Stereoselective Radical Research Articles

Total Synthesis of (−)‐Histrionicotoxin through a Stereoselective Radical Translocation–Cyclization Reaction

AbstractStereoselective total syntheses of (−)‐histrionicotoxin and (−)‐histrionicotoxin 235A are described. The 1‐azaspiro[5.5]undecane skeleton was constructed diastereoselectively by a radical translocation–cyclization reaction involving a chiral cyclic acetal; the use of tris(trimethylsilyl)silane was crucial for the high diastereoselectivity. The cyclization product was converted into (−)‐histrionicotoxin 235A through a one‐pot partial‐reduction–allylation reaction of a derivative containing an unprotected lactam. Finally, two terminal alkenes were transformed into enynes with the 1,3‐amino alcohol protected as an oxathiazolidine oxide to complete the total synthesis of (−)‐histrionicotoxin.

Total Synthesis of (-)-Histrionicotoxin through a Stereoselective Radical Translocation-Cyclization Reaction.

Stereoselective total syntheses of (-)-histrionicotoxin and (-)-histrionicotoxin 235A are described. The 1-azaspiro[5.5]undecane skeleton was constructed diastereoselectively by a radical translocation-cyclization reaction involving a chiral cyclic acetal; the use of tris(trimethylsilyl)silane was crucial for the high diastereoselectivity. The cyclization product was converted into (-)-histrionicotoxin 235A through a one-pot partial-reduction-allylation reaction of a derivative containing an unprotected lactam. Finally, two terminal alkenes were transformed into enynes with the 1,3-amino alcohol protected as an oxathiazolidine oxide to complete the total synthesis of (-)-histrionicotoxin.

Stereoselective Radical Cyclization Cascades Triggered by Addition of Diverse Radicals to Alkynes To Construct 6(5)-6-5 Fused Rings.

Cascade radical cyclization of alkynyl ketones with various carbon- and heteroatom-centered radical precursors via a sequential radical addition/1,5-H radical shift/5-exo-trig/radical cyclization process was realized for the first time. This method provides a strategically novel and step-economical protocol for diversity-oriented synthesis of a wide range of carbocyclic and heterocyclic 6(5)-6-5 fused ring systems with three contiguous stereocenters, including a quaternary carbon in high yields with excellent chemo- and diastereoselectivity.

Alkanethiol-promoted stereoselective radical rearrangement of 7-methylidene-6,8-dioxabicyclo[3.2.1]octanes to 2-acetyl-3,4-dihydropyrans

Alkanethiol-promoted stereoselective radical rearrangement of 7-methylidene-6,8-dioxabicyclo[3.2.1]octanes to 2-acetyl-3,4-dihydropyrans

A novel β-(oxy)alkyl radical during copper(I)-mediated stereoselective synthesis of (Z)-ene-1,4-diones in a reaction of 2,2,2-trichloro-1-phenylethanone

A novel β-(oxy)alkyl radical derived from trichloro methyl compound containing neither a suitably located C–C multiple bond nor a leaving group or a H-atom at the β-position of the radical in a reaction of 2,2,2-trichloro-1-phenyl-ethanone with 2mol equiv each of CuCl and bpy in refluxing DCE under a N2 atm underwent intramolecular heterolysis (just like formation of intact radical cation–anion pair) during stereoselective radical dimerization to Z-ene-1,4-dione along with small amount of reductive dechlorination product. The stereochemistry was established by X-ray diffraction spectroscopy of various solid crystalline products.

ChemInform Abstract: Synthesis of α‐Chiral Butyrolactones by Highly Stereoselective Radical Transfer or Sequential Asymmetric Alkylations: Concise Preparation of Leupyrrin Moieties.

AbstractTwo novel stereoselective methods are reported for the synthesis of chiral butyrolactones (VIII)/(IX) and (XVI).

Synthesis of α-Chiral Butyrolactones by Highly Stereoselective Radical Transfer or Sequential Asymmetric Alkylations: Concise Preparation of Leupyrrin Moieties.

Inspired by the bioactive natural metabolites leupyrrin A1 and B1 , two novel stereoselective methods for the highly concise synthesis of densely substituted α-chiral butyrolactones are reported. The first approach relies on an innovative three-step Ti(III) -catalyzed radical reaction that proceeds with excellent chemo-, regio-, and stereoselectivity. The alternative route utilizes sequential asymmetric alkylations and enables asymmetric synthesis of the authentic α-tetrasubstituted butyrolactone motif of the leupyrrins in only four steps from commercially available substrates.

Organic Photocatalytic Cyclization of Polyenes: A Visible-Light-Mediated Radical Cascade Approach.

A visible-light-mediated, organic photocatalytic stereoselective radical cascade cyclization of polyprenoids is described. The desired cascade cyclization products are achieved in good yields and high stereoselectivities with eosin Y as photocatalyst in hexafluoro-2-propanol. The catalyst system is also suitable for 1,3-dicarbonyl compounds, which require only catalytic amounts of LiBr to promote the formation of the corresponding enols.

Odorless, One-Pot Regio- and Stereoselective Iodothiolation of Alkynes with Sodium Arenesulfinates under Metal-Free Conditions in Water.

A newly developed regio- and stereoselective radical addition of alkyne under metal-free conidtions has been disclosed. This chemistry, in which odorless sodium arenesulfinates in place of thiols are used as the sulfur reagent, provides an efficient, one-pot approach for the generation of β-iodoalkenyl sulfides, which can be easily further functionalized to derive various alkenes and alkynyl sulfides rendering this methodology attractive to both synthetic and medicinal chemistry.

Stereoselective Radical C-H Alkylation with Acceptor/Acceptor-Substituted Diazo Reagents via Co(II)-Based Metalloradical Catalysis.

Co(II)-based metalloradical catalysis has been, for the first time, successfully applied for asymmetric intramolecular C-H alkylation of acceptor/acceptor-substituted diazo reagents. Through the design and synthesis of a new D2-symmetric chiral amidoporphyrin as the supporting ligand, the Co(II)-based metalloradical system, which operates at room temperature, is capable of 1,5-C-H alkylation of α-methoxycarbonyl-α-diazosulfones with a broad range of electronic properties, providing the 5-membered sulfolane derivatives in high yields with excellent diastereoselectivity and enantioselectivity. In addition to complete chemoselectivity toward allylic and allenic C-H bonds, the Co(II)-based metalloradical catalysis for asymmetric C-H alkylation features a remarkable degree of functional group tolerance.

A computational study on Lewis acid-catalyzed diastereoselective acyclic radical allylation reactions with unusual selectivity dependence on temperature and epimer precursor.

In stereoselective radical reactions, it is accepted that the configuration of the radical precursor has no impact on the levels of stereoinduction, as a prochiral radical intermediate is planar, with two identical faces, independently of its origin. However, Sibi and Rheault (J. Am. Chem. Soc. 2000, 122, 8873-8879) remarkably obtained different selectivities in the trapping of radicals originated from two epimeric bromides, catalyzed by chelating Lewis acids. The selectivity rationalization was made on the basis of different conformational properties of each epimer. However, in this paper we show that the two epimers have similar conformational properties, which implies that the literature proposal is unable to explain the experimental results. We propose an alternative mechanism, in which the final selectivity is dependent on different reaction rates for radical formation from each epimer. By introducing a different perspective of the reaction mechanism, our model also allows the rationalization of different chemical yields obtained from each epimer, a result not rationalized by the previous model. Adaptation to other radical systems, under different reaction conditions, is also possible.

Stereoselective Radical Azidooxygenation of Alkenes

Radical azidooxygenation of various alkenes is described. A readily prepared N3-iodine(III) reagent acts as a clean N3-radical precursor. Radical generation is achieved with TEMPONa acting as a mild organic reducing reagent. The C-radical generated after N3-radical addition is efficiently trapped by in situ generated TEMPO. Yields are good to excellent, and for cyclic systems azidooxygenation occurs with excellent diastereoselectivity.

