First synthesis of (+)-myxothiazol A

First convergent synthesis of (+)-myxothiazol A (1) was achieved based on modified (one-pot) Julia olefination between (3,5R)-dimethoxy-(4R)-methyl 6-oxo-(2E)-hexenamide (2), corresponding to left-side of the final molecule, and E-4-2′-(1S,6-dimethylheptadiene)-(2,4′-bis-thiazole)-4-methybenzothiazole sulfone (4) corresponding to right-side.     

Total synthesis of (+)-piericidin A1 and (−)-piericidin B1

The convergent syntheses of (+)-piericidin A₁ 1 and (?)-piericidin B₁ 2 have been achieved based on classical Julia-Lythgoe olefination between 4-hydroxy-5,6-dimethoxy-3-methyl-2-[5-oxo-3-methyl-pent-(2E)-enyl]-pyridine 3 corresponding to the left half of the final molecule, and chiral phenyl sulfones, (4R,5R)-2,4,6-trimethyl-5-methoxy-1-phenylsulfonyl-octa-(2E,6E)-diene 20 and (4R,5R)-5-tert-butyldimethylsiloxy-2,4,6-trimethyl-1-phenylsulfonyl-octa-(2E,6E)-diene 33, corresponding to the right halves. The construction of the two stereogenic centers in the right half of piericidins was achieved based on lipase-catalyzed hydrolysis of methyl (2,3)-anti-3-acetoxy-2,4-dimethyl-hex-(4E)-enoate (±)-22.     

Total synthesis of (R)-(+)-goniothalamin and (R)-(+)-goniothalamin oxide: first application of the sulfoxide-modified Julia olefination in total synthesis

A short and efficient synthesis of (R)-(+)-goniothalamin 1 and (R)-(+)-goniothalamin oxide 2 is described. During this approach, the sulfoxide-modified Julia olefination was used as a key step to connect aldehyde 5 to sulfoxide 6. The desired styryl-containing adduct is obtained in good yield and with excellent E/Z selectivity.     

Synthetic studies aimed at the elucidation of the stereostructure of the aggregation pheromone, 2-methyl-6-(4′-methylenebicyclo[3.1.0]hexyl)hept-2-en-1-ol,produced by the male stink bug Erysarcoris lewisi

The male-produced aggregation pheromone of the stink bug Erysarcoris lewisi Distant was shown to be one of the two diastereomers of (2Z,6R)-2-methyl-6-(4′-methylenebicyclo[3.1.0]hexyl)hept-2-en-1-ol by synthesizing and bioassaying (2E,6R)-, (2E,6S)-, (2Z,6R)-, and (2Z,6S)-isomers. These were synthesized from the enantiomers of citronellal by employing an intramolecular α-ketocarbene addition to a double bond and the E-selective or Z-selective olefination of a formyl group as the key steps. A reliable method was developed for the preparation of ethyl 2-(di-o-tolylphosphono)propanoate, Ando’s reagent for Z-selective olefination.     

Enantioselective total synthesis of (2R,3R,6R)-N-methyl-6-(deca-1′,3′,5′-trienyl)-3-methoxy-2-methylpiperidine, an insecticidal alkaloid

An insecticidal piperidine alkaloid, (2R,3R,6R)-N-methyl-6-(deca-1′,3′,5′-trienyl)-3-methoxy-2-methylpiperidine, was efficiently synthesized in a stereoselective manner starting from d-alanine. Chiral center at C-6 was controlled by hydrogenation of imine and side chain was introduced by Julia olefination. The absolute configuration of natural product was determined to be 2R, 3R, 6R.     

Lipase mediated resolution of 1,3-butanediol derivatives: chiral building blocks for pheromone enantiosynthesis. Part 3

(R,S)-1,3-Butanediol 5 was kinetically resolved by enzymatic acetylation with vinyl acetate under the presence of Chirazyme™ L-2, c–f, yielding (S)-1-O-acetyl-1,3-hydroxybutane 6 and (R)-1,3-di-O-acetyl-1,3-butanediol 7 with enantiomeric excesses of 91% (E=67.3). Compounds 6 and 7 were easily transformed into the corresponding (S)-3-O-(2-methoxyethoxymethyl)-3-hydroxybutanal 10 and (R)-3-benzyloxybutanal 19, through a protection–deprotection and functional group interchange methodology. Subsequent reaction of 10 and 19 with 3-(methoxycarbonylpropionylmethylene)triphenylphosphorane afforded methyl (E,S)-8-O-(2-methoxyethoxymethyl)-4-oxo-5-nonenoate 12 and (E,R)-8-benzyloxy-4-oxo-5-nonenoate 20. The alkenes 19 and 20 were then catalytically hydrogenated to the corresponding saturated esters 13 and 21. Treatment of 13 and 21 with 1,2-ethanedithiol/F₃B·OEt₂ afforded dithioketals 14 and 22, which were respectively reduced to (S)-1,8-dihydroxy-4-nonanone ethylidenedithioketal 15 and (R)-8-O-benzyl-1,8-dihydroxy-4-nonanone ethylidenedithioketal 23. Finally, deprotection of 15 by catalytic hydrogenation under acidic conditions gave the expected (5S,7S)-(−)-7-methyl-1,6-dioxaspiro[4.5]decane 1. The (5R,7R)-(+)-1 enantiomer was analogously prepared from 23. Both compounds were formed by this procedure with an e.e. of 91%.     

