Endocyclic Olefin Research Articles

Mechanism and Origins of Regio- and Stereoselective Alkylboration of Endocyclic Olefins Enabled by Nickel Catalysis.

The Ni-catalyzed alkylboration of endocyclic olefins is a stereo- and regioselective approach for the synthesis of boron-containing compounds. We report a detailed density functional theory (DFT) study to elucidate the mechanism and origins of the stereo-, chemo-, and regioselectivity of alkylboration of endocyclic olefins enabled by nickel catalysis. The alkylboration proceeds via the migratory insertion of alkenes, β-H elimination of the Ni(II) complex, subsequent migratory insertion leading to a new Ni(II) complex, combined with an alkyl radical, and reductive eliminations. The electronic effects of the endocyclic olefins synergistically control the regioselectivity toward the C1- and C2-position boration. In C1-position boration, a more electron-deficient carbon atom tends to combine with an electron-rich -Bpin group and leads to C1-position boration products. The stereoselectivity is influenced by the solvent effect, and the interaction between the substrate and Ni-catalyzed groups, the low-polarity solvent 1,4-dioxane, and a favorable steric hindrance effect result in the cis-alkylboration product. Chemoselectivity toward 1,3-alkylboration results from the steric hindrance effects of the -Bpin group.

Nickel Catalyzed Regio- and Stereoselective Alkylboration of Endocyclic Olefins

Nickel Catalyzed Regio- and Stereoselective Alkylboration of Endocyclic Olefins

Regio- and Stereoselective Alkylboration of Endocyclic Olefins Enabled by Nickel Catalysis.

Whereas there is a significant interest in the rapid construction of diversely substituted saturated heterocycles, direct and modular access is currently limited to the mono-, 2,3-, or 3,4-substitution pattern. This Communication describes the straightforward and modular construction of 2,4-substituted saturated heterocycles from readily available materials in a highly stereo- and regioselective manner, which sets the stage for numerous readily accessible drug motifs. The strategy relies on chain walking catalysis.

Pot-Economical Total Synthesis of Clinprost.

The pot-economical synthesis of clinprost is reported, in which the core bicyclo[3.3.0]octenone structure was synthesized by two key steps: an asymmetric domino Michael/Michael reaction catalyzed by diphenylprolinol silyl ether and an intramolecular Horner-Wadsworth-Emmons reaction. The trisubstituted endocyclic alkene was selectively introduced by 1,4-reduction followed by trapping of the generated enolate with Tf2NPh and subsequent utilization of the Suzuki-Miyaura coupling reaction. Chiral, nonracemic clinprost was synthesized in seven pots with a 17% total yield and excellent enantioselectivity.

Controlled Living Cascade Polymerization To Make Fully Degradable Sugar-Based Polymers from d-Glucose and d-Galactose.

Monomers derived from glucose and galactose, which contain an endocyclic alkene (in the sugar ring) and a terminal alkyne, underwent a cascade polymerization to prepare new polymers with the ring-opened sugar incorporated into the polymer backbone. Polymerizations were well-controlled, as demonstrated by a linear increase in molecular weight with monomer-to-initiator ratio and generally narrow molecular weight dispersity values. The living nature of the polymerization was supported by the preparation of a block copolymer from two different sugar-based monomers. The resulting polymers were also fully degradable. They underwent fast and complete depolymerization to small molecules under acidic conditions.

ChemInform Abstract: Medium‐Sized Heterocycle Synthesis by the Use of Synergistic Effects of Ni—NHC and γ‐Coordination in Cycloisomerization.

