Asymmetric and Geometry‐Selective α‐Alkenylation of α‐Amino Acids

Abstract

Both E‐ and Z‐N′‐alkenyl urea derivatives of imidazolidinones may be formed selectively from enantiopure α‐amino acids. Generation of their enolate derivatives in the presence of K+ and [18]crown‐6 induces intramolecular migration of the alkenyl group from N′ to Cα with retention of double bond geometry. DFT calculations indicate a partially concerted substitution mechanism. Hydrolysis of the enantiopure products under acid conditions reveals quaternary α‐alkenyl amino acids with stereodivergent control of both absolute configuration and double bond geometry.

s-Block cooperative catalysis: alkali metal magnesiate-catalysed cyclisation of alkynols

“…As the reaction is inverse rst order in the substrate (vide infra) it is assumed that the incoming alkynol coordinates to the single Mg centre rather than the K centres, which are already highly coordinated by bonding to the crown ether. In previous studies, the addition of 18-crown-6 has had both benecial 78,81 and detrimental 77 impacts on synthetic performance. Here we observe an improvement in catalytic activity, which could perhaps be explained by the 18-crown-6 that sufficiently satises the coordination environment of the metal, preventing further excess alcohol coordination from occurring which would cause inhibition.…”

“…Although the crown ether is oen expected to sequester the alkali metal there are examples of structures where the alkali metal is still able to maintain p-interactions with the anionic moiety whilst coordinated by the crown ether. [71][72][73][74][75][76][77][78] Thus it appears that under these conditions, the addition of the crown ether must tune the coordination environment of K, modifying its Lewis acidity to generate a more catalytically competent species (vide infra). Therefore, these results suggest that the alkali metal is intimately involved and plays a key role in the cyclisation reaction.…”

Concerted Nucleophilic Aromatic Substitution Reactions

“…Prior to this most recent work, Clayden et al had studied extensive alternative applications of these aryl transfer reactions,notably in ring-expansion reactions, [78][79][80][81][82][83] for example,with substrates 240, 241. [78] Although Hammett plots are not reported for these series,t he analogy to the amino acid cases just discussed make it highly likely that they follow cS N Ar pathways through transition states 244.Inthe presence of dimethylpropyleneurea (DMPU) and lithium diisopropylamide (LDA), ring-expanding isomerisation was effected with excellent retention of stereochemistry.Thus,the initially generated benzyllithium is configurationally stable for the period needed to carry out the rearrangement.…”

Konzertierte nukleophile aromatische Substitutionen

“…In diesen Fällen übernimmt der Schritt der Enolatbildung diese Rolle. Dieser jüngsten Arbeit vorangehend haben Clayden et al umfangreiche alternative Anwendungen dieser Aryltransferreaktionen untersucht, insbesondere bei Ringexpansionsreaktionen,[78][79][80][81][82][83] z. B. mit den Substraten 240 und 241 [78].…”