Divergent Strategy for Diastereocontrolled Synthesis of Small- and Medium-Ring Architectures

Nitrogen and oxygen medium rings, in particular nine-membered rings, epitomize a unique area of chemical space that occurs in many natural products and biologically appealing compounds. The scarcity of 8- to 12-membered rings among clinically approved drugs is indicative of the difficulties associated with their synthesis, principally owing to the unfavorable entropy and transannular strain. We report here a scandium triflate-catalyzed reaction that allows for a modular access to a diverse collection of nine-membered ring heterocycles in a one-pot cascade and with complete diastereocontrol. This cascade features an intramolecular addition of an acyl group-derived enol to a α,β-unsaturated carbonyl moiety, leading to N- and O-derived medium-ring systems. Computational studies using the density functional theory support the proposed mechanism. Additionally, a one-pot cascade leading to hexacyclic chromeno[3',4':2,3]indolizino[8,7-b]indole architectures, with six fused rings and four contiguous chiral centers, is reported. This novel cascade features many concerted events, including the formation of two azomethine ylides, [3 + 2]-cycloaddition, 1,3-sigmatropic rearrangement, Michael addition, and Pictet-Spengler reaction among others. Phenotypic screening of the resulting oxazonine collection identified chemical probes that regulate mitochondrial membrane potential, adenosine 5'-triphosphate contents, and reactive oxygen species levels in hepatoma cells (Hepa1-6), a promising approach for targeting cancer and metabolic disorders.

“Drug-likeness” properties of natural compounds

Our previous work was focused on the fundamental physical and chemical concepts behind “drug-likeness” and “natural product (NP)-likeness”. Herein, we discuss further details on the concepts of “drug-likeness”, “lead-likeness” and “NP-likeness”. The discussion will first focus on NPs as drugs, then a discussion of previous studies in which the complexities of the scaffolds and chemical space of naturally occurring compounds have been compared with synthetic, semisynthetic compounds and the Food and Drug Administration-approved drugs. This is followed by guiding principles for designing “drug-like” natural product libraries for lead compound discovery purposes. In addition, we present a tool for measuring “NP-likeness” of compounds and a brief presentation of machine-learning approaches. A binary quantitative structure–activity relationship for classifying drugs from nondrugs and natural compounds from nonnatural ones is also described. While the studies add to the plethora of recently published works on the “drug-likeness” of NPs, it no doubt increases our understanding of the physicochemical properties that make NPs fall within the ranges associated with “drug-like” molecules.

Traceless Solid-Phase Organic Synthesis

Traceless solid-phase synthesis represents an ultimate sophisticated synthetic strategy on insoluble supports. Compounds synthesized on solid supports can be released without a trace of the linker that was used to tether the intermediates during the synthesis. Thus, the target products are composed only of the components (atoms, functional groups) inherent to the target core structure. A wide variety of synthetic strategies have been developed to prepare products in a traceless manner, and this review is dedicated to all aspects of traceless solid-phase organic synthesis. Importantly, the synthesis does not need to be carried out on a linker designed for traceless synthesis; most of the synthetic approaches described herein were developed using standard, commercially available linkers (originally devised for solid-phase peptide synthesis). The type of structure prepared in a traceless fashion is not restricted. The individual synthetic approaches are divided into eight sections, each devoted to a different methodology for traceless synthesis. Each section consists of a brief outline of the synthetic strategy followed by a description of individual reported syntheses.

Configuration-Dependent Medium-Sized Ring Formation: Chiral Molecular Framework with Three-Dimensional Architecture

This report describes a configuration-dependent [6 + 8 + 5] fused ring formation via a tandem cyclic N-acyliminium nucleophilic addition reaction. Cyclization of the acyclic precursor prepared on a solid phase using l-Ser and a racemic mixture of Fmoc- trans-2-aminocyclohexanecarboxylic acid predominantly yielded the cyclic diastereomer with the (1 R,2 R)-2-aminocyclohexane moiety rather than the tricyclic diastereomer from the (1 S,2 S)-enantiomer. In contrast, the model compound prepared with d-Ser predominantly cyclized with the (1 S,2 S)-2-aminocyclohexanecarboxylic acid substrate. The outcome of the cyclization was not influenced by the type of resin, the spacer, or the N-substituent. The analogous synthesis of the [6 + 7 + 5] fused ring system yielded inseparable diastereomers in a 1:0.6 ratio.

Polymer-Supported Syntheses of Heterocycles Bearing Oxazine and Thiazine Scaffolds

In this review, we summarize synthetic approaches to preparing single or fused oxazine and thiazine derivatives using solid-phase synthesis (SPS). The literature survey revealed that diverse compounds bearing variously functionalized 1,2-oxazine, 1,3-oxazine, or 1,4-oxazine scaffolds and the corresponding thiazines are accessible by SPS. The latest contributions involving the stereoselective polymer-supported syntheses of morpholines indicate that the field is continuing to expand.

Nα-Amino acid containing privileged structures: design, synthesis and use in solid-phase peptide synthesis

Fmoc-protected Nα-amino acid containing heterocyclic privileged structures, O-(1-methyl-5-oxo-2,5-dihydro-1H-pyrrol-3-yl)-l-serine and O-((S)-5-oxo-2,3,5,7a-tetrahydro-1H-pyrrolizin-7-yl)-l-serine, were synthesized on the solid phase from simple commercially available building blocks under mild conditions. The amino acid side-chain is composed of tetramic acid, a natural product derived privileged structure. The key transformation was the formation of cyclic enol ethers via nonclassical Wittig olefinations of the esters. Solid-phase synthesis represents a method of choice, particularly for the synthesis of peptides. This route is compatible with traditional Merrifield solid-phase peptide synthesis (SPPS), as documented on the preparation of the pentapeptide Leu-enkephalin amide H-Tyr-Gly-Gly-Phe-Leu-NH2 with Phe or Tyr replaced by a novel amino acid.

Traceless Solid-Phase Synthesis of [6,7,8 + 5,6,7]-Fused Molecular Frameworks

We report two synthetic strategies for traceless solid-phase synthesis of molecular scaffolds comprising 6- to 8-membered rings fused with 5- to 7-membered rings. Traceless synthesis facilitated preparation of target molecules without any trace of polymer-supported linkers. The cyclization proceeded via acid-mediated tandem N-acylium ion formation followed by the nucleophilic addition of O- and C-nucleophiles. The presented synthetic strategy enabled, through the use of simple building blocks without any conformational preferences, the evaluation of the predisposition of different combinations of ring sizes to form fused ring molecular scaffolds. Compounds with any combination of [6,7 + 5,6,7] ring sizes were accessible with excellent crude purity. The 8-membered cyclic iminium was successfully fused only with the 5-membered cycle and larger fused ring systems were not formed, probably due to their instability.

Fused Ring Molecular Scaffold with 3D Architecture for Constrained Peptidomimetics: Polymer-Supported Stereoselective Synthesis of Tetrahydrobenzo[e]pyrazino[2,1-c][1,2,4]thiadiazinone 6,6-dioxide via N-Acyl Iminiums

3,4,4a,5-Tetrahydrobenzo[e]pyrazino[2,1-c][1,2,4]thiadiazin-1(2H)-one 6,6-dioxides, molecular scaffolds with 3D architecture, were synthesized on solid supports via tandem N-acyl iminium ion cyclization followed by nucleophilic addition. The modular synthesis proceeded under mild conditions using commercially available building blocks and provided crude products with respectable purity. The synthesized compounds are applicable as fused nitrogenous heterocyclic compounds in drug discovery and as constrained peptidomimetics incorporated into a peptide backbone.