Combinatorial synthesis of natural product-like molecules using a first-generation spiroketal scaffold

Short Scalable Route to Bis-morpholine Spiroacetals and Oxazepane Analogues: Useful 3D-Scaffolds for Compound Library Assembly

sp3-Rich molecular scaffolds incorporating nitrogen heterocycles represent important starting points for assembling compound screening libraries and drug discovery. Herein, we report a four-step synthesis of a conformationally well-defined sp3-rich scaffold incorporating two morpholine rings embedded within a spiroacetal framework. The synthesis involves the intermediacy of a 2-chloromethyl-substituted morpholine, accessed from epichlorohydrin and readily available β-aminoalcohols. Base-mediated dehydrochlorination affords an exocyclic enol ether, from which the second morpholine ring is constructed in two steps. Scaffold synthesis is high-yielding and can be performed on a large scale. The methodology allows ready substitution of one-or both- of the morpholine rings for 1,4-oxazepanes and the generation of 6,7- and 7,7-spiroacetal analogues, which are virtually unexplored in drug discovery. Substituted 6,6-systems can be prepared and, in some instances, undergo acid-mediated anomerization to deliver the scaffolds in high diastereoselectivity. The two amine functionalities embedded in the 6,6- and 6,7-spiroacetal scaffolds were sequentially functionalized to provide a diverse physical compound library. These library compounds occupy a similar chemical space to small-molecule drugs that have been approved for clinical application by the Food and Drug Administration yet are structurally dissimilar and may therefore act upon novel targets, representing attractive starting materials for drug discovery.

Stereoselective Synthesis of 1,6,9-Tri-oxaspiro[4.5]decanes From d-Glucose: Novel Structural Motifs of Spiroacetal Natural Products

Spiroacetals are the central structural core element of numerous natural products and are essential for their biological activity. A typical structural representative of a spiroacetal is the bicyclic 1,6-dioxaspiro[4.5]decane ring system. It represents the complete or partial structure of many biologically potent natural products such as the Paravespula pheromone 1, the antibiotic (+)-monensin A 2, the anticancer agent (−)-berkelic acid 3, the antimitotic ingredient spirastrellolide F, characterized after methylation as (+)-methyl ester 4, and the marine toxin (−)-calyculin A 5. In these compounds, the 1,6-dioxaspiro[4.5]decane ring system is found in either spiro ( R)-6 or ( S) - 6 configuration. The corresponding 1,6,9-trioxaspiro[4.5]decane framework ( S)-7 and ( R)-7 with opposite chirality at the spiro center due to an additional oxygen atom at position 9 in the pyran portion has so far not been found in living organisms or been synthesized. To close this gap and enable structure–activity relationship studies, potentially leading to novel antibiotics and selective anticancer agents, we have developed an efficient and stereocontrolled route to the ( R)- and ( S)-configurated 1,6,9-trioxaspiro[4.5]decane ring system leading to oxa analog motifs of the above natural products.

Re2O7-Catalyzed Approach to Spirocyclic Ether Formation from Acyclic Precursors: Observation of Remote Stereoinduction

Ketones that are flanked by an allylic alcohol and an alkene isomerize to spirocyclic ethers in the presence of Re2O7 through allylic alcohol transposition, oxocarbenium ion formation, and Prins cyclization. These processes provide significant increases in molecular complexity, with multiple stereocenters being set relative to a stereocenter in the substrate. Stereoselectivity arises from the initial reversible steps being more rapid than the final step, thereby allowing for thermodynamically controlled stereochemical equilibration prior to product formation.

Bicyclic acetals: biological relevance, scaffold analysis, and applications in diversity-oriented synthesis

The chemoinformatics analysis of fused, spiro, and bridged bicyclic acetals is instrumental for the DOS of natural product-inspired molecular collections.

