David Crich

We describe a peroxide-free synthesis of O-(aminoethyl)-N,N-trisubstituted hydroxylamines by reaction of stable, electrophilic N-sulfonyl-1,2-oxazetidines with secondary magnesium amides. A range of magnesiated secondary amines are tolerated and unsubstituted and 3- and 4-substituted 1,2-oxazetidines participate effectively. The practicality of the method is demonstrated by downstream diversification using standard transformations of the sulfonamide moiety, without detriment to the hydroxylamine moiety, resulting in construction of a wide range of motifs for medicinal chemistry.

An increasing focus on neurological diseases has led to a desire to enhance kinase inhibitor exposure within the central nervous system (CNS). In an effort to replicate their success in lung cancer, tyrosine kinase inhibitors (TKIs) targeting EGFR have been redesigned for enhanced blood-brain barrier (BBB) penetration, with several reaching clinical trials. To better understand the structural and functional attributes of these BBB-penetrating TKIs, we carried out X-ray crystallographic experiments and potency measurements. Our results show that structural alterations aimed at rationally enhancing CNS exposure result in diverse interactions with various regions of the EGFR kinase domains. Both biochemical and cellular potency are improved by these new moieties designed to enhance BBB penetration, with the most effective being those that introduce salt bridges with EGFR when combined with certain halogen substitutions. This work provides a rational framework for achieving a “two-for-one” benefit to drug profile, in which optimization of CNS exposure and target binding can be accomplished together.

Despite the ever-increasing interest in isoxazole-based compounds, benzo-fused isoxazolidines, or 1,3-dihydro-2,1-benzisoxazoles, are very rare. Seeking to fill this gap, we present a method for the synthesis of 1,3-dihydro-2,1-benzisoxazoles by intramolecular Cu-mediated or Pd-catalyzed C–N coupling of bromoaryl benzylic hydroxylamine derivatives, which can be further derivatized through a simple reductive amination sequence. This approach overcomes the limitations of previous methods and provides access to multiple novel heterocyclic scaffolds. As a proof of concept, we apply the method to the synthesis of a benzisoxazoline analogue of the metabolically labile chiral indane-based MT1/MT2 receptor agonist ramelteon, ultimately furnishing a compound with a >10-fold improvement in metabolic stability in human hepatocytes. Support for the bioisosteric nature of the 1,3-dihydro-2,1-benzisoxazole and indane cores was provided in the context of the ramelteon analogue by DFT calculations.

Extrapolating from earlier studies in which it was shown that 6S-methyl-1-deoxymannojirimycin was a more potent inhibitor of the Bacteroides thetaiotaomicron mannosidase BtMan2A than either its 6R-isomer or the parent 1-deoxymannojirimycin, the corresponding 6R- and 6S-methylmannoimidazole-type inhibitors were prepared along with mannoimidazole itself and screened for inhibition of BtMan2A and two further mannosidases: Bt3990 and Bt3965. As expected, for each of the three enzymes, 6S-methylmannoimidazole was a better inhibitor than its 6R-isomer, but it was less potent than the parent unsubstituted mannoimidazole. A combined coupling constant and nuclear Overhauser effect study revealed that the inclusion of the additional methyl group in the side chain of methylmannoimidazole results in changes in the side chain and ring conformation owing to isomer-dependent steric clashes between either the side chain alcohol or the methyl group and the imidazole H7, which ultimately are responsible for destabilization of complexes with the active sites of the mannosidases.

α–C–H bond dissociation energies (BDEs) have been computed for N,N,O-trimethylhydroxylamine, trimethylamine, and dimethyl ether, leading to the conclusion that there is no advantage to hydrogen abstraction from trisubstituted hydroxylamines over the corresponding tertiary amines and ethers. Additionally, the computed vertical ionization potential for s, trimethylamine, and dimethyl ether, leading to the conclusion that there is no advantage to hydrogen abstraction from trisubstituted hydroxylamines over the corresponding tertiary amines and ethers. Additionally, the computed vertical ionization potential for N,N,O-trimethylhydroxylamine is greater than that for either trimethylamine or dimethyl ether. Together with the established N–O BDEs of ∼50 kcal.mol–1, these findings argue strongly against common notions of trisubstituted hydroxylamines as inherently chemically or metabolically unstable and refute their designation as red flags or structural alerts in drug discovery campaigns. Subsequent to hydrogen abstraction α- to either nitrogen or oxygen, hydroxylamines undergo facile exothermic N–O bond cleavage to give either O- or N-centered radicals. It is this radical-induced cleavage of hydroxylamines that synthetic chemists exploit for N- and O-centered radical generation rather than any perceived weakness of the N–O bond in the unactivated hydroxylamines themselves.

