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Zinc alcoholates homoallylic

The zinc-based Simmons-Smith type procedures frequently require rather harsh conditions in order to provide acceptable cyclopropane yields. Also, the discrimination between allylic alcohols, homoallylic alcohols and olefins without a hydroxyl group is often not very pronounced. These drawbacks are avoided by a new method which substitutes samarium metal (or samarium amalgam) for zinc (Table 4)43. This cnahlcs only allylic alcohols to be cyclo-propanated under very mild conditions, even for highly crowded substrates. The hydroxy-directed diastereofacial selectivity is good to excellent for cyclic olefins. Due to this property, the method has been applied to the stereoselective synthesis of 1,25-dihydroxycholecalciferol44. [Pg.986]

Jeong and co-workers utilized a cobalt-alkyne complex to enhance enantioselectivity of the addition of bis (homoallyl)zinc to propargyl aldehydes 68 by the exaggeration of steric environment. The reaction provided optically enriched propargyl alcohol 69 in the presence of a chiral ligand and titanium tetra(isopropoxide) in excess. Adduct 69 was subjected to PKR to yield optically enriched bicyclic compounds 70 (Equation (39)). ... [Pg.358]

The addition of substituted allylic zinc reagents to aldehydes is usually unselective" . Furthermore, the direct zinc insertion to substituted allylic halides is complicated by radical homocoupling reactions. Both of these problems are solved by the fragmentation of homoallylic alcohols. Thus, the ketone 166 reacts with BuLi providing a lithium alcoholate which, after the addition of ZnCl2 and an aldehyde, provides the expected addition product... [Pg.317]

Failure for Grignard reagents to undergo similar additions to homoallylic amines may be explained by the greater stability of the complexes between an amine and zinc rather than magnesium. By contrast, addition of allylzinc bromide to allyl or homoallyl alcohols proceeded considerably less efficiently compared to amines and the regioselectivity was reversed (equation 55)81. [Pg.892]

Tetrakis(triphenylphosphine)-palladium(O), 289 Tin(IV) chloride, 300 Trimethylamine N-oxide, 325 Zinc amalgam, 347 Homoallylic alcohols By addition reactions of allyl to carbonyl groups... [Pg.391]

Isoprenylation of Isopropenyl groups.1 Cyclic terpenes substituted by an isopropenyl group can be converted into sesquiterpenes with the bisabolane skeleton by addition of HOC1 under biphasic conditions to give an allylic chloride (10, 208-209) followed by a zinc-induced reaction with isovaleraldehyde to form an isomeric mixture of homoallylic alcohols. [Pg.135]

A procedure has been developed for the palladium-catalysed a-arylation of amides by aryl bromides using the zinc enolates of the amides. The reaction works well with bromoarenes carrying a variety of ring substituents and with bromopyridine. In addition, the reaction has been shown to be effective with morpholine amides to give products which are precursors for aldehydes and ketones.39 A new method has been reported for the allylation of aryl halides using homoallyl alcohols as the allyl source the palladium-catalysed reaction, which may be both stereo- and regio-sepecific, uses a retro-allylation reaction to form a a-allyl(aryl)palladium intermediate.40... [Pg.181]

This reaction can also be used to prepare homoallylic alcohols. Reaction of 3-butene-2-ol (1) with ClSi(CH3)3 and Nal in acetonitrile followed by a reaction with a ketone and zinc provides a homoallylic tertiary alcohol (2) in 52% yield. [Pg.86]

Chiral homoallylic alcohols and amines were prepared by using a four-component condensation reaction based on a zinc homologation followed by a trapping with an aldehyde or an imine.301 A regio- and stereospecific carbocupration reaction on alkynyl sulfoxide provides the corresponding metallated /3,/3-dialkylated a,/ -ethylenic sufoxide 98. Addition of aldehydes or imines, followed by addition of the bis(iodomethyl)zinc carbenoid, provides adducts 99 in good overall yield and with very high diastereoselectivity (Scheme 36). [Pg.130]

The Simmons-Smith reaction often starts very slowly and proceeds quite slowly at room temperature. The method of the preparation of active zinc-copper couple has been improved 93, 196, 298, 376, 443, 466). The use of zinc dust/cuprous chloride instead of zinc-copper couple gives higher yields in the Simmons-Smith reaction 407). The reaction with homoallylic alcohols of low reactivity was facilitated by use of excess Simmons-Smith reagent and distillation of ether to a halfvolume followed by heating in a stainless steel tube 182). The Simmons-Smith reaction was successfully applied to vinyl trimethylsilyl ethers to prepare, after methanolysis, cyclopropyl alcohols unobtainable by other methods 109). Here, the zinc-silver couple was used instead of the zinc-copper couple, and the quenching of the reaction mixture of the Simmons-Smith reaction was improved by using pyridine under nonaqueous conditions. [Pg.90]

Allyliczinc halides reagents have been prepared from sterically hindered homoallylic alcohols, using a novel fragmentation reaction of the corresponding zinc aUcoxide (Scheme 6). The interest of this reaction resides in the absence of the Wurtz homocoupling product, usually present in high proportion when the direct reaction of zinc with allylic halides is used. [Pg.5213]

Lewis acids, such as SnCU, also catalyze the reaction, in which case the species that adds to the alkenes is H2C O SnCl4. " Montmorillonite KIO clay containing zinc (IV) has been used to promote the reaction. " The reaction can also be catalyzed by peroxides, in which case the mechanism is probably a free-radical one. Other transition metal complexes can be used to form homoallylic alcohols. A typical example is the reaction of methylenecyclohexane with an aryl aldehyde to give 89. [Pg.1394]

The presence of a heteroatom in the olefin at an allylic or homoallylic position considerably stablizes the zinc organometallic compound obtained after the hydrozincation of the olefin. Thus allylic alcohols and amines are especially well suited as substrates for this hydrozincation procedure. [Pg.477]

Zinc-mediated Barbier-t) e addition of 238, followed by Luche s procedure obtained a mixture of the homoallylic alcohol diastereomers 239 and 240. Alcohol 239 was carried through benzoylation, deketalization, silylation, and ozonolysis, to produce C-branched y-lactone 241. Benzoylation of 240 followed by hydroboration, PCC oxidation, debenzoylation, and alkaline-promoted cyclization directly formed C3-branching 2-deoxyfuranose 242 [87] (O Scheme 63). A novel method for stereoselective synthesis of 4 -a-carbon-substituted nucleosides, through epoxidation of 4, 5 -unsaturated nucleosides and SnCU-promoted epoxy ring opening, was... [Pg.338]

Tandem reactions. Homoallylic alcohols are obtained when ozonides are treated with zinc and ally lie bromides. Zinc performs two tasks to reduce the ozonides and to form nucleophilic agents. [Pg.279]

Step provides the homoallylic alcohol, predominantly as the anti isomer. The high diastereoselectivity is attributed to the generation of pure ( )-2-butenylzinc in the presence of the electrophile. These reactions can proceed in high yield even with only a catalytic (10 mol%) amount of zinc. [Pg.376]

ICP-MS analysis of the homoallyl alcohols formed in these reactions indicates negligible tin and cerium contamination. The resin can be recovered and regenerated with allyl bromide and zinc or allylmagnesium bromide and reused several times without notable loss of activity. [Pg.661]

Zero-valent indium can also be generated in situ by reduction of indium(III) halide with metallic zinc or aluminum [141] or even tin [142]. Thus,)3-trifluoro-methylated homoallylic alcohols have been prepared in high yields and with excellent stereoselectivity using indium powder [143] or a mixture of InClj/Sn [144]. [Pg.25]

Keywords homoallyl alcohol, homopropargyl alcohol, zinc powder... [Pg.87]


See other pages where Zinc alcoholates homoallylic is mentioned: [Pg.59]    [Pg.59]    [Pg.59]    [Pg.101]    [Pg.314]    [Pg.216]    [Pg.410]    [Pg.218]    [Pg.161]    [Pg.317]    [Pg.767]    [Pg.888]    [Pg.893]    [Pg.160]    [Pg.127]    [Pg.17]    [Pg.158]    [Pg.94]    [Pg.94]    [Pg.109]    [Pg.468]    [Pg.1003]    [Pg.1316]    [Pg.180]    [Pg.180]    [Pg.372]    [Pg.374]    [Pg.374]    [Pg.1003]   
See also in sourсe #XX -- [ Pg.59 ]

See also in sourсe #XX -- [ Pg.59 ]

See also in sourсe #XX -- [ Pg.59 ]




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