Diversity-Oriented Synthesis of N,N,O-Trisubstituted Hydroxylamines from Alcohols and Amines by N-O Bond Formation

Atypical N-Alkyl to N-Noralkoxy Switch in a Dual cSRC/BCR-ABL1 Kinase Inhibitor Improves Drug Efflux and hERG Affinity

We describe an atypical amine bioisostere, the trisubstituted hydroxylamine, that upon incorporation into an approved dual cSRC/BCR-ABL1 kinase inhibitor yields 9, a compound that retains potent biological activity and couples it with improved drug efflux and hERG affinity at the expense of only a 2 atomic mass unit increase in molecular weight. Contrary to the common expectation for hydroxylamines in medicinal chemistry, 9 is well tolerated in vivo and lacks the mutagenicity and genotoxicity so often ascribed to lesser substituted hydroxylamines. A matched molecular pair (MMP) analysis suggests that the beneficial properties conferred by the N-alkyl to N-noralkoxy switch arises from a reduction in basicity of the piperazine unit. Overall, these results lend additional support to the use of trisubstituted hydroxylamines as bioisosteres of N-alkyl groups that are not involved in key polar interactions.

Discovery of a Hydroxylamine-Based Brain-Penetrant EGFR Inhibitor for Metastatic Non-Small-Cell Lung Cancer

Metastases to the brain remain a significant problem in lung cancer, as treatment by most small-molecule targeted therapies is severely limited by efflux transporters at the blood-brain barrier (BBB). Here, we report the discovery of a selective, orally bioavailable, epidermal growth factor receptor (EGFR) inhibitor, 9, that exhibits high brain penetration and potent activity in osimertinib-resistant cell lines bearing L858R/C797S and exon19del/C797S EGFR resistance mutations. In vivo, 9 induced tumor regression in an intracranial patient-derived xenograft (PDX) murine model suggesting it as a potential lead for the treatment of localized and metastatic non-small-cell lung cancer (NSCLC) driven by activating mutant bearing EGFR. Overall, we demonstrate that an underrepresented functional group in medicinal chemistry, the trisubstituted hydroxylamine moiety, can be incorporated into a drug scaffold without the toxicity commonly surmised to accompany these units, all while maintaining potent biological activity and without the molecular weight creep common to drug optimization campaigns.

Recent Advances in the Synthesis of Di- and Trisubstituted Hydroxylamines

As an underrepresented functional group in bioorganic and medicinal chemistry, the hydroxylamine unit has historically received little attention from the synthetic community. Recent developments, however, suggest that hydroxylamines may have broader applications such that a review covering recent developments in the synthesis of this functional group is timely. With this in mind, this review primarily covers developments in the past 15 years in the preparation of di- and trisubstituted hydroxylamines. The mechanism of the reactions and key features and shortcomings are discussed throughout the review.

Bis(arylsulfonyl) Peroxide-Mediated Difunctionalization of Cyclic Enol Ethers

A bis(arylsulfonyl) peroxide-mediated 1,2-difunctionalization of cyclic enol ethers is reported. Bis(nosyl) peroxide selectively sulfonylates the 3-position of enol ethers, generating an oxocarbenium ion that is trapped by a carboxylic acid nucleophile at the 2-position. The reaction proceeds in a good yield and tolerates a variety of cyclic enol ethers, including glycals as well as various carboxylic acids, which act as the oxocarbeium ion trapping reagents.

A redox-enabled strategy for intramolecular hydroamination

Metal- or acid-catalyzed intramolecular hydroamination and Cope-type intramolecular hydroamination, a distinct concerted approach using hydroxylamines, typically suffer from significant synthetic limitations. Herein we report a process for intramolecular hydroamination that uses a redox-enabled strategy relying on efficient in situ generation of hydroxylamines by oxidation, followed by Cope-type hydroamination, then reduction of the resulting pyrrolidine N-oxide. The steps are performed sequentially in a single pot, no catalyst is required, the conditions are mild, the process is highly functional group tolerant, and no chromatography is generally required for isolation. A robustness screen and a gram-scale example further support the practicality of this approach.

The N,N,O-Trisubstituted Hydroxylamine Isostere and Its Influence on Lipophilicity and Related Parameters

The influence of substitution of an N,N,O-trisubstituted hydroxylamine (-NR-OR'-) unit for a hydrocarbon (-CHR-CH2-), ether (-CHR-OR'-), or amine (-NR-CHR'-) moiety on lipophilicity and other ADME parameters is described. A matched molecular pair analysis was conducted across five series of compounds, which showed that the replacement of carbon-carbon bonds by N,N,O-trisubstituted hydroxylamines typically leads to a reduction in logP comparable to that achieved with a tertiary amine group. In contrast, the weakly basic N,N,O-trisubstituted hydroxylamines have greater logD 7.4 values than tertiary amines. It is also demonstrated that the N,N,O-trisubstituted hydroxylamine moiety can improve metabolic stability and reduce human plasma protein binding relative to the corresponding hydrocarbon and ether units. Coupled with recent synthetic methods for hydroxylamine assembly by N-O bond formation, these results provide support for the re-evaluation of the N,N,O-trisubstituted hydroxylamine moiety in small-molecule optimization schemes in medicinal chemistry.

Highly efficient C(CO)-C(alkyl) bond cleavage in ketones to access esters over ultrathin N-doped carbon nanosheets

Selective oxidative cleavage of the C(CO)–C bond in ketones to access esters is a highly attractive strategy for upgrading ketones. However, it remains a great challenge to realize this important transformation over heterogeneous metal-free catalysts. Herein, we designed a series of porous and ultrathin N-doped carbon nanosheets (denoted as CN-X, where X represents the pyrolysis temperature) as heterogeneous metal-free catalysts. It was observed that the fabricated CN-800 could efficiently catalyze the oxidative cleavage of the C(CO)–C bond in various ketones to generate the corresponding methyl esters at 130 °C without using any additional base. Detailed investigations revealed that the higher content and electron density of the graphitic-N species contributed to the excellent performance of CN-800. Besides, the high surface area, affording active sites that are more easily accessed, could also enhance the catalytic activity. Notably, the catalysts have great potential for practical applications because of some obvious advantages, such as low cost, neutral reaction conditions, heterogeneous nature, high efficiency, and broad ketone scope. To the best of our knowledge, this is the first work on efficient synthesis of methyl esters via oxidative esterification of ketones over heterogeneous metal-free catalysts.

Ultrathin and metal-free N-doped carbon nanosheets showed high activity and selectivity for oxidative esterification of ketones via C(CO)–C bond cleavage to access methyl esters.

Synthesis of 10-Aza-9-oxakalkitoxin by N-O Bond Formation

We describe a formal synthesis of 10-aza-9-oxakalkitoxin, the hydroxalog of the cytotoxic marine natural product kalkitoxin, that features Mukaiyama Markovnikov silyl peroxidation of a terminal alkene and N-O bond formation as the central enabling steps.

Rapid, One-Step Synthesis of α-Ketoacetals via Electrophilic Etherification

Herein, we report a rapid, one-step synthesis of α-ketoacetals via electrophilic etherification of α-alkoxy enolates and monoperoxyacetals. Methyl, primary, and secondary α-ketoacetals were obtained in 44-63% yields from tetrahydropyranyl substrates; using methyl tetrahydropyranyl, alkyl tetrahydropyranyl, or methyl tetrahydrofuranyl peroxyacetals, however, methyl and primary products were isolated in 66-90% yields. The present method is applied to C-O bond formation at tertiary carbons, via alkyl and methyl peroxyacetals, with yields of 25-65%. Intermolecular "alkoxyl" transfer, from peroxyacetal to α-alkoxy enolate, relies heavily on decreased steric bulk surrounding the peroxide bond and site of etherification; additionally, we found the α-OCH₃ group to be critical in ensuring product formation. α-Ketoacetals demonstrated excellent reactivity, as selective, nucleophilic attack at the unprotected carbonyl furnished α-hydroxy acetals in 80-100% yields; subsequent hydrolysis of the foregoing compounds provided α-hydroxy aldehydes in yields of 58-90%.

