Pheromone synthesis. Part 264: Synthesis of the core 3-oxabicyclo[3.3.0]octane structures of gomadalactones A,B and C,the components of the contact sex pheromone of the white-spotted longicorn beetle,Anoplophora malasiaca

The core bicyclic cyclopentanelactone structures of gomadalactones A, B and C with α-hydroxyketone system were synthesized from (R)-pulegone, employing deconjugation of an α,β-unsaturated lactone as the key step. Comparison of the CD spectra of the synthetic compounds with those of the natural products confirmed the absolute configuration of the natural pheromone components as proposed in 2007. X-ray crystallographic analysis of the model compound of gomadalactone B core structure was carried out.     

Radical mediated stereoselective synthesis of meso-7,11-dimethylheptadecane, a female sex pheromone component of the spring hemlock looper and the pitch pine looper

meso-7,11-Dimethylheptadecane, a female sex pheromone component of the spring hemlock looper and the pitch pine looper, was synthesized from ethyl 2-(bromomethyl)propenoate in nine steps and 14% overall yield. The key step in the synthesis is the highly diastereoselective chelation-controlled radical reaction of diethyl 4-benzyloxy-2,6-dimethyleneheptanedioate with pentyl iodide performed in the presence of 6 equiv of MgBr₂·OEt₂.     

The geometric isomers of methyl-2,4,6-decatrienoate, including pheromones of at least two species of stink bugs

All eight geometric isomers of methyl 2,4,6-decatrienoate were synthesized from readily accessible starting materials by fully exploiting Wittig-type olefinations, and taking advantage of an easy separation of 2E and 2Z unsaturated esters. The aggregation pheromone of the brown-winged green bug, Plautia stali, methyl (E,E,Z)-2,4,6-decatrienoate (also a cross-attractant for the brown marmorated stink bug, Halyomorpha halys), was expediently produced in two easy steps from (E)-4,4-dimethoxy-2-butenal in 55% yield. The sex pheromone of the red-shouldered stink bug, Thyanta pallidovirens, methyl (E,Z,Z)-2,4,6-decatrienoate, was conveniently synthesized from 2,4-octadiyn-1-ol in 32% yield using in situ manganese dioxide oxidation-Wittig condensation in a key step.     

Synthesis of the sex pheromone of the obscure mealybug, the first example of a new class of monoterpenoids

A diastereoselective synthesis of (1R^∗,2R^∗,3S^∗)-1-acetoxymethyl-2,3,4,4-tetramethylcyclopentane, the sex pheromone of the obscure mealybug Pseudococcus viburni, is described. Key steps included the polyphosphoric acid-catalyzed cyclization of isobutyl methacrylate to form the core five-membered ring, and diastereoselective quenching of an enolate intermediate to give the thermodynamically less favored cis orientation of vicinal methyl groups in a cyclopentanone intermediate.     

Ab initio calculations on blocked Pro-Ala dipeptides

The effect of the chirality of the amino acid at position i + 2 on a β-turn was investigated by a grid scan ab initio calculation on the Ac- -Pro- -Ala-NH₂ and Ac- -Pro- -Ala-NH₂ blocked dipeptides. Th6-31G basis set was used to estimate the effect of the alanyl side chain on the conformation of the peptide backbone in a blocked dipeptide as a simple, but complete model for a reverse turn. This study provides a quantum mechanical evaluation of the ability of the NH at the i + 3 residue to form the H-bond that closes the 10 membered ring which stabilizes the turn. The lowest energy of all 64 probed conformations of the -Ala containing peptide corresponded to a good type II β-turn with a hydrogen bond distance between the acetyl oxygen and the amide terminal hydrogen of 2.21 Å. A comparison with the nonblocked dipeptide ab initio study indicates that the presence of the end blocks enhances the propensity of the -Ala-containing dipeptide for a type II β-turn, but does not seem to enhance the propensity of the -Ala-containing dipeptide for a type I β-turn. The energies and geometric parameters for the lowest four optimized conformations identified by the grid scan search for each molecule have been calculated.     

