An Expedient Route for the Stereoselective Construction of Bridged Polyheterocyclic Ring Systems Using the Tandem "Pincer" Diels-Alder Reaction

Investigations into a Stoichiometrically Equivalent Intermolecular Oxidopyrylium [5 + 2] Cycloaddition Reaction Leveraging 3-Hydroxy-4-pyrone-Based Oxidopyrylium Dimers

Oxidopyrylium [5 + 2] cycloaddition reactions are powerful strategies for constructing complex bicyclic architectures. However, intermolecular cycloadditions of oxidopyrylium ylides are limited due to competing dimerization processes; consequently, high equivalents of dipolarophiles are often used to help intercept the ylide prior to dimerization. Recent studies by our lab have revealed that oxidopyrylium dimers derived from 3-hydroxy-4-pyrones are capable of reverting back to ylides in situ and as a result can be used as clean oxidopyrylium ylide sources. The following manuscript investigates intermolecular cycloaddition reactions between 3-hydroxy-4-pyrone-derived oxidopyrylium dimers and stoichiometrically equivalent ratios of alkyne dipolarophiles under thermal conditions. With certain reactive alkynes, pure cycloadducts can be obtained following a simple evaporation of the solvent, which is a benefit of the completely atom-economical reaction conditions. However, when less reactive alkynes are used the yields suffer due to a competing dimer rearrangement. Finally, when reactive-yet-volatile alkynes are used, such as methyl propiolate, competing 2:1 ylide/alkyne cycloadducts are observed. Intriguingly, these complex cycloadducts, which can be obtained in good yields from the pure cycloadducts, form with high regio- and stereoselectivities; however, both the regio-and stereoselectivities differ remarkably based on the source of the oxidopyrylium ylide.

An IMDAF approach to annellated 1,4:5,8-diepoxynaphthalenes and their metathesis reaction leading to novel scaffolds displaying an antiproliferative activity toward cancer cells

A 3,5a-epoxyfuro[2,3,4-de]isoquinoline scaffold, the product of ROCM of 1,4:5,8-diepoxynaphthalenes, is a promising antiproliferative agent toward breast and prostate human cancer cell lines.

Synthesis and ethylene-promoted metathesis of adducts of tandem [4+2]/[4+2] cycloaddition between bis-furyl dienes and maleic acid derivatives

A series of 1,4:5,8-diepoxynaphthalenes, annellated with carbo- and heterocycles, was synthesized based on the tandem intermolecular/intramolecular [4+2] cycloaddition of bis-furyl dienes with moderately to highly reactive cyclic dienophiles (maleic anhydride and maleinimides).

Crystal structure and Hirshfeld analysis of di-tert-butyl 2,2'-[(ethyl-aza-nedi-yl)bis-(methyl-ene)]bis-(1H-pyrrole-1-carboxyl-ate)

The title compound, C22H33N3O4, crystallizes in the triclinic space group P with two mol-ecules in a unit cell. The two pyrrole rings are essentially planar (r.m.s. deviation = 0.002 Å) and they form a dihedral angle of 81.24 (10)° with each other. The crystal packing is stabilized by C-H⋯π inter-actions and π-π stacking inter-actions, forming a three-dimensional network. The Hirshfeld surface analysis and two-dimensional fingerprint plots reveal that the most important contributions for the crystal packing are from H⋯H (74.3%), C⋯H/H⋯C (11.5%) and O⋯H/H⋯O (9.1%) contacts.

Crystal structure and Hirshfeld surface analysis of dimethyl (1R*,3aS*,3a1 R*,6aS*,9R*,9aS*)-3a1,5,6,9a-tetra-hydro-1H,4H,9H-1,3a:6a,9-di-epoxy-phenalene-2,3-di-carboxyl-ate

The title di-epoxy-phenalene derivative, C17H18O6, comprises a fused cyclic system containing four five-membered rings (two di-hydro-furan and two tetra-hydro-furan) and one six-membered ring (cyclo-hexa-ne). The five-membered di-hydro-furan and tetra-hydro-furan rings adopt envelope conformations, and the six-membered cyclo-hexane ring adopts a distorted chair conformation. Two methyl carboxyl-ate groups occupy adjacent positions (2- and 3-) on a tetra-hydro-furan ring. In the crystal, two pairs of C-H⋯O hydrogen bonds link the mol-ecules to form inversion dimers, enclosing two R 2 2(6) ring motifs, that stack along the a-axis direction and are arranged in layers parallel to the bc plane.

