Silylstannylation of Allenes and Silylstannylation−Cyclization of Allenynes. Synthesis of Highly Functionalized Allylstannanes and Carbocyclic and Heterocyclic Compounds

Palladium-Catalyzed Remote Diborylative Cyclization of Dienes with Diborons via Chain Walking

A novel method for catalytic remote bismetalation of alkene substrates by the addition of dimetal reagents is accomplished by using chain walking. In the presence of a palladium catalyst, the reaction of various 1,n-dienes and diborons were converted into cyclopentane derivatives with two boryl groups at remote positions via facile regioselective transformation of an unactivated sp³ C-H bond to a C-B bond. Sequential construction of three distant bonds, which is difficult to achieve by any method, was accomplished for the reactions of 1,n-dienes (n ≥ 7).

Palladium-catalyzed silaborative carbocyclizations of 1,6-diynes

An addition/cyclization reaction of 1,6-diynes was developed for the synthesis of highly substituted 1,2-dialkylidenecycloalkanes. In this work, 1,6-diynes reacted with (dimethylphenylsilyl)pinacol-borane in the presence of a palladium catalyst to afford 1,2-dialkylidenecycloalkanes bearing silyl and boryl groups with a (Z,Z)-configuration in good to excellent yields. Moreover, the corresponding products could be easily converted into other synthetically useful compounds. This protocol provides an efficient and practical method of heteroelement-element linkage addition to the unsaturated 1,6-diynes.

Ethynylglycine synthon, a useful precursor for the synthesis of biologically active compounds: an update: part I: preparations of ethynylglycine synthon

The ethynylglycine synthon {(R)-2,2-dimethyl-3-(tert-butoxycarbonyl)-4-ethynyloxazolidine} is a chiral compound with valuable synthetic interest. An update on the different routes for its synthesis is reviewed and discussed.

Borostannylation of alkynes and enynes. Scope and limitations of the reaction and utility of the adducts

The utility of the hetero-bismetallating reagent 1,3-dimethyl-2-trimethylstannyl-2-bora-1,3-diazacyclopentane (1) has not been fully realized because of the hydrolytic instability of the products derived from catalyzed vicinal syn-additions to alkynes. The isolation of a variety of such adducts derived from alkynes (and also from hitherto unreported additions to 1,3-enynes) as stable boron pinacolates is reported. Examples of the applications of resulting products in tandem cross-coupling reactions and as dienes in Diels-Alder reactions are illustrated.

A new context for palladium mediated B-addition reaction: an open door to consecutive functionalization

Palladium metal-catalyzed boron addition to unsaturated carbon-carbon bonds provides an efficient and convenient route for the preparation of organoboranes, which are versatile intermediates for organic synthesis. Palladium complexes are responsible for the exclusive catalytic performance and eventually allow access to selectively functionalized molecules by catalytic consecutive tandem sequences. The final objective is to find suitable palladium complexes that make it possible to perform a one-pot sequential reaction (B-addition/functionalization) by means of a multifaceted palladium catalyst. Mechanistic insights into the concatenated reactions through B chemistry are required if one wants to understand the experimental results.

Palladium-catalyzed asymmetric silaboration of allenes

An enantioselective silaboration of allenes was achieved using an achiral silylborane in the presence of a palladium catalyst bearing a chiral monodentate phosphine ligand. (R)-2-Bis(3,5-dimethylphenyl)phosphino-1,1'-binaphthyl gave the highest enantioselectivities in the addition of (diphenylmethylsilyl)pinacolborane to the internal C=C bond of terminal allenes at 0 degrees C, giving the corresponding beta-borylallylsilanes in high yields with high enantiomeric excesses. The enantioselectivity depended on the bulkiness of substituents of allenes: the enantiomeric excesses were found to be 91-93% ee (R = tert- and sec-alkyl), 88-90% ee (R = aryl), and 80-82% ee (R = prim-alkyl and Me) at 0 degrees C. Perfect chirality transfer was observed in the intramolecular cyclization reactions of the functionalized allylsilanes, affording highly enantioenriched cyclic alkenylboranes, which underwent Suzuki-Miyaura coupling with aryl halides.

Asymmetric multicomponent reactions: diastereoselective synthesis of substituted pyrrolidines and prolines

A novel diastereoselective synthesis of substituted pyrrolidines has been developed. Asymmetric multicomponent reactions of optically active phenyldihydrofuran, N-tosyl imino ester, and silane reagents in a one-pot operation afforded highly substituted pyrrolidine derivatives diastereoselectively. The reaction is quite efficient and constructed up to three stereogenic centers in a single operation.

GaCl3-Catalyzed Allenyne Cycloisomerizations to Allenenes

Cycloisomerizations of allenynes to allenenes have been studied in the presence of catalytic amounts of [Au(PPh3)]SbF6 in dichloromethane or GaCl3 in toluene. Both catalytic systems are quite effective for terminal 1,6-allenynes. However, they showed different reactivities toward allenynes with di-substituents at the allenic terminal carbon. For the GaCl3-catalyzed reactions, allenenes were obtained in reasonable to high yields. However, for a Au(I)-catalyzed reaction, a triene was obtained in a poor yield. Thus, GaCl3 serves as an effective catalyst for the cycloisomerization of allenynes bearing a terminal alkyne to give cyclic allenenes in reasonable to high yields.

Highly efficient [2 + 2] intramolecular cyclizations of allenynes under microwave irradiation: construction of fused bicyclic compounds

A palladium [2 + 2] cycloaddition of 1,6- and 1,7-allenyne carboxylates and microwave-mediated [2 + 2] cycloaddition of various 1,n-allenynes were developed and, particularly, the microwave irradiated [2 + 2] cycloaddition of allenynes can provide a simple, general and eco-friendly synthetic method to fused bicyclo[m,2,0]alkadienes.

Tuning the Acceptors in Catalyzed Cyclizations Initiated by Allenes. Silylstannylation/Cyclization of Allene-Aldehydes for Synthesis of Polyalkylated Indolizidines Including 223A Congeners

Starting from succinamide and 1,2-heptadiene-4-ol, a racemic allene-aldehyde substrate, 20, suitable for R(3)SiSnR'(3)-mediated cyclization was synthesized in six steps and in 21% yield. Stereoselective cyclization (relative cis configuration at the new stereogenic centers of the homoallyl alcohol generated) proceeded smoothly, giving a mixture of indolizidinols bearing five contiguous stereocenters in a combined yield of 80%. Relative configurations of each of the products were unequivocally established by a combination of 2D NMR experiments and single-crystal X-ray analysis. The major indolizidinol obtained in 32% yield was elaborated into indolizidine 5,8-epi-indolizidine 223A via a five-step reaction sequence in 32% overall yield. The second major component (24%) of the key cyclization yielded, in four steps, indolizidine 6,8-epi-223 in 14% yield. Even though revision of the initially postulated structure foiled our original synthetic plans for the natural product, indolizidine 223A, the new stereoselective cyclization strategy and several selective transformations of the indolizidine derivatives reported here may find further applications for the synthesis of highly alkylated indolizidine and other related alkaloids.