Search

A first practical Ni(0)-catalyzed highly stereoselective methyl-carboxylation of homopropargylic alcohols with ZnMe2and CO2for the efficient synthesis of α-alkylidene-γ-butyrolactones is described. The reaction may be applied to other alkynols. [ABSTRACT FROM AUTHOR]

Treatment of [PdI(Ph)(PPh 3)] 2with allenes CH 2CCHR (R = CMe 3, CO 2Et, P(O)(OEt) 2, and SO 2Ph) in dichloromethane at room temperature produces a mixture of cis and trans isomers of the π-allyl palladium complexes PdI(η 3-CH 2C(Ph)CHR)(PPh 3) in which the R group is anti to the Ph group. The disubstituted allenes MeCHCCHR (R = P(O)(OEt) 2and SO 2Ph) similarly react with [PdI(Ph)(PPh 3)] 2to give the π-allyl palladium complexes PdI(η 3-MeCHC(Ph)CHR)(PPh 3) in which the R group is anti and the Me group is syn to the Ph group. PdI(Ph)(dppe) alone was found to be unreactive toward allenes such as CH 2CCHSO 2Ph and MeCHCCHSO 2Ph at room temperature. In contrast, in the presence of TlPF 6, PdI(Ph)(dppe) readily reacts with allenes CH 2CCHR (R = CMe 3, CO 2Et, COPh, and SO 2Ph) and MeCHCCHSO 2Ph to give the π-allyl palladium complexes [Pd(η 3-CH 2C(Ph)CHR)(dppe)]PF 6and [Pd(η 3-MeCHC(Ph)CHR)(dppe)]PF 6, respectively. Although mechanistically possible, vinyl complexes were not observed as the insertion products in all cases. The substituents of allenes appear to have no effect on the reaction pathways, at least for the allenes used in this study. The insertion reactions involving PdI(Ph)(PR 3)(allene) have been studied by computational chemistry using the model complex PdI(Ph)(MeCHCCHSO 2H)(PH 3). [ABSTRACT FROM AUTHOR]

Efficient synthetic routes to construct multisubstituted benzo[1,2-b:5,4-b′]difurans and benzo[1,2-b:4,5- b′]difurans from bis(allyloxy)bis(alkynyl)benzenes or bis(alkynyl)dihydroxybenzenes and allylic halides utilizing palladium-catalyzed double annulations is reported. By further applying a double ring-closing metathesis reaction, pentacyclic compounds were also prepared. [ABSTRACT FROM AUTHOR]

Cyclometallation of two unsaturated carbon-carbon bonds usually requires the application of low-valent metal catalysts, which could cleave the propargylic ester linkage. Thus, it is desirable to identify a catalyst which could undergo cyclometallation without cleaving the propargylic ester linkage. In this paper, we used trans-RhCI(CO)(PPh3)2 to realize the cyclometallation of propargylic 2,3-dienoates. The substituents at the 4-position of allenoate moiety nicely control the reaction pathway: when the 4-position of propargylic 2,3-dienoate 1 was monosubstituted with an aryl group, the bicyclic intermediate 7 formed by the cyclometallation could highly selectively undergo carbometalation with the alkyne moiety in the second molecule of propargylic 2,3-dienoate 1 to afford metallabicyclic intermediates 8a or 8b. Subsequent reductive elimination would afford 9, which could undergo an intramolecular Diels-Alder reaction resulting in the formation of polycyclic bis(δ-lactone)-containing structures 2. The intermediate could be trapped by adding 3-methoxyprop-1-yne affording cyclization-aromatization product 4p highly selectively. If the substituent at the 4-positon of the 2,3-allenoate moiety has a β-H atom, sequential unimolecular cyclometallation/β-H elimination/reductive elimination occurs to afford cross-conjugated 5(Z)-alkylidene-4-alkenyl-5,6-dihydropyran-2-ones. The Z-stereochemistry of the exo double bond was determined by the cyclometallation. Some of the α,β-unsaturated δ-lactones could be easily converted to other synthetically useful compounds via reduction reaction, hydrogenation, and iodination/coupling protocol. [ABSTRACT FROM AUTHOR]

