The first example of enantioselective allyl transfer from a linear homoallylic alcohol to an aldehyde

[reaction: see text] The first example of enantioselective linear homoallylic alcohol transfer reaction was revealed. In all cases, the whole rearrangement is thermodynamically favorable and a steric effect is the driving force of this reaction. The preservation of the stereocenter and olefin geometry together with the isolation of gamma-adduct homoallylic alcohols in one isomeric form have warranted the proposed mechanism.     

Efficient allylation of aldehydes promoted by carboxylic acids

A variety of carboxylic acids have been screened for mediating the allylation of aldehydes with allytributyltin in different solvents. A novel, general, and practical method of allylation of aldehydes promoted by carboxylic acids under mild reaction conditions has been developed. Among them, p-nitrobenzoic acid afforded high to quantitative yields of the homoallylic alcohol products, and can be easily recovered after workup by aqueous HCl. Glyoxylic acid self-catalyzed the allylation without adding any other promoter or catalyst to give the corresponding allylation product in good yield. The regioselectivity of the crotylation of aldehydes is tunable by controlling the acidity of the carboxylic acids. The crotylation of aldehydes produced the alpha-adduct as major products in moderate to good yields with CF(3)CO(2)H as a promoter. A possible mechanism for the allylation is also discussed.     

Reactivity of γ-chloro-gem-trichloroalkanes with chromous chloride

CrCl₂-mediated condensation of γ-chloro-gem-trichloroalkanes with aldehyde generates homoallylic alcohols through a hydride rearrangement followed by a Nozaki-Hiyama allylation.     

Lewis Acid-Mediated Decarboxylative Allylation of Enol Carbonates

A homoallylic ketone can be transformed and functionalized by various synthetic reactions, and thus, is regarded as one of the representative building blocks in organic chemistry. An additional route to access homoallylic ketones, namely, a Lewis acid-mediated decarboxylative allylation of cyclic enol carbonates, prepared by fixation of carbon dioxide onto propargyl alcohols, was developed in this work. The treatment of a cyclic enol carbonate with a Lewis acid in the presence of an allylsilane resulted in the formation of a homoallylic ketone. It was found that the title reaction proceeded well by the combined use of zirconium tetrachloride with allyltrimethylsilane. The allylation occurred with high regioselectivity and the corresponding homoallylic ketones were obtained in good-to-high yields. A reaction mechanism involving the decarboxylative formation of an oxyallyl cation equivalent is proposed.     

Barbier-type reaction mediated with tin nano-particles in water

Tin nano-particles are employed in the Barbier-type allylation reaction of carbonyl compounds in water to afford the corresponding homoallylic alcohols in good yields. The in situ generated allylation intermediates, allyltin(II) bromide and diallyltin dibromide, have been directly observed by using ¹H NMR. A mechanism is proposed based on this observation.     

Ring-closing metathesis as a tool for the efficient preparation of chiral spirocyclic ethers from homoallylic alcohols

The preparation of chiral spirocyclic ethers via allylic etherification/olefin metathesis of homoallylic alcohols is investigated. This reaction sequence transforms the enantiopure substrate alcohols, synthesized by a one-pot asymmetric rhodium-catalyzed conjugate addition/metal-mediated allylation procedure, into the desired spiro ethers with full conversions and in excellent isolated yields. The synthetic sequence provides an efficient means for a rapid construction of functionalized spiro ethers in a stereoselective manner.     

Diastereoselective production of homoallylic alcohols bearing quaternary centers from gamma-substituted allylic indiums and ketones

Highly stereoselective In-employed addition of gamma-substituted allylic halides (cyclohexenyl halides, cinnamyl halides, and ethyl 4-bromocrotonate) to ketones is established to produce homoallyl alcohols bearing quaternary centers. The reactivity patterns and relative stability of allylic indiums were studied. The addition of water characteristically affected the reactions. Cyclohexenyl indium addition was completely disturbed, but a clear reaction was observed in the cinnamyl and crotonate-indium addition. In the case of ethyl 4-bromocrotonate, an interesting conversion of a gamma-adduct into an alpha-adduct was observed in anhydrous conditions.     