This paper describes a new approach in transition-metal-catalyzed unsymmetric cycloisomerization for medium-sized heterocycles. The steric and electronic effects of an NHC-NiH catalyst and γ-heteroatom chelation were used together as a basis for 1,n-diene termini differentiation and for n(γ)-exo-trig (head-to-tail) product selectivity. Heterocycles bearing an exocyclic methylene such as oxepines, thiepines, siloxepines, and oxocanes were synthesized from the corresponding 1,n-dienes by a fine-tuning of the NHC properties. The implication of the underlying hypothesis was further demonstrated in a competition experiment in which strained oxepines were formed preferentially over other competing oxa-/carbocycles. Under more forcing physical conditions and the use of a suitable NHC ligand, the exocyclic methylene products were isomerized further into endocyclic olefin products regioselectively in one pot.

Effects of chemical complexity on the autoxidation mechanisms of endocyclic alkene ozonolysis products: from methylcyclohexenes toward understanding α-pinene.

Formation of highly oxidized, multifunctional products in the ozonolysis of three endocyclic alkenes, 1- methylcyclohexene, 4-methylcyclohexene, and α-pinene, was investigated using a chemical ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometer with a nitrate ion (NO3(-)) based ionization scheme. The experiments were performed in borosilicate glass flow tube reactors at room temperature (T = 293 ± 3 K) and at ambient pressure. An ensemble of oxidized monomer and dimer products was detected, with elemental compositions obtained from the high-resolution mass spectra. The monomer product distributions have O/C ratios from 0.8 to 1.6 and can be explained with an autocatalytic oxidation mechanism (=autoxidation) where the oxygen-centered peroxy radical (RO2) intermediates internally rearrange by intramolecular hydrogen shift reactions, enabling more oxygen molecules to attach to the carbon backbone. Dimer distributions are proposed to form by homogeneous peroxy radical recombination and cross combination reactions. These conclusions were supported by experiments where H atoms were exchanged to D atoms by addition of D2O to the carrier gas flow. Methylcyclohexenes were observed to autoxidize in accordance with our previous work on cyclohexene, whereas in α-pinene ozonolysis different mechanistic steps are needed to explain the products observed.

ChemInform Abstract: Ready Access to Alkylidenecyclopentenones by Nazarov Cyclization/β‐Elimination of 2‐Hydroxyalkyl‐1,4‐dien‐3‐ones.

Cross-conjugated dienones bearing α-hydroxyalkyl substituents readily undergo Nazarov cyclization followed by deprotonation of the resulting cyclopentenyl cation, and β-elimination of the exocyclic hydroxy group to form alkylidenecyclopentenones in moderate yield. The deprotonation step occurs with complete regioselectivity opposite to the hydroxyalkyl substituent, with preferential introduction of an endocyclic olefin. The transformation is tolerant of a variety of substituents and occurs under relatively mild conditions.

Rhodium-catalyzed hydroformylation of ketal-masked β-isophorone: computational explanation for the unexpected reaction evolution of the tertiary Rh-alkyl via an exocyclic β-elimination derivative.

Ketal-masked β-isophorone (7,9,9-trimethyl-1,4-dioxaspiro[4.5]dec-7-ene) is an interesting case study of Rh-catalyzed hydroformylations not only for the competition between secondary and tertiary rhodium alkyls but also for the unexpected isomerization of the tertiary Rh-alkyl to the exocyclic olefin which undergoes hydroformylation, yielding the acetaldehyde derivative (2) of 7,9,9-trimethyl-1,4-dioxaspiro[4.5]decane. Although experimental results at 100 °C pointed to reaction reversibility, the reason for this kind of behavior was however obscure. A thorough density functional theory (DFT) computational investigation of the various transition states (TS) and intermediates along the reaction pathways making use of B3P86 hybrid functionals and the 6-31G* basis set, coupled to effective core potentials for Rh in the LanL2DZ valence basis set, has been carried out to shed some light on the reaction mechanism. The TS barrier heights, based on alkyl-Rh TS free energies, computed under the hypothesis of nonreversibility were in favor of a normal hydroformylation reaction (III:II = 70:30). While the endocyclic olefins produced skew or twisted arrangements of the six-membered ring similarly to the CO insertion TS that can be even higher than the alkyl-Rh ones, grid-point calculations during the potential energy surface (PES) scan produced the much more stable chair conformation for the exocyclic olefin complex. The relevant TS were found to be very favorable as well, thus explaining the preference for the exocyclic arrangement of the tertiary intermediate, for which the reaction is therefore entirely reversible and invariably proceeds to the acetaldehyde derivative (2). Conversely for the secondary isomers, the reaction is only partially reversible, thus enriching the tertiary fraction and producing the secondary aldehyde (3) in a very limited amount.

