents is different. While ma?lione displays the usual syn (or cis) relative stereochemistry of the bromo­hydroxy vicinal group, isorigidol shows an anti (or trans) arrangement. The 8-Br and 9-OH groups are both in equatorial positions in isorigidol, while the 9-OH group is axial in ma?lione, as in most chamigrenes. The absolute configurations of the chiral centers were determined as 6S, 8S and 9R in ma?lione, and 3R, 6S, 8S and 9S in isorigidol.     

Facile determination of the absolute stereochemistry of hydroxy fatty acids by GC: application to the analysis of fatty acid oxidation by a P450BM3 mutant

The determination of the absolute stereochemistry of hydroxy fatty acid methyl esters as their (S)-ibuprofen esters is possible via standard gas chromatographic techniques. Analyses of various racemic and nonracemic standards and mixtures from enzymic oxidation show excellent resolution of the resultant diastereomers, with the (S,S)-diastereomers eluting first in all cases studied. The stereochemistry of the oxidation of dodecanoic acid by P450_BM3, which has not been previously reported, was determined by this method and indicated a preference for (R)-hydroxylation. The sensitivity of this technique allows the analysis of very small quantities of product, which has revealed that the oxidation of dodecanoic and hexadecanoic acids by the T268A mutant of P450_BM3 display the same stereochemical efficiency and produce (R)-hydroxy fatty acids in the same manner as wildtype P450_BM3, despite the poor coupling efficiency of these substrates. This stereochemistry implies that hydroxylation catalysed by the T268A mutant of P450_BM3 occurs through residual levels of the normal hydroxylating species.     

Synthesis of mono- and dihydroxy-substituted 2-aminocyclooctanecarboxylic acid enantiomers

(1R,2S,6R)-2-Amino-6-hydroxycyclooctanecarboxylic acid (?)-10 was synthesized from (1R,2S)-2-aminocyclooct-5-enecarboxylic acid (+)-2 via an iodolactone intermediate, while (1R,2S,3R,4S)-2-amino-5,6-dihydroxycyclooctanecarboxylic acid (?)-12 was prepared by using the OsO₄-catalyzed oxidation of Boc-protected amino ester (?)-5. The stereochemistry and relative configurations of the synthesized compounds were determined by 1D and 2D NMR spectroscopy (based on 2D NOE cross-peaks and ³J(H,H) coupling constants) and X-ray crystallography.