An enantioselective total synthesis of the stilbenolignan (−)-aiphanol and the determination of its absolute stereochemistry

The title natural product (−)-aiphanol has been prepared by total synthesis. A key step involved the asymmetric dihydroxylation of (E)-3,5-dimethoxy-4-(methoxymethoxy)cinnamyl alcohol with the AD-mix-β to give triol (1R,2R)-1-(3′,5′-dimethoxy-4′-methoxymethoxyphenyl)-2,3-dihydroxypropanol, the absolute stereochemistry of which was confirmed by single-crystal X-ray analysis of a readily available bromo-derivative. These studies have established that the naturally occurring enantiomer of aiphanol possesses the (S)-configuration at each of C-2′ and C-3′.     

Synthesis of 4-(2-hydroxy-1-methyl-5-oxo-1H-imidazol-4(5H)-ylidene)-5-oxo-1-aryl-4,5-dihydro-1H-pyrrole-3-carboxylates, a new triazafulvalene system

(2E,3Z)-2-(1-Methyl-2,5-dioxoimidazolidin-4-ylidene)-3-[(arylamino- or heteroarylamino)methylene]succinate 5 obtained by [2+2] cycloaddition of (5Z)-5-[(dimethylamino)methylene]-3-methylimidazolidine-2,4-dione (1) and dimethyl acetylenedicarboxylate (2) followed by substitution of the dimethylamino group with aromatic or heteroaromatic amines, afforded by heating in ethanol in the presence of potassium hydroxide, potassium salts 6. Acidification of 6 with hydrochloric acid afforded mixtures of (E)- and (Z)-isomers of methyl 4-(2-hydroxy-1-methyl-5-oxo-1H-imidazol-4(5H)-ylidene)-5-oxo-1-phenyl-4,5-dihydro-1H-pyrrole-3-carboxylates. On the other hand, alkylation of compounds 6 with methyl iodide or benzyl bromide produced the corresponding methyl (E)-4-(2-methoxy- or 2-benzyloxy-1-methyl-5-oxo-1H-imidazol-4(5H)-ylidene)-5-oxo-1-phenyl-4,5-dihydro-1H-pyrrole-3-carboxylates 9, derivatives of a new triazafulvalene system.     

Synthesis of 4,6-(and 5,7-)dimethoxy-3,3-dimethyl-1-indanones

Grignard reaction of ethyl 3-(3,5-dimethoxyphenyl)-propionate (4) followed by cyclodehydration of the carbinol (5) with conc H₂SO₄ gave 4,6-dimethoxy-3,3-dimethylindane (6). Oxidation of the indane (6) with CrO₃-pyridine complex in methylene chloride gave 4,6-dimethoxy-3,3-dimethylindan-1- one (1) in high yield. Conjugate addition of methyl magnesium iodide to methyl α-cyano-β-methyl-3,5-dimethoxycinnamate (11), prepared from 3,5-dimethoxyacetophenone (10) by Knoevenagel condensation, resulted in methyl 2-cyano-3-(3,5-dimethoxyphenyl)-3,3-dimethylpropionate (12). Refluxing the ester (12) with aq DMSO containing sodium chloride gave the corresponding nitrile (15) which underwent Höesch reaction to yield 5,7-dimethoxy-3,3-dimethylindan-1-one (2).     

A concise synthesis of (+)-cassiol

A synthesis of the anti-ulcerogenic compound (+)-cassiol 1b with 43% overall yield has been achieved. This short and efficient synthesis features the one-pot Julia olefination reaction of lactol (S)-2 with sulfone 3b through the key intermediate (−)-4b.     

