Spirodiepoxides: heterocycle synthesis and mechanistic insight

CRYSTAL STRUCTURE OF 3-tert-BUTYL- PYRAZOLO[5,1-с][1,2,4]TRIAZINE-3,4-DIYL DICARBOXYLATES

By single crystal X-ray diffraction the structures of (trans-)7-R¹-8-methyl-3-tert-butyl-3,4-dihydropyrazolo[5,1-с][1,2,4]triazine-3,4-diyl dibenzoates 3а, 4а (R¹ = H, Br) and diacetate 3b (R¹ = H) are studied for the first time. The compounds are synthesized by oxidative bromination of respective 1,4-dihydropyrazolo[5,1-c][1,2,4]triazines in the presence of carboxylic acids. Dicarboxylates adopt a twist-conformation with the equatorial (3a) or axial (3b, 4a) orientation of tert-butyl groups relative to the heterocycle. In the case of diacetate 3b, a decrease in the thermal motion of the O4–C16–C17 fragment along with a decrease in the unit cell volume are observed on cooling from 300 K to 200 K. Bond lengths, torsion angles, and molecular packings in the crystals are discussed.

Synthesis and crystal structures of 7,8-bromo (dibromo)-3-tert-butylpyrazolo[5,1-c][1,2,4]triazines

Single crystal structures in a series of 7-bromo-, 8-bromo-, and 7,8-dibromo-3-tert-butylpyrazolo[5,1-c][1,2,4]triazin-4(1H)-ones have been investigated by X-ray diffraction. Novel 7-bromo- and 7,8-dibromo-3-tert-butylpyrazolo[5,1-c][1,2,4]triazines were synthesized by reduction of triazine carbonyl with dehydrative aromatization in acidic media, and their XRD structural features were compared with that of the 4-oxo analogs. The lengths and bond angles and the packing of molecules in crystals have been considered. Non-valence interactions for some of the studied compounds were observed. Correlations between the presence of bromine atoms at different positions and structural features are determined.

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The online version contains supplementary material available at 10.1007/s11224-021-01768-0.

Highly Diastereoselective Synthesis of Syn-1,3-Dihydroxyketone Motifs from Propargylic Alcohols via Spiroepoxide Intermediates

Syn dihydroxyketone motifs are embedded in a wide range of biologically active natural products, however the development of stereoselective synthetic methods to assemble these structures has proven a challenging task. We report a highly diastereoselective method for the synthesis of syn dihydroxyketones from propargylic alcohols, with wide scope for application in natural product synthesis. The reaction sequence involves regioselective cyclisation of propargylic alcohols with incorporation of a triketone to give enol dioxolanes that are then diastereoselectively epoxidised to form unusual spiroepoxide intermediates. Hydrolysis affords syn dihydroxyketones as essentially single diastereisomers. The reaction sequence is operationally simple, of wide substrate scope, and remarkably can be efficiently carried out as a one-pot process with no loss of overall yield or diastereoselectivity.

Ground-state dioxygen undergoes metal-free [3 + 2]-annulations with allenes and nitrosoarenes under ambient conditions

The cycloadditions of molecular dioxygen with neutral π-bond motifs rely heavily on singlet-state ¹O₂, whereas ground state ³O₂ is chemically inactive. Here we report novel [3 + 2]-annulations among ground-state ³O₂ (1 bar), allenes, and nitrosoarenes at low temperatures, efficiently yielding dioxygen-containing oxacycles. With less hindered 1-arylallene derivatives, these dioxygen species undergo skeletal rearrangement to 3-hydroxy-1-ketonyl-2-imine oxides. These cycloadditions represent valuable one-pot O,N,O-trifunctionalizations of allenes. Our EPR experiments confirm the presence of 1,4-diradical intermediates from an allene/nitrosoarene mixture, which manifest the hidden diradical properties of nitrosoarenes.

Complete stereodivergence in the synthesis of 2-amino-1,3-diols from allenes

Herein, we describe studies to understand how both reagent and substrate control can be effectively employed in the stereodivergent oxidative amination of allenes, with transfer of the axial chirality of an enantioenriched precursor to point chirality in each possible diastereomeric 2-amino-1,3-diol product.

The conversion of allenes to strained three-membered heterocycles

The addition of heteroatoms to an allenic double bond yields strained heterocycles that serve as scaffolds for further useful transformations.

Modular functionalization of allenes to aminated stereotriads

Nitrogen-containing stereotriads, compounds with three adjacent stereodefined carbons, are commonly found in biologically important molecules. However, the preparation of molecules bearing these motifs can be challenging. Herein, we describe a modular oxidation protocol which converts a substituted allene to a triply functionalized amine of the form C-X/C-N/C-Y. The key step employs a Rh-catalyzed intramolecular conversion of the allene to a strained bicyclic methylene aziridine. This reactive intermediate is further elaborated to the target products, often in one reaction vessel and with effective transfer of the axial chirality of the allene to point chirality in the stereotriad.

Total synthesis of (±)-rocaglamide via oxidation-initiated Nazarov cyclization

This article describes the evolution of a Nazarov cyclization-based synthetic strategy targeting the anticancer, antiinflammatory, and insecticidal natural product (±)-rocaglamide. Initial pursuit of a polarized heteroaromatic Nazarov cyclization to construct the congested cyclopentane core revealed an unanticipated electronic bias in the pentadienyl cation. This reactivity was harnessed in a successful second-generation approach using an oxidation-initiated Nazarov cyclization of a heteroaryl alkoxyallene. Full details of these two approaches are given, as well as the characterization of undesired reaction pathways available to the Nazarov cyclization product. A sequence of experiments that led to an understanding of the unexpected reactivity of this key intermediate is described, which culminated in the successful total synthesis of (+)-rocaglamide.

Direct entry to erythronolides via a cyclic bis[allene]

The complexity and low tractability of antibiotic macrolides pose serious challenges to addressing the problem of resistance through semi- or total synthesis. Here we describe a new strategy involving the preparation of a complex yet tractable macrocycle and the transformation of this macrocycle into a range of erythronolide congeners. These compounds represent valuable sectors of erythromycinoid structure space and constitute intermediates with the potential to provide further purchase in this space. The routes are short. The erythronolides were prepared in three or fewer steps from the macrocycle, which was prepared in a longest linear sequence of 11 steps.

Spirodiepoxide-based cascades: direct access to diverse motifs

Allene epoxide formation/opening reaction sequences enabled direct access to diverse products. Described here are a single flask procedure for allene preparation and allene oxidation/derivatization reactions that give, among others, diendiol, diyndiol, α'-hydroxy-γ-enone, dihydrofuranone, butenolide, and δ-lactone products.

Hydrogen-bonding catalysis and inhibition by simple solvents in the stereoselective kinetic epoxide-opening spirocyclization of glycal epoxides to form spiroketals

Mechanistic investigations of a MeOH-induced kinetic epoxide-opening spirocyclization of glycal epoxides have revealed dramatic, specific roles for simple solvents in hydrogen-bonding catalysis of this reaction to form spiroketal products stereoselectively with inversion of configuration at the anomeric carbon. A series of electronically tuned C1-aryl glycal epoxides was used to study the mechanism of this reaction based on differential reaction rates and inherent preferences for S(N)2 versus S(N)1 reaction manifolds. Hammett analysis of reaction kinetics with these substrates is consistent with an S(N)2 or S(N)2-like mechanism (ρ = -1.3 vs ρ = -5.1 for corresponding S(N)1 reactions of these substrates). Notably, the spirocyclization reaction is second-order dependent on MeOH, and the glycal ring oxygen is required for second-order MeOH catalysis. However, acetone cosolvent is a first-order inhibitor of the reaction. A transition state consistent with the experimental data is proposed in which one equivalent of MeOH activates the epoxide electrophile via a hydrogen bond while a second equivalent of MeOH chelates the side-chain nucleophile and glycal ring oxygen. A paradoxical previous observation that decreased MeOH concentration leads to increased competing intermolecular methyl glycoside formation is resolved by the finding that this side reaction is only first-order dependent on MeOH. This study highlights the unusual abilities of simple solvents to act as hydrogen-bonding catalysts and inhibitors in epoxide-opening reactions, providing both stereoselectivity and discrimination between competing reaction manifolds. This spirocyclization reaction provides efficient, stereocontrolled access to spiroketals that are key structural motifs in natural products.