Diastereoselective radical addition to γ-alkyl-α-methylene-γ-butyrolactams and the synthesis of a chiral pyroglutamic acid derivative

The cis- and trans-stereoselective radical additions to α-methylene-γ-alkyl- γ-lactams were investigated and the scope and limitation of the reaction were also revealed. This stereoselective radical reaction was used for synthesis of chiral pyroglutamic acid derivatives starting from a commercially available chiral amino acid.

ChemInform Abstract: Stereoselective Radical Amination of Electron‐Deficient C(sp3)—H Bonds by Co(II)‐Based Metalloradical Catalysis: Direct Synthesis of α‐Amino Acid Derivatives via α‐C—H Amination.

AbstractTreatment of sulfamoyl azides with the Co complex COP allows an efficient, regio‐ and stereoselective access to cyclic sulfamides.

Formal Synthesis of (−)‐Cephalotaxine

AbstractA formal synthesis of (−)‐cephalotaxine (1) by means of a highly stereoselective radical carboazidation process is reported. The synthesis begins with the protected (S)‐cyclopent‐2‐en‐1‐ol derivative 10 and uses the concept of self‐reproduction of a stereogenic center (Schemes 5 and 6). For this purpose, the double bond adjacent to the initial chiral center in 10 is converted into an acetonide after stereoselective dihydroxylation. The initial alcohol function is used to build an exocyclic methylene group suitable for the carboazidation process 8→7 (Scheme 7). Finally the protected diol moiety is converted back to an alkene (14→15→6) and used for the formation of ring B via a Heck reaction (6→(−)‐16; Scheme 8).

Stereoselective radical amination of electron-deficient C(sp3)-H bonds by Co(II)-based metalloradical catalysis: direct synthesis of α-amino acid derivatives via α-C-H amination.

The cobalt(II) complex of 3,5-Di(t)Bu-IbuPhyrin, [Co(P1)], is an effective catalyst for intramolecular amination of electron-deficient C-H bonds, including those adjacent to electron-withdrawing CO(2)R, C(O)NR(2), C(O)R, and CN groups, in excellent yields with high regio- and stereoselectivity. The [Co(P1)]-catalyzed amination system provides a direct method for the synthesis of α-amino acid derivatives from the corresponding carboxylate precursors.

Expedient Construction of the [7–5–5] All-Carbon Tricyclic Core of the Daphniphyllum Alkaloids Daphnilongeranin B and Daphniyunnine D

A synthetic strategy for the construction of the [7-5-5] all-carbon tricyclic core of numerous calyciphylline A-type Daphniphyllum alkaloids has been developed using a key intramolecular Pauson-Khand reaction. A subsequent base-mediated double-bond migration and a regio- and stereoselective radical late stage allylic oxygenation provide access to the substitution patterns of daphnilongeranin B and daphniyunnine D.

ChemInform Abstract: Total Synthesis and Absolute Configuration Determination of (+)‐Subincanadine F.

The first asymmetric synthesis of indole alkaloid (+)-subincanadine F was successfully accomplished with the uncommon 7-endo-trig stereoselective radical cyclization as the key step and its absolute configuration was thus assigned.

ChemInform Abstract: Synthesis of 3α-Alkoxy-4β-substituted-2-azetidinones from L( +)-Tartaric Acid.

Abstract Tartaric acid, regioselective saponification, Pig Liver Esterase, 3-methoxy-2-azetidinone, N -Hydroxy-2-thiopyridone, radical decarboxylation. The synthesis of 3α-alkoxy-4β-azetidinones from L(+) tartaric acid is described. Regioselective saponification and methylation along with a stereo selective radical decarboxylative alkylation are key steps leading to optically pure trans-β-lactams.

ChemInform Abstract: Utilization of Selenium-Directed (2 + 2) Cycloadditions: Concise Synthesis of (.+-.)-Fragranol.

The reaction of 2-(phenylseleno)prop-1ene 1 and methyl vinyl ketone 2 in the presence of EtAlCl2 gave the [2 + 2] cycloadducts 3a and 3b. A radical substitution of the adducts 3a and 3b with allytributyltin gave the allylated cyclobutane product, which was transformed into the cyclobutane natural product, (±)-fragranol. Stereoselective radical substitution using the ethylene glycol ketals of compounds 3a and 3b was also achieved.