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Homer-Wadsworth-Emmons olefinations

In 2000, the Merlic research group reported concise total syntheses of calphostins A and D and the first total syntheses of calphostins B and C [33]. This original approach to the naphthalene fragment relied on a chromium carbene intermediate to obtain the required regioselection. The synthesis began with phosphonate 39 (three steps), followed by a Homer-Wadsworth-Emmons olefination of 39 with... [Pg.164]

The same group recently disclosed a related free radical process, namely an efficient one-pot sequence comprising a homolytic aromatic substitution followed by an ionic Homer-Wadsworth-Emmons olefination, for the production of a small library of a,/3-unsaturated oxindoles (Scheme 6.164) [311]. Suitable TEMPO-derived alkoxy-amine precursors were exposed to microwave irradiation in N,N-dimethylformam-ide for 2 min to generate an oxindole intermediate via a radical reaction pathway (intramolecular homolytic aromatic substitution). After the addition of potassium tert-butoxide base (1.2 equivalents) and a suitable aromatic aldehyde (10-20 equivalents), the mixture was further exposed to microwave irradiation at 180 °C for 6 min to provide the a,jS-unsaturated oxindoles in moderate to high overall yields. A number of related oxindoles were also prepared via the same one-pot radical/ionic pathway (Scheme 6.164). [Pg.213]

The formation of allenes by Wittig or Homer-Wadsworth-Emmons-olefination of ketenes is well established [1] for more recent examples, see (a) T. Minami,... [Pg.91]

Phenylalanine-derived oxazolidinone has heen used in O Scheme 52 as a chiral auxiliary for as)rmmetric cross-aldolization (Evans-aldol reactions [277,278,279,280,281,282,283,284, 285]). The 6-deoxy-L-glucose derivative 155 has heen prepared by Crimmins and Long [286] starting with the condensation of acetaldehyde with the chlorotitanium enolate of O-methyl glycolyloxazohdinethione 150. A 5 1 mixture is obtained from which pure 151 is isolated by a single crystallization. After alcohol silylation and subsequent reductive removal of the amide, alcohol 152 is obtained. Swem oxidation of 152 and subsequent Homer-Wadsworth-Emmons olefination provides ene-ester 153. Sharpless asymmetric dihydroxylation provides diol 154 which was then converted into 155 (O Scheme 60) (see also [287]). [Pg.901]

Related reactions Horner-Wadsworth-Emmons olefination, Homer-Wadsworth-Emmons olefination - Still-Gennari modification, Julia-Lithgoe olefination, Takai-Utimoto olefination, Tebbe oiefination, Wittig reaction, Wittig reaction - Schiosser modification ... [Pg.650]

The reaction of ethyl 4-(bromofluoromethyl)benzoate with triethyl phosphite at 200°C produces the corresponding a-fluorobenzylphosphonate in 68% yield. After Homer-Wadsworth-Emmons olefination, this gives fiuorinated aromatic retinoic acid analogues. - ... [Pg.76]

The stabilized a-substituted cyanomethylphosphonate carbanion resulting from the addition of potassium diethyl cyanomethylphosphonate to one equivalent of acrylonitrile in THF/HMPA appears to be a useful reagent in Homer-Wadsworth-Emmons olefination reaction. Thus, the reaction with aromatic aldehydes is completely stereoselective and produces ( )-l-aryl-2,4-dicyano-1-butenes in high yields (71-83%, Scheme 6.34).- ... [Pg.277]

However, further synthetic investigations have shown that with piperidine/AcOH as catalyst, the reaction proceeds by several competitive processes to give complex mixtures containing the desired phosphonate, the Homer-Wadsworth-Emmons olefination product, the straight-condensation product, and other byproducts. [Pg.373]

The reaction has been investigated with MeOH, - z-PrOH, - t-BuOH, " phenols, " cyclic allylic alcohols, - homoallylic alcohols, 3,4-hydroxybenzoate, and mono Z-butyldimethyl-silyl protected diols. A subsequent Homer-Wadsworth-Emmons olefination with a range of aldehydes affords the corresponding enol ethers. ... [Pg.440]

The rest of the synthesis of pumiliotoxin is straightforward involving a Homer-Wadsworth-Emmons olefination (chapter 15) and a hydrogenation that accomplishes reductive animation and the control of another chiral centre. This synthesis is also discussed in chapter 21. [Pg.615]

With this strategy in mind, Keck s construction of the 2,6-cii-tetrahydropyran unit commenced with the asymmetric allylation [185] of aldehyde 2.275 with allystannane 2.276 promoted by BEMOL titanium tetraisopropoxide (BITIP) to furnish the homoaUyl alcohol 2.274 (95 %, 95 % ee) [186]. The formation of the key 2,6- s-tetrahydropyran 2.277 was accomplished by the Hosomi-Sakurai-Prins cychzation in the presence of TMSOTf in 85 % yield as a single diaste-reomer. SUyl deprotection and oxidation, followed by Homer-Wadsworth-Em-mons olefination proceeded to provide 2.278, which was further elongated to the phosphonoacetate 2.279. Exposure of the resulting phosphonoacetate to NaHMDS after desilylation and subsequent oxidation afforded the macrocycle 2.280 via the Homer-Wadsworth-Emmons olefination protocol. [Pg.123]

Complex molecules have also been synthesized by sequential use of the orga-nocatalytic a-amination of aldehydes by azodicarboxylates catalyzed by some of the systems shown above and by other reaction processes, such as the Passerini reaction [40], Homer-Wadsworth-Emmons olefination [41], Wittig olefination [42], and allylation reaction followed by a ring-closing metathesis [43], In addition, the combination of catalysts in cycle-specific organocascade processes [44] has allowed the synthesis of chiral complex molecules with good results. [Pg.763]

Reaction of [2- " C]acetone with vinylmagnesium bromide (Figure 6.75, Procedure A) provided 2-methyl-3-[2- " C]buten-2-ol (257), which upon treatment with PBra rearranged to give 3,3-dimethyl[3- " C]allyl bromide (258). The latter was used for the alkylation of ethyl acetoacetate, thereby extending the carbon skeleton of the /3-keto ester by a labeled five-carbon (isoprenyl) unit. The initially formed alkylated intermediate 259 was not isolated but immediately saponified and decarboxylated to give ketone 260. Subsequent Homer-Wadsworth-Emmons olefination and reduction of the separated trans-ester 261 converted 260 into [T- Clgeraniol 12621. [Pg.339]


See other pages where Homer-Wadsworth-Emmons olefinations is mentioned: [Pg.110]    [Pg.41]    [Pg.619]    [Pg.232]    [Pg.287]    [Pg.468]    [Pg.173]    [Pg.372]    [Pg.356]    [Pg.329]    [Pg.95]    [Pg.11]    [Pg.36]    [Pg.168]    [Pg.74]    [Pg.82]    [Pg.96]    [Pg.47]    [Pg.93]    [Pg.20]    [Pg.111]    [Pg.218]    [Pg.257]    [Pg.173]    [Pg.62]    [Pg.70]   
See also in sourсe #XX -- [ Pg.177 ]




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