Allylation of Carbonyl Compounds Mediated by Aluminum/Fluoride Salts in Water

A novel mediator (Al/KF) has been developed and employed in the Barbier‐type alkylations of various aldehydes and ketones with alkyl halide in water. The carbonyl compounds could be effectively converted into corresponding homoallylic alcohol in good yields only when allyl bromides or substituted allyl bromides were used as halides. Aromatic aldehydes could afford homoallylic alcohols in high yields, unfortunately, the allylation of aromatic aldehyde substituted by nitro‐ or amino‐group could not proceed smoothly, and the allylation yields of ketones and aliphatic carbonyl compounds were lower under the same condition. The diastereoselectivity and regioseletivity of the reaction have also been studied, the predominant products preferred the erythro‐ or anti‐isomer in dominant γ‐adduct by using Al/KF mediated allylation of benzaldehydes with cinnamyl bromide and ethyl 4‐bromo‐2‐butenoate in water.     

Alpha-phosphono lactone analogues of cytidine and cytosine arabinoside diphosphates: synthesis via ring closing metathesis

alpha-Phosphono lactone derivatives of the nucleosides cytidine and cytosine arabinoside have been prepared from the corresponding nucleoside aldehydes. The stereochemical outcome of allylation reactions with these aldehydes was found to be dependent upon both the choice of protecting groups for the 2'- and 3'-hydroxyl groups and, to some extent, the nature of the Lewis acid catalyst employed. Ultimately, conditions were found that favored either the 5'R or 5'S diastereomer from different cytidine aldehydes, and gave some stereoselectivity in additions to an aldehyde derivative of ara-C. The resulting homoallylic alcohols were used as substrates in attempted Knovenagel and Horner-Wadsworth-Emmons condensations, but elimination was found to predominate over lactone formation under the conditions employed. The desired alpha-phosphono lactones could be prepared through a reaction sequence that included ring-closing metathesis on acrylate esters of the homoallylic alcohols, followed by reduction of the resulting alpha,beta-unsaturated lactones and carbon-phosphorus bond formation on enolates generated from the saturated lactones.     

A recyclable electrochemical allylation in water

To develop environmentally benign processes for C-C bond formation, electrochemistry is applied in a tin-mediated allylation reaction in water. In this electrochemical process, the corresponding homoallylic alcohols are prepared in excellent yields, while both tin salt and water can be recycled and electrode materials are not consumed.     

Highly efficient three-component synthesis of protected homoallylic amines by bismuth triflate-catalyzed allylation of aldimines

Bismuth triflate catalyzes the allylation of a variety of in situ generated protected aldimines using aldehydes, primary carbamates, and allyltrimethylsilane in a three-component reaction. The reaction proceeds rapidly and affords the corresponding protected homoallylic amine in good yield (up to 86%). Scope and limitations of the aldehyde and carbamate components are reported.     

New route to alpha-adducts of homoallylic alcohols by an acid-catalyzed stereospecific allyl-transfer reaction from gamma-adducts

Allylation of aldehydes by an allyl-transfer reaction from the gamma-adducts of homoallylic alcohols has been successfully carried out to give the corresponding alpha-adducts regiospecifically. The reaction proceeds via a hemiacetal (11), derived from an aldehyde and the homoallylic alcohol, followed by a six-membered cyclic transition state (2-oxonia[3.3]-sigmatropic rearrangement) in the presence of a Lewis acid. Moreover, the gamma-adducts are restructured into the corresponding alpha-adducts via a similar transition state by an acid catalyst, in which chirality in both anti- and syn-gamma-adducts is stereospecifically transferred to the corresponding E- and Z-alpha-adducts, respectively, with > 98% ee.     

SnCl2／Cu-Mediated Carbonyl Allylation Reaction in Water： Scope, Selectivity and Mechanism

Copper was found to be able to promote the SnC12-mediated carbonyl allylation reactions in water, giving the corresponding homoallylic alcohol products in very high yields. Detailed studies showed that the reaction could be applied to a variety of carbonyl compounds including those with hydroxyl, amino and nitro groups. It was also found that this reaction showed good regioselectivities for some substrates. Furthermore, carefully controled experiments and in situ NMR measurements provided important insights into the mechanism of the newly developed reaction.     

