Reaction of enol ethers with zinc carbenoid reagents. Cyclopropanation and subsequent isomerization to allylic ethers

The reactions of enol ethers (ROCC, R = alkyl, silyl) with zinc carbenoid reagents were found to give allylic ethers in several cases along with the expected cyclopropyl ethers. The ratio of these two products was highly dependent on the concentration of the reaction mixture. Thus, the selective formation of each product was conveniently attained by merely changing the amounts of the solvent used. Zinc iodide, a by-product of the reaction, plays a key role in the present cyclopropyl to allylic rearrangement. Ring-opened ionic intermediates are proposed.     

Cobalt- and rhodium-catalyzed cross-coupling reaction of allylic ethers and halides with organometallic reagents

Reactions of 2-alkenyl methyl ether with phenyl, trimethylsilylmethyl, and allyl Grignard reagents in the presence of cobalt(II) complexes are discussed. The success of the reactions heavily depends on the combination of the substrate, ligand, and Grignard reagent. In the reaction of cinnamyl methyl ether, the formation of the linear coupling products predominates over that of the relevant branched products. In the cobalt-catalyzed allylation of allylic ethers, addition of a diphosphine ligand can change the regioselectivity, mainly providing the corresponding branched products. Rhodium complexes catalyze the reactions of allylic ethers and halides with allylmagnesium chloride and allylzinc bromide, respectively, in which the branched coupling product is the major product.     

Efficient and stereoselective synthesis of allylic ethers and alcohols

[Structure: see text] A short and efficient synthesis of allylic TBS ethers and allylic alcohols has been developed, based upon a unique Kocienski-Julia olefination reaction. Allylic alcohols and allylic ethers are obtained in good to excellent yields and with high (E)-selectivity. The conditions are mild and the procedure is broadly applicable.     

High diversity on simple substrates: 1,4-dihalo-2-butenes and other difunctionalized allylic halides for copper-catalyzed S(N)2' reactions

Enantioselective allylic alkylation with an organomagnesium reagent catalyzed by copper thiophene carboxylate (CuTC) was carried out on difunctionalized substrates, such as commercially available 1,4-dichloro-2-butene and 1,4-dibromo-2-butene, and on similar compounds of higher substitution pattern of the olefin for the formation of all-carbon chiral quaternary centers. The high regioselectivity obtained throughout the reactions favored good regiocontrol for the addition of phenyl Grignard reagents. Other difunctionalized substrates (allylic ethers and allylic alcohols) also underwent asymmetric S(N)2' substitution.     

Ruthenium-catalyzed chemoselective N-allyl cleavage: novel Grubbs carbene mediated deprotection of allylic amines

A novel application of the Grubbs carbene complex has been discovered. The first examples of the catalytic deprotection of allylic amines with reagents other than palladium catalysts have been achieved through Grubbs carbene mediated reaction. Significantly, the catalytic system directs the reaction toward the selective deprotection of allylic amines (secondary as well as tertiary) in the presence of allylic ethers. A variety of substrates, including enantiomerically pure multifunctional piperidines, are also usable. The new method is more convenient, chemoselective, and operationally simple than the palladium-catalyzed method. The current mechanistic hypothesis invokes a nitrogen-assisted ruthenium-catalyzed isomerization, followed by hydrolysis of the enamine intermediate. We believe that the reactive species involved in the reaction may be an Rubond;H species rather than the Grubbs carbene itself. Thus, the isomerization may occur according to the hydride mechanism. The synthetic utility of this ruthenium-catalyzed allyl cleavage is illustrated by the preparation of indolizidine-type alkaloids.     

Zirconium-mediated SN2' substitution of allylic ethers: regio- and stereospecific formation of protected allylic amines

A new zirconium-mediated, regio- and stereospecific SN2' substitution of allylic ethers with a nitrogen nucleophile has been developed. Cbz-protected amine products were isolated in high yield from reactions with a wide range of Z allylic ethers. A mechanism of the allylic substitution consistent with the results of the kinetics and kinetic isotope effect studies was proposed.     