Synthesis of optically active β′-hydroxy-β-enaminoketones via enzymatic resolution of carbinols derived from 3,5-disubstituted isoxazoles

The preparation of several enantiomerically pure β′-hydroxy-β-enaminoketones from the corresponding isoxazolic carbinols, which have been obtained by enzymatic kinetic resolution of the racemic β-hydroxyisoxazoles catalyzed by lipases, is described. The enzymatic transesterification of racemic (±)-5-(2-hydroxypropyl)-3-methylisoxazole 3a, and racemic (±)-5-(2-hydroxy-2-p-tolylethyl)-3-methylisoxazole 3d, has been studied with respect to the influence of experimental variables such as the used enzyme, the acylating agent or the solvent on the enantioselectivity of the reaction. After the reductive cleavage of the isoxazolic ring of the enantiopure carbinols, (R)- and (S)-2-amino-4-oxo-2-hepten-6-ol, (R)- and (S)-5, and (R)-2-amino-6-p-tolyl-4-oxo-2-hexen-6-ol, (R)-7 with an enantiomeric excess >98% were obtained.     

Asymmetric synthesis of (R)-(+)-6-(1,4-dimethoxy-3-methyl-2-naphthyl)-6-(4-hydroxyphenyl)hexanoic acid as a key intermediate for a neurodegenerative disease agent

An asymmetric synthesis of (R)-(+)-6-(1,4-dimethoxy-3-methyl-2-naphthyl)-6-(4-hydroxyphenyl)hexanoic acid 2 as a key intermediate for a neurodegenerative disease agent 1 has been developed. A key reaction was an asymmetric hydrogenation of hindered acrylic acid 13 catalyzed by the Rh-JOSIPHOS system in the presence of a base to afford a chiral acid up to 93% ee.     

Total synthesis of optically active liverwort sesquiterpenes,trifarienols A and B,using phenylethylamine as a chiral auxiliary

The imine of (rac)-2,3-dimethylcyclohexanone 10a with (S)-(−)-phenylethylamine was reacted with methyl acrylate to yield methyl (1′S,6′R)-3-(1′,6′-dimethyl-2′-oxocyclohexyl)propanoate 4a in 26% (97% ee) after hydrolysis. When (2RS,3R)-2,3-dimethylcyclohexanone 10b was used, the same product 4b was obtained in 59% yield (>99.5% ee) after hydrolysis. When (2RS,3R)-2,3-dimethylcyclohexanone 10b and (R)-(+)-phenylethylamine were used, the reaction underwent in only 5% yield, the products being 4c, 12, 13, 14, and 15. Thus, the reaction of 3b with methyl acrylate is a matched case, while that of 3c is a mismatched case. These phenomena are explained by the nonbonded interaction of methyl acrylate with chiral phenylethylamine and the methyl group at the 6′-position of the cyclohexanone ring in the transition state. The propanoate product 4b was successfully transformed into liverwort sesquiterpene (+)-trifarienol A 1 and (−)-trifarienol B 2 in 10 steps. We have developed an HPLC method to determine the ees of 2,2-disubstituted and 2,2,3-trisubstituted cyclohexanones using the corresponding pentafluorophenyl esters.     

Synthetic transformations of isoquinoline alkaloids. Synthesis of 1-halo derivatives of <Emphasis Type="Italic">endo</Emphasis>-ethenotetrahydrothebaine and their behavior in the heck reaction

Bromination of endo-ethenotetrahydrothebaine derivatives having a pyrrolidine ring fused at the C⁷-C⁸ bond, namely 1′-substituted 4,5α-epoxy-6α,14-etheno-3,6-dimethoxy-17-methyl-2′,5′,7β,8β-tetrahydro-1′H-14α-pyrrolo[3′,4′:7,8]morphinan-2′,5′-diones, 1′-aryl-4,5α-epoxy-6α,14-etheno-3,6-dimethoxy-17-methyl-2′,5′,7β,8β-tetrahydro-1′H-14α-pyrrolo[3′,4′:7,8]morphinans, and 4,5α-epoxy-6α,14-etheno-2′α-hydroxy-3,6-dimethoxy-17-methyl-1′-phenyl-2′,5′,7β,8β-tetrahydro-1′H-14α-pyrrolo[3′,4′:7,8]morpphinan-5′-one, with molecular bromine in formic acid smoothly afforded the corresponding 1-bromo derivatives. Iodination of 4,5α-epoxy-6α,14-etheno-3,6-dimethoxy-17-methyl-1′-phenyl-2′,5′,7β,8β-tetrahydro-1′H-14α-pyrrolo[3′,4′:7,8]-4,5α-epoxy-6α,14-etheno-3,6-dimethoxy-17-methyl-1′-phenyl-2′,5′,7β,8β-tetrahydro-1′H-14α-pyrrolo[3′,4′:7,8]-morphinan-2′,5′-dione with iodine(I) chloride gave 4,5α-epoxy-6α,14-etheno-1-iodo-3,6-dimethoxy-17-methyl-1′-phenyl-2′,5′,7β,8β-tetrahydro-1′H-14α-pyrrolo[3′,4′:7,8]morphinan-2′,5′-dione. The resulting 1-halo derivatives were brought into the Heck reaction with acrylic acid esters to obtain 1-[(E)-2-(alkoxycarbonyl)ethenyl]-substituted compounds. Demethylation of the 6-methoxy group in 1-bromo-endo-ethenotetrahydrothebaines was accomplished using boron(III) bromide in chloroform.     

