Palladium-Catalyzed Synthesis of Endocyclic Allenes and Their Application in Stereoselective [2 + 2]Cycloaddition with Ketenes

Synthesis of Chiral Endocyclic Allenes and Alkynes via Pd-Catalyzed Asymmetric Higher-Order Dipolar Cycloaddition

A Pd-catalyzed asymmetric higher-order dipolar cycloaddition between allenyl carbonates and azadienes is achieved by exploiting novel alkylidene-π-allyl-Pd dipoles. This research provides a modular platform for the synthesis of challenging chiral endocyclic allenes bearing a medium-sized heterocyclic motif and a centrally chiral stereocenter in good yields with high enantio- and diastereoselectivities (29 examples, up to 97% yield, 97:3 er and >19:1 dr). Experimental and computational studies elucidate the possible reaction mechanism and the observed stereochemical results. Based on the mechanistic understanding, a new π-propargyl-Pd dipole was designed to further extend the success of the higher order dipolar cycloaddition strategy to the synthesis of 10-membered endocyclic alkynes from propargyl carbonates and azadienes (13 examples, up to 98% yield and 94.5:5.5 er).

Stereo- and regiocontrol in intermolecular [2+2] cycloadditions between diarylketenes and allenamides to access substituted α-methylenecyclobutanones

The development of intermolecular [2+2] cycloadditions between allenamides and diarylketenes is described. α-Aryldiazo arylketones are employed as ketene precursors that react smoothly with allenamides in the presence of a P(C6F5)3 promoter. High diastereoselectivity (dr > 20 : 1) with endo-regioselectivity can be achieved when two aryl groups of ketenes have opposite electronic properties. The role of P(C6F5)3 is to increase the reaction yields, but also enhancing stereoselectivity.

Synthesis of Chiral Endocyclic Allenes by Palladium-Catalyzed Asymmetric Annulation Followed by Cope Rearrangement

Catalytic asymmetric synthesis of chiral endocyclic allenes remains a challenge in allene chemistry owing to unfavored tension and complex chirality. Here, we present a new relay strategy merging Pd-catalyzed asymmetric [3+2] annulation with enyne-Cope rearrangement, providing a facile route to chiral 9-membered endocyclic allenes with high efficiency and enantioselectivity. Moreover, theoretical calculations and experimental studies were performed to illustrate the critical, but unusual Cope rearrangement that allows for the complete central-to-axial chirality transfer.

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.

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.

Stereocontrolled synthesis and functionalization of cyclobutanes and cyclobutanones

In the last decade a certain number of new cyclobutane and cyclobutanone synthesis and functionalization protocols have been published. Organo- and biocatalyzed eco-friendly approaches to cyclobutane-containing molecules have been developed with interesting results. Also, successful new total synthesis of bioactive compounds and drugs have been recently reported where a four membered ring represented the key intermediate. Therefore, the rising interest in this field represents a great point of discussion for the scientific community, disclosing the synthetic potential of strained four membered ring carbocyclic compounds. Herein we report a critical survey on the literature concerning the enantiocontrolled synthesis and functionalization of cyclobutane derivatives, with particular attention to metal-free, low impact methodologies, published during the period 2000-2013.

Subtle electronic effects in metal-free rearrangement of allenic alcohols

A general and stereoselective rearrangement of allenic alcohols to (E,E)-1,3-dien-2-yl triflates and chlorides was developed under metal-free conditions. Subtle electronic effects of the alkyl and aryl substituents on the carbon bearing the hydroxyl group has a profound impact on the reaction rate and efficiency such that vinyl triflates were obtained from electron-deficient substrates and trimethylsilyl triflate whereas vinyl chlorides were generated with an electron-rich substrate and trimethylsilyl chloride.

Formal intermolecular [2 + 2] cycloaddition reaction of allenamide [corrected] with alkenes via gold catalysis

An efficient method was developed to construct the densely functionalized cyclobutane[corrected] adducts through formal intermolecular cycloaddition of allenamides [corrected] with electron-rich olefins via gold catalysis, in which vinyl ethers/amides and electron-rich styrenes worked very well. In addition, a series of allenamide [corrected] dimerization products were prepared from the same allenamide [corrected] substrates.

Toward an integrated route to the vernonia allenes and related sesquiterpenoids

The synthesis of a model endocyclic allene related to the vernonia allenes is described. Fragmentation of a suitable decalin derivative gave the simplified germacrane scaffold. Computational analysis of this and related substrates provides insight into the stereoelectronic requirements of C-C fragmentation. The overall strategy to access these and other sesquiterpenes and the key steps in the present sequence are also discussed.

How easy are the syntheses of allenes?

This short review highlights some of the recent important developments on the synthesis of allenes and its applications on the synthesis of some allenic natural products and allenic-based optoelectronic materials.

TfOH-catalyzed tandem cyclopropane ring enlargement/C-C formation/etherification of alkynylcyclopropanes and 1,3-diketones to cyclobutane-fused dihydrofurans

A new type of TfOH-catalyzed tandem cyclopropane ring enlargement/C-C formation/etherification reaction between alkynylcyclopropanes and 1,3-diketones to cyclobutane-fused dihydrofurans is described. The reaction tolerates a range of aryl and alkyl substituents with moderate to good yields.