Diversity-Oriented Synthesis of Natural-Product-like Libraries Containing a 3-Methylbenzofuran Moiety for the Discovery of New Chemical Elicitors

Natural products are a major source of biological molecules. The 3-methylfuran scaffold is found in a variety of plant secondary metabolite chemical elicitors that confer host-plant resistance against insect pests. Herein, the diversity-oriented synthesis of a natural-product-like library is reported, in which the 3-methylfuran core is fused in an angular attachment to six common natural product scaffolds-coumarin, chalcone, flavone, flavonol, isoflavone and isoquinolinone. The structural diversity of this library is assessed computationally using cheminformatic analysis. Phenotypic high-throughput screening of β-glucuronidase activity uncovers several hits. Further in vivo screening confirms that these hits can induce resistance in rice to nymphs of the brown planthopper Nilaparvata lugens. This work validates the combination of diversity-oriented synthesis and high-throughput screening of β-glucuronidase activity as a strategy for discovering new chemical elicitors.

Turning Spiroketals Inside Out: A Rearrangement Triggered by an Enol Ether Epoxidation

Spiroketals organize small molecule structures into well-defined, three-dimensional configurations that make them good ligands of proteins. We recently discovered a tandem cycloisomerization-dimerization reaction of alkynyl hemiketals that delivered polycyclic, enol-ether-containing spiroketals. Here we describe rearrangements of those compounds, triggered by epoxidation of their enol ethers that completely remodel their structures, essentially turning them "inside out". Due to the high level of substitution on the carbon skeletons of the substrates and products, characterization resorted to X-ray crystallography and advanced computation and NMR techniques to solve the structures of representative compounds. In particular, a new proton-detected ADEQUATE NMR experiment (1,1-HD-ADEQUATE) enabled the unequivocal assignment of the carbon skeleton of one of the new compounds. Solution of the structures of the representative compounds allowed for the assignment of product structures for the other compounds in two separate series. Both the rearrangement and the methods used for structural determination of the products are valuable tools for the preparation of characterization of new small molecule compounds.

Stereoselective synthesis of 1,6-dioxaspirolactones from spiro-cyclopropanecarboxylated sugars: total synthesis of dihydro-pyrenolide D

A one-pot protocol for the stereoselective construction of γ-spiroketal γ-lactone frameworks from sugar derived spiro-cyclopropanecarboxylic acids involving a ring enlargement and cyclization reaction is revealed.

Possibilities of quercetin insertion into poly(N,N-dimethylacrylamide-co-3, 9-divinyl-2, 4, 8, 10-tetraoxaspiro (5.5) undecane) network

The study presents the possibility of coupling quercetin into poly(N,N-dimethylacrylamide-co-3, 9-divinyl-2, 4, 8, 10-tetraoxaspiro (5.5) undecane) (PDMA-co-U) gel network through a semi-imprinted procedure in order to obtain a responsive antioxidant composite. A continuous magnetic field was used during quercetin insertion for prearranging the compounds - macromolecular matrix and quercetin - and improving the coupling efficiency of the flavonoid structure into the polymeric network. The new supramolecular systems were characterized by FTIR, SEM and thermogravimetric analyses. The dependence of the swelling degree equilibrium and particle size distribution of the studied chemical structures were also evaluated related to the experimental conditions of preparation.

Polymer supported synthesis of a natural product-inspired oxepane library

Natural product inspired compound collections are prevalidated due to the evolutionary selection of the natural product scaffolds. Their synthesis requires the development of novel strategies amenable to formats suitable for library build-up. We describe a method for the synthesis of an oxepane library inspired by the core structure of oxepane natural products endowed with multiple bioactivities. Core aspects of the strategy are the establishment of a one-pot method employing different immobilized scavengers, the employment of an enyne ring closing reaction and diversification by means of different transformations, for example, cycloadditions and cross-metathesis reactions. In total, a collection of 115 oxepanes was obtained in 5-6-step reaction sequences.