Diaryliodonium Salt-Based Synthesis of N-Alkoxyindolines and Further Insights into the Ishikawa Indole Synthesis

A diaryliodonium salt-based strategy enabled the first systematic synthesis of rarely accessible N-alkoxyindolines. Mechanistic analyses suggested that the reaction likely involves reductive elimination of iodobenzene from iodaoxazepine via a four-membered transition state, followed by Meisenheimer rearrangement. Substrates with N-carbamate protection afforded indole in a manner similar to that of the Ishikawa indole synthesis. Preinstallation of a stannyl group as an iodonium salt precursor greatly expanded the substrate scope, and further mechanistic insights are discussed.

Synthesis of O-tert-Butyl-N,N-disubstituted Hydroxylamines by N-O Bond Formation

The reaction of magnesium amides with tert-butyl 2,6-dimethyl perbenzoate in tetrahydrofuran at 0 °C provides a method for the synthesis O-tert-butyl-N,N-disubstituted hydroxylamines by direct N-O bond formation with a broad functional group tolerance. Less sterically hindered magnesium amides require ortho,ortho-disubstitution on the perester electrophile component, whereas sterically encumbered magnesium amides perform comparably with either tert-butyl perbenzoate or tert-butyl 2,6-dimethyl perbenzoate. A reaction mechanism is presented to account for the observed reactivity.

The Bond Dissociation Energy of the N-O Bond

Bond dissociation energy (BDE) has been calculated for a series of compounds that contain N-O bonds. These structures encompass model N,N,O-trisubstituted hydroxylamines that include O-methoxy, O-acyl, and O-phenyl hydroxylamines. The calculations used three accurate composite methods, CBS-QB3, CBS-APNO, and G4 methods and the computationally more affordable M06-2X/6-311+G(3df,2p) density functional theory (DFT) functional. The calculated N-O single-bond BDEs are 5-15 kcal/mol higher than a generic N-O BDE of 48 kcal/mol quoted in the literature and in textbooks. The M06-2X DFT functional provides BDEs that are in excellent agreement with the higher-level composite methods. We also provided a comparison of the N-O BDE for pyridine-N-oxide to simple trialkylamine oxides. Based on an experimental BDE of 63.3 ± 0.5 kcal/mol for pyridine-N-oxide, our best estimate gives 56.7 ± 0.9 kcal/mol N-O BDE for trimethylamine-N-oxide and 59.0 ± 0.8 kcal/mol for triethylamine-N-oxide.

En Route to the Transformation of Glycoscience: A Chemist's Perspective on Internal and External Crossroads in Glycochemistry

Carbohydrate chemistry is an essential component of the glycosciences and is fundamental to their progress. This Perspective takes the position that carbohydrate chemistry, or glycochemistry, has reached three crossroads on the path to the transformation of the glycosciences, and illustrates them with examples from the author's and other laboratories. The first of these potential inflexion points concerns the mechanism of the glycosylation reaction and the role of protecting groups. It is argued that the experimental evidence supports bimolecular SN2-like mechanisms for typical glycosylation reactions over unimolecular ones involving stereoselective attack on naked glycosyl oxocarbenium ions. Similarly, it is argued that the experimental evidence does not support long-range stereodirecting participation of remote esters through bridged bicyclic dioxacarbenium ions in organic solution in the presence of typical counterions. Rational design and improvement of glycosylation reactions must take into account the roles of the counterion and of concentration. A second crossroads is that between mainstream organic chemistry and glycan synthesis. The case is made that the only real difference between glycan and organic synthesis is the formation of C-O rather than C-C bonds, with diastereocontrol, strategy, tactics, and elegance being of critical importance in both areas: mainstream organic chemists should feel comfortable taking this fork in the road, just as carbohydrate chemists should traveling in the opposite direction. A third crossroads is that between carbohydrate chemistry and medicinal chemistry, where there are equally many opportunities for traffic in either direction. The glycosciences have advanced enormously in the past decade or so, but creativity, input, and ingenuity of scientists from all fields is needed to address the many sophisticated challenges that remain, not the least of which is the development of a broader and more general array of stereospecific glycosylation reactions.

Synthesis of trisubstituted hydroxylamines by a visible light-promoted multicomponent reaction

A green and efficient route for the synthesis of trisubstituted hydroxylamines from β-keto ester, 2-nitrosopyridine and aryldiazoacetates has been reported. This multicomponent reaction occurred under mild conditions without catalysts or additives.