A convenient approach to cyclic enol phosphates via ring-closing metathesis

A strategy employing the second generation Grubbs catalyst in order to facilitate the generation of a variety of cyclic enol phosphates, including 2H-chromen-4-yl, 1,2-dihydroquinolin-4-yl, 2H-thiochromen-4-yl, 2H-thiochromen-4-yl 1,1-dioxide, and benzofuran-2-yl enol phosphate scaffolds is described. This work represents the first case of an olefin metathesis reaction in which one of the groups participating in the metathesis event is an enol phosphate moiety.     

Ring-closing metathesis of α-ester-substituted enol ethers: application to the shortest synthesis of KDO

Ring-closing metathesis reactions of α-ester-substituted enol ethers are described. In the case of unsubstituted terminal olefins, isomerization prior to cyclization was observed as an undesired side reaction, which could not be completely inhibited. Furthermore, this methodology was applied to a formal synthesis of KDO, which now represents the shortest synthetic pathway to KDO and its deoxy analogue. Interestingly, in this route olefin isomerization was not observed, presumably due to the increased steric environment of the double bond. Finally, an efficient two-step conversion to transform an alcohol into an α-alkoxy acrylate is also described.     

Enantioselective synthesis of cyclic enol ethers and all-carbon quaternary stereogenic centers through catalytic asymmetric ring-closing metathesis

The first examples of catalytic asymmetric ring-closing metathesis (ARCM) reactions of enol ethers are reported. To identify the most effective catalysts, various chiral Mo- and Ru-based catalysts were screened. Although chiral Ru catalysts (those that do not bear a phosphine ligand) promote ARCM in some cases, such transformations proceed in <10% ee. In contrast, Mo-based alkylidenes give rise to efficient ARCM and deliver the desired products in the optically enriched form. Thus, Mo-catalyzed enantioselective transformations allow access to various five- and six-membered cyclic enol ethers in up to 94% ee from readily available achiral starting materials. The first examples of catalytic ARCM that lead to the formation of all-carbon quaternary stereogenic centers are also disclosed. Mechanistic models that offer a plausible rationale for the identity of major enantiomers as well as the observed levels of enantioselectivity are provided. Representative examples demonstrate that the enol ether moiety and the unreacted alkene of the ARCM products can be discriminated with excellent site selectivity (>98%).     

Phthalideisoquinoline enol lactones

:(Z)-Narceine enol lactone ($\text{10}$) is thennodynamically less stable than its geometric isomer $\text{11}$, and solvolyses faster in methanol to provide keto ester $\text{9}$. In the less hindered hydrastine series, however, the order is reversed, and it is the E isoner $\text{4}$ which is less stable than the Z analog $\text{3}$ and which solvolyses faster to keto ester $\text{6}$.     

Ring-opening metathesis copolymerization employing ruthenium-based metathesis catalysts

The copolymerization of both high- and low-strain cyclic olefins employing three ruthenium-based metathesis catalysts is desribed. The effect of the ligand environment as well as the nature of the carbene on the copolymerizations is discussed.     

Polyfluorinated enol acetates

The synthesis of polyfluorinated enol acetates has been performed by reductive dechlorination of chloropolyfluoroketones with zinc in Ac₂O. Under these conditions, hexafluoroacetone is preferably reduced at the carbonyl group.Translated fromIzyestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 174–176, January, 1994.     

Alkyne metathesis

This review discusses the emergence of alkyne metathesis as a valuable synthetic tool applicable in the synthesis of complex molecules and polymer science.     

Enantioselective RH-catalyzed hydrogenation of enol acetates and enol carbamates with monodentate phosphoramidites

[reaction: see text] Monodentate phosphoramidites, in particular PipPhos and its octahydro analogue, are excellent ligands for the rhodium-catalyzed asymmetric hydrogenation of aromatic enol acetates, enol carbamates, and 2-dienol carbamates up to 98% ee. These latter substrates were hydrogenated selectively to the carbamates of the allyl alcohol.     

A general synthesis of substituted indoles from cyclic enol ethers and enol lactones

[reaction: see text] X = CH2, C[double bond]O, R2 = H, alkyl. A general method was developed for the one-pot synthesis of highly functionalized indoles from simple, commercially available aryl hydrazines and cyclic enol ethers. Enol lactones were also used as substrates, affording substituted indole acetic acid or indole propionic acid derivatives. This procedure affords 2,3-disubstituted indoles as single regioisomers from the appropriately substituted enol ether or enol lactone. This method was highlighted in the efficient synthesis of the antimigraine drug sumitriptan and the antiinflammatory drug indomethacin.     

Aqueous olefin metathesis

According to popular belief, oxygen and water are the natural enemies of organometallic reactions and therefore must be excluded rigorously from the reaction vessel. This belief is founded in the case of the highly reactive nucleophilic metal alkylidene complexes that were used in early catalytic olefin metathesis. However, owing to the high stability of the ruthenium carbene complexes introduced by Grubbs, metathesis in water has become reality.     

Ketonization of incarcerated acetophenone enol

A free-standing simple enol has been generated inside a carceplex. Rates of ketonization under various conditions were determined; ketonization is extremely slow as compared to the rate in solution. Complexed water is required, and the mechanism proceeds via formation of an enolate followed by protonation at carbon by the same molecule of water that removed the proton from the enol. Acid or base retards ketonization by removing water from the cavity.     

Alkenol-alkyne cross metathesis

Allyl alcohols and their homologues were used in the enyne cross metathesis to prepare hydroxy-functionalized dienes. An isomerization was found to occur under prolonged heating, and a method for conversion to ( E)-diene product is also reported.     

Deracemization of silyl enol ethers

The deracemization by enantioselective protonation of silyl enol ethers was tested using 2,2-dimethyl 5-phenyl 1,3-dioxolan 4-one 1. The results obtained, especially with pantolactone as a chiral proton donor, are better than when the deracemization is carried out with the lithium enolate of 1.     

Palladium-catalysed vinylation of enol triflates

Enol triflates react with olefinic products in the presence of triethylamine and a palladium acetate-triphenylphosphine catalyst to give conjugated dienes in good yield and under mild conditions.     

Regiospecific ureidoalkylation of silyl enol ethers

Site-specific ureidoalkylation of silyl enol ethers can be achieved by their reactions with chloromethylcarbamates at ?78° under the influence of titanium tetrachloride.