Super silyl stereo-directing groups for complete 1,5-syn and -anti stereoselectivities in the aldol reactions of beta-siloxy methyl ketones with aldehydes

In this communication, we report that substrate-controlled 1,5-syn and -anti stereoinduction in the aldol reaction of beta-tris(trialkylsilyl)siloxy methyl ketones can be achieved with high diastereoselectivities. Tris(trialkylsilyl)silyl groups are easily prepared and play an important role in the selectivities of these reactions. Furthermore, all of the 1,3,5-triol stereoisomers can easily be prepared from beta-siloxy methyl ketones in no more than three steps.

Natural Products as a Foundation for Drug Discovery

Natural products have contributed to the development of many drugs for diverse indications. While most U.S. pharmaceutical companies have reduced or eliminated their in-house natural product groups, new paradigms and new enterprises have evolved to carry on a role for natural products in the pharmaceutical industry. Many of the reasons for the decline in popularity of natural products are being addressed by the development of new techniques for screening and production. This overview aims to inform pharmacologists of current strategies and techniques that make natural products a viable strategic choice for inclusion in drug discovery programs.

An iodoetherification-dehydroiodination strategy for the synthesis of complex spiroketals from dihydroxyalkene precursors

Dihydroxyalkenes or their monoprotected alcohol derivatives are transformed to 5,5- and 5,6-spiroketals through a sequence involving an initial iodocyclization, followed by a silver triflate mediated spiroketalization step on the derived hydroxy-iodoether.

Design, synthesis, and biological evaluation of a scaffold for iGluR ligands based on the structure of (−)-dysiherbaine

The design and synthesis of four 2,2-disubstituted dihydrobenzofurans that are structurally related to several glutamate-containing natural products, including (-)-dysiherbaine, is described. Biological evaluation of these analogs shows that one is a KA receptor antagonist and another is an NMDA receptor agonist.

Stereocontrolled synthesis of spiroketals via Ti(Oi-Pr)4-mediated kinetic spirocyclization of glycal epoxides with retention of configuration

A Ti(Oi-Pr)4-mediated kinetic spiroketalization reaction has been developed for the stereocontrolled target- and diversity-oriented synthesis of spiroketals. In contrast to most existing methods for spiroketal synthesis, this reaction does not rely upon thermodynamic control over the stereochemical configuration at the anomeric carbon. Stereochemically diverse glycals are first alkylated at the C1-position to introduce a hydroxyl-bearing side chain, then epoxidized stereoselectively. Treatment with Ti(Oi-Pr)4 leads to an unusual kinetic epoxide-opening spirocyclization (spirocycloisomerization) with retention of configuration at the anomeric carbon. The reaction is proposed to proceed via a chelation-controlled mechanism and has been used to form five-, six-, and seven-membered rings with stereochemically diverse substituents. This approach may also be useful for the related intermolecular beta-mannosidation reaction. This Ti(Oi-Pr)4-mediated spirocyclization is stereochemically complementary to our previously reported MeOH-induced spirocyclization, which proceeds with inversion of configuration, and together, these reactions provide comprehensive access to systematically stereochemically diversified spiroketals.

Chemical variation of natural product-like scaffolds: design and synthesis of spiroketal derivatives

The design and synthesis of spiroketal structures and their chemical modification, leading to a collection of new small molecules for biological evaluation as orally-bioavailable lead compounds is described. Both [6,5]- and [6,6]-membered ring spiroketal units have been prepared in a stereochemically-varying fashion starting from commercially available (R)- or (S)-glycidol, in ten, eleven and twelve linear steps, in overall yields of 45, 40 and 20%, respectively. Further elaboration according to Lipinski's guidelines has given a collection of structurally-diverse, new spiroketal derivatives in high yields and with high purity.

Stereocontrolled Synthesis of Spiroketals via a Remarkable Methanol-Induced Kinetic Spirocyclization Reaction

A methanol-induced kinetic spiroketalization reaction has been developed for the stereocontrolled target- and diversity-oriented synthesis of spiroketals. In contrast to existing methods for spiroketal synthesis, this reaction does not depend on thermodynamic product stability or require axial attack of an oxygen nucleophile. Stereodiverse glycals are alkylated at the C1 position with side chains bearing protected hydroxyl groups. After alcohol deprotection, the glycal is epoxidized stereoselectively, then the side chain hydroxyl is spirocyclized with inversion of configuration at the anomeric carbon by addition of excess MeOH at -63 degrees C. This spirocyclization reaction appears to proceed by MeOH hydrogen-bonding catalysis and has been used to form five- and six-membered rings with stereoisomeric substituents. In some cases, the stereocomplementary spiroketals can be also obtained by classical acid-catalyzed equilibration.

Stereocontrolled Synthesis of a Complex Library via Elaboration of Angular Epoxyquinol Scaffolds

We have accomplished the synthesis of a complex chemical library via elaboration of angular epoxyquinol scaffolds with distinct skeletal frameworks. The key strategy involves highly stereocontrolled [4 + 2] Diels-Alder cycloadditions of chiral, nonracemic epoxyquinol dienes to generate the scaffolds. Further scaffold diversification involves hydrogenation, epimerization, dehydration, and condensation of the carbonyl group with alkoxyamine and carbazate building blocks. Further elaboration of the scaffolds also provided new skeletal frameworks using hydroxyl-directed Diels-Alder cycloaddition and reductive N-N bond cleavage. The overall process afforded 244 highly complex and functionalized compounds. Preliminary biological screening of the library uncovered six compounds which showed significant inhibition of Hsp 72 induction.

Towards the combinatorial synthesis of spongistatin fragment libraries by using asymmetric aldol reactions on solid support

By relying on asymmetric boron-mediated aldol reactions, solid phase methodology for the stereoselective synthesis of highly substituted spiroacetals was developed and applied to the preparation of a complex AB-spiroacetal subunit of the antimitotic agent spongistatin 1 (altohyrtin A).

Libraries from natural product-like scaffolds

Natural products are an attractive source of varied structures that exhibit potent biological activities, and desirable pharmacological profiles. Since the relatively recent advent of high-throughput organic synthesis in the drug discovery process, several design approaches have been applied to the construction of screening libraries. Libraries of natural-product derivatives, natural-product-like compounds prepared by total synthesis, and libraries derived from natural-products are several types that have been reported.

Combinatorial synthesis of natural products

Combinatorial syntheses allow production of compound libraries in an expeditious and organized manner immediately applicable for high-throughput screening. Natural products possess a pedigree to justify quality and appreciation in drug discovery and development. Currently, we are seeing a rapid increase in application of natural products in combinatorial chemistry and vice versa. The therapeutic areas of infectious disease and oncology still dominate but many new areas are emerging. Several complex natural products have now been synthesised by solid-phase methods and have created the foundation for preparation of combinatorial libraries. In other examples, natural products or intermediates have served as building blocks or scaffolds in the synthesis of complex natural products, bioactive analogues or designed hybrid molecules. Finally, structural motifs from the biologically active parent molecule have been identified and have served for design of natural product mimicry, which facilitates the creation of combinatorial libraries.

Solution-phase library synthesis of furanoses

The solution-phase synthesis of amido-, urea-, and aminofuranoses was achieved. Alkylated furanose aldehydes were treated with primary amines in the presence of sodium triacetoxyborohydride to give secondary amines. Subsequent acylation with acid chlorides and isocyanates afforded amidofuranoses and ureafuranoses, respectively. Second, reductive amination of furanose aldehydes with secondary amines yielded tertiary amines. The resulting acetonides were treated with alcohols in the presence of acid to yield mixed acetals. In the library syntheses, functionalized scavenger resins were used in the purification of intermediates and products.