Novel synthesis of the ABC rings of solanoeclepin A.

A stereocontrolled synthesis of the ABC rings of solanoeclepin A has been achieved. The seven-membered ring B was synthesized by an intramolecular Prins-ene reaction between an aldehyde and an enyne-dicobalthexacarbonyl complex. The acetylene in this synthesis plays multiple roles: to join the A and C rings, to allow stereoselective cyclization via dicobalthexacarbonyl complexation, and to facilitate Nicholas cation stabilization followed by deprotonation to form an endo-cyclic olefin (Nicholas-Prins cyclization).

Synthesis of 19‐Nor‐Vitamin D A‐Ring Synthons via Ring‐Closing Olefin Metathesis

Abstract19‐Nor‐vitamin D A‐ring synthons were obtained from linear olefins by means of ring‐closing olefin metathesis followed by palladium‐catalyzed isomerization of the endocyclic olefin product to an exocyclic olefin.

Medium-sized heterocycle synthesis by the use of synergistic effects of Ni-NHC and γ-coordination in cycloisomerization.

This paper describes a new approach in transition-metal-catalyzed unsymmetric cycloisomerization for medium-sized heterocycles. The steric and electronic effects of an NHC-NiH catalyst and γ-heteroatom chelation were used together as a basis for 1,n-diene termini differentiation and for n(γ)-exo-trig (head-to-tail) product selectivity. Heterocycles bearing an exocyclic methylene such as oxepines, thiepines, siloxepines, and oxocanes were synthesized from the corresponding 1,n-dienes by a fine-tuning of the NHC properties. The implication of the underlying hypothesis was further demonstrated in a competition experiment in which strained oxepines were formed preferentially over other competing oxa-/carbocycles. Under more forcing physical conditions and the use of a suitable NHC ligand, the exocyclic methylene products were isomerized further into endocyclic olefin products regioselectively in one pot.

Ready Access to Alkylidenecyclopentenones by Nazarov Cyclization/β‐Elimination of 2‐Hydroxyalkyl‐1,4‐dien‐3‐ones

AbstractCross‐conjugated dienones bearing α‐hydroxyalkyl substituents readily undergo Nazarov cyclization followed by deprotonation of the resulting cyclopentenyl cation, and β‐elimination of the exocyclic hydroxy group to form alkylidenecyclopentenones in moderate yield. The deprotonation step occurs with complete regioselectivity opposite to the hydroxyalkyl substituent, with preferential introduction of an endocyclic olefin. The transformation is tolerant of a variety of substituents and occurs under relatively mild conditions.

N-Heterocyclic Carbene Catalyzed Intramolecular Hydroacylation of Alkynylphosphonates

Two exocyclic and endocyclic olefin tautomers of chromone phosphonates were obtained in good to excellent yields at different temperatures by utilizing an N-heterocyclic carbene catalyst in the intramolecular reactions between the formyl group and alkynylphosphonates. The exocyclic olefins can isomerize to the endocyclic derivatives completely when treated with a thiazolium salt precatalyst and potassium carbonate at 30 °C for three to five hours. In addition, the chromone phosphonate derivatives were also applied in the one-pot synthesis of benzopyranopyridine phosphonates in moderated yields.

Catalytic asymmetric hydrogenation of heterocyclic ketone-derived hydrazones, pronounced solvent effect on the inversion of configuration

An enantioselective hydrogenation of hydrazones derived from heterocyclic ketones was developed with up to 85% ee. The enantiomeric purity was enriched to >99% ee by crystallization from EtOAc in >80% yield. Optimization studies have revealed a notable solvent effect that resulted in inversion of enantioselectivity from 85% ee in MeOH to −27% ee in DCE. The hydrazone geometry and possible hydrogenation via endocyclic alkene were examined as possible factors for the inversion of enantioselectivity.

The biotransformation of methadone in man: synthesis and identification of a major metabolite.

Abstract After administration of methadone to man, only two basic urinary excretion products were detected, these were unchanged drug and 2-ethyl-1, 5-dimethyl-3, 3-diphenyl-1-pyrroline. The hydriodide of the latter was synthesized and its endocyclic alkene structure was confirmed from infrared and nuclear magnetic resonance spectral data. This compound was shown to be identical with the hydriodide of the basic product formed by reaction of 1, 5-dimethyl-3, 3-diphenylpyrrolid-2-one with ethyl-lithium.

A Synthetic Route to Fully Substituted Chiral Cyclopentylamine Derivatives: Precursors of Carbanucleosides

Removal of silyl protection from D-glucose derived substrate 6 afforded 7, which upon acetonide deprotection followed by reaction with N-benzylhydroxylamine furnished two isomeric isoxazolidinocyclopentane derivatives via spontaneous cyclization of an in situ generated nitrone. The methyl xanthate derivative of the tertiary hydroxyl group of one isomer was isolated and subjected to radical deoxygenation reaction to form epimeric products, while with the other isomer it underwent spontaneous 1,2-elimination to form a mixture of the two possible endocyclic olefins. Hydrogenolytic cleavage of the isoxazolidine rings of the purified products followed by insertion of 5-amino-4-chloropyrimidine moiety and purine ring construction smoothly afforded structurally unique carbanucleoside analogues. Various spectroscopic methods on the synthesized compounds and X-ray analysis on one important intermediate were used to assign the structures and stereochemistry of the products.

A Nine-Step Total Synthesis of (−)-Platencin

Within 1 year, platencin, a recently discovered antibiotic, has become a highly competitive synthetic target, due to its promising bioactivity and its unusual complex molecular architecture. Herein, a particularly concise total synthesis of platencin starting from inexpensive perillaldehyde is described. The key features of this approach are (1) a highly diastereoselective Diels-Alder reaction with Rawal's dieneforming the first all-carbon quaternary center, (2) a ring-closing metathesis to generate the strained tricylic skeleton, (3) a hydration/dehydration strategy to efficiently shift the endocyclic alkene to the exoposition, and (4) a 1,4-addition of a hindered ketone enolate to methyl acrylate to create the second all-carbon quaternary center. In view of the brevity (nine linear steps) and the overall yield of 10%, our synthesis compares favorably with all the previous ones.

Microwave-assisted regioselective olefinations of cyclic mono- and di-ketones with a stabilized phosphorus ylide

A number of cyclic mono- and di-ketones underwent regioselective olefination with (carbethoxyethylidene)triphenylphospharane under controlled microwave heating. The Wittig reaction of 4-substituted cyclohexanones or 1,2- and 1,4-cyclohexanediones with the ylide at 190 °C for 20 min in MeCN afforded the exocyclic olefins in >94:6 isomer ratios. On the other hand, the same reactions carried out at 230 °C for 20 min in the presence of 20 mol % DBU furnished the endocyclic olefins in >83:17 isomer ratios. The base-mediated isomerization of the exocyclic olefins into the endocyclic isomers was primarily driven by thermodynamic stability of the products and the effect of ring structures on deconjugation was examined.

Access to Macrocyclic Endocyclic and Exocyclic Allylic Alcohols by Nickel‐Catalyzed Reductive Cyclization of Ynals.

AbstractFor Abstract see ChemInform Abstract in Full Text.