N -Acyliminium ion chemistry and palladium catalysis: a useful combination to obtain bicyclic heterocycles

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.

Palladium-Catalyzed Highly Stereoselective Synthesis of N-Aryl-3-arylmethylisoxazolidines via Tandem Arylation of O-Homoallylhydroxylamines

The palladium-catalyzed tandem arylation of O-homoallylhydroxylamines with 2 equiv of aryl bromides was examined. With Pd2(dba)3 (1 mol %) as the catalyst, Xantphos (2 mol %) as the ligand, and NaOt-Bu as the base, the reactions of O-homoallylhydroxylamines with aryl bromides via sequential N-arylation/cyclization/C-arylation in toluene afforded the corresponding N-aryl-3-arylmethylisoxazolidines in good yields with excellent diastereoselectivity.

Pd-Cu Bimetallic Catalyzed Domino Cyclization of α-Allenols Followed by a Coupling Reaction: New Sequence Leading to Functionalized Spirolactams

A novel regioselective metal-catalyzed spirocyclization of alpha-allenols-cross coupling (Heck, Sonogashira, and Suzuki) reaction sequence, leading to potentially bioactive spirocyclic lactam derivatives has been developed. Precursors for the tandem spirocyclization-coupling reaction, alpha-allenols 2 a-d were obtained starting from alpha-oxolactams 1 a-c via indium-mediated Barbier-type carbonyl-allenylation reaction in aqueous media by using our previously described methodologies.

Gold catalysis in organic synthesis: efficient cycloisomerization of alpha-aminoallenes to 3-pyrrolines

[reaction: see text] The gold(III) chloride-catalyzed cycloisomerization of various alpha-aminoallenes gave the corresponding 3-pyrrolines in good to high chemical yields. An interesting dependence of the chirality transfer and reactivity on the N-protecting group was observed. The 3-pyrrolines are highly useful intermediates for the synthesis of functionalized pyrrolines, pyrrolidines, and other natural products.

PdII-Catalyzed Oxidative Dimeric Cyclization-Coupling Reaction of 2,3-Allenoic Acids: An Efficient Synthesis of Bibutenolide Derivatives

Three sets of convenient catalytic systems have been developed for the oxidative dimeric cyclization coupling of differently substituted 2,3-allenoic acids catalyzed by Pd(II), affording bibutenolides that are not otherwise readily available. The advantages and disadvantages of these systems are discussed. Although the diastereoselectivity for the bicyclization of racemic 2,3-allenoic acids is low, excellent diastereoselectivity was realized in the bicyclization reaction of optically active 2,3-allenoic acids, leading to the optically active bibutenolides in high yields and ee. Based on a mechanistic study, it is believed that the reaction may proceed by means of a double oxypalladation and reductive elimination to yield butenolide 3 and Pd(0) species, which may be reoxidized to the catalytically active Pd(II) species in the presence of alkyl iodide/air, metallic iodide/air, or benzoquinone.

Pd(II)-catalyzed coupling cyclization of 2,3-allenoic acids with allylic halides. An efficient methodology for the synthesis of beta-allylic butenolides

An effective method for the synthesis of beta-allyl polysubstituted butenolides from the easily available allylic halides and 2,3-allenoic acids is described. By using this method optically active butenolides can be obtained. According to the results presented in this paper, the reaction may proceed via three consecutive steps: cyclic oxypalladation of the allene, insertion of the C=C bond in allylic halides, and beta-dehalopalladation.

Studies on Pd(II)-catalyzed coupling-cyclization of alpha- or beta-amino allenes with allylic halides

The palladium-catalyzed coupling-cyclization of alpha- or beta-amino allenes with allylic halides leading to 3-allylic 2,5-dihydropyrroles and 1,2,3,6-tetrahydropyridines, respectively, was studied. The starting materials are easily available. The skeletons of both two classes of products were established by the X-ray diffraction studies of 7i and 9b. Through the study of the reaction of 2b with 3-chloro-1-butene, 1-chloro-2-butene, and pi-allyl palladium species and the stereochemical outcome of the coupling cyclization of (S)-2m and (R)-2n, it is believed that the current transformation most likely proceeded via a Pd(II)-catalyzed pathway, although a Pd(0) pathway cannot be completely excluded.

Efficient synthesis of 4-(2'-alkenyl)-2,5-dihydrofurans and 5,6-dihydro-2H-pyrans via the Pd-catalyzed cyclizative coupling reaction of 2,3- or 3,4-allenols with allylic halides

In the absence of a base, palladium(II) catalysts, such as PdCl(2), PdCl(2)(CH(3)CN)(2), Pd(OAc)(2), and [(pi-C(3)H(5))PdCl](2), can catalyze the cyclizative coupling reaction of 2,3- or 3,4-allenols with allylic halides in DMA at room temperature to provide 2,5-dihydrofurans and 5,6-dihydro-2H-pyrans, respectively, in moderate to good yields. Under similar reaction conditions, nonsubstituted 2,3-allenol 1s affords bimolecular cyclizative coupling product 5s as the major product. The scope of the reaction and its mechanism have been studied briefly. On the basis of the experimental results, the transformation was believed to proceed via a divalent palladium-catalyzed pathway.