Protonation-Assisted Conjugate Addition of Axially Chiral Enolates: Asymmetric Synthesis of Multisubstituted β-Lactams from α-Amino Acids

A Computational Study on the Reaction Mechanism of Stereocontrolled Synthesis of β-Lactam within [2]Rotaxane

The macrocycle effect of [2]rotaxane on the highly trans-stereoselective cyclization reaction of N-benzylfumaramide was extensively investigated by various computational methods, including DFT and high-level DLPNO-CCSD(T) methods. Our computational results suggest that the most favorable mechanism of the CsOH-promoted cyclization of the fumaramide into trans-β-lactam within [2]rotaxane initiates with deprotonation of a N-benzyl group of the interlocked fumaramide substrate by CsOH, followed by the trans-selective C-C bond formation and protonation by one amide functional group of the macrocycle. Our distortion/interaction analysis further shows that the uncommon trans-stereoselective cyclization forming β-lactam within the rotaxane may be attributed to a higher distortion energy (mainly from the distortion of the twisted cis-fumaramide conformation enforced by the rotaxane). Our systematic study should give deeper mechanistic insight into the reaction mechanism influenced by a supramolecular host.

Novel Strategies for Enantio- and Site-Selective Molecular Transformations

A strategy for symmetric synthesis based on dynamic chirality of enolates (memory of chirality) has been developed. Asymmetric alkylation, conjugate addition, aldol reaction, and arylation via C-N axially chiral enolate intermediates are described. Asymmetric alkylation and conjugate addition via C-O axially chiral enolate intermediates with a half-life of racemization as short as approx. 1 s. at -78 °C have been accomplished. Organocatalysts for asymmetric acylation and site-selective acylation have been developed. Kinetic resolution of racemic alcohols via remote asymmetric induction by the catalyst is shown. Catalyst-controlled site-selective acylation of carbohydrates and its application to total synthesis of natural glycoside are described. Chemo-selective monoacylation of diols and selective acylation of secondary alcohols with reversal of inherent reactivity are also discussed. Geometry-selective acylation of tetrasubstituted alkene diols is achieved, where acylation takes place independent from the steric environments of the substrates.

Challenges and Opportunities in the Post-Synthetic Modification of Interlocked Molecules

Several strategies have been successfully utilised to obtain a wide range of interlocked molecules. However, some interlocked compounds are still not obtained directly and/or efficiently from non-interlocked components because the requisites for self-assembly cannot always be enforced. To circumvent such a synthetic problem, a strategy that consists of synthesizing an isolable and storable interlocked building block in a step that precedes its modification is an appealing chemical route to more sophisticated interlocked molecules. Synthetic opportunities and challenges are closely linked to the fact that the mechanical bond might greatly affect the reactivity of a functionality of the encircled axle, but that the interlocked architecture needs to be preserved during the synthesis. Hence, the mechanical bond plays a fundamental role in the strategy employed. This Review focuses on the challenging post-synthetic modifications of interlocked molecules, sometimes through cleavage of the axle's main chain, but always with conservation of the mechanical bond.

Azetidin-2-ones: structures of anti-mitotic compounds based on the 1-(3,4,5-tri-meth-oxy-phen-yl)azetidin-2-one core

A series of related substituted 1-(3,4,5-tri-meth-oxy-phen-yl)azetidin-2-ones have been characterized: 3-(4-fluoro-phen-yl)-4-(4-meth-oxy-phen-yl)-1-(3,4,5-tri-meth-oxy-phen-yl)azetidin-2-one, C25H24FNO5 (1), 3-(furan-2-yl)-4-(4-meth-oxy-phen-yl)-1-(3,4,5-tri-meth-oxy-phen-yl)azetidin-2-one, C23H23NO6 (2), 4-(4-meth-oxyphen-yl)-3-(naphthalen-1-yl)-1-(3,4,5-tri-meth-oxy-phen-yl)azetidin-2-one, C29H27NO5 (3), 3-(3,4-di-meth-oxy-phen-yl)-4-(4-meth-oxy-phen-yl)-1-(3,4,5-tri-meth-oxy-phen-yl)azetidin-2-one, C27H29NO7 (4) and 4,4-bis-(4-meth-oxy-phen-yl)-3-phenyl-1-(3,4,5-tri-meth-oxy-phen-yl)azetidin-2-one, C32H31NO6 (5). All of the compounds are racemic. The lactam and 3,4,5-tri-meth-oxy-phenyl rings are approximately co-planar and the orientation of the lactam and the 4-meth-oxy-phenyl substituent is approximately orthogonal. The chiral centres, although eclipsed by geometry, have torsion angles ranging from -7.27 to 13.08° for the 3 position, and -8.69 to 13.76° for the 4 position of the β-lactam. The structures display intra-molecular C-H⋯O bonding between the 3,4,5-tri-meth-oxy-phenyl ring and the lactam ketone. Further C-H⋯O inter-actions are observed and form either an opposing meth-oxy 'buckle' to join two mol-ecules together or a cyclic dimer.

