Frequencies and integral intenstities of the lactone and ester carbonyls of natural guaianolides

The frequencies and integral intensities of the IR bands of the stretching vibrations of lactone and acyclic ester carbonyls at C-8 of ten natural guaianolides and their derivatives, and of three model compounds have been studied. The most probable interpretation of the change in the value of Aν_C=O and ν_C=O of the lactone carbonyl have been suggested. In elegin and some of its derivatives, acroptilin, chlorohyssopifolin B, and eleganin, a lowering of the values of Aν_C=0 and ν_C=0 in comparison with dihydroelegin, hexahydroelegin, and the hydroxylactone of cynaropicrin is caused by nonvalent interactions of the lactone carbonyl with the α-exocyclic double bond. The increase in Aν_C=O and ν_C=O in dihydroelegin, hexahydroelegen, and the hydroxylactone of cynaropicrin is due to the angular strain of the γ-lactone ring. It has been shown that in an acyclic side chain at C-8 an increase in the integral intensity and in the frequency of the α,β-unsaturated ester carbonyl relative to guaianolides in which the terminal vinyl group at C-17 is absent is due to a conjugation effect.     

Reduction of the natural sesquiterpene lactone leucomisin

The hydrogenation of the natural sesquiterpene lactone leucomisin has been investigated; a nontrivial procedure for the qualitative and quantitative analysis of the reaction products by the HPLC method has been developed.Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 73–78, January–February, 1995. Original article submitted October 18, 1994.     

Total synthesis of the antibacterial polyketide natural product thailandamide lactone

Stereoselective total synthesis of the structurally intriguing polyketide natural product thailandamide lactone was accomplished, and done so using a convergent approach for the first time to the best of our knowledge. The key features of this synthesis included use of a Crimmins acetate aldol reaction, Evans methylation, Urpi acetal aldol reaction, Sharpless asymmetric epoxidation and subsequent γ-lactonization for the installation of six asymmetric centers and the use of the Negishi reaction, Julia-Kocienski olefination, cross metathesis, HWE olefination and intermolecular Heck coupling for construction of a variety of unsaturated linkages. Pd(i)-based Heck coupling was introduced, for the first time to the best of our knowledge, quite efficiently to couple the major eastern and sensitive western segments of the molecule. The antibacterial activity of thailandamide lactone was also evaluated.

A convergent strategy for the total synthesis of the structurally intriguing polyketide natural product thailandamide lactone has been developed for the first time. The antibacterial activity of the molecule has also been disclosed.     

Synthesis of a lactone natural product found in Greek tobacco

A short and stereoselective synthesis of (3R^∗,4R^∗,7R^∗)-3,7-epoxy-4,8-dimethyl-8-nonen-4-olide, a natural product isolated from cured tobacco leaves, has been completed in six steps and 22% overall yield. A samarium(II) iodide mediated reductive cyclisation has been used to construct the functionalised tetrahydropyranol core and subsequent sequential stereoselective addition of methyllithium and lactonisation furnished the natural product.     

Studies on the manganese-mediated isomerization of alkynyl carbonyls to allenyl carbonyls

[reaction: see text] A study of the role of base in the isomerization of manganese-coordinated conjugated alkynyl carbonyls to the corresponding allenyl carbonyls is described. The use of phosphine additives indicates that manganese requires a ligand prior to isomerization with amine bases. A series of amine bases were also examined for their efficacy in this isomerization reaction revealing a strong dependence on pK(a). By contrast, potassium tert-butoxide led to rapid isomerization in the absence of added manganese ligand.     

An intermediate for the total synthesis of guaianolides

A synthesis of an intermediate (5) for the construction of quaianolidesis described. The relative stereochemistry at C-1 and C-7 is established with complete stereocontrol.     

Circular dichroism of guaianolides and the products of their amination

The presence of an -oriented hydroxy group at C₈ in a guaianolide promotes the stereospecificity of the amination of an exomethylene bond linked with the -lactone grouping. The addition of morphiline and of piperidine to the exomethylene bond of the lactone ring of a guaianolide containing an -oriented hydroxy group at C₈ takes place with the preferential formation of the 11S isomer.Institute of Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 722–724, November–December, 1984.     

Circular dichroism of guaianolides and the products of their amination

The presence of an α-oriented hydroxy group at C₈ in a guaianolide promotes the stereospecificity of the amination of an exomethylene bond linked with the γ-lactone grouping. The addition of morphiline and of piperidine to the exomethylene bond of the lactone ring of a guaianolide containing an α-oriented hydroxy group at C₈ takes place with the preferential formation of the 11S isomer.     

Improved enantioselective synthesis of natural striatenic acid and its methyl ester

This letter describes the improved and efficient enantioselective synthesis of natural striatenic acid, isolated from Cheilolejeunea serpentina, and its methyl ester starting from a readily available enantiopure building block.     

Coumarin-derived discodermolide analogues possessing equivalent antiproliferative activity to the natural product--a further simplification of the lactone region

Analogues of discodermolide in which the complete C-1 to C-7 fragment is replaced with a coumarin moiety display equivalent potency to that of the natural product.     

Theoretical study on the mechanism of iron carbonyls mediated isomerization of allylic alcohols to saturated carbonyls

The conversion of allylic alcohols to enols mediated by Fe(CO)(3) has been studied through density functional theoretical calculations. From the results obtained a complete catalytic cycle has been proposed in which the first intermediate is the [(allyl alcohol)Fe(CO)(3)] complex. This intermediate evolves to the [(enol)Fe(CO)(3)] complex through two consecutive 1,3-hydrogen shifts involving a pi-allyl hydride intermediate. The highest Gibbs energy transition state corresponds to the partial decoordination ot the enol ligand prior to the coordination of a new allyl alcohol molecule that regenerates the first intermediate. Alternative processes for the [(enol)Fe(CO)(3)] complex such as [Fe(CO)(3)]-mediated enol-aldehyde transformation and enol isomerization have also been considered. The results obtained show that the former process is unfavourable, whereas the enol isomerization may compete with the enol decoordination step of the catalytic cycle.     

Furans as intermediates for the synthesis of oxygenated natural products. Asymmetric synthesis of Prelog-Djerassi lactone.

A facile, asymmetric synthesis of the methyl ester of Prelog-Djerassi lactone ($\text{3}$) has been completed in nine steps from furfuraldehyde ($\text{4}$).