The influence of the organolithium reagent nature on the possibility of the O→C<Subscript>sp</Subscript> migration of the R<Subscript>3</Subscript>Si group in propynes HC≡CCH<Subscript>2</Subscript>OSiR<Subscript>3</Subscript>

A possibility of the O→C_sp 1,4-migration of the R₃Si group in silyl ethers of terminal acetylenic alcohols upon treatment with organolithium reagents (RLi) was studied. In the case of 3-trimethylsilyloxypropyne, depending on the nature of RLi, the heterolysis of the Si—O bond occurs either by the action of acetylide formed as a result of deprotonation with the formation of 3-trimethylsilylprop-2-yn-1-ol trimethylsilyl ether, or by the action of the metalation agent with the formation of propargyl alcohol. The realization of the O→C_sp 1,4-migration of the Me₃Si group requires the use of mild organolithium reagents (lithium hexamethyldisilazanide and diisopropylamide). Silyl ethers having steric hindrance at the carbon atom bonded to the reaction center or around the silicon atom do not react with the studied organolithium reagents.     

Sequential catalytic isomerization and allylic substitution. Conversion of racemic branched allylic carbonates to enantioenriched allylic substitution products

A catalytic protocol for the conversion of readily accessible racemic, branched aromatic allylic esters to branched allylic amines, ethers, and alkyls has been developed. Palladium-catalyzed isomerization of branched allylic esters to terminal allylic esters, followed by sequential iridium-catalyzed allylic substitution, gave the branched allylic products in good yield with high regioisomeric and enantiomeric selectivity. Both electron-rich and electron-poor branched allylic esters gave products in >90% ee. High enantiomeric excesses were also observed for the products from the reactions of 2-thienyl acetates and dienyl carbonates.     

Copper-mediated displacements of allylic THP ethers on a bisphosphonate template

The copper-mediated displacement of allylic THP ethers by Grignard reagents has been examined in a system that contains a geminal bisphosphonate ester. With Grignard reagents derived from several aromatic halides or benzyl bromide the displacement proceeds in attractive yields, but more mixed results were obtained from reactions with alkyl halides. In addition to its role as a nucleophile, the Grignard reagent also appears to deprotonate the bisphosphonate to generate an anionic intermediate. Formation of this anion appears to limit competitive nucleophilic attack at the phosphonate group and provides an intermediate that can be trapped by reaction with an electrophilic reagent such as methyl iodide to access a more substituted system.     

Regio- and enantioselective iridium-catalyzed intermolecular allylic etherification of achiral allylic carbonates with phenoxides

An enantioselective and regioselective iridium-catalyzed allylic etherification is described. The reaction of sodium and lithium aryloxides with achiral (E)-cinnamyl and terminal aliphatic allylic electrophiles in the presence of 2 mol % of an iridium-phosphoramidite complex provides chiral allylic aryl ethers in high yields and excellent levels of regio- and enantioselectivity. Lithium aryloxides containing a single substituent at an ortho, meta, or para position as well as sterically hindered phenoxides were tolerated. Reactions in THF displayed the most suitable balance of rate, regio-, and enantioselectivity. High ee's were also observed for the products from the reaction of alkyl (E)-allylic carbonates.     

Ether formation from allylic alcohols catalyzed by samarium trichloride

The behaviour of various allylic alcohols in the presence of catalytic amounts of SmCl₃ (by heating in 1,2-dichloroethane) has been studied. Diallyl ethers are obtained in many cases in good yields. Mixed allyl alkyl ethers are also prepared if 2–5 equivalents of an aliphatic alcohol is present. The reactions are interpreted as proceeding through a pseudo allylic carbonium intermediate initiated by a preliminary complexation of the allyl hydroxyl to the samarium ion.     

Stereoselective and diversity-oriented synthesis of trisubstituted allylic alcohols and amines

Stereoselective and diversity-oriented synthesis of trisubstituted olefins was achieved by using ortho-diphenylphosphanyl benzoate (o-DPPB) as a directing group for allylic substitution. The starting point of this methodology was a set of α-methylene aldehydes derived from Baylis-Hillman adducts. Subsequent addition of different organometallic reagents led to a variety of allylic alcohol substrates. After introduction of the reagent-directing o-DPPB group, copper-mediated allylic substitution with a wide range of Grignard reagents enabled the stereoselective construction of a large number of E-configured trisubstituted allylic alcohols and amines in excellent yields and stereoselectivities. Remarkable is the synthetic flexibility, which allows a wide range of permutations starting from an aldehyde followed by successive introduction of the substituents R(2) and R(3) from organometallic Grignard based reagents. Thus, starting from only a few precursors, a diversity-oriented synthesis of stereodefined trisubstituted allylic alcohols and amines becomes possible.     

Reactivity of RCu,BF3 and R2 CuLi,BF3 towards allylic acetals and ethers

Organocopper and cuprate reagents associated with Lewis acids, are highly reactive towards allylic acetals and ethers. Displacements of the alkoxy group occurs by SN₂' attack according to the various parameters of the reaction.     

