- A concise synthesis of (+)-conagenin and its isomer using chiral tricyclic iminolactones
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An efficient synthesis of (+)-conagenin, a novel immunomodulator produced by Streptomyces roseosporus, has been achieved via the shortest route at present. At the same time, 2-epiconagenin was synthesized according to the same methodology.
- Wang, Hai-Fei,Ma, Guo-Hua,Yang, Shao-Bo,Han, Rong-Gang,Xu, Peng-Fei
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p. 1630 - 1635
(2008/12/21)
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- ENANTIOSELECTIVE SYNTHESIS OF NON-PROTEINOGENIC AMINO ACIDS VIA METALLATED BIS-LACTIM ETHERS OF 2,5-DIKETOPIPERAZINES
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Bis-lactim ethers 1 of 2,5-diketopiperazines contain a chiral inducing center, an acidic CH-bond and two sites susceptible to hydrolysis.They react with BuLi to give Li compounds of type 4, 15, 29 or 32, which possess a prochiral C atom.They readily add electrophiles (such as alkylating agents or carbonyl compounds) with unusually high diastereoface differentiation.In many cases the d.e-value (d.e. = diastereomeric excess = asymmetric induction) of the adduct exceeds 95percent.On hydrolysis the adducts are cleaved liberating the chiral auxiliary (used to build up the bis-lactim ether 1) and the target molecules, the optically active amino acid methyl esters of type 8, 19, 25 or 36.The two amino acid esters are separable either by fractional distillation or (eventually after further hydrolysis to amino acids) by chromatography.Transition state models are discussed that could explain the exceptionally high asymmetric induction and the predictability of the induced configuration.
- Schoellkopf, Ulrich
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p. 2085 - 2092
(2007/10/02)
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- Asymmetric Syntheses via Heterocyclic Intermediates, X. - Enantioselective Synthesis of (2R)-2-Methylserines
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Aldehydes and ketones react with the lithiated bislactim ether 3 of cyclo-(L-Ala-L-Ala) with 81 to >95percent asymmetric induction (d.e. = diastereomeric excess) at C-3; (R) configuration is formed predominantly. - A model concept for the asymmetric induction is proposed. - With aldehydes or unsymmetrical ketones C-7 of the adducts 6 becomes a chiral center, too. (R) configuration is induced here with d. e. 47 - 73percent.Hydrolysis of the addition products 6 (0.25 N HCl, room temperature) gives L-Ala-OCH3 and (2R)-2-methylserine methyl esters 7.Both compounds can be separated either at the ester stage by distillation or - if 7 is thermolabile - after further hydrolysis at the amino acid stage.
- Schoellkopf, Ulrich,Groth, Ulrich,Hartwig, Wolfgang
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p. 2407 - 2418
(2007/10/02)
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