Synthesis of (+)-(S)-Streptenol A and Biomimetic Synthesis of (2R,4S)- and (2S,4S)-2-(Pent-3-enyl)piperidin-4-ol

(+)-(S)-Streptenol A is synthesized by coupling a 1,3-dithiane with an optically pure epoxide. The absolute configuration of (+)-(S)-streptenol A is thereby correlated with that of (S)-malic acid. Stereoselective reduction of an oxime that could easily be prepared from streptenol A gave the (3S,5R)- and (3S,5S)-aminostreptenols, and after cyclization, configurationally pure 2,4-functionalized piperidine alkaloids.     

Stereoselektive Hydrid-Reduktion von Tetronsäurederivaten. Synthese verzweigtkettiger Tetrofuranosen

Stereoselective Hydride Reduction of Tetronic Acid Derivatives, Synthesis of Branched-Chain Tetrofuranoses The 3-methoxymethyl derivatives of 2-methyl-D , L -threofuranose ( 10a ) and 2-deoxy-2-methyl-D ,L -erythrofuranose ( 11a ) are prepared starting from 2-methyl-tetronic acid ( 1 ). The key step is the stereoselective reduction of the 3-oxo-function of 2-chloro-2-methyl-3-oxo-γ-butyrolacton ( 2 ) by sodiumborohydride, which proceeds predominantly anti with respect to the C,Cl-bond. The configuration of the reduction products has been established by ¹H- and ¹³C-NMR.-spectroscopy.     

Diastereoselective michael addition of nitrogen and sulfur-nucleophiles to α,β-unsaturated δ-thiolactams

5,6-Dihydropyridine-2-thiones 2 are synthesized from 5,6-dihydropyridin-2-ones 1 and Lawesson reagent. Stereoselective Michael-like addition of amines, methylhydrazine or functionalized thiols affords trans piperidine-2-thiones 5 with the corresponding heterosubstituent in position 4 as major products. The configuration of the adducts 5 was determined by nmr-techniques.     

Stereoselective reduction of α-substituted β-keto esters by hydrostannane/organotin triflate

Stereoselective reduction of α-substituted β-keto esters is achieved by the combined use of hydrostannane/organotin triflate. syn-Aldols are obtained with more than 90% selectivities.     

Stereoselektive reduktive Dimerisierung von α-Cyan-β-(4-pyridyl)acrylsäurederivaten

Stereoselective Reductive Dimerisation of α-Cyano-β-(4-pyridyl)acrylic Acid Derivatives Catalytic hydrogenation of the α-substituted β-(4-pyridyl)acrylonitriles 3 and 4 (see Scheme 3) yields via stereoselective reductive dimerization the substituted cyclo-pentene derivatives 7 and 8 (see Scheme 4 and 5) instead of the expected dihydro-products 5 and 6 . The mechanism of this reaction is discussed. The structure and relative configuration of 10 have been established by X-ray single crystal analysis.     

A Novel Stereoselective Total Synthesis of (+)‐5‐epi‐Cytoxazone

Abstract

Stereoselective synthesis of a potent cytokine modulator cytoxazone isomer has been achieved from 2,3‐O‐isopropylidene D‐glyceraldehyde involving Grignard reaction, subsequent formation of an azide followed by reduction to the aminodiol, and finally cyclization of the N‐Boc diol.     

Facile Synthesis of 7‐epi‐Taxane and Its Derivatives and Preliminary Evaluation of Anticancer Activity

7‐epi‐Taxane has been achieved efficiently in gram scale from natural taxane via inversion of the 7‐hydroxyl group simply using Ag₂O as catalyst and DMF as solvent. The catalyst could be quantitatively recovered by filtration without loss of catalytic activity. This condition is also applicable to the direct epimerization of taxane derivatives (e.g., docetaxel and paclitaxel) to 7‐epi‐taxane derivatives (e.g., 7‐epi‐docetaxel and 7‐epi‐paclitaxel). Furthermore, 33 ester derivatives of 7‐epi‐taxane with different amino acid moieties at the position of C‐13 were successfully synthesized via esterification without protecting C‐7‐OH. Bioassay results revealed that compounds 13 and 18 have good selectivity against prostatic cancer cell line DU145, with IC₅₀ value as low as 15.9 nmol/L for 18 .     

Ionic hydrogenation-directed stereoselective construction of C-20(H) stereogenic center in steroid side chains: Scope and limitations

Stereoselective synthesis of steroidal C-20 tertiary alcohols with n-butyl, vinyl, furyl, thienyl, thiazolyl, aryl and pyridyl side chains via Grignard reaction or organolithium reagents have been realized starting from readily available 16-dehydropregnenolone acetate. The ionic hydrogenation of steroidal C-20 tertiary alcohols having furyl, methylfuryl, thienyl, phenyl and 4-methoxyphenyl side chains, resulted into the deoxygenated product with C-20 natural configuration in excellent yields. However, the alkyl, thiazolyl and pyridyl incorporated steroidal C-20 tertiary alcohols were failed under the same reaction condition. The scope of ionic hydrogenation is further highlighted through the stereoselective reduction of steroidal C-20,21-ene compounds with furyl, thienyl and 4-methoxyphenyl side chains gave the saturated compounds with C-20 natural configuration.     

Stereoselective Synthesis of Tricyclo[6.2.2.02,7]dodecane-3,6,9,10-tetrol via Selective Reduction of α,β-Unsaturated 1,4-DIKetone

Stereoselective synthesis of tricyclo[6.2.2.0^2,7]dodecane-3,6,9,10-tetrol was developed starting from p-benzoquinone. The endo selective Diels–Alder cycloaddition of p-benzoquinone and 1,3-cyclohexadiene afforded the corresponding bicyclic diketone. The synthesis of the title compound was based on the cycloadduct by selective reduction with NaBH₄, acetylation with AcCl, and hydroxylation with OsO₄-NMO.

[Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for the following free supplemental resource: Full experimental and spectral details.]     

Stereoselective Synthesis of (−)‐(1R,1′R,5′R,7′R)‐1‐Hydroxy‐exo‐brevicomin and (+)‐exo‐Brevicomin from 3,4,6‐Tri‐O‐acetyl‐D‐glucal

Stereoselective syntheses of (?)‐(1R,1′R,5′R,7′R)‐1‐hydroxy‐exo‐brevicomin ( 1 ) and (+)‐exo‐brevicomin ( 2 ) were accomplished from 3,4,6‐tri‐O‐acetyl‐D ‐glucal ( 5 ; Schemes 2 and 3). Chemoselective reduction, Grignard reaction, Barton? McCombie deoxygenation, and ketalization were used as key steps.     

Taxane Diterpenoids from Taiwanese Yew Taxus sumatrana

Thirty seven taxanes were characterized from the leaves and twigs of Taiwanese yew (Taxus sumatrana, Taxaceae). Four of these metabolites are new and designated as sumataxins A–D ( 1 – 4 ). Compound 1 possesses an 11(15→1),11(10→9)‐di‐abeo‐taxane skeleton with an unusual spiro‐connected 2,2‐dimethyl‐1,3‐dioxolane ring at C(4), whereas compound 2 has a rare β‐OH orientation at C(13) of taxane diterpene ester. In addition, sumataxin C ( 3 ) is formulated as an 11(15→1)‐abeo‐taxane with a 4,5‐acetonide ring skeleton. Compound 4 is the first metabolite with a 4,20‐epoxy‐taxane structure. The structures of all the taxanes were established by spectroscopic methods. All compounds were evaluated for anti‐HSV‐1 and PBMC activities. Compound 9 exhibited significant enhancement of cell proliferation on peripheral blood mononuclear cells.     

Stereoselective synthesis of 4-substituted 1,2,3,4,10,10a-hexahydropyrazino[1,2-<Emphasis Type="Italic">a</Emphasis>]indoles

4-Substituted 1,2,3,4-tetrahydropyrazino[1,2-a]indoles were synthesized from 2-cyanoindole. (R)-4-Methyl-1,2,3,4-tetrahydropyrazino[1,2-a]indole was obtained by the Mitsunobu reaction. Stereoselective reduction of 4-substituted 1,2,3,4-tetrahydropyrazino[1,2-a]indoles gave 4-substituted 1,2,3,4,10,10a-hexahydropyrazino[1,2-a]indoles. (4R, 10aR)-4-Methyl-1,2,3,4,10,10a-hexahydropyrazino[1,2-a]indole was synthesized.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 221–225, January, 2005.     

Synthesis of Cis- and Trans-1-methylcyclohexane-1,4-diols and Their 4-Hemisuccinate Esters

Stereoselective syntheses of the previously uncharacterized cis- and trans-1-methylcyclohexane-1,4-diols and their conversion to 4-hemisuccinate esters are described.     