A highly regioselective Cu(I)-catalyzed anti-carbometallation of secondary terminal propargylic alcohols with 1° alkyl or aryl Grignard reagents affording 2-substituted allylic alcohols was developed. By using this method, optically active allylic alcohols can be prepared from the optically active propargylic alcohols without obvious loss of the enantiopurity. The cyclic organometallic intermediate formed may undergo an iodination or a Pd(0)-catalyzed coupling reaction to afford stereo-defined allylic alcohols. [ABSTRACT FROM AUTHOR]

Discusses the advances on the chemistry of allenes. Factors that led to the development of allene chemistry; Ability of allenes to demonstrate reactivities and selectivities; Mechanism of action of allenes and reactions with other organic compounds.

The PdCl2-catalyzed cyclocarbonylation reaction of propargylic amines with CuCl2 and benzoquinone afforded (E)-α-chloroalkylidene-β-lactams in moderate to good yields. The formation of the corresponding Z-isomers or five-membered products was not observed. The reaction of the readily available optically active prop argylic amines provides a convenient synthesis of the corresponding (E)-α-chloroalkylidene-β-lactams with high ee values. The structure and the stereochemistry of the products were established by the X-ray single-crystal diffraction study of (E)-6d and (E)-6e, which indicates that the stereoselectivity in this reaction is different from what was observed with propargylic alcohols. A rationale for this reaction was proposed. [ABSTRACT FROM AUTHOR]

Examines the control of regioselectivity in Pd(0)-catalyzed coupling-cyclization. Determination on the organic halides by steric and electronic effects; Regioselectivity of reaction; Stability of palladium intermediates.

An efficient cascade reaction of 2,3-allenoic acids with dialkyl azodicarboxylate in the presence of PPh3 affording 1,2-bis(alkoxycarbonyl)-pyrazol-3-ones in moderate yields has been developed. The reaction may proceed via the intramolecular Michael addition of the expected 2,3-allenoyl hydrazines. [ABSTRACT FROM AUTHOR]

A highly regio- and stereoselective palladium(0)-catalyzed addition of organoboronic acids with 1,2-allenylphosphonates in the presence of HOAc forming tri- or tetrasubstituted 1(E)-alkenylphosphonates is reported in this paper. The stereo-selectivity is much higher than the reported cases. The effects of different R¹, R², and R³ were studied. A mechanism of this reaction is proposed on the basis of our previous study. [ABSTRACT FROM AUTHOR]

Under different reaction conditions, 1,1-diarylpropadienes and 1,3-diarylpropynes can be prepared respectively by the sequential lithiation of 1-aryl-l-propynes, transmetalation, and the corresponding Pd(0)-catalyzed cross-coupling with aryl halides. A rationale for the formation of 1,3-diarylpropynes is proposed. [ABSTRACT FROM AUTHOR]

1,5-Bisallenes may undergo radical, carbometallation, cyclometallation, cyclization, and consecutive cascade reactions to form many different 5-7-membered rings, showing diversified reactivities. There is no doubt that the chemistry of 1,5-bisallenes would be enriched by further study. [ABSTRACT FROM AUTHOR]

A three-component Pd(0)-catalyzed reaction of 1,5-bisallenes with organic halides in the presence of primary amines was observed to afford stereodefined not readily available ten-membered cyclic compounds highly chemo- and regioselectively. A mechanism involving two π-allylic palladium intermediates was proposed to account for the observed regio- and stereoselectivity. [ABSTRACT FROM AUTHOR]

An efficient and highly regioselective route to synthesize polysubstituted 1,3,4-alkatrien-2-yl cyclopropane derivatives via Pd(0)-catalyzed highly regioselective coupling cyclization of (2,3-butadienyl)malonate or bis(phenylsulfonyl)methane with propargylic carbonates was reported. The reaction proceeded smoothly under neutral conditions to afford the products in 73−96% yields. The products may be efficiently converted to cyclopentenone derivatives via a catalytic Pauson−Khand reaction under ambient conditions. [ABSTRACT FROM AUTHOR]

