The key factors for carbonyl‐stabilised ammonium ylide‐mediated epoxidation reactions were systematically investigated by experimental and computational means and the hereby obtained energy profiles provide explanations for the observed experimental results. In addition, we were able to identify the first tertiary amine‐based chiral auxiliary that allows for high enantioselectivities and high yields for such epoxidation reactions.
The use of readily accessible ammonium ylides for (asymmetric) transformations, especially cyclization reactions, has received considerable attention over the past two decades. A variety of highly enantioselective protocols to facilitate annulation reactions have recently been introduced as an alternative to other common methods including S-ylide-mediated strategies. It is the intention of this short review to provide an introduction to this field by highlighting the potential of ammonium ylides for (asymmetric) cyclization reactions as well as to present the limitations and challenges of these methods.
The
first (4 + 1)-annulation of o-quinone methides
with α-branched allenoates as C1 synthons has been developed.
This operationally simple protocol gives access to highly functionalized
dihydrobenzofurans in an unprecedented fashion with excellent diastereoselectivities
and high yields.
A novel high yielding, enantio- and diastereoselective protocol for the synthesis of α-allylated highly functionalised β-amino acid derivatives by adding isoxazolidin-5-ones to MBH carbonates under asymmetric phase-transfer catalysis has been developed.
The first highly asymmetric catalytic synthesis of densely functionalized dihydrobenzofurans is reported, which starts from
ortho
‐hydroxy‐containing
para
‐quinone methides. The reaction relies on an unprecedented formal [4+1]‐annulation of these quinone methides with allenoates in the presence of a commercially available chiral phosphine catalyst. The chiral dihydrobenzofurans were obtained as single diastereomers in yields up to 90 % and with enantiomeric ratios up to 95:5.
Ionene polymers were found to be versatile phase-transfer catalysts for a variety of different reactions like a-alkylation reactions or Michael addition reactions. Reasonably low catalyst loadings allowed us to obtain the target products under operationally simple biphasic conditions at ambient temperature and the ionenes can be recovered and reused by simple filtration.Ionene polymers constitute an important class of polyelectrolytes [1] in which quaternary ammonium groups form part of the polymer backbone. [2] These polymers are typically prepared either by chain or step polymerization of suitable monomers (e. g., self-polyaddition of aminoalkylhalides, Menshutkin reaction between bis-tertiary amines and activated dihalides) or by cationic functionalization of reactive precursor polymers. [3] The first ionene was synthesized by Marvel and co-workers in 1930, [4] and since then ionenes have been widely studied for a number of applications in chemistry, biology, physics, medicine and materials science. [2] Some of us have recently described the synthesis of new ionene polymers containing DABCO and N,N'-(x-phenylene)dibenzamide (x = ortho-/meta-/para-) and their intriguing self-assembly properties in aqueous medium (Scheme 1). [5] On the other hand, the use of quaternary ammonium salts as phase-transfer catalysts (PTCs) has emerged as one of the fundamental principles in catalysis, which found widespread applications on laboratory scale and for industrial processes [6] by using either achiral or chiral ammonium salt catalysts. [7] Based on our interests in the design and development of new (asymmetric) PTCs [8] and the above-mentioned functional ionenes, [5] we report here the first evidences of the catalytic potential of some ammonium ionene polymers for different fundamental reactions under phase-transfer conditions.We put our main focus on the alkylation of glycine Schiff base 1 under biphasic conditions. This transformation, which gives access to a variety of natural and non-natural amino acids in either a racemic [9] or an enantioenriched manner, [10] turned out to be one of the benchmark reactions for novel PTCs in the past. Scheme 2 shows a simplified mechanistic illustration for the possible function of ionene polymers A as phase-transfer catalysts for this transformation (as these polymers show a rather limited solubility in organic solvents and aqueous solutions an interfacial mechanism seems to be most likely but it must be pointed out that presently no further experimental proof therefore is available). Table 1 gives an overview of the most significant results obtained in a thorough screening of the alkylation of 1 with benzylbromide (2) by using the three regioisomeric ionenes A under different biphasic conditions. The reactions proceeded [a] M.
A straightforward phosphine-catalyzed formal [4+2] annulation between α-branched allenoates and arylidene azlactones has been developed to access highly functionalized spirocyclohexenes. This cyclization favors the γ-addition of the phosphine-activated allenoates over a β’-addition pathway. Detailed computational studies support the proposed mechanism and provide a reasonable explanation for the observed regioselectivity and the noted effect of the catalyst.
The use of carbonyl‐stabilized ammonium‐ and sulfonium ylides allows for the synthesis of highly‐functionalized trifluoroacetyl‐substituted cyclopropanes. It turned out that the diastereoselectivities strongly depend on the nature of the chosen ylide and the employed base. The products could be obtained in good yields under operationally simple conditions.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.