Unnatural α-amino ethyl esters from diethyl malonate or ethyl β-bromo-α-hydroxyiminocarboxylate

We have explored here the scope of the age-old diethyl malonate-based accesses to α-amino esters involving Knoevenagel condensations of diethyl malonate on aldehydes, reductions of the resulting alkylidenemalonates, the preparation of the corresponding α-hydroxyimino esters and their final reduction. This synthetic pathway turned out to be general although some unexpected limitations were encountered. The synthetic modifications of some of the intermediates - using Suzuki-Miyaura coupling or cycloadditions - before undertaking the oximation step - provided accesses to further α-amino esters. Moreover, other pathways to α-hydroxyimino esters were explored including an attempt to improve the cycloadditions between ethyl β-bromo-α-hydroxyiminocarboxylate and various alkylfuranes.

Diels–Alder reactions between hexafluoro-2-butyne and bis-furyl dienes: kinetic versus thermodynamic control

The tandem [4+2] cycloaddition between hexafluoro-2-butyne and bis-furyl diens can lead to “pincer” or “domino”-adducts.

Atom-Economical and Metal-Free Synthesis of Multisubstituted Furans from Intramolecular Aziridine Ring Opening

Multisubstituted furans were prepared from dialkyl 2-(aziridin-2-ylmethylene)malonate and/or 1,3-dione through aziridine ring opening by the internal carbonyl oxygen with the assistance of BF3·OEt2, followed by aromatization. This synthetic method is free from any metal and is atom-economical with all of the atoms in the starting material retained in the final product.

Crystal structures and conformations of two Diels-Alder adduct derivatives: 1,8-bis-(thio-phen-2-yl)-14-oxa-tetra-cyclo-[6.5.1.0(2,7).0(9,13)]tetra-deca-2(7),3,5-trien-10-one and 1,8-diphenyl-14-oxa-tetra-cyclo[6.5.1.0(2,7).0(9,13)] tetra-deca-2,4,6-trien-10-one

The title compounds, C21H16O2S2 (I) and C25H20O2 (II), are products of a tandem 'pincer' Diels-Alder reaction consisting of [2 + 2] cyclo-additions between benzo[c]furan and cyclo-penta-none. Each comprises a fused tetra-cyclic ring system containing two five-membered rings (in envelope conformations with the O atom as the flap) and six-membered rings (in boat conformations). In addition, two thio-phene rings in (I) and two phenyl rings in (II) are attached to the tetra-cyclic ring system. The cyclo-penta-none ring adopts a twisted conformation in (I) and an envelope conformation in (II). In (I), the thio-phene rings are positionally disordered over two sets of sites, with occupancy ratios of 0.901 (2):0.099 (2) and 0.666 (2):0.334 (2). In (II), the oxygen atom of the cyclo-penta-none ring is rotationally disordered over two sites with an occupancy ratio of 0.579 (4):0.421 (4). The mol-ecular structure of (I) is stabilized by an intra-molecular C-H⋯O hydrogen bond, which generates an S(7) ring motif. In the crystal, the mol-ecules are linked via weak C-H⋯O hydrogen bonds, which generate R (2) 2(16) ring motifs in (I) and C(8) chains in (II). In both structures, the crystal packing also features C-H⋯π inter-actions. The crystal studied of compound (I) was twinned by non-merohedry. The twin component is related by the twin law [-1 0 0 -0.101 1 -0.484 0 0 -1] operated by a twofold rotation axis parallel to the b axis. The structure of (I) was refined with a twin scale factor of 0.275 (2).

Aqueous-phase deactivation and intramolecular [2 + 2 + 2] cycloaddition of oxanorbornadiene esters

Both inter- and intramolecular degradation pathways were identified for the aqueous phase deactivation of oxanorbornadiene (OND) electrophiles, and propargylic OND esters were found to undergo facile intramolecular [2 + 2 + 2] homo-Diels-Alder cycloaddition in polar media.

Ethyl 1,3,10,12-tetraphenyl-19,20-dioxahexacyclo[10.6.1.13,10.02,11.04,9.013,18]icosa-4(9),5,7,13(18),14,16-hexaene-2-carboxylate

The title compound, C₄₅H₃₄O₄, is the product of a tandem ‘pincer’ Diels–Alder reaction consisting of two consecutive [4 + 2] cyclo­additions between two 2-benzofuran units and ethyl propiolate. The mol­ecule comprises a fused hexa­cyclic system containing four five-membered rings, which are in the usual envelope conformation, and two six-membered rings. In addition, four phenyl rings are attached to the hexa­cyclic system. The packing is stabilized by C—H⋯π inter­actions.

