Phosphonium ylides olefination with

NMR spectroscopic studies f111,13C, and 31P) are consistent with the dipolar ylide structure and suggest only a minor contribution from the ylene structure.234 Theoretical calculations support this view.235 The phosphonium ylides react with carbonyl compounds to give olefins and the phosphine oxide. 

Phosphonium ylides react with carbonyl compounds to give olefins whereas sulfonium ylides aflFord epoxides. In their behavior toward car-... 

Alkenes (olefins) from reaction of phosphonium ylides with aldehydes or ketones... 

The jS-oxido-ylides synthesis of trisubstituted olefins has also been applied to the synthesis of farnesol (127). The phosphonium salt (123) with the aldehyde (124) and formaldehyde gave the hydroxy farnesol derivative (125) which was transformed into farnesol (127) and into (126), a position isomer of Cj juvenile hormone. 

Olefination Reactions Involving Phosphonium Ylides. The synthetic potential of phosphonium ylides was developed initially by G. Wittig and his associates at the University of Heidelberg. The reaction of a phosphonium ylide with an aldehyde or ketone introduces a carbon-carbon double bond in place of the carbonyl bond. The mechanism originally proposed involves an addition of the nucleophilic ylide carbon to the carbonyl group to form a dipolar intermediate (a betaine), followed by elimination of a phosphine oxide. The elimination is presumed to occur after formation of a four-membered oxaphosphetane intermediate. An alternative mechanism proposes direct formation of the oxaphosphetane by a cycloaddition reaction.236 There have been several computational studies that find the oxaphosphetane structure to be an intermediate.237 Oxaphosphetane intermediates have been observed by NMR studies at low temperature.238 Betaine intermediates have been observed only under special conditions that retard the cyclization and elimination steps.239... 

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. 

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. 

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 trimethylsilylated ylides (1), easily generated from trimethyl chlorosilane and ylides, react with aldehydes 2 to form vi-nylsilanes 3 (2,3). The vinylphosphonium silanolates 4 are also formed. Compounds 3 are versatile reagents for further reactions (4). The ylide J (with R1 =H) reacts with aldehydes 2 to give the dienes j). The oxidation of with the adduct 6, from triphenyl-phosphite and ozone, gives access to a generaT synthesis of acyl-silanes (trimethylsilylketones) (2). The silylated ylides react to form phosphonium salts 7 with halogen compounds. The salts 7.can be desilylated by fluorine ions. The disubstituted ylides JO Tormed can be converted in statu nascendi with aldehydes V[ into the tris-substituted olefin J2 (2,3). In the case of R3-I, vinyl... 

For the synthesis of 313 Hammond and Descoins also used a Wittig reaction 205). Thus, starting from l-hepten-3-ol 319 the phosphonium salt 320 is formed via several steps. The ylide of this salt is olefinated with atdehyde 305 to 311. Reduction of 311 and subsequent acetylation afford the pheromone 313 (Scheme 58) 205). 

Carlson et al. obtained (Z)-9-tricosene 382 by reaction of nonanal 380 with the ylide generated from the phosphonium salt 381 and butyllithium in DMSO 219>220> (Scheme 68). Bestmann et al. 221> synthesized the (Z)-olefin 382 by olefination of tetra-decanal 384, with the ylide generated by treatment of the corresponding phosphonium salt 383 with potassium in HMPA 341 (Scheme 68). 

For the preparation of the second component 421 of the cockroach pheromone Burgsthaler et al. 228) also used a Wittig reaction. Lithium acetylide is alkylated with the two halides 414 and 416 and the resulting alkynyl bromide 417 converted into the phosphonium salt. Olefination of the corresponding ylide with 9-bromo-2-nonanone 418 gives a (Z)/( )-mixture of olefin 419 which is converted into the pheromone 3,ll-dimethyl-29-hydroxynonacosan-2-one 421 by aeetoaeetate synthesis, hydrogenation, hydrolysis, and decarboxylation 228) (Scheme 73). 

Reaction of the ylide, generated from the phosphonium salt 578 (from the alcohol 577 and phosphine hydrobromide 504), with the polyene aldehyde 542 gives aleuria-xanthin acetate 579218). The methyl ester of the naturally occurring bixin 586 is formed by a combination of some carbonyl olefinations 279). The acetoxyaldehyde 580 is olefinated with methoxycarbonylmethylene-triphenylphosphorane 67 to the ( )-unsaturated ester 581. The latter is converted into the phosphonium salt 582 upon treatment with triphenylphosphine hydrobromide 504. The corresponding ylide of 582 is reacted with the dialdehyde 539 to the polyene aldehyde ester 583. The latter is reduced and converted into phosphonium salt 584. The corresponding ylide is now reacted in a third carbonyl olefination with 585 to give the methyl ester 586 279> (Scheme 98). 

Reactions of Phosphonium Ylides. - 2.3.1 Reactions with Carbonyl Compounds. This year we are able to report several variations of the traditional Wittig olefination which employ the addition of catalysts to effect the reaction. For example, Lebel et al. have reported a new salt-free process for the methyl-enation of aldehydes, in which the phosphorane is generated in situ from triphenylphosphine and a diazo precursor with either a rhodium- or rhenium-based catalyst (Scheme 6). It was found that the most effective combination of catalyst and diazo-compound were Wilkinson s catalyst [RhCl(PPh3)3] and... 

The cyclohexanone (S3), an intermediate for the synthesis of thromboxane antagonists, has been prepared by a combination of phosphine oxide- and phosphonium ylide-based olefinations.30 Reaction of the lactone (50) with methoxymethyldiphenylphosphine oxide anion gave a poorly characterized adduct (presumably (51)) which on reduction with sodium borohydride, followed by treatment with sodium hydride gave the vinyl ether (52) in 80% overall yield from (SO) (Scheme 8). Further modification gave the required cyclohexