Tertiary Carbon Centers Research Articles

Total Synthesis of Cryptotrione

AbstractThe total synthesis of cryptotrione (1) was enabled by substrate‐controlled diastereoselective construction of the bicyclo[3.1.0]hexene framework through platinum‐catalyzed enyne cycloisomerization and Lewis acid induced polyene cyclization to construct the abietane‐type tricyclic diterpene skeleton. The stereogenic tertiary carbon center in the side chain was installed in a diastereodivergent manner by conjugate addition reactions.

Total Synthesis of Cryptotrione.

The total synthesis of cryptotrione (1) was enabled by substrate-controlled diastereoselective construction of the bicyclo[3.1.0]hexene framework through platinum-catalyzed enyne cycloisomerization and Lewis acid induced polyene cyclization to construct the abietane-type tricyclic diterpene skeleton. The stereogenic tertiary carbon center in the side chain was installed in a diastereodivergent manner by conjugate addition reactions.

Efficient Assembly of Molecular Complexity Enabled by Palladium‐Catalyzed Heck Coupling/C(sp2)−H Activation/ C(sp3)−H Activation Cascade

AbstractA palladium‐catalyzed [2+2+1] annulation among 3‐iodochromones, benzyl bromides, and norbornene has been developed. This annulation consists of a domino sequence involving Heck coupling/C(sp2)−H activation/C(sp3)−H activation, affording a variety of complex chromone derivatives bearing five contiguous tertiary carbon centers in up to 94% yield and 99:1 dr. Interestingly, the diastereoselectivity could be switched by fine‐tuning the solvent, in which endo isomer and exo isomer were obtained using mesitylene/CH3CN and mesitylene, respectively.magnified image

Ni(II)/tBu-SMI-PHOX catalyzed enantioselective addition of arylboronic acids to cyclic N-sulfonyl aldimines

An efficient Ni(ClO4)2·6H2O/SMI-PHOX catalyzed enantioselective addition of arylboronic acids to cyclic N-sulfonyl aldimines is envisioned to afford excellent enantioselectivities (up to 99% ee) in high yields (up to 98%). This protocol offers new opportunities for the synthesis of chiral benzosultams containing a stereogenic tertiary carbon center. The highlights of this method include mild reaction conditions, the use of cheap metal catalyst and a wide substrate scope.

Nickel-Catalyzed α-Carbonylalkylarylation of Vinylarenes: Expedient Access to γ,γ-Diarylcarbonyl and Aryltetralone Derivatives.

We report a Ni-catalyzed regioselective α-carbonylalkylarylation of vinylarenes with α-halocarbonyl compounds and arylzinc reagents. The reaction works with primary, secondary, and tertiary α-halocarbonyl molecules, and electronically varied arylzinc reagents. The reaction generates γ,γ-diarylcarbonyl derivatives with α-secondary, tertiary, and quaternary carbon centers. The products can be readily converted to aryltetralones, including a precursor to Zoloft, an antidepressant drug.

Enantioselective Dehydroxyhydrogenation of 3-Indolylmethanols by the Combined Use of Benzothiazoline and Chiral Phosphoric Acid: Construction of a Tertiary Carbon Center.

The chiral indole is an important structure in organic chemistry. We have developed an enantioselective hydrogen transfer reaction of indolylmethanol, which is characterized by the combined use of benzothiazoline and a newly synthesized chiral phosphoric acid. The reaction furnished indoles bearing a chiral tertiary carbon center at the 3-position in high to excellent yields and with excellent enantioselectivities, most of which are greater than 95% ee. The chiral indole was converted into an inhibitor of leukotriene production while retaining excellent ee.

Light-Triggered Catalytic Asymmetric Allylic Benzylation with Photogenerated C-Nucleophiles.

Herein is reported the asymmetric allylic benzylation of Morita–Baylis–Hillman (MBH) carbonates with 2-methylbenzophenone (MBP) derivatives as nonstabilized photogenerated C-nucleophiles. The dual activation of both reaction partners, chiral Lewis-base activation of the electrophile and light activation of the nucleophile, enables the stereoselective installation of benzyl groups at the allylic position to forge tertiary and quaternary carbon centers.

NiH‐Catalyzed Migratory Defluorinative Olefin Cross‐Coupling: Trifluoromethyl‐Substituted Alkenes as Acceptor Olefins to Form gem‐Difluoroalkenes

AbstractWe report a NiH‐catalyzed migratory defluorinative coupling between two electronically differentiated olefins. A broad range of unactivated donor olefins can be joined directly to acceptor olefins containing an electron‐deficient trifluoromethyl substituent in both intra‐ and intermolecular fashion to form gem‐difluoroalkenes. This migratory coupling shows both site‐ and chemoselectivity under mild conditions, with the formation of a tertiary or quaternary carbon center.

NiH-Catalyzed Migratory Defluorinative Olefin Cross-Coupling: Trifluoromethyl-Substituted Alkenes as Acceptor Olefins to Form gem-Difluoroalkenes.

We report a NiH-catalyzed migratory defluorinative coupling between two electronically differentiated olefins. A broad range of unactivated donor olefins can be joined directly to acceptor olefins containing an electron-deficient trifluoromethyl substituent in both intra- and intermolecular fashion to form gem-difluoroalkenes. This migratory coupling shows both site- and chemoselectivity under mild conditions, with the formation of a tertiary or quaternary carbon center.

Stereospecific nucleophilic substitution at tertiary and quaternary stereocentres.

Nucleophilic substitution reactions have always been considered as one of the most powerful reactions for the creation of carbon–carbon or carbon–heteroatom bonds in organic synthesis. In contrast to secondary carbons, the steric shielding of tertiary carbons retards a concerted, stereospecific nucleophilic substitution, and ionizing pathways often lead to nonselective substitution due to ion pair dissociation. In this minireview, we will detail pioneering contributions and more recent achievements emphasizing the feasibility of nucleophilic substitution on tertiary stereocentres under certain conditions, with inversion of configuration. The development of these transformations at tertiary centres are of remarkable added value to practitioners in the field of complex molecule synthesis. A stereoselective substitution at a quaternary carbon stereocentre with inversion of configuration is also discussed in the case of a three-membered ring.

Facilitating the transmetalation step with aryl-zincates in nickel-catalyzed enantioselective arylation of secondary benzylic halides

Nickel-catalyzed asymmetric cross-coupling of secondary alkyl electrophiles with different nucleophiles represents a powerful strategy for the construction of chiral tertiary carbon centers. Yet, the use of aryl Grignard reagents or aryl zinc halides in many reactions typically resulted in low enantioselectivity, mainly due to their slow transmetalation step in the catalytical cycle and consequently the requirement of relatively high temperature. Here we report that the use of lithium aryl zincate [Ph2ZnBr]Li facilitates the transmetalation step of the nickel-catalyzed cross-coupling reaction. Based on this discovery, a highly enantioselective construction of fluoroalkyl-substituted stereogenic center by a nickel-catalyzed asymmetric Suzuki-Miyaura coupling of α-bromobenzyl trifluoro-/difluoro-/mono- fluoromethanes with a variety of lithium aryl zincates [Ph2ZnBr]Li that were in situ generated from the reaction of lithium organoboronate with 1.0 equivalent of ZnBr2 was described.

Reductive Fragmentation of Tetrazoles: Mechanistic Insights and Applications toward the Stereocontrolled Synthesis of 2,6-Polysubstituted Morpholines.

A new methodology to synthesize 2,6-di-, and 2,2',6-trisubstituted morpholines via the reduction of oxabicyclic tetrazoles under mild conditions is described. The reaction proved successful for a wide range of tetrazoles, including sterically encumbered ones harboring gem-substituents on tertiary carbon centers. The mechanism for the decades-old reduction of tetrazoles to secondary amines is elucidated.

Copper-catalyzed cyanoisopropylation of beta-keto esters using azos: synthesis of beta-dicarbonyls bearing an alfa-tertiary nitrile moiety

A copper-catalyzed α-cyanoisopropylation reaction of β-keto esters using azo compounds as cyanoalkylating agents is presented, providing an easy access to β-dicarbonyls containing an α-tertiary nitrile moiety with adjacent tertiary and quaternary carbon centers. It is remarkable because a tremendous steric conflict is involved during reaction. Such nitriles were otherwise unavailable, and the reaction features simple and relatively mild conditions and good functional-group tolerance.

Metal-free synthesis of gem-silylboronate esters and their Pd(0)-catalyzed cross-coupling with aryliodides.

A transition-metal-free method for the synthesis of gem-silylboronate esters with arylboronic acids and trimethylsilyldiazomethane (TMSCHN2) has been developed. This transformation is a straightforward homologation of arylboronic acids and features wide substrate scope and good functional-group tolerance. The gem-silylboronate esters undergo efficient Suzuki-Miyaura cross-coupling with aryliodides and the silyl group of the product can be further functionalized. Tertiary carbon centers with different substituents can be constructed successfully by selective and sequential functionalization.

Total Synthesis of an Anticancer Natural Product (±)‐Peharmaline A and Its Analogues

First total synthesis of a rare β‐carboline–vasicinone hybrid alkaloid (±)‐peharmaline A has been accomplished in just 3 steps starting from known compounds. Stereoselective Pictet–Spengler reaction to nitrogenated tertiary carbon center and one‐pot construction of the tricyclic skeleton of vasicinone are the highlights of present synthesis. We have also synthesized structurally close analogues of the natural product by following the developed route.

Application of Hantzsch Ester and Meyer Nitrile in Radical Alkynylation Reactions.

The first example of constructing a Csp3-Csp bond with substituted Hantzsch ester and Meyer nitrile is reported. When benziodoxole-activated alkyne was applied as the alkynyl donor, products containing Csp3-Csp bonds involving primary, secondary, and tertiary carbon centers were achieved in up to 97% yields. K2S2O8 was the optimum radical initiator in this reaction.

Silver/NBS-Catalyzed Synthesis of α-Alkylated Aryl Ketones from Internal Alkynes and Benzyl Alcohols via Ether Intermediates.

The silver hexafluoroantimonate/ N-bromosuccinimide (NBS)-catalyzed synthesis of α-alkylated aryl ketones with a tertiary carbon center from internal alkynes and benzyl alcohols is reported. This reaction proceeds via the etherification of benzyl alcohols with an in situ generated benzyl bromide, formed by the reaction of benzyl alcohol with a catalytic amount of NBS and AgSbF6. Ag-catalyzed C-O cleavage of the ether leads to a tolyl radical, which undergoes addition to the alkyne, ultimately leading to the α-alkylated aryl ketone products.

Impacts of Stereoisomerism on Molecular Packing and Charge Transport of Imide-Fused Corannulene Derivatives

Two chiral tertiary carbon centers bearing one mesityl group at each center are introduced into the molecular backbone of imide-fused corannulene derivatives to produce four stereoisomers (i.e., (S, S), (R, R), (R, S), or (S, R) configurations on two chiral carbons) in one pot, which are separated into two portions through column chromatography over silica gel. Portion 1, containing a pair of enantiomers ((S, S) and (R, R)), adopts layered packing in the crystal. Portion 2, consisting of a pair of mesomers ((R, S) and (S, R)), exhibits columnar packing in their cocrystal. Theoretical calculations are performed on these two packing motifs, revealing that Portion 1 displays hole-dominated transport, whereas Portion 2 shows electron-dominated transport.

Hypervalent iodine(III)-mediated decarboxylative acetoxylation at tertiary and benzylic carbon centers.

The decarboxylative acetoxylation of carboxylic acids using a combination of PhI(OAc)2 and I2 in a CH2Cl2/AcOH mixed solvent is reported. The reaction was successfully applied to two types of carboxylic acids containing an α-quaternary and a benzylic carbon center under mild reaction conditions. The resulting acetates were readily converted into the corresponding alcohols by hydrolysis.

Heck Reaction of Electronically Diverse Tertiary Alkyl Halides.

The efficient Pd-catalyzed Heck reaction of diverse tertiary alkyl halides with alkenes has been developed. Unactivated tertiary alkyl halides efficiently react at room temperature under visible light irradiation with no exogenous photosensitizers required. For activated tertiary alkyl halides, the same catalytic system works well without light. These methods offer a general access to electronically diverse alkenes possessing quaternary and functionalized tertiary allylic carbon centers. The substituents at these centers include alkyl-, carbalkoxy-, tosyl-, phosphonyl-, and boronate groups. It was also shown that the end-game mechanism of this transformation may vary depending on the type of the substrates used.