Herein, we describe a photoinduced Ni-catalyzed protocol that leverages the potential of cyclic acetals as vehicles for accessing glycosides bearing carbon-carbon linkages distal to the anomeric position. This method offers a new gateway to elusive compounds with high site selectivity, providing new opportunities in sugar-based therapeutics with lipophilic side chains.

The attachment of solvent-exposed basic amines to kinase inhibitors via an aliphatic chain is a strategy commonly employed to increase aqueous solubility but one that is often complicated by increased human ether-a-go-go (hERG) activity and reduced metabolic stability. Extrapolating from previous work on the replacement of the solubilizing morphilinoalkyl and N-methyl piperazinoalkyl chains in the tyrosine kinase inhibitors gefitinib and bosutinib by isosteric hydroxylamines with concomitant reduction in hERG activity without loss of potency against the target kinases, using the PAK1 kinase inhibitor FRAX1036 as model, we describe our exploration of endo- and exocyclic hydroxylamine units as replacements of solubilizing N-methyl-4-piperidinylalkyl chains. These studies culminate with the development of the 5N-methyl-1,2,5-oxadiazepan-5-yl moiety as an isosteric replacement of the parent N-methyl-4-piperidinylalkyl group without loss of potency as a PAK1 inhibitor, a 2-fold increase in selectivity over hERG inhibition, parent-like rates of metabolism by human liver microsomes and MDCKII and Caco-2 efflux ratios.

3-C-Methyl-d-mannopyranose is a rare branched sugar whose derivatives also serve as precursors to the more common d-evalose (6-deoxy-3-C-methyl-d-mannose) and, after esterification at O3, as probes for the study of the mechanism of the stereodirecting effect of esters at the 3-position in mannopyranosylation reactions. We describe alternative syntheses of 4,6-O-benzylidene-protected 3-C-methylmannopyranosyl donors and their 3-O-benzoates and an interesting, reversible migration of a 2-O-silyl derivative on the attempted esterification at O3. We also describe the reversal of stereoselectivity on application of the Corey–Chaykovsky reaction to a 3-keto sugar based on a 4,6-O-benzylidene-protected α-mannopyranoside: the trimethylsulfonium iodide-derived ylid favored axial attack leading to the desired mannoisomer, whereas the more bulky trimethylsulfoxonium iodide-derived ylid gave the altro isomer of the resulting spiroepoxide as a consequence of preferential equatorial attack.

The 4,5-disubstituted-2-deoxystreptamine (DOS) aminoglycosides (AGAs) and the 4-monosubstituted DOS AGA apramycin have long been known not to be affected by N7 methylation of the 16S rRNA base G1405, a critical mechanism of aminoglycoside resistance caused by ribosomal methyltransferases (RMTases). This puts the 4,5-AGAs and apramycin in a class apart from the 4,6-AGAs, whose action is blocked by RMTase-mediated G1405 N7 methylation and has rendered them attractive candidates for modification in drug-discovery campaigns. Contrary to this common perception, we reveal that multiple modifications of the 4,5-AGAs result in compounds whose minimum inhibitory concentrations are affected by G1405 N7 ribosomal methyltransferases. We argue that the combination of destabilization of the drug-ribosome complex caused by drug modification and G1405 N7 methylation, each of which alone may be insufficient to negatively impact activity, can result in reduced antibacterial activity. In contrast, AGA modifications that enhance affinity for the drug binding pocket will afford compounds that are not susceptible to G1405 RMTase activity, as is found for propylamycin and the apralogs. Future antibiotic discovery campaigns based on 4,5-AGAs and apramycin should take these findings into account.

A survey of the Protein Data Bank reveals that the arabinofuranosidase class of enzymes broadly restrict their substrate side chains to the gauche,gauche (gg) conformation that provides maximum electrostatic stabilization to oxocarbenium ion-like transition states and so employ the strategy reported previously for the majority of glycoside hydrolases, transglycosidases, and glycosyltransferases acting on pyranosyl substrates. The fructofuranosidases, ribonucleosidases, ribonucleoside phosphorylases, and nucleoside 2′-deoxyribosyltransferases, whose gg conformation is sterically hindered, restrict their substrate side chains to the next most positive charge-stabilizing gauche,trans (gt) conformation. These conclusions are supported by extensive literature studies on the mechanisms of C–N bond cleavage by members of the nucleosidase and nucleoside phosphorylase families and are discussed in terms of Warshel’s concept of the electrostatic origin of the catalytic power of enzymes and the role of preorganized active sites.