(−)‐(1R,2S,2′R,5R)‐2‐(1‐Hydroxyprop‐2‐yl)‐5‐methylcyclohexanol

Stereoselective hydroboration of (?)‐isopulegol and subsequent fractional crystallization furnishes the title compound, C₁₀H₂₀O₂. The relative configuration of the stereogenic centres has been assigned by means of X‐ray diffraction analysis since the monoterpenediol is employed as a versatile chiral building block in stereospecific natural product synthesis.     

Stereoselektive Bildung von Oxaspiropentanen und Spiropentylketonen. Zur Stereochemie der nucleophilen Substitution am Cyclopropan

Stereoselective Formation of Oxaspiropentanes and of Spiropentyl Ketones. On the Stereochemistry of Nucleophilic Substitution at Cyclopropanes In an intramolecular S_N2-type substitution with formation of the spiro[2.2]-pentane skeleton, the highly strained transition state 2 with one three membered ring in an equatorial/equatorial and one in an apical/equatorial position of a pentacoordinated carbon atom ought to be - at least formally - involved. Yet, several examples of such processes are known. With the endo/exo isomeric bromohydrines 4 and 5 and bromoketones 13 and 14 this reaction is now shown to occur with clean inversion of configuration at the cyclopropane carbon atom. Unambiguous configurational assignments are made by X-ray crystal structure determinations of the 7′exo-bromo-benzoate 6 (from the bromohydrine 4 ), of the endo-sulfonate 12 (from oxaspiropentane 7 , formed from 4 ), of the 7exo-bromonorcarane 13 , and of its cyclization product 15 . - A comparison of the qualitatively measured rates of epoxide ring formation from the closely related systems 18 and 20 proves the expected slower substitution at the cyclopropane carbon atom as compared to the open chain analogue.     

Stereoselektive Synthese von Pyrrolizidin-Alkaloiden aus Nitroalkanonen

Stereoselective Synthesis von Pyrrolizidin Alkaloids from Nitroalkanones Reduction of 5-nitropentadecane-2,8-dione ( 11 ), synthesized by a Michael reaction from nitromethane, methyl vinyl ketone, and dec-1-en-3-one, gave, depending on the conditions, two epimeric 3-heptyl-2,3,5,6,7,7a-hexahydro-5-methyl-1H-pyrrolizines as the main products: Catalytic hydrogenation (Pd/C) afforded the expected 7aα-hydro epimer 1b with cis-orieted H-atoms at C(3), C(5), and C(7a). NaBH₃CN/NH₄OAc reduction of the nitrodione 11 yielded all four diastereoisomers with the 7aβ-hydro epimer 1a as the predominant component; 1a is a pheromone of the cryptic thief ant Solenopsis sp. The N-atom of the pyrrolizidine ring stems from NH₄OAc exclusively as shown by reduction of 11 with NaBH₃CN/(¹⁵N)H₄OAc.     

Stereoselektive Synthese von Cyclopropylkohlenhydraten

Stereoselective synthesis of cyclopropylsugars Use of the phase-transfer catalysis significantly increased the yield of the reactions leading to the known compounds 2 and 3 . Stereoselective cyclopropylation of 3 and 4 and 8 , was brought about using in the first instance the Simmons-Smith's reagent, whereas the reaction of dichlorocarbene led to 6 easily dechlorinated to 8 . Both diastereoisomeric cyclopropylsugars 4 and 8 were transformed into the corresponding ethyl glycosides.     

ZEOLITE MEDIATED STEREOSELECTIVE SYNTHESIS OF γ-ALKYLIDENEBUTENOLIDES

ABSTRACT

Stereoselective synthesis of Z-γ-alkylidenebutenolides has been achieved through H-ZSM 5 mediated dehydration.     

Stereospecific epoxidation of anthracyclinones: a route to 8(r)-methoxydaunomycinone

Stereoselective epoxidation of $\text{1}$, the 7,8-unsaturated derivative of daunomycinone, followed by $\text{trans}$ opening of the epoxide $\text{2}$, afforded 8(R)-methoxydaunomycinone $\text{6a}$; its configuration at C-8 was determined by chemical correlation and PMR studies.     

Stereoselective reaction between alkyl propiolates and phenols in the presence of sodium azide in tert-butyl alcohol

Abstract  

Stereoselective reaction of various substituted phenols with alkyl propiolates in the presence of a catalytic amount of sodium azide in tert-butyl alcohol at reflux temperature leads to alkyl (Z)-3-phenoxy-2-propenoates in good yields.