Synthesis of 2-Unsubstituted Pyrrolidines and Piperidines from Donor-Acceptor Cyclopropanes and Cyclobutanes: 1,3,5-Triazinanes as Surrogates for Formylimines.

A synthetic procedure to access 2-unsubstituted pyrrolidines and piperidines is presented. In the presence of MgI2 as Lewis acid, donor-acceptor cyclopropanes or corresponding cyclobutanes were treated with 1,3,5-triazinanes, leading to the five- or six-membered ring systems under mild conditions in yields up to 93%. This protocol tolerates a great variety of functional groups and thus provides an efficient entry to this class of pyrrolidines and piperidines.

Ring-Opening 1-Amino-3-aminomethylation of Donor-Acceptor Cyclopropanes via 1,3-Diazepanes.

The first ring-opening reaction of donor-acceptor cyclopropanes to give diamines is reported. For this reaction, a 1,3-bisfunctionalization was developed using cyclopropanes, triazinanes, and Sc(OTf)3 as the catalyst, followed by treatment with acid. The reaction proceeds under very mild conditions and tolerates many functional groups. Moreover, a library of various 1,3-diazepanes, which arise as intermediates of the first formal aza-[4+3]-cycloaddition reaction with donor-acceptor cyclopropanes, was synthesized.

Stereocontrolled Total Synthesis of (-)-Stemaphylline.

Homologation of readily available α-boryl pyrrolidines with metal carbenoids is especially challenging even when good leaving groups (Cl- ) are employed. By performing a solvent switch from Et2 O to CHCl3 , efficient 1,2-metalate rearrangement of the intermediate boronate occurs with both halide and ester leaving groups. The methodology was used in the total synthesis of the Stemona alkaloid (-)-stemaphylline in just 11 steps (longest linear sequence), with high stereocontrol (>20:1 d.r.) and 11 % overall yield. The synthesis also features a late-stage lithiation-borylation reaction with a tertiary amine containing carbenoid.

Ring-Opening 1,3-Halochalcogenation of Cyclopropane Dicarboxylates.

Donor-acceptor cyclopropanes with two geminal carboxylic esters are reacted with chalcogenyl chlorides and bromides to afford ring-opened products bearing the halogen atoms in the 1-position, adjacent to the donor, and the chalcogenyl residue in the 3-position next to the two acceptor groups. A variety of different donors (e.g., aryl, N, and O) are used. The stereospecificity of the reaction is demonstrated by using a chiral starting material.

Ring-Opening Regio-, Diastereo-, and Enantioselective 1,3-Chlorochalcogenation of Cyclopropyl Carbaldehydes.

meso-Cyclopropyl carbaldehydes are treated in the presence of an organocatalyst with sulfenyl and selenyl chlorides to afford 1,3-chlorochalcogenated products. The transformation is achieved by a merged iminium-enamine activation. The enantioselective desymmetrization reaction, leading to three adjacent stereocenters, furnished the target products in complete regioselectivity and moderate to high diastereo- and enantioselectivities (d.r. up to 15:1 and e.r. up to 93:7).

[3 + 3]-Cycloaddition of Donor-Acceptor Cyclopropanes with Nitrile Imines Generated in Situ: Access to Tetrahydropyridazines.

Donor-acceptor cyclopropanes are reacted under the influence of a Lewis acid with hydrazonyl chlorides to afford tetrahydropyridazines. Formally, this transformation can be regarded as a [3 + 3]-cycloaddition of three-membered rings and nitrile imines generated in situ. This efficient method provides fast access to a variety of structurally diverse pyridazine derivatives. The structure of a typical product was confirmed by X-ray crystallography.

[4+3] Cycloaddition of Donor-Acceptor Cyclopropanes with Amphiphilic Benzodithioloimine as Surrogate for ortho-Bisthioquinone.

Donor-acceptor cyclopropanes were reacted with amphiphilic benzodithioloimine to give seven-membered heterocycles with two sulfur atoms. Formally, this transformation can be regarded as a [4+3] cycloaddition reaction of the three-membered ring and ortho-bisthioquinone. The benzodithioloimine serves as a surrogate for this highly reactive diene. The structure of the products was confirmed by X-ray crystallography. Broad signals in (13) C NMR studies suggest that several conformers, slowly interconverting on the NMR timescale, are present at room temperature.

Exploiting amphiphilicity: facile metal free access to thianthrenes and related sulphur heterocycles.

Benzodithioloimines are reacted with arynes or alkynes substituted with electron-withdrawing groups to afford the corresponding thianthrene or benzo[b][1,4]dithiine derivatives. The transformation takes place under mild reaction conditions without any transition metal. Furthermore, the reaction mode could be expanded to 2-thiocyanatopyrroles yielding pyrrolothiazoles.

Activation of aryl thiocyanates followed by aryne insertion: access to 1,2-thiobenzonitriles.

Palladium-catalyzed activation of carbon-sulfur bonds allows aryne insertion into aryl thiocyanates to generate new C-SAr and C-CN bonds in one step. The readily available starting materials make this method efficient in generating a variety of 1,2-thiobenzonitriles. By choosing an oxygen atmosphere the yields are increased and side reactions are minimized.

Pd-catalyzed three-component coupling of terminal alkynes, arynes, and vinyl cyclopropane dicarboxylate.

A palladium-catalyzed three-component coupling involving in situ generated arynes, terminal alkynes, and vinyl cyclopropane dicarboxylate has been developed. The process demonstrates the first example of aryne chemistry combined with the ring opening of vinyl cyclopropanes. This efficient method using readily available starting materials generates two new carbon-carbon bonds in one pot.

Ring-opening 1,3-dichlorination of donor-acceptor cyclopropanes by iodobenzene dichloride.

Donor-acceptor cyclopropanes are reacted with iodobenzene dichloride to afford ring-opened products bearing chlorine atoms in the 1- and 3-positions, adjacent to the donor and acceptor groups. A variety of different donors (e.g., alkyl, aryl, N, and O) and acceptor moieties (e.g., ketones, diesters, and dinitriles) are used.