- Stereoselective synthesis of (1R)- and (1R,2S)-1-aryl-2-alkylamino alcohols from (R)-cyanohydrins
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Hydrogenation of (R)-cyanohydrins (R)-1 with LiAlH4 occurs without racemization to give the (R)-2-amino alcohols (R)-3. (1R,2S)-2-Amino alcohols (1R,2S)-4 are obtained with high diastereoselectivity by addition of methyl Grignard to O-silyl protected cyanohydrins (R)-2 and subsequent hydrogenation with NaBH4. The N-alkylated 2-amino alcohols (R)-8 and (1R,2S)-9 can be prepared either by reductive alkylation of the corresponding 2-amino alcohols (R)-3 and (1R,2S)-4, respectively, or by a transimination reaction of the Grignard addition products with primary amines and subsequent hydrogenation with NaBH4. The lower diastereoselectivity of hydrogenation in case of the N-alkylmino compounds in comparison to the N-unsubstituted imines is explained by a weaker chelating effect.
- Effenberger, Franz,Gutterer, Beate,Jaeger, Juergen
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p. 459 - 467
(2007/10/03)
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- Regio-, Stereo-, and Enantioselectivity in the Electrophilic Reactions of 2-Amino-4-phenyl-3-butenenitriles
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The allylic anion generated from 2-(N-methylanilino)-4-phenyl-3-butenenitrile (1) reacted with iodomethane and 3-bromo-1-chloropropane in THF/HMPA to give γ-substituted products exclusively, predominantly in the 2Z-configuration.Substitution of the N-methylanilino group with the methyl ether of L-(-)-ephedrine resulted in the generation of chiral aminonitrile 2.Lithiated 2 reacted sluggishly in THF with allyl bromide and benzyl chloride in the absence of HMPA to give the γ-substituted products predominantly in the 2Z-configuration, with little diastereoselectivity.The aminonitrile 3 prepared from cinnamaldehyde, L-(-)-ephedrine, and KCN was lithiated with 2 equiv of LDA resulting in high facial selectivity (76 - 100 percent diastereomeric excess) in alkylations in the presence of HMPA and LiI.Lithiated 3 reacted with propionaldehyde, benzaldehyde, and p-bromobenzaldehyde at -78 deg C to give the γ-addition products, predominantly in the 4R-configuration (64 -78 percent de).The stereochemistry of the isomers of each product was determined by chemical correlation and spectral methods including analyses of the CD spectra and X-ray diffraction.A bicyclic transition state C wherein lithium is chelated by both amino and alkoxy groups is proposed to interpret the observed stereoselectivity.
- Chang, Chih-Jung,Fang, Jim-Min,Liao, Li-Fan
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p. 1754 - 1761
(2007/10/02)
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- N-ALKYLOXAZABOROLIDINES DERIVED FROM EPHEDRINES
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The synthesis of oxazaborolidines from amides derived from ephedrine and pseudoephedrine by treatment with BH3-THF is reported.The reaction affords chiral oxazaborolidines with nitrogen atom substituents of different steric requirements enlarging the pote
- Tlahuext, Hugo,Contreras, Rosalinda
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p. 727 - 730
(2007/10/02)
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- (-)-N-FORMYLNOREPHEDRINE FROM CATHA EDULIS
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Key Word Index - Catha edulis; Celastraceae; (-)-N-formylnorephedrine; (-)-N,O-diformylnorephedrine; formylation; conformation; cisoid; transoid; NMR. The alkaloidal fraction of Catha edulis yielded upon repeated chromatography (-)-N-formylnorephedrine whose (1)H NMR and (13)C NMR spectra suggested the presence of cisoid (major) and transoid forms (minor).The identity of the isolated compound was established by comparison with the major product obtained by formylating (-)-norephedrine; the minor product was found to be (-)-N,O-diformylnorephedrine.
- Al-Meshal, Ibrahim A.,Nasir, Mohammad,El-Feraly, Farouk S.
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p. 2241 - 2242
(2007/10/02)
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