1,721,131 research outputs found
Synthesis of 8,5-fused bicyclic lactam by stereo- and regioselective radical cyclization
No full textA conformationally restricted dipeptide mimetic was synthesized via a regio- and stereoselective radical cyclization of an α-N-acetyl acrylamide
Biosynthesis of mycophenolic acid. Oxidation of 6-farnesyl-5,7-dihydroxy-4- methylphthalide in a cell-free preparation from Penicillium brevicompactum
No full textThe conversion of 6-farnesyl-5,7-dihydroxy-4-methylphthalide (5) into mycophenolic acid (1) proceeds through the hydroxy-ketone (11) by oxidation of the central double bond
Chiral acyl anion and enolonium ion equivalents. Asymmetric synthesis of α-methoxy-aldehydes
No full textThe reaction of the lithium derivative of (+)-(S)-p-tolyl p-tolylthiomethyl sulphoxide [(+)-(S)-(1)] with benzaldehyde and phenylacetaldehyde in tetrahydrofuran at -78 °C gives, after methylation of the hydroxy-group, reduction of the sulphoxide, and hydrolysis of the resulting dithioacetal, the corresponding α-methoxy-aldehydes, (-)-(R)-2-methoxy-2-phenylacetaldehyde and (+)-(R)-2-methoxy-3-phenylpropionaldehyde, enantiomeric excess 70 and 46%, respectively. The reaction of (+)-(S)-(1) with benzaldehyde in tetrahydrofuran at room temperature in the presence of Triton B gives the new optically pure reagent (E)-(+)-(S)-1-p-tolysulphinyl-1-p-tolylthio-2-phenylethylene [(+)-(S)-(2)], which is a chiral enolonium equivalent. The addition of sodium methoxide to (+)-(S)-(2) in methanol gives, after reduction of the sulphoxide and hydrolysis of the resulting dithioacetal, (+)-(S)-2-methoxy-2- phenylacetaldehyde, enantiomeric excess 60%. The unfavourable equilibrium in addition of methoxide to (+)-(S)-(2) means that the latter is not very useful as a chiral enolonium equilvalent in reactions of this type
"Asymmetric Synthesis of Protected alpha-Hydroxyaldehydes via Reduction of alpha-Arylthio-beta-oxosulphoxides
No full textStereoselective Synthesis of t -Butyl 2-Amino-2,5-dideoxy-L-lyxo-pentanoate: Formal Synthesis of L-Daunosamine
No full textEnantiomerically pure t-Butyl 2-amino-2,5-dideoxy---pentanoate 4c is synthesized via the highly diastereoselective MgBr2 mediated addition of dibenzylamino acetate silylketene acetal 2 to O-benzyl lactic aldehyde 3. The synthesis of γ-lactone 5c, a known intermediate in the synthesis of L-daunosamine and L-vancosamine, is also described
Biosynthesis of ascochitine and synthesis of its biogenetic precursors
No full text' Enzymic trap ' experiments show that the main biosynthetic pathway to ascochitine (1) involves the direct reduction of the enzyme-bound ester into aldehyde (4). Therefore the complete pathway has now been assessed. Various synthetic approaches have been tried to synthesize the precursors: the use of 1,3-dithians, of an organolithium equivalent to the Friedel-Crafts reagent, and of phase-transfer reactions allowed for a good introduction of the label
Acetogenin synthesis. Organocopper reagents, anions of 1,3-dithians and of protected cyanohydrins as intermediates in ketide side-chain synthesis
No full textThe synthesis is reported of 3-s-butyl-4-methyl-3,6,8-trihydroxy-3,4- dihydroisocoumarin (1a) and 3-(1-methyl-prop-1-enyl)-4-methyl-3,6,8-trihydroxy- 3,4-dihydroisocoumarin (1b) using organocopper reagents and anions of 1,3-dithians for compound (1a), and anions of 1,3-dithians and of O-trimethylsilylcyanohydrins for compound (1b). The synthetic methods used represent new means for attaining an extensive range of acetogenins of natural origin and of biogenetic intermediates
ASYMMETRIC-SYNTHESIS OF 3,4-CIS-SUBSTITUTED BETA-LACTAMS VIA CHIRAL NOREPHEDRINE-DERIVED OXAZOLIDINES
No full textA diastereo- and enantioselective approach to functionalized 3,4-cis-β-lactams 9 and 16 was developed based on the use of chiral norephedrine-derived oxazolidines. The key-steps in the synthesis of 9 (Scheme 1) are the potassium hypochlorite epoxidation of aldehyde 1 and the lithium dimethylcuprate addition to acid 2, both steps proceed regio- and stereoselectively (98%) and in high yield. Standard synthetic methods and the Miller hydroxamate procedure for NC cyclization were used to complete the synthesis of the target cis-β-lactwn 9 (98% enantiomeric excess). In the synthesis of cis-substituted 3-amino-2-azetidinone 16 (Scheme 4) the key-step is the aqueous ammonia opening of epoxy acid 2 which proceeds regio- and stereoselectively (98%). The Miller-type cyclization of hydroxamate 15a under Mitsunobu conditions failed to give 16a in a yield higher than 35%
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