Regiochemical control in the metal-catalyzed transposition of allylic silyl ethers

A novel mode of regiochemical control over the allylic [1,3]-transposition of silyloxy groups catalyzed by Re2O7 has been developed. This strategy relies on a cis-oriented vinyl boronate, generated from the Alder-ene reaction of homoallylic silyl ethers and alkynyl boronates, to trap out the allylic hydroxyl group. The resulting cyclic boronic acids are excellent partners for cross-coupling reactions. High chirality transfer is observed for the rearrangement of enantioenriched allylic silyl ethers.     

Diastereoselectivity of the reactions of organolithium reagents with protected erythrulose oximes

The addition of organolithium reagents to the CN bond of several erythrulose-derived chiral (E)- and (Z)-ketoxime ethers has been shown to be highly diastereoselective in the case of the (E)-isomers. Chelated and nonchelated transition states have been proposed to rationalize these results, with additional support of computational methods.     

Copper catalyzed asymmetric synthesis of chiral allylic esters

The complex derived from Taniaphos ligand 4 and CuBr*Me2S catalyzes the asymmetric addition of Grignard reagents to 3-bromopropenyl esters 1 to provide allylic esters 2 in high yields and high chemio-, regio-, and enantioselectivities. The work demonstrates that allylic asymmetric alkylation (AAA) can be done on substrates bearing a heteroatom at the gamma-position. The method is a practical route to chiral, nonracemic allylic alcohols. The use of functionalized substrates 1 or Grignard reagents leads to more complex products 2, which can be further manipulated as demonstrated in conversion to (S)-5-ethyl-2(5H)-furanone 6 and (S)-benzoic acid-cyclopent-2-enyl ester 7.     

Rhodium-catalyzed asymmetric coupling reaction of allylic ethers with arylboronic acids

An asymmetric allylic substitution of simple allylic ethers with arylboronic acids in the presence of a rhodium(I)/(R)-DTBM-SEGPHOS catalyst has been developed. The reactions proceeded smoothly at room temperature to give the corresponding branch products with excellent regioselectivities and good to excellent enantioselectivities.     

Direct preparation of allylic zirconium reagents from zirconocene-olefin complexes and alkenes

A novel method for preparation of allylic zirconium reagents directly from 1-alkenes via zirconocene-olefin complex has been developed. Selective transfer of the hydride of zirconocene allyl hydride complex, a tautomer of zirconocene-olefin complex, to diisopropyl ketone generates the corresponding zirconocene alkoxide allyl. The allylic zirconium reagents formed effects stereoselective allylation of aldehyde at 25 degrees C and -78 degrees C to provide syn- and anti-homoallyl alcohols, respectively. The anti-isomer is formed via a six-membered chair transition state under kinetic control. The syn-selectivity can be rationalized by considering isomerization of the anti-adduct by a retroallylation process.     

Synthesis of allylic and homoallylic alcohols from unsaturated cyclic ethers using a mild and selective C-O reduction approach

Unsaturated cyclic ethers can be mildly and selectively reduced with catalytic amounts of B(C(6)F(5))(3) in the presence of an alkylsilane. The allylic position is preferentially reduced with minimal or no scrambling of olefin geometry. For electronically equivalent substrates, steric factors guide the reducing agent to the least substituted site.     

Catalytic asymmetric generation of (Z)-disubstituted allylic alcohols

A one-pot method for the direct preparation of enantioenriched (Z)-disubstituted allylic alcohols is introduced. Hydroboration of 1-halo-1-alkynes with dicyclohexylborane, reaction with t-BuLi, and transmetalation with dialkylzinc reagents generate (Z)-disubstituted vinylzinc intermediates. In situ reaction of these reagents with aldehydes in the presence of a catalyst derived from (-)-MIB generates (Z)-disubstituted allylic alcohols. It was found that the resulting allylic alcohols were racemic, most likely due to a rapid addition reaction promoted by LiX (X = Br and Cl). To suppress the LiX-promoted reaction, a series of inhibitors were screened. It was found that 20-30 mol % tetraethylethylenediamine inhibited LiCl without inhibiting the chiral zinc-based Lewis acid. In this fashion, (Z)-disubstituted allylic alcohols were obtained with up to 98% ee. The asymmetric (Z)-vinylation could be coupled with tandem diastereoselective epoxidation reactions to provide epoxy alcohols and allylic epoxy alcohols with up to three contiguous stereogenic centers, enabling the rapid construction of complex building blocks with high levels of enantio- and diastereoselectivity.