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A new synthetic method providing both enantiomers by heteroconjugate addition strategy rs described for stereocontrolled synthesis of optically active compounds from sugar chirons. Preparation includes introduction of phenylthioac...
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A new synthetic method providing both enantiomers by heteroconjugate addition strategy rs described for stereocontrolled synthesis of optically active compounds from sugar chirons. Preparation includes introduction of phenylthioacetylene, acidic epimerization via dicobalthexacarbonyl complex, hydrosilylation and oxidation. Addition of carbon nucleophiles to the heteroolefins extended at the C-1 position yielded the product with high stereoselectivity. Mode of addition was switchable via alpha- or beta-chelation control. [References: 18]
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Iodoetherification of alcohol 4 and several ether derivatives 12-15 afforded bis-tetrahydrofurans 8 and 9. The ratio of 8:9 depended on the nature of the ether substituent with 8 being the major product for R=H, SiMe(3), and Si(t)...
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Iodoetherification of alcohol 4 and several ether derivatives 12-15 afforded bis-tetrahydrofurans 8 and 9. The ratio of 8:9 depended on the nature of the ether substituent with 8 being the major product for R=H, SiMe(3), and Si(t)BuPh(2) whereas 9 was favoured when R=dichlorobenzyl. Attempts to effect ring expansion of 8 afforded ketone 16 wherein hydride migration had occurred. Iodoetherification of trisubstituted hydroxyalkene 17 afforded predominantly iodoether 21. Attempts to increase the amount of iodoether 22 formed led to a 1:1 ratio of 21:22 using trimethylsilyl ether 23. Treatment of iodoether 21 and 22 with Ag2CO3 in aqueous acetone proceeded stereospecifically affording the ring expansion products 27 and 28 respectively. Pyran 28 possesses the same stereochemistry as that present in the polyeher antibiotic salinomycin. [References: 16]
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Methods have been developed to synthesize tri- and pentapeptide thioesters containing one or more p-(hydroxyphenyl)glycine (pHPG) residues and l-serine, some where the latter is O-phosphorylated, O-acetylated, or exists as a β-la...
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Methods have been developed to synthesize tri- and pentapeptide thioesters containing one or more p-(hydroxyphenyl)glycine (pHPG) residues and l-serine, some where the latter is O-phosphorylated, O-acetylated, or exists as a β-lactam. Selection of orthogonal protection strategies and development of conditions to achieve seryl O-phosphorylation without β-elimination and to maintain stereochemical control, especially simultaneously at exceptionally base-labile pHPG α-carbons, are described. Intramolecular closure of a seryl peptide to a β-lactam-containing peptide and the syntheses of corresponding thioester analogues are also reported. Modification of classical Mitsunobu conditions is described in the synthesis of the β-lactam- containing products, and in a broadly useful observation, it was found that simple exclusion of light from the P(OEt)_3-mediated Mitsunobu ring closure afforded yields of >95%, presumably owing to reduced photodegradation of the azodicarboxylate used. These sensitive potential substrates and products will be used in mechanistic studies of the two nonribosomal peptide synthetases NocA and NocB that lie at the heart of nocardicin biosynthesis, a family of monocyclic β-lactam antibiotics.
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Formylation of (-)-menthone (11) with LDA and HCO2CH2CF3 avoids loss of configurational integrity at the isopropyl group, giving hydroxymethylenementhone 12. Lithium 2,2,6,6-tetramethylpiperidide- induced intramolecular cyclopropa...
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Formylation of (-)-menthone (11) with LDA and HCO2CH2CF3 avoids loss of configurational integrity at the isopropyl group, giving hydroxymethylenementhone 12. Lithium 2,2,6,6-tetramethylpiperidide- induced intramolecular cyclopropanation of derived unsaturated terminal epoxide 17 (and chlorohydrin 16), efficiently generates a substituted tricyclo[4.4.0.01,5]decan-4-ol 18, which is used in a concise synthesis of (-)-cubebol (1). In contrast, isopropyl group inversion during formylation of menthone with NaOMe and HCO2Et led, by a similar strategy, to syntheses of 7-epicubebol (33) and (from(t)-menthone) of naturally occurring (-)-10-epicubebol (39), confirming the original structural assignment. Computational studies support the origin of the inversion as being rate-determining formylation of cis-enolate 27 from amixture of rapidly interconverting enolates. In the synthesis of 7-epicubebol (33), allylic tertiary C-H insertion is observed as a significant competing reaction in the intramolecular cyclopropanation of unsaturated terminal epoxide 22.
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We describe a stereocontrolled method that converts tertiary allylic alcohols in pyrrolidine-2-carboxylic acid derivatives prepared from 3-(S)-hydroxy-L-proline to all-syn 3,4-disubstituted L-prolines. A study of various parameter...
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We describe a stereocontrolled method that converts tertiary allylic alcohols in pyrrolidine-2-carboxylic acid derivatives prepared from 3-(S)-hydroxy-L-proline to all-syn 3,4-disubstituted L-prolines. A study of various parameters to optimize the reductive rearrangement of tertiary allylic alcohols to tetrasubstituted olefins was conducted.
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[2+2]Cycloaddition of CSI to 5-O-protected-3-O-allenyl-1,2-O-isopropylidene-#alpha#-D-xylofuranoses 8 and 9 gave respective #beta#-lactams 10-13 having an exo-propylidene group,in moderate stereoselectivity. Compounds 10,11 and te...
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[2+2]Cycloaddition of CSI to 5-O-protected-3-O-allenyl-1,2-O-isopropylidene-#alpha#-D-xylofuranoses 8 and 9 gave respective #beta#-lactams 10-13 having an exo-propylidene group,in moderate stereoselectivity. Compounds 10,11 and tetracyclic cephams 14,15 obtained from 10 and 11 or 12 and 13,were used as substrates for a variety of transformations leading to the introduction of isoproppyl,hydroxyisopropyl,oxygen and nitrogenfunctions,#alpha# to the #beta#-lactam carbonyl group.These reactions proceeded in high stereoselectivity with control of the absolute configuration of the cephams formed.
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Reactions of enantiopure (SR)-[(p-tolylsulfinyl)methyll-p-quinol with ArAlMe_2reagents allowed a highlydiastereoselective 1,4-addition of the aryl group with an efficient desymmetrization of the prochiralcyclohexadienone moiety. T...
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Reactions of enantiopure (SR)-[(p-tolylsulfinyl)methyll-p-quinol with ArAlMe_2reagents allowed a highlydiastereoselective 1,4-addition of the aryl group with an efficient desymmetrization of the prochiralcyclohexadienone moiety. The asymmetric synthesis of phenyl-substituted polyoxygenated cyclohexanederivatives was achieved by combining this reaction with a stereoselective reduction and elimination ofthe β-hydroxysulfoxide, after oxidation to sulfone, to recover a carbonyl group, and a stereoselectiveepoxidation.
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Functionalised bicyclo[4.4.0] system 5, and spiro[5.4] system 10 were synthesised from the bispropargyl compound 3 and the monopropargyl compound 6 by tin-mediated radical cyclization. (C) 1997 Elsevier Science Ltd. [References: 20]
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We have developed an organocatalyzed asymmetric cascade sulfa-Michael-aldol reaction between 2-mercaptoindole-3-carbaldehydes and enals, which provides efficient access to the stereocontrolled construction of dihydrothiopyrano[2,3...
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We have developed an organocatalyzed asymmetric cascade sulfa-Michael-aldol reaction between 2-mercaptoindole-3-carbaldehydes and enals, which provides efficient access to the stereocontrolled construction of dihydrothiopyrano[2,3-b]indole skeletons. Under the catalysis of chiral diphenylprolinol TMS ether, the reactions ran smoothly to give the corresponding synthetically useful and pharmaceutically valuable dihydrothiopyrano[2,3-b]indoles in high yields and with 64-96% ee.
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An efficient and highly stereocontrolled approach for the synthesis of the quinuclidine incorporated lobelane analogues, endo,endo- and exo,exo-2,6-cis-diphenethyl-1-azabicyclo-[2.2.2]octane (2 and 3), has been developed. Analogue...
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An efficient and highly stereocontrolled approach for the synthesis of the quinuclidine incorporated lobelane analogues, endo,endo- and exo,exo-2,6-cis-diphenethyl-1-azabicyclo-[2.2.2]octane (2 and 3), has been developed. Analogues 2 and 3 were designed to mimic the axial and equatorial geometry, respectively, of the vesicular monoamine transporter-2 (VMAT2) inhibitor, lobelane. The exo,exo analogue 2 had comparable affinity to lobelane and had greater affinity than the endo,endo analogue 3 at the tetrabenazine binding site onVMAT2, indicating that the preferred binding mode of lobelane is likely the extended conformation.
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