A facile one-pot stereoselective synthesis of trisubstituted (E)-2-methylalk-2-enoic acids from unactivated Baylis-Hillman adducts and a simple access to some important insect pheromones

An efficient one-pot stereoselective synthesis of trisubstituted (E)-2-methylalk-2-enoic acids has been accomplished by treatment of unactivated Baylis-Hillman adducts, 3-hydroxy-2-methylenealkanoates, with Al-NiCl₂·6H₂O in methanol at room temperature followed by hydrolysis. The method has been applied to the synthesis of three important insect pheromones, (4S,2E)-2,4-dimethyl-2-hexenoic acid, (+)-(S)-manicone and (+)-(S)-normanicone.     

Unsymmetrical double Wittig olefination on the syntheses of insect pheromones. Part 1: Synthesis of 5,9-dimethylpentadecane, the sexual pheromone of Leucoptera coffeella

An expeditious three-step synthesis of a mixture of stereoisomers of 5,9-dimethylpentadecane 1, the sexual pheromone of the coffee leaf miner Leucoptera coffeella, is described. The route employs an unsymmetrical double Wittig olefination to build the carbon skeleton of the molecule, as the key reaction. The bis-phosphonium salt 3, derived from 1,3-dibromopropane 2, reacted ‘one-pot’ with the ketones 2-octanone and 2-hexanone, affording the asymmetric diene 4. This was readily hydrogenated over Pd/C, furnishing pheromone 1 in 54% overall yield. Synthetic 1 showed high biological activity when tested in field experiments.     

Syntheses of 5‐Hexadecanolide, 6‐Acetoxy‐5‐hexadecanolide and Tanikolide

Total syntheses of 5‐hexadecanolide ( 1 ), 6‐acetoxy‐5‐hexadecanolide ( 2 ) and tanikolide ( 3 ) are described. 1‐Bromoundecane ( 4 ) and 5‐benzyl‐1‐pentanal ( 5 ) were chosen as starting materials. Wittig olefination and Grignard addition 4 and 5 afforded the 16‐carbon skeleton, which under went a series of functional group transformations to give δ‐lactonederivatives 1, 2 and 3.     

Pheromone synthesis. Part 245: Synthesis and chromatographic analysis of the four stereoisomers of 4,8-dimethyldecanal, the male aggregation pheromone of the red flour beetle, Tribolium castaneum

All four stereoisomers of 4,8-dimethyldecanal (1) were synthesized from the enantiomers of 2-methyl-1-butanol and citronellal. Enantioselective GC analysis enabled separation of (4R,8R)-1 and (4R,8S)-1 from a mixture of (4S,8R)-1 and (4S,8S)-1, when octakis-(2,3-di-O-methoxymethyl-6-O-tert-butyldimethylsilyl)-γ-cyclodextrin was employed as a chiral stationary phase. Complete separation of the four stereoisomers of 1 on reversed-phase HPLC at −54 °C was achieved after oxidation of 1 to the corresponding carboxylic acid 12 followed by its derivatization with (1R,2R)-2-(2,3-anthracenedicarboximido)cyclohexanol, and the natural 1 was found to be a mixture of all the four stereoisomers.     

Stereoselective synthesis of the obscure mealybug pheromone by hydrogenation of a tetrasubstituted alkene precursor

An improved diastereoselective synthesis of (1R^∗,2R^∗,3S^∗)-1-acetoxymethyl-2,3,4,4-tetramethylcyclopentane 1, the sex pheromone of the obscure mealybug, Pseudococcus viburni, is described. The key step was diastereoselective catalytic hydrogenation of the tetrasubstituted double bond in 2,3,4,4-tetramethyl-cyclopent-2-enone 4 to give the thermodynamically less favored cis-2,3,4,4-tetramethyl-cyclopentanone 3a.