Applications of Chiral Three-membered Rings for Total Synthesis: A Review

Protonation of P-Stereogenic Phosphiranes: Phospholane Formation via Ring Opening and C-H Activation

Protonation of cyclopropanes and aziridines is well-studied, but reactions of phosphiranes with acids are rare and have not been reported to result in ring opening. Treatment of syn-Mes*PCH₂CHR (Mes* = 2,4,6-(t-Bu)₃C₆H₂, R = Me or Ph, syn-1-2) or anti-Mes*PCH₂CHPh (anti-2) with triflic acid resulted in regiospecific anti-Markovnikov C-protonation with ring opening and cyclophosphination of a Mes* ortho-t-Bu group to yield the phospholanium cations [PH(CH₂CH₂R)(4,6-(t-Bu)₂-2-CMe₂CH₂C₆H₂)][OTf] (R = Me or Ph, 3-4), which were deprotonated with NEt₃ to give phospholanes 5-6. Enantioenriched or racemic syn-1 both gave racemic 3. The byproduct [Mes*PH(CH₂CH₂Me)(OH)][OTf] (7) was formed from syn-1 and HOTf in the presence of water. Density functional theory calculations suggested that P-protonation followed by ring opening and hydride migration to C yields the phosphenium ion, [Mes*P(CH₂CH₂Me)][OTf], which undergoes C-H oxidative addition of an o-t-Bu methyl group. This work established a new reactivity pattern for phosphiranes.

Asymmetric Epoxidation of Olefins with Sodium Percarbonate Catalyzed by Bis-amino-bis-pyridine Manganese Complexes

Asymmetric epoxidation of a series of olefinic substrates with sodium percarbonate oxidant in the presence of homogeneous catalysts based on Mn complexes with bis-amino-bis-pyridine ligands is reported. Sodium percarbonate is a readily available and environmentally benign oxidant that is studied in these reactions for the first time. The epoxidation proceeded with good to high yields (up to 100%) and high enantioselectivities (up to 99% ee) using as low as 0.2 mol. % catalyst loadings. The epoxidation protocol is suitable for various types of substrates, including unfunctionalized alkenes, α,β-unsaturated ketones, esters (cis- and trans-), and amides (cis- and trans-). The reaction mechanism is discussed.

Synthesis of N-methylene phosphonate aziridines: reaction scope and mechanistic insights

Treatment of N-carbamoyl aziridines by the diethyl phosphite anion affords either α-methylene-phosphonate or gem-bisphosphonate derivatives containing an aziridine motif depending on the nature of the base used.

Unusual Transformations of Strain-Heightened Oxetanes

Oxetanes are important motifs for drug discovery and are valuable templates in organic synthesis. Much of their use as synthetic intermediates exploits their inherent strain, often resulting in chain extensions at the expense of the heterocycle. Modifications on the carbon alpha to the oxygen of oxetanes, such as the C═O of β-lactones, extend the modes of reactivity. Nevertheless, the outcomes are still largely predictable. On the other hand, other alpha modifications, such as a ═CH₂, a spiro-oxiranyl moiety, or a spiro-cyclopropyl group, increase strain and open pathways not available to simple oxetanes or β-lactones. Methods in generating 2-methyleneoxetanes, 1,5-dioxaspiro[3.2]hexanes, and 4-oxaspiro[2.3]hexanes have been developed by us and others. To date, reactions of these systems have sometimes been predictable, but often the outcomes have been unexpected. This has provided fertile ground for thinking about what controls reactivity and what other reaction pathways might be accessible to these strain-heightened oxetanes.This Account summarizes the published literature on the most straightforward approaches to 2-methyleneoxetanes, dioxaspirohexanes, and oxaspirohexanes and on their reactivity. In contrast to simple oxetanes, reactions of 2-methyleneoxetanes with nucleophiles at C4 release an enolate rather than an alkoxide. Also, 2-methyleneoxetanes can be converted to homopropargyl alcohols or undergo a silicon accelerated isomerization/electrocyclic ring opening, processes accessible only because of the exocyclic double bond. In addition, oxetane oxocarbenium ions, derived from protonation of the enol ether, can react with nucleophiles to provide 2,2-disubstituted oxetanes. Oxaspirohexanes are readily prepared by Simmons-Smith cyclopropanation of 2-methyleneoxetanes. These unusual systems undergo a variety of substituent dependent rearrangements in the presence of the Lewis acid BF₃·Et₂O. In addition, upon treatment with Zeise's dimer, oxaspirohexanes are transformed to synthetically useful 3-methylenetetrahydrofurans. Dioxaspirohexanes are easily accessed by dimethyldioxirane oxidation of 2-methyleneoxetanes. Predictably, dioxaspirohexanes react with many nucleophiles to give α-functionalized-β'-hydroxy ketones. Unexpectedly, 2,2-disubstituted oxetanes can also be selectively produced. This latter pathway has led to further unusual transformations, illuminating computational studies, and novel routes to biologically relevant molecules.