Total synthesis of (4R,5S,6E,14S)- and (4R,5S,6E,14R)-cystothiazoles F

Total synthesis of (4R,5S,6E,14S)- and (4R,5S,6E,14R)-cystothiazoles F 3 was achieved from the chiral bithiazole-type primary alcohols [(S)- and (R)-4-ethoxycarbonyl-2′-(1-hydroxymethylethyl)-2,4′-bithiazoles 8], which were obtained based on the enzymatic resolution of racemic alcohol 8 and its acetate 9. From a direct comparison by means of chiral HPLC between natural cystothiazole F 3 and synthetic compounds [(4R,5S,6E,14S)- and (4R,5S,6E,14R)-cystothiazoles 3], natural cystothiazole F 3 was found to be a 33:67 diastereomeric mixture [(4R,5S,6E,14S)-3:(4R,5S,6E,14R)-3 = 33:67].     

Synthesis of enantiomerically pure (E)-1,1,3,3,6,6-hexamethyl-1-sila-4-cycloheptene and its absolute configuration

The title compound 1, a highly strained (E)-cycloalkene, was prepared in enantiomerically pure form from the corresponding trans-1,2-diol 4 via the thionocarbonate 5. The racemic 4 was separated by enantioselective HPLC on an amylose tris(3,5-dimethylphenylcarbamate) column. The absolute configuration of 1 was determined by circular dichroism spectroscopy in connection with theoretical calculations; the (+)-enantiomer has the (S)- and the (−)-enantiomer the (R)-configuration.     

Efficient preparation of (1′R)-(−)-1-(2′-hydroxy-1′-phenylethyl)piperidin-2-one: synthesis of (2′S,3R)-(+)-stenusine

An efficient oxidation of (2′R)-(−)-2′-phenyl-2′-(piperidin-1-yl)ethanol 2 with bromine to generate the corresponding piperidin-2-one 3 in 96% is described. In addition, starting from 3, (2′S,3R)-(+)-stenusine 8 was synthesized in 70% overall yield. The X-ray analysis of piperidine 6·HCl is also reported.     

Synthesis of (−)-(1′S,4aS,8aR)- and (+)-(1′S,4aR,8aS)-4a-ethyl-1-(1′-phenylethyl)-octahydroquinolin-7-ones

A synthesis of the enamine (−)-(1′S)-5-ethyl-1-(1′-phenylethyl)-1,2,3,4-tetrahydropyridine 4 and its application in a synthesis of (−)-(1′S,4aS,8aR)- and (+)-(1′S,4aR,8aS)-4a-ethyl-1-(1′-phenylethyl)-octahydroquinolin-7-ones 5 and 6 is described. In addition, an X-ray study of 6 is reported. Finally, the preparation of (+)-(4aS,8aR)-4a-ethyl-octahydroquinolin-7-one 7 is described.     

Kinetic resolution of a dihydrobenzofuran-type neolignan by lipase-catalysed acetylation

The kinetic resolution of 3,5′-dimethoxy-4′,7-epoxy-8,3′-neolignane-4,9,9′-triol 1 by lipase-catalysed acetylation in an organic solvent was investigated. Ten different lipases were screened for enantioselectivity in the reaction. The enantiomeric excess (e.e.) of the products was strongly dependent on the type of lipase used. After optimisation of the reaction conditions for Candida cylindracea lipase, the e.e. and yield of the reaction was improved greatly and, in some cases, the enantiomerically pure starting material 1 could be isolated, albeit in a low yield, with the acetylation affording predominantly (2R,3S)-1 and the (2S,3R)-esters.     

Chemoenzymatic total synthesis of cryptocaryalactone natural products

A new chemoenzymatic route for the preparation of cryptocaryalactones natural products using a kinetic resolution process as the key step is described. Novozyme-435 catalyzed hydrolysis of the prochiral (±)-monoester 7 afforded the precursors of cryptocaryalactones with high enantiomeric excess and excellent yields. The compounds (4S,6S)-7 and (4R,6R)-7 were converted to (+)-(6R,2′S)-cryptocaryalactone (1) and (?)-(6S,2′R)-cryptocaryalactone (2), respectively by employing Wittig-olefination, lactonization and acylation reactions.