Memory of Chirality Concept in Asymmetric Intermolecular Michael Addition of α-Amino Ester Enolates to Enones and Nitroalkenes

A highly stereoselective asymmetric intermolecular conjugate addition of α-amino ester derivatives to cyclic enones via the memory of chirality (MOC) concept in high yields with excellent diastereo- and enantioselectivity (dr >99:1, up to 99% ee) is reported. The applicability and the generality of the strategy was demonstrated by its further exploration to acyclic α,β-unsaturated ketone and aromatic nitroalkenes, resulting in the formation of δ-keto-α-amino ester derivative and γ-nitro-α-amino ester derivatives, respectively, with excellent ee and dr.

Stereocontrolled Synthesis of β-Lactams within [2]Rotaxanes: Showcasing the Chemical Consequences of the Mechanical Bond

The intramolecular cyclization of N-benzylfumaramide [2]rotaxanes is described. The mechanical bond of these substrates activates this transformation to proceed in high yields and in a regio- and diastereoselective manner, giving interlocked 3,4-disubstituted trans-azetidin-2-ones. This activation effect markedly differs from the more common shielding protection of threaded functions by the macrocycle, in this case promoting an unusual and disfavored 4-exo-trig ring closure. Kinetic and synthetic studies allowed us to delineate an advantageous approach toward β-lactams based on a two-step, one-pot protocol: an intramolecular ring closure followed by a thermally induced dethreading step. The advantages of carrying out this cyclization in the confined space of a benzylic amide macrocycle are attributed to its anchimeric assistance.

Divergent, stereoselective access to heterocyclic α,α-quaternary- and β(2,3,3)-amino acid derivatives from a N-Pmp-protected Orn-derived β-lactam

A suitably protected Orn-derived (3S,4S)-β-lactam was used as common intermediate in the synthesis of conformationally constrained (3S,4S)-2-oxoazepane α,α- and (2S,3S)-2-oxopiperidine-β(2,3,3)-amino acid derivatives. Compared to alternative procedures using an N-p-methoxybenzyl group at the 2-azetidinone, the incorporation of a p-methoxyphenyl moiety is crucial for the excellent stereochemical outcomes in the preparation of these heterocyclic amino acids. Chemoselective 7- or 6-exo-trig cyclization was achieved through alternative sequences of Pmp-deprotection/Boc-activation, followed by inter- and intramolecular β-lactam ring opening, respectively.

Experimental and theoretical studies on the rearrangement of 2-oxoazepane α,α-amino acids into 2'-oxopiperidine β(2,3,3) -amino acids: an example of intramolecular catalysis

Enantiopure β-amino acids represent interesting scaffolds for peptidomimetics, foldamers and bioactive compounds. However, the synthesis of highly substituted analogues is still a major challenge. Herein, we describe the spontaneous rearrangement of 4-carboxy-2-oxoazepane α,α-amino acids to lead to 2'-oxopiperidine-containing β(2,3,3) -amino acids, upon basic or acid hydrolysis of the 2-oxoazepane α,α-amino acid ester. Under acidic conditions, a totally stereoselective synthetic route has been developed. The reordering process involved the spontaneous breakdown of an amide bond, which typically requires strong conditions, and the formation of a new bond leading to the six-membered heterocycle. A quantum mechanical study was carried out to obtain insight into the remarkable ease of this rearrangement, which occurs at room temperature, either in solution or upon storage of the 4-carboxylic acid substituted 2-oxoazepane derivatives. This theoretical study suggests that the rearrangement process occurs through a concerted mechanism, in which the energy of the transition states can be lowered by the participation of a catalytic water molecule. Interestingly, it also suggested a role for the carboxylic acid at position 4 of the 2-oxoazepane ring, which facilitates this rearrangement, participating directly in the intramolecular catalysis.