Dimethyl 11,13-dimethyl-16-[1,2-bis-(methoxy-carbon-yl)ethen-yl]-12-oxo-16,17-dioxa-18-aza-hexa-cyclo-[7.5.1.1.1.1.0]octa-deca-2,7-diene-2,3-dicarboxyl-ate

The title compound, C₂₇H₂₉NO₁₁, is a product of the tandem ‘domino’ Diels–Alder reaction. The mol­ecule comprises a fused hexa­cyclic system containing four five-membered rings (two dihydro­furan and two tetra­hydro­furan) in the usual envelope conformations and two six-membered rings (tetra­hydro­pyridinone and piperidine) adopting slightly flattened boat and chair conformations, respectively. The dispositions of the carboxyl­ate substituents relative to each other are determined by both steric reasons and inter­molecular C—H⋯O hydrogen bonding and attractive anti­parallel C=O⋯C=O inter­actions [C⋯O = 2.995 (2) Å].

Application of a 6pi-1-azatriene electrocyclization strategy to the total synthesis of the marine sponge metabolite ageladine A and biological evaluation of synthetic analogues

A 12-step synthesis of the angiogenesis inhibitory marine metabolite ageladine A is reported. The key steps include a 6pi-1-azatriene electrocyclization for formation of the pyridine ring and a Suzuki-Miyaura coupling of N-Boc-pyrrole-2-boronic acid with a chloroimidazopyridine. In addition, an assessment of the biological activity of a variety of synthetic analogues of ageladine A prepared during this synthesis is described.

Total synthesis of ageladine A, an angiogenesis inhibitor from the marine sponge Agelas nakamurai

[reaction: see text] A 12-step total synthesis of the tricyclic heteroaromatic marine metabolite ageladine A has been achieved using a 6pi-azaelectrocyclization and a Suzuki-Miyaura coupling of N-Boc-pyrrole-2-boronic acid with a chloropyridine as key steps.

Diastereoselective Syntheses of New Analogues of the Farnesyltransferase Inhibitor RPR 130401

The access to several benzo[f]perhydroisoindolic analogues of farnesyltransferase inhibitors from a single dienic precursor is reported. An initial [4 + 2] cycloaddition between diphenylisobenzofuran6 and pyrrolines 11, 14, and 15 led to either the syn or the anti isomers, depending on the mode of activation of the cycloaddition. The syn diastereomers were isolated in 90% de under 12 kbar at room temperature, while their anti counterparts were obtained with the same selectivity by warming the reaction mixture to 110 degrees C in toluene at atmospheric pressure. Both syn and anti adducts were separately N-deprotected, and the resulting amines reacted with an activated ester derived from the acid (20) to afford the final targets (5). Two new analogues (5a and 5b) of the FT inhibitor RPR 130401 were thus synthesized in a mere three-step synthetic scheme with overall yields from 30 to 60%.

An enantioselective double Diels-Alder approach to the tetracyclic framework of colombiasin A

The complex tetracyclic carbon skeleton of colombiasin A is conveniently accessed through an enantioselective intermolecular Diels-Alder-sulfoxide elimination-intramolecular Diels-Alder (DA-E-IMDA) sequence.

Synthesis and Antifungal Activity Evaluation of 3-Hetaryl-2,5-dihydrofuran-2-ones. An Unusual Fragmentation of the Oxazole Ring via 2,3-Selenoxide Shift

In continuing the studies on the synthesis and evaluation of antifungal activity of the analogues of (-)incrustoporine, the replacement of the phenyl moiety at C3 of the furanone ring with a hetaryl substituent was considered. Thus, a series of 5-alkyl-3-hetaryl-2,5-dihydrofuran-2-ones with the thienyl, furyl and thiazolyl moieties attached to C3 was synthesized, and the compounds subjected to antifungal activity screening. In the preparation of compounds containing the oxazolyl fragment, the [2,3]-sigmatropic rearrangement led to the fragmentation of the oxazole ring, resulting in the formation of 3-(1-benzamido-2-oxoethylidene)-5-methyltetrahydrofuran-2-one. Somewhat surprisingly, the antifungal efficiency of the derivatives was lower in comparison with analogues containing a substituted phenyl at C3.

Exploring the Reactivity of Dioxacyclic Compounds as a Route to Polysubstituted Decalins and Fused Polycycles

We have described a chemo-, regio-, and stereocontrolled methodology for the simple and efficient synthesis of a large variety of cis-decalins and related fused polycyclic systems with control at up to six stereocenters, based on the sequential ring-opening of dioxacyclic templates. We have established that the most useful feature of the reactivity of the dioxacyclic compounds toward the nucleophilic ring-opening reaction is that the first ring-opening reaction is significantly faster than the second allowing the sequential transformation of the oxabicyclic moieties. The flexibility of the sequential ring-opening process and its limitations have been demonstrated and a new enantioselective mode of opening was reported. The enantioselective base-induced desymmetrization was successfully applied to a thiadioxapentacycle to give the product in >95% ee using a chiral lithium amide base.