Juergen Liebscher

3-Ylidenepiperazine-2,5-diones as versatile organic substrates

3-Ylidenepiperazine-2,5-diones and 3,6-diylidenepiperazine-2,5-diones are cyclic dipeptides consisting of one or two didehydroamino acid moieties, respectively. Some compounds of this series occur in nature. They can easily be synthesised by several methods also in optically active form and are prone to addition reactions to the C–C double bond by electrophiles (enamine reactivity), nucleophiles (Michael reactivity), radicals, oxidising reagents or 1,3-dipoles, usually in a stereoselective manner. The resulting adducts can further be transformed to natural products and analogues or serve as precursors for interesting α-amino or α-keto acid derivatives by cleavage of the diketopiperazine ring.

Unusual CC Bond Migration in 3-Ylidene-2,5-piperazinediones

Treatment of 3-alkylidene or 3-benzylidene-2,5-piperazinediones 6 with catalytic amounts of acid gives rise to the formation of isomers 7 by migration of the CC bond into the alkyl substituent at position 6, or results in mixtures of racemic 7 and E isomers 8. The existence of tautomeric equilibria is discussed.

Guanidinium-tagged Organocatalysts for Direct Aldol Reactions

Hexasubstituted guanidinium moieties have been used as ionic liquid tags for organocatalyts for the first time. Such conjugates were obtained in the (S)-proline and (S)-pyrrolidine-2-ylmethyl series by alkylation reactions of pentasubstituted guanidines. The resulting guanidinium-tagged organocatalysts were applied to asymmetric direct aldol reactions providing high stereoselectivities and yields and good recyclability, and thus performed better than (S)-proline itself Graphical Abstract Guanidinium-tagged Organocatalysts for Direct Aldol Reactions

Synthesis of optically active 3,4,5,6-tetrahydro-2H-1,4-thiazin-3-ones and their benzo analogues by ring transformation of glycidic esters

Chiral cis and trans glycidic esters 9 substituted by alkyl or ethoxycarbonyl react with cysteamine or o-aminothiophenol 10 by stereoselective ring transformation to non-aromatic optically active 2-(α-hydroxyalkyl)-1,4-thiazin-3-ones 12. The attack of the mercapto function at the α-position of the glycidate occurs predominantly by inversion of configuration. As compared with known reactions of aryl-substituted glycidates with o-aminothiophenol or cysteamine preferentially giving thiazepinones, a remarkable effect of the substituent in the glycidate on the regiochemistry was found.

Novel tandem cyclisations at the piperazinedione ring via cope rearrangement

Novel rearrangements of 3-ylidene-piperazine-2,5-dione (1) were achieved affording Cope rearrangement products 3 and 4 under neutral conditions or tricyclic piperazine-2,5-diones 5, rac-6 and 7 by additional tandem cyclisation in formic acid. Cope rearrangement products 3 and 4 were transformed into new quaternary α-amino acids 9 and 11 by hydrogenation and hydrolysis.

Twisted Structure of a Substituted 2-Chloro-3-(4-chloro-2-methyl-1,3-oxazol-5-yl)-1H-indole

2,2,2-Trichloroethyl 2-chloro-3-(4-chloro-2-methyl-1,3-oxazol-5-yl)-H-indole-1-carboxylate, C 15 H 9 Cl 5 N 2 O 3 , the first synthetic example of an oxazolylindole with a Cl atom at the ortho position of each ring, exhibits a dihedral angle of 45.6 (1)° between the heteroaromatic ring systems.