A PdCl2/NaI-catalyzed homodimeric coupling-cyclization reaction of 2,3-allenols was observed to provide an efficient route to 4-(1′,3′-dien-2′-yl)-2,5-dihydrofuran derivatives. By using the easily available optically active starting materials, 2,5-dihydrofurans with high enantiopurity may be prepared. A Pd(II)-catalyzed mechanism was also discussed. [ABSTRACT FROM AUTHOR]

3-(Methoxycarbonyl)-2-halo-1,3(Z)-dienes were prepared highly stereoselectively via SN2′-type addition-elimination reactions of 3-(methoxycarbonyl)-1,2-allen-4-ols with MX. These products may easily undergo Negishi or Sonogashira coupling reactions to yield a series of stereodefined polysubstituted (E)-1,3-dienes. [ABSTRACT FROM AUTHOR]

We have previously reported an efficient synthesis of β-phenylselenium-substituted butenolides via electrophilic cyclization of 2,3-allenoates with PhSeC1 in aqueous MeCN. However, when 2,3-allenoates were treated with PhSeCl in MeCN, 3-phenylseleno-4-oxo-2(E)-alkenoates were formed unexpectedly. The addition of Li2CO3 improved the yield and the selectivity of the reaction. A possible mechanism involving a decomposition of selenate esters was proposed. [ABSTRACT FROM AUTHOR]

Highly regio- and stereoselective reactions of readily available 2-(methoxycarbonyl)-2,3-allenols 1with oxalyl chloride in the presence of Et3N or DMSO afforded methyl 2-(ethynyl)alk-2(E)-enoates (E)-2 and 2-(1′-chlorovinyl)alk-2(Z)-enoates (Z)-3, respectively, in moderate to good yields. [ABSTRACT FROM AUTHOR]

In this paper, it was reported that double 1,2-addition reaction of 2,3-allenoates with allyl magnesium chloride at room temperature in the absence of any transition metal catalyst provides an efficient method for the synthesis of tertiary α-allenols. The optically active allenol could be prepared from the reaction of the optically active 2,3-allenoate without obvious racemization of the axial chirality. Under different reaction conditions, cyclization reactions of α-allenol 2i prepared have been studied for the synthesis of different 2,5- dihydrofuran derivatives. [ABSTRACT FROM AUTHOR]

Two sets of reaction conditions were established to enable the highly regio- and stereoselective iodohydroxylation of 1,2-allenylic diphenyl phosphine oxides, yielding (E)-2-iodo-3-hydroxy-1-alkenyl diphenyl phosphine oxides with very high stereoselectivity. The scope of this reaction was examined extensively. Notably, studies on the reactivity of optically active substrates indicated that the axial chirality in the starting allenes may be efficiently transferred to the center chirality of the products with no discernible loss of enantiopurity. Due to the importance of phosphine-containing compounds, both as reagents and ligands, this reaction shows potentials in organic synthesis. Investigations using ESI-MS technology on the 18O-labeled product, which was prepared using 18O-water as the solvent, indicated that the 18O atom was bound to phosphorus in the final product and the oxygen atom of the hydroxyl comes from the phosphinyl functionality of the allene reactant. These results provided solid evidence for the formation of a five-membered cyclic intermediate from the neighboring group participation of the diphenylphosphinyl group. To the best of our knowledge, this is the first time that the neighboring group participation of this type of group was observed. [ABSTRACT FROM AUTHOR]

An efficient cascade reaction of 2,3-allenoic acids with dialkyl azodicarboxylate in the presence of PPh3 affording 1,2-bis(alkoxycarbonyl)-pyrazol-3-ones in moderate yields has been developed. The reaction may proceed via the intramolecular Michael addition of the expected 2,3-allenoyl hydrazines. [ABSTRACT FROM AUTHOR]

We have developed a convenient method for the efficient monofluorination via the electrophilic fluorocyclization reaction of 2,3-allenoic acids with Selectfluor in MeCN in the presence of 10 equiv of H2O or even in pure water to afford β-fluorobutenolides in moderate to high yields. [ABSTRACT FROM AUTHOR]