Nucleophilic Transformations of Lewis Acid-Activated Disubstituted Epoxides with Catalyst-Controlled Regioselectivity

Due to their inherent ring strain and electrophilicity, epoxides are highly attractive building blocks for fundamental organic reactions. However, controlling the regioselectivity of disubstituted epoxide transformations is often particularly challenging. Most Lewis acid-mediated processes take advantage of intrinsic steric or electronic substrate bias to influence the site of nucleophilic attack. Therefore, the scope of many of these systems is frequently quite limited. Recent efforts to generate catalysts that can overcome substrate bias have expanded the synthetic utility of these well-known reactions. In this Perspective, we highlight various regioselective transformations of disubstituted epoxides, emphasizing those that have inspired the production of challenging, catalyst-controlled processes.

A Chiral Macrocyclic Tetra-N-Heterocyclic Carbene Yields an "All Carbene" Iron Alkylidene Complex

The first chiral macrocyclic tetra-N-heterocyclic carbene (NHC) ligand has been synthesized. The macrocycle, prepared in high yield and large scale, was ligated onto palladium and iron to give divalent C2 -symmetric square planar complexes. Multinuclear NMR and single crystal X-ray diffraction demonstrated that there are two distinct NHCs on each ligand, due to the bridging chiral cyclohexane. Oxidation of the iron(II) complex with trimethylamine N-oxide yielded a bridging oxo complex. Diazodiphenylmethane reacted with the iron(II) complex at room temperature to give a paramagnetic diazoalkane complex; the same reaction yielded the "all carbene" complex at elevated temperature. Electrochemical measurements support the assignment of the "all carbene" complex being an alkylidene. Notably, the diazoalkane complex can be directly transformed into the alkylidene complex, which had not been previously demonstrated on iron. Finally, a test catalytic reaction with a diazoalkane on the iron(II) complex does not yield the expected cyclopropane, but actually the azine compound.

Synthesis and biological evaluation of cyanoaziridine phosphine oxides and phosphonates with antiproliferative activity

This work reports an efficient diastereoselective synthetic methodology for the preparation of phosphorus substituted cyanoaziridines through the nucleophilic addition of TMSCN, as cyanide source, to the C-N double bond of 2H-azirine derivatives. The aziridine ring, in these novel cyanoaziridines, can be activated by simple N-tosylation or N-acylation. In addition, the cytotoxic effect on cell lines derived from human lung adenocarcinoma (A549) and human embryonic kidney (HEK293) was also screened. N-H and N-Substituted cyanoaziridines showed excellent activity against the A549 cell line in vitro. Moreover, selectivity towards cancer cell (A549) over (HEK293), and non-malignant cells (MCR-5) has been observed.

A Chimeric Styrene Monooxygenase with Increased Efficiency in Asymmetric Biocatalytic Epoxidation

The styrene monooxygenase (SMO) system from Pseudomonas sp. consists of two enzymes (StyA and StyB). StyB catalyses the reduction of FAD at the expense of NADH. After the transfer of FADH2 from StyB to StyA, reaction with O2 generates FAD-OOH, which is the epoxidising agent. The wastage of redox equivalents due to partial diffusive transfer of FADH2 , the insolubility of recombinant StyB and the impossibility of expressing StyA and StyB in a 1:1 molar ratio reduce the catalytic efficiency of the natural system. Herein we present a chimeric SMO (Fus-SMO) that was obtained by genetic fusion of StyA and StyB through a flexible linker. Thanks to a combination of: 1) balanced and improved expression levels of reductase and epoxidase units, and 2) intrinsically higher specific epoxidation activity of Fus-SMO in some cases, Escherichia coli cells expressing Fus-SMO possess about 50 % higher activity for the epoxidation of styrene derivatives than E. coli cells coexpressing StyA and StyB as discrete enzymes. The epoxidation activity of purified Fus-SMO was up to three times higher than that of the two-component StyA/StyB (1:1, molar ratio) system and up to 110 times higher than that of the natural fused SMO. Determination of coupling efficiency and study of the influence of O2 pressure were also performed. Finally, Fus-SMO and formate dehydrogenase were coexpressed in E. coli and applied as a self-sufficient biocatalytic system for epoxidation on greater than 500 mg scale.