Wittig-type olefination

Wittig-type olefination has been reported. The reaction is thought to involve the salt (47) and shows little stereoselectivity. 2.2.2 Ketones.- The stereochemistry of olefination of 2,3-epoxy- and protected 2-hydroxy cyclohexanones with ethylidenetriphenyl-... 

A series of conjugated polyenes capped with chromophores and containing an androstane spacer were synthesized by Wittig or Wittig-type olefinations from epi-androsterone 5150. For example, vinyl carboxaldehyde 52, prepared from 51 in 60% yield as shown in equation 32, was treated with 9-anthrylmethylphosphonium bromide and n-butyllithium to give diene 53. Exocyclic diene 53 was subsequently oxidized to vinyl carboxaldehyde 54. The androsterone vinyl aldehyde intermediate could either be treated with a tetraphenylporphyrinpolyenyl phosphonium ylide, or, as shown below, the phosphonium salt of the androsterone (55) could be reacted with TPP polyeneal 56. The desired all-(E) isomer, 57, was obtained from the ( )/(Z)-isomeric mixture by chromatographic purification. 

It is difficult to obtain cross-coupling products of two different carbonyl compounds by an intermolecular version of the McMurry reaction. Examples that use excess amounts of one carbonyl component are few. "" When one carbonyl component is replaced by a 1,1-dihalo compound or dithioacetal and the alternative is reduced with a low-valent metal such as low-valent titanium or chromium(ii), cross-coupling products, that is, Wittig-type olefins, are produced in high yields. Because the alternative approach is described elsewhere, we concentrate on only its important features here. 

Wittig-type olefinations can also be performed using selenoaldehydes. Phosphorus ylides initially attack the carbon atom of the selenocarbonyl functionality.405 Aromatic selenoketones undergo reductive dimerization with organo-lithium reagents probably via an electron transfer mechanism.406 Also the addition of organolithium reagents takes... 

Wittig-type olefination. These complexes convert esters to vinyl ethers and ketones to olefins in 78-100% yield. They are somewhat less reactive than the corresponding Ti complexes.1 In the olefination of aldehydes, ketones, and thioketones, the (Z)-isomer is... 

Methyl 3- /7/-D-daunosaminide 154 has been derived from d-149 via a Wittig-type olefination using (2-thiazolylmethylene)triphenylphosphorane (Scheme 13.53). A 1 1 mixture of (E)- and (Z)-alkenes is obtained, which is isomerized in the presence of iodine into a 9 1 mixture of ( )-152 and (Z)-152. Methylation of the thiazole moiety increases the electrophilicity of the alkene, which then accepts nucleophiles such as benzylamine. The adduct is treated with NaBH4 to give a thiazolidine. Acetylation and mercury-mediated hydrolysis of the thiazolidine ring generates 153, which, on acidic treatment in methanol, yields the A-benzyl 3- /7/-D-daunosaminide 154 [99]. 

Takai, K. Kakiuchi,T. Kataoka,Y. Utimoto, K., A Novel Catalytic Effect of Lead on the Reduction of a Zinc Carbenoid with Zinc Metal Leading to a Geminal Dizinc Compound. Acceleration of the Wittig-Type Olefination with the RCHX2-TiCl4-Zn Systems by Addition of Lead. /. Org. 

Dibutyltelluronium benzyhde, generated by treatment of the telluronium salt with potassium /-butoxide, behaves similarly to the above-stabilized ylides, undergoing Wittig-type olefinations with aromatic aldehydes. ... 

The Corey-Winter olefin synthesis has been performed on p-lactam derivatives.18 This olefination was employed in cases where traditional Wittig-type olefination of the corresponding aldehyde led to disappointing results. In one example, treatment of 40 with carbonyldiimidazole and exposure of the resulting thionocarbonate to P(OMe)3 afforded olefin 41 in 81% yield. 

There are other organometallic derivatives that give Wittig-type olefination reactions, with titanium derivatives being the most commonly used. The initial addition reaction to the carbonyl gives a transient metal-ated alcohol that leads to an alkene. An example is the Tebbe reagent, which exists as a bridged methylene species (see 683), where Cp is cyclopentadienyl. 9 The aluminum species can be varied to include chloride... 

Other organometallic reagents have been reported to give Wittig-type olefination reactions with ketones and aldehydes. Typical examples are 700, 86 701,587 702,588 703,589 and 704.590 -phis list is certainly not exhaustive and is probably not completely representative. It does, however, illustrate some of the reagent types that have appeared and continue to appear. 

Takai, K. et ah, A novel catalytic effect of lead on the reduction of a zinc carbenoid with zinc metal leading to a geminal dizinc compound. Acceleration of the Wittig-type olefination with the RCHX2-TiChj-Zn systems by addition of lead, J. Org. Chem., 59, 2668, 1994. 

A concise synthesis of substituted stUbenes from propargylic phosphonium salts by a cobalt-catalyzed Diels-Alder/Wittig olefination reaction has been described (Figure 1.4) [37]. It was shown that the cobalt(I)Diels-Alder reaction of propargylic phosphonium salts and alkyne-fimctionalized phosphonium salts with 1,3-dienes led to dihydroaromatic phosphonium salt intermediates that were directly used in a one-pot Wittig-type olefination reaction with aldehydes. Subsequent oxidation led to styrene- and stUbene-type products with the formation of three new carbon-carbon bonds. The reaction gives predominantly the Econfigured products. 

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