Aldehydes Wittig reagent

The Wittig reaction uses phosphorus ylides (called Wittig reagents) to convert aldehydes and ketones to alkenes... 

The aldehyde functionality present in 3-phenyl-2H-azirine-2-carbox-aldehyde reacts selectively with amines and with Qrignard and Wittig reagents to give a variety of substituted azirines. These azirines have been used, in turn, to prepare a wide assortment of heterocyclic rings such as oxazoles, imidazoles, pyrazoles, pyrroles, and benzazepins. ... 

Aldehydes can be prepared by the Wittig reaction using (methoxymethylene)-triphenylphosphorane as the Wittig reagent and then hydrolyzing the product with acid. For example,... 

Allyltin compounds can be prepared by simple modifications of the usual reaction involving allyl Grignard reagents (139), by the 1,4-addition of trialkyltin hydrides to 1,3-dienes 140,141), or by the reaction of an aldehyde or ketone with the appropriate, tin-carrying, Wittig reagents (142). 

The Wittig reagent (56) is best protected as an ester and reacts chemo-selectively with the aldehyde rather than the less reactive ketone in (57),... 

Specific enol equivalents will be needed for both synthons (61) and (66), Since (61) is to give a double bond but (66) is to give an alcohol, the logical choices are a Wittig reagent - actually (67) - for (61) and a Reformatsky reagent for (66). The ester to aldehyde conversion (65 63) Is easiest by over-reduction and re-... 

Piperazine linker 77 was treated with propargyltriphenylphosphine bromide to provide a resin-bound Wittig reagent (Scheme 40) [89]. Base treatment followed by aldehyde addition produced a resin-bound 2-amino-butadiene which was implemented in [4 + 2] cycloadditions. Alternatively, treatment with 3% TFA in CH2CI2 released a,(J-unsaturated methylke-tones in high yields. 

Methylenation of ol-(N-Boc-amino) aldehydes. Methylenation of these chiral aldehydes with the Wittig reagent or with CH2Ir-Zn-TiCl4 (13, 114) is accompanied by extensive racemization. However, the neutral reagent 1 obtained from CH2I2, Zn, and A1(CH3), converts these aldehydes to the protected allylamines in 40-75% yield and in >99% ee. 

E Selective Wittig reagents. The reaction of 1 with lithium in THF provides LiDBP, which on reaction with an alkyl halide (2 equiv.) and NaNH2 in THF gives a salt-free ylide such as 2 or 3, formed by reaction with ethyl iodide or butyl iodide, respectively. These ylides react readily with aldehydes at —78°, but the intermediate oxaphosphetanes are unusually stable and require temperatures of 70-110° for conversion to the phosphine oxide and the alkene, which is obtained in E/Z ratios of 6-124 1. Highest (E)-selectivity is observed with a-branched aldehydes. 

The Wittig reaction consists in the replacement of carbonyl oxygen of aldehydes and ketones by a methylene group with the aid of phosphine-methylenes resulting in the formation of cis or trans olefines. The reaction proceeds through the nucleophilic addition of Wittig reagent (phosphine methylene) across the > C = O bond and formation of an intermediate cyclic. 

The migration of a C=C bond to form a C=N bond was also observed with hydro-xylamine [78, 79], hydrazine [80, 81] and primary amines [82]. The /f iminylphos-phine oxide formed in the reaction may serve as a Wittig reagent in the presence of a base to react with a ketone or an aldehyde leading to ,/fun saturated alkenyl-imines 153 (Scheme 10.74). The phosphorus group can be a phosphonium salt as well as a phosphonate. 

Olefination of the Aldehyde 178 using a stabilized Wittig reagent followed by protecting group chemistry at the lower branch and reduction of the a,p-unsaturated ester afforded the allylic alcohol 179 (Scheme 29). The allylic alcohol 179 was then converted into an allylic chloride and the hydroxyl function at the lower branch was deprotected and subsequently oxidized to provide the corresponding aldehyde 161 [42]. The aldehyde 161 was treated with trimethylsilyl cyanide to afford the cyanohydrin that was transformed into the cyano acetal 180. The decisive intramolecular alkylation was realized by treatment of the cyano acetal 180 with sodium bis(trimethylsi-lyl)amide. Subsequent treatment of the alkylated cyano acetal 182 with acid (to 183) and base afforded the bicyclo[9.3.0]tetradecane 184. 

Polymer Wittig reagents are useful for the conversion of an aldehyde or ketone to an alkene according to the sequence in Eq. 9-70 [Ford, 1986a,b],... 

Scheme 5 shows a formal explanation of this conceptual idea for alkanes developed by Wilhams [60] the alcohol is first oxidized into the aldehyde affording a hydride complex. The aldehyde is then condensed with the Wittig reagent to form the unsaturated compound, which becomes hydrogenated by the hydride complex, thus regenerating the catalyst. 

Alkenes are obtained from selective hydrogenation of alkynes (see Section 5.3.1), and the reaction of a phosphorus ylide (Wittig reagent) with an aldehyde or a ketone (see Section 5.3.2). 

The reaction of both aldehydic groups in 3,5-diformyl-4-ethyl-4//-pyran (85b) and in analogous 4//-thiopyrans 38b and 339 with a carbocyclic Wittig reagent was successfully explored in the syntheses of hetero[17]annulene 608 (Scheme 37).85 Analogous 3,5-diformyl derivative 85d reacted with phenyl-hydrazines to give Schiff bases 609.418 Hydroxylamine and 85d at room... 

Urns-Epoxides. This unstable ylide (1), when generated as formulated above, reacts with an aliphatic aldehyde at —78° to give a fram-epoxide with almost complete stereoselectivity. The stereochemical selectivity is markedly dependent on the base and also on the counterion of the arsonium salt. Optimum selectivity for the trans-epoxide is obtained with conditions similar to those that induce cis-olefination in Wittig reactions.2 Stereoselection is not so high with aromatic aldehydes. The reagent also reacts with ketones to form trisubstituted epoxides. 

Aldehyde or ketone Wittig reagent (an ylide) Alkene Tripheny I phosph ine oxide... 

The hindered, stabilized Wittig reagent is unable to react with ketones, but reacts slowly with normal unhindered aldehydes at elevated temperature. However, it reacts at a reasonable rate with the highly reactive starting a-ketoaldehyde at 0°C. No reaction occurs on the... 

Selective reactions. Wittig reactions of 1 with an aldehyde are possible in the presence of keto, ester, and amino groups.2 Wittig reagents react under normal conditions with acid chlorides. The stabilized ylid I reacts preferentially with the acid chloride group of 4-lormylhcnzoyl chloride (2) lo give 3 as the major product. ... 

Unlike Wittig reagents, 1 reacts with epoxides to form a y-oxido ylide (b), which reacts with an aldehyde to form a /ran.v-homoallylic alcohol. 

The corresponding Wittig reagent, CHj PPhj, reacts smoothly with both aldehydes and ketones to give methylenated products In high yield but with one subtle limitation. The problem cannot be detected with aldehydes because they react rapidly even at temperatures as low as -78°C, but ketones react more slowly, and an adjacent enolizable chiral center can be epimerized as a result of competitive reversible enolization. This limitation of the Wittig... 

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