Allene functionalization via bicyclic methylene aziridines

The oxidative functionalization of olefins is a common method for the formation of vicinal carbon-heteroatom bonds. However, oxidative methods to transform allenes into synthetic motifs containing three contiguous carbon-heteroatom bonds are much less developed. This paper describes the use of bicyclic methylene aziridines (MAs), prepared via intramolecular allene aziridination, as scaffolds for functionalization of all three allene carbons.

Oxidation-initiated Nazarov cyclization of vinyl alkoxyallenes

A mild method for the diastereoselective formation of C(4), C(5)-disubstituted cyclopentenones has been developed, involving formation of a pentadienyl cation via diastereoselective oxidation of a vinyl alkoxyallene. Conrotatory electrocyclization provides the cyclopentenone product. The broad scope, mild conditions, and uncommon substitution pattern accessible through this transformation make it a useful addition to the existing repertoire of cyclopentenone synthetic methods.

Spirodiepoxide strategy to the C ring of pectenotoxin 4: synthesis of the C1-C19 sector

The synthesis of a C1-C19 precursor to pectenotoxin 4 is presented. The strategy employed the functionalized allene shown. Key features include: olefin metathesis of two simple fragments to prepare the left portion of the allene-precursor, diastereoselective propargylation of an epoxy aldehyde to form the right portion, use of the DMDO-stable m-fluorobenzyl ether, and an allene spirodiepoxidation/C-ring formation cascade.

Spirodiepoxides: synthesis of epoxomicinoids

Three closely related analogues of epoxomicin have been synthesized. Allene-derived spirodiepoxides were key intermediates. Spirodiepoxide formation and stereochemical dependence on solvent, oxidant, and allene structure were cataloged. The facial selectivity of the first epoxidation of 1,3-disubstituted and trisubstituted allenes was found to be >20:1 with dimethyldioxirane in chloroform. For stereogenic allenes, the facial selectivity of the second oxidation is dependent primarily on allene structure and secondarily on solvent and oxidant. For the acyclic systems evaluated this ratio was as high as 8:1. A conformational model is advanced to account for these observations.

Modeling a macrocyclic bis[spirodiepoxide] strategy to erythronolide A

A concise route to functionalized 14-membered macrolides related to erythronolide A was achieved. Key steps include the simultaneous formation of bis[allenic] substrates, efficient macrolactonization, highly stereoselective oxidation to the corresponding bis[spirodiepoxide], and nucleophilic spirodiepoxide opening. The structure and reactivity of these macrolides, and the strategy that led to their evaluation, are discussed.

Asymmetric synthesis, structure, and reactivity of unexpectedly stable spiroepoxy-beta-lactones including facile conversion to tetronic acids: application to (+)-maculalactone A

A novel class of small spirocyclic heterocycles, spiroepoxy-beta-lactones (1,4-dioxaspiro[2.3]-hexan-5-ones), is described that exhibit a number of interesting reactivity patterns. These spiroheterocycles, including an optically active series, are readily synthesized by epoxidation of ketene dimers (4-alkylidene-2-oxetanones) available from homo- or heteroketene dimerization. An analysis of bond lengths in these systems by X-ray crystallography and comparison to data for known spirocycles and those determined computationally suggest that anomeric effects in these systems may be more pronounced due to their rigidity and may contribute to their surprising stability. The synthetic utility of spiroepoxy-beta-lactones was explored, and one facile rearrangement identified under several conditions provides a three-step route from acid chlorides to optically active tetronic acids, ubiquitous heterocycles in bioactive natural products. The addition of various nucleophiles to these spirocycles leads primarily to addition at C5 and C2. The utility of an optically active spiroepoxy-beta-lactone was demonstrated in the concise, enantioselective synthesis of the antifouling agent, (+)-maculalactone A, which proceeds in five steps from hydrocinnamoyl chloride by way of a tetronic acid intermediate.

Silyl-substituted spirodiepoxides: stereoselective formation and regioselective opening

A short synthesis of the natural product epi-citreodiol and the method developed to gain access to this target are described. Key advances focus on silyl substituted allenes. Upon exposure to dimethyldioxirane, spirodiepoxides form with high face selectivity and subsequently react at the silyl terminus.