Wittig olefin formation

Fluonnated ylides have also been prepared in such a way that fluonne is incorporated at the carhon P to the carbamonic carbon Vanous fluoroalkyl iodides were heated with tnphenylphosphine in the absence of solvent to form the necessary phosphonium salts Direct deprotonation with butyUithium or hthium dusopropy-lamide did not lead to yhde formation, rather, deprotonation was accomparued by loss of fluonde ion Flowever deprotonation with hydrated potassium carbonate in thoxane was successful and resulted in fluoroolefin yields of45-S0% [59] (equation 54) P-Fluorinated ylides may also be prepared by the reaction of an isopropyli-denetnphenylphosphine yhde with a perfluoroalkanoyl anhydnde The intermediate acyl phosphonium salt can undergo further reaction with methylene tnphenylphosphorane and phenyUithium to form a new yhde, which can then be used in a Wittig olefination procedure [60] (equation 55) or can react with a nucleophile [6/j such as an acetyhde to form a fluonnated enyne [62] (equation 56)... 

Y (as confirmed by results with triafulvenes (see p. 101)). With phosphonium ylides 2-pyrone formation competes with Wittig olefination of the cyclopropenone carbonyl group. 

Olefin formation from aldehydes and phosphonates. Workup is more advantageous than the corresponding Wittig reaction because the phosphate by-product can be washed away with water. 

The Wittig olefination of the tricyclic y-oxo ester 231f affords the 6-methylene tricycle 244. Treatment of the latter with diluted hydrochloric acid results in the formation of the 6-methyl-2-oxobicyclo[3.2.1]oct-6-ene derivative 240f in 94% yield (Scheme 70) [106,108]. This transformation maybe realized also in a one-pot procedure giving compound 240n in 70% yield (Scheme 70) [104b]. The overall yield in the step-by-step preparation of this compound, as shown in Schemes 65 and 68, was only 55% starting from the chloro ester 1-Me. 

With the fully functionalized heterocyclic core completed, synthetic attention next focused on introduction of the 3,5-dihydroxyheptanoic acid side-chain. This required initial conversion of the ethyl ester of 35 to the corresponding aldehyde through a two-step reduction/oxidation sequence. In that event, a low-temperature DIBAL reduction of 35 provided primary alcohol 36, which was then oxidized to aldehyde 37 with TRAP. Subsequent installation of the carbon backbone of the side-chain was accomplished using a Wittig olefination reaction with stabilized phosphonium ylide 38 resulting in exclusive formation of the desired -olefin 39. The synthesis of phosphonium ylide 38 will be examined in Scheme 12.5 (Konoike and Araki, 1994). 

The Wittig reaction is a very important method for olefin formation. The stereochemistry about the new carbon-carbon double bond is the Z (or less stable) isomer. This unusual stereoselectivity indicates that product formation is dominated by kinetic control during formation of the oxaphosphetane. 

In the synthesis of an oxepine from 2,3,4-tri-O-benzyl-D-xylose, l,6-anhydro-3,4,5-tri-0-benzyl-2-deoxy-D-xylosept-l-enitol, the three-step sequence (Wittig olefination, vinyl ether formation, and RCM) was used <2004CAR1163>. 

Wittig olefination reaction ( the phosphorus way ) has been a very popular reaction in organic synthesis. However, it is now in competition with Peterson/Chan olefination reaction327 ( the silicon way ). Formally, this latter involves the formation of a (3-silyl heteroatomic anion, which in the absence of an electrophile undergoes a (3-shift of the silyl moiety to the heteroatom (usually oxygen) with final elimination of silylated heteroatomic anion and formation of the olefin. 

A brominated polystyrene is reacted with sodium diphenylphosphan to form the polymeric Wittig reagent. Reaction with a halide and a base form the ylid which reacts with the carbonyl compound to the olefin. Wittig olefination can be made stereoselective (33). The formation of cis-olefins is accomplished in salt-free solu-... 

The required olefin alcohols were prepared via a slight adaptation of the literature protocols (9). Kinetic furanoside formation was followed by benzylation (10a —> 11a) and hydrolysis produced the lactols 12a that were subsequently purified by column chromatography. Wittig olefination then... 

Access to derivatives with protecting groups other than benzyl was also desired. Tri-O-acetyl-D-glucal (13) was deacetylated and benzylated with p-methoxybenzyl chloride to give 14. Cleavage of the olefin in 14 gave formate aldehyde 15. Hydrolysis of the formate ester led to lactol 16 and Wittig olefination then furnished the PMB protected olefin alcohol Id (Scheme 3). 

The Wittig olefination of 1,5-oxazocine 416 with [(methoxy)methylene]triphenylphosphorane led to the formation of the enol ether 431a (88% yield) as an inseparable mixture of E and Z isomers (2.5 1 ratio). Instead, when the reaction was carried out using methyltriphenylphosphorane as Wittig reagent, the as 3,5-disubstituted diastereoisomer 431b was exclusively formed in 85% yield (Scheme 84) <1998JOC3492>. 

A keto group was extensively used in olefinations, providing a convenient access to natural-type oxonine products. Chemoselective formation of silyl enol ether of oxonine 171 (Scheme 34) followed by Wittig olefination, deprotection, and diastereoselective methylation afforded acetate 172 in good yield <2004JA1642>. 

Quaternary phosphonium salts are organophosphorous compounds used as Wittig olefination reagents, phase transfer catalysts, electrolytes, ionic liquids, and as surface active reagents. Their preparation involves the C-P bond formation in tertiary phosphines. We envisaged that addition of phosphines to unsaturated compounds should be preferable as compared to the conventional method using a substitution reaction of organohalogen compounds (Scheme 1). In this chapter, we describe our recent study on this subject. 

Phosphoranium salts have been shown to be very useful reagents in the Wittig olefination reaction The formation proceeds easily both when alkylphosphanes and arylphosphancs are utilized as reagents-... 

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