Efficient synthesis of 3-chloromethyl-2(5H)-furanones and 3-chloromethyl-5,6-dihydropyran-2-ones via the PdCl2-catalyzed chlorocyclocarbonylation of 2,3- or 3,4-allenols

A mild and efficient methodology involving PdCl2-catalyzed chlorocyclocarbonylation of 2,3- or 3,4-allenols with CuCl2 for the synthesis of 3-chloromethyl-2(5H)-furanones and 3-chloromethyl-5,6-dihydropyran-2-ones was developed. This reaction proceeded in a highly regioselective manner, i.e., the chlorine atom was introduced to the terminal position of the allene moiety while the lactone linkage was formed between the center carbon atom of the allene moiety and the hydroxyl oxygen, which was established by the X-ray single crystal diffraction study of gamma-lactone 3p. The highly optically active 3-chloromethyl-2(5H)-furanones could be easily prepared from the readily available optically active 2,3-allenols. A mechanism for this reaction was proposed.     

Efficient synthesis of 4-(3'-furanyl)butenolide derivatives via PdII-catalyzed oxidative heterodimeric cyclization reaction of 2,3-allenoic acids and 1,2-allenyl ketones

The oxidative cyclization/dimerization reaction between two classes of allenes with different functionalities was reported to provide an efficient route to polysubstituted 4-(3'-furanyl)-2(5H)-furanones, which are not readily available from the known methods. The highly optically active butenolides could be easily formed from the optically active 2,3-allenoic acids, which was obtained conveniently through chiral resolution with optically active amines, that is, cinchonidine or alpha-methyl benzylamine. A mechanistic study showed that the reaction proceeded via a matched double oxypalladation-reductive elimination process. The Pd(II) species may be regenerated via the subsequent cyclometallation of two equivalents of 1,2-allenyl ketones with Pd(0) and protonlysis of Pd enolates formed with the in situ generated HCl.     

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.     

An efficient synthesis of 3-aryl-2-aryl(or methyl)sulfanylbenzo[b]thiophenes via cyclization of aryl 2-aryl(or methyl)sulfanylmethylsulfanylphenyl ketones

A convenient method for the preparation of 2-aryl(or methyl)sulfanylbenzo[b]thiophenes has been developed. Thus, aryl 2-aryl(or methyl)sulfanylmethylsulfanylphenyl ketones, easily prepared from readily available aryl 2-sulfanylphenyl ketones or 2-chloro-5-nitrophenyl phenyl ketone, are treated with LDA in 1,2-dimethoxyethane (DME) to give 3-aryl-2-aryl(or methyl)sulfanylmethylsulfanyl-2,3-dihydrobenzo[b]thiophen-3-ols, which in turn were dehydrated with thionyl chloride to afford 3-aryl-2-aryl(or methyl)sulfanylbenzo[b]thiophenes in reasonable overall yields.     

Pd(CH3CN)2Cl2-catalyzed oxidative heterodimerization reaction of 2,3-allenamides and 1,2-allenyl ketones: an efficient synthesis of 4-(furan-3'-yl)-2(5H)-furanimines

The Pd(II)-catalyzed oxidative heterodimerization reaction of 2,3-allenamides and 1,2-allenyl ketones was studied. It provides an efficient route for the synthesis of the polysubstituted 4-(furan-3'-yl)-2(5H)-furanimines, which are not readily available from the known methods. Due to the application of benzoquinone, the loadings of both the palladium catalyst and ketone have been greatly reduced for the oxidative heterodimerization of 2,3-allenamides and 1,2-allenyl ketones in acetic acid.