Platinum- and gold-catalyzed cycloisomerization reactions of hydroxylated enynes

Exposure of enynes containing a hydroxyl group at one of the propargylic positions to catalytic amounts of either PtCl2 or (PPh3)AuCl/AgSbF6 results in a selective rearrangement with formation of bicyclo[3.1.0]hexan-3-one derivatives. The same products are obtained by a "one-pot" process on treatment of an alkynal with allylchlorodimethylsilane (4) and PtCl2 via a reaction cascade involving an initial platinum-catalyzed allylation followed by the cycloisomerization of the homoallylic alcohol formed in situ. This novel skeletal reorganization process was implemented into a concise total synthesis of the terpenes sabinone (18) and sabinol (19). Furthermore it is shown that conversion of the hydroxylated enynes into the corresponding acetates followed by reaction with a cationic gold catalyst formed from (PPh3)AuCl and AgSbF6 opens entry into isomeric products bearing the ketone function at the C-2 position of the bicyclo[3.1.0]hexane skeleton. The outcome of a deuterium labeling experiment and the analysis of the stereochemical course of the cycloisomerization reaction are consistent with the formation of cyclopropylmethyl platinum carbene species as reactive intermediates.     

A [3,3]-sigmatropic process catalysed by acetate. The decarboxylative Claisen rearrangement

Allylic tosylacetates and tosylmalonates undergo acetate-catalysed decarboxylative Claisen rearrangement in the presence of N,O-bis(trimethylsilyl)acetamide. The homoallylic sulfones formed in these transformations correspond to the products of regiospecific allylation of sulfone-stabilised carbanions. A mechanistic rationale is proposed.     

Indium(I)/CuFe2O4 Reagent for Allylation of Carbonyls and Epoxide Rearranged Carbonyls

Russian Journal of General Chemistry - Indium(I)/CuFe2O4 reagent for carbonyl allylation and epoxide rearranged carbonyl allylation is proposed for formation of homoallylic alcohols. The In(I)...     

Enantioselective allylation of aldehydes catalyzed by new bifunctional bisoxazoline-metal complexes

The asymmetric allylation reactions of aldehydes catalyzed by a 10 mol % bisoxazoline complex bearing a phosphine oxide moiety, which was newly designed and synthesized from l-serine, afforded the corresponding homoallylic alcohols in 48-74% yields with 39-86% ee. The reaction proceeds with the dual activation of the aldehyde and allylsilane by the Lewis acid and base of the catalyst. The evidence for the activation of the allylsilane was clarified by the ³¹P NMR spectra.     

Diastereoselective allylation of α-ketoamides bearing camphor N-tosylpyrazolidinone auxiliary: efficient synthesis of highly optically active two stereoisomers

Complementary allylation conditions were developed for the synthesis of both diastereomers of tertiary homoallylic alcohols. Treatment of camphor N-tosylpyrazolidinone derived α-ketoamides with allyltributylstannane afforded both the individual homoallylic alcohols in high optical purity (up to 98% de) when the reaction was carried out in the presence of Sn(OTf)₂ and PdCl₂, respectively. The stereochemical outcome and reversal of stereoselectivity in the reaction are proposed based on ¹³C NMR and FTIR studies.     

Metal-catalyzed rearrangement of homoallylic ethers to silylmethyl allylic silanes in the presence of a Di-tert-butylsilylene source

[reaction: see text] In examining the scope of the di-tert-butylsilylene transfer to gem-disubstituted alkenes to form silacyclopropanes, we discovered an unprecedented reaction of homoallylic ethers. When silylene transfer was performed at room temperature or above, two di-tert-butylsilylene units were incorporated into the molecule, and complete rearrangement of the carbon backbone occurred. This report describes the scope of this unique reaction as well as the mechanistic studies conducted that led to a proposed mechanism.     

Convergent and stereodivergent synthesis of complex 1-aza-7-oxabicyclo[2.2.1]heptanes

A convergent and stereodivergent pathway to highly substituted 1-aza-7-oxabicyclo[2.2.1]heptanes is described. It begins with a coupling reaction involving allylic alcohol, aldehyde, and LiHMDS to produce stereodefined primary homoallylic amines. Subsequent N-oxidation and condensation with formaldehyde or glyoxylate defines a convenient entry to densely functionalized homoallylic nitrones whose intramolecular annulation can be controlled to deliver one of two distinct heterocyclic skeletons, each with ≥20:1 stereoselection. Control of the stereochemistry in these reactions results from both control of the nitrone geometry and selective partitioning of the reaction pathway between direct [3 + 2] cycloaddition and tandem [3,3] rearrangement/[3 + 2] cycloaddition.