Preparation of C2-Symmetric Allenes by Palladium-Catalyzed Double-Nucleophilic Substitution on 3-Bromopenta-2,4-dienyl Acetate

Core Structure-Oriented Asymmetric α-Allenylic Alkylation of Amino Acid Esters Enabled by Chiral Aldehyde/Palladium Catalysis

Aiming at the reported chiral synthons leading to manzacidins A and D, here we report a highly efficient catalytic asymmetric α-allenylic alkylation reaction of NH2-unprotected amino acid esters that is promoted by combined chiral aldehyde/palladium catalysis. Fifty examples of unnatural α,α-disubstituted amino acid esters are reported with good-to-excellent yields and stereoselectivities. Based on this methodology, a key intermediate leading to manzacidin C and its other three stereoisomers is prepared accordingly.

Palladium-Catalyzed Three-Component Coupling of 1,1-Dibromoalkenes, Vinylzinc Chloride, and Soft Nucleophiles: One-Pot Synthesis of 1,3-Disubstituted Allenes

The three-component coupling reaction of 1,1-dibromoalkenes 1, vinylzinc chloride 2, and carbon soft nucleophiles 3 was realized in the presence of the catalytic palladium/Xantphos species, and 1,3-disubstituted allenic products 5 were obtained in yields of up to 77%. The successive two palladium-catalyzed processes, namely the cross-coupling reaction and the allylic substitution, assembled 5 from the easily accessible starting compounds.

Palladium-Catalyzed Sequential Twofold Nucleophilic Substitution on 3-Bromopenta-2,4-dienyl Phosphate: Preparation of C1- and C2-Symmetric Doubly Functionalized Allenes

Readily available 3-bromopenta-2,4-dienyl esters (1x, acetate; 1y, benzoate; 1z, diethyl phosphate) were applied to the palladium-catalyzed reaction with various soft nucleophiles. The reaction proceeded through the twofold nucleophilic substitution via formal S_N2'- and S_N2-processes, giving the various doubly functionalized allenes 2 in good yields. In the reactions of carboxylates 1x and 1y, the first substitution took place at the C-Br bond to form (allenyl)methyl ester intermediates 3. Because the second substitution on 3 proceeded faster than the first substitution on 1x or 1y, 3 was not isolable, and C₂-symmetric allenes 2 were obtained even in the presence of remaining 1x and 1y. On the other hand, the phosphate moiety was more reactive than the C-Br moiety in 1z. The initial products from 1z were 5-Nu-3-bromopenta-1,3-dienes 4, which were less reactive than 1z. Monosubstitution products 4 were isolable, and the stepwise introduction of two different Nu groups in C₁-symmetric allenes 2 was realized starting with 1z under the controlled reaction conditions. By the use of a chiral palladium catalyst, axially chiral doubly functionalized allenes were obtained up to 95% ee.

Bis(phenylsulfonyl)methane mediated synthesis of olefins via a halogen elimination and double bond migration

An effective dehydrochlorination of bis(phenylsulfonyl)alkane to prepare alkene building blocks is developed. The elimination together with double bond migration results in a variety of β,γ-unsaturated bis(phenylsulfonyl)olefins in good yields with only E geometry. The following chemical diversification represents an easy and straightforward access to a series of alkene building blocks.

Palladium-Catalyzed SN2'-Cyclization of Ambivalent (Bromoalkadienyl)malonates: Preparation of Medium- to Large-Membered Endocyclic Allenes

A palladium-catalyzed reaction for preparing various endocyclic allenes was developed. The substrates for the reaction were readily available ω-(pronucleophile-tethered)-3-bromo-1,3-alkadienes, and a palladium-catalyst facilitated their unimolecular S_N2'-cyclization in the presence of potassium tert-butoxide to give the corresponding 9- to 16-membered endocyclic allenes in fair yields of up to 67% together with the dimeric 16- to 32-membered endocyclic bis-allenes and other oligomeric/polymeric intermolecular reaction products. For higher yields of the monomeric endocyclic allenes, the reaction needed to be conducted under high-dilution conditions. Using a chiral palladium catalyst, axially chiral endocyclic allenes were obtained in up to 70% ee.

Syntheses and Biological Evaluation of Costunolide, Parthenolide, and Their Fluorinated Analogues

Inspired by the biosynthesis of sesquiterpene lactones (SLs), herein we report the asymmetric total synthesis of the germacrane ring (24). The synthetic strategy features a selective aldol reaction between β,γ-unsaturated chiral sulfonylamide 15a and aldehyde 13, as well as the intramolecular α-alkylation of sulfone 21 to construct a 10-membered carbocylic ring. The key intermediate 24 can be used to prepare the natural products costunolide and parthenolide (PTL), which are the key precursors for transformation into other SLs. Furthermore, the described synthetic sequences are amenable to the total synthesis of SL analogues, such as trifluoromethylated analogues 32 and 45. Analogues 32 and 45 maintained high activities against a series of cancer cell lines compared to their parent PTL and costunolide, respectively. In addition, 32 showed enhanced tolerance to acidic media compared with PTL. To our surprise, PTL and 32 showed comparable half-lives in rat plasma and in the presence of human liver microsomes.

Direct FeX3-based stereocontrolled access to (Z)-3-alkenyl-oxindoles from allenols

Iron trihalides (FeCl(3) and FeBr(3)) smoothly promote the halogenation/rearrangement of 2-indolinone-tethered allenols to efficiently afford 3-halodienyl-oxindoles with good yield and total selectivity. Also, 2-halo-1,3-dienes are synthetically interesting building blocks for the preparation of functionalized 3-alkenyl-oxindoles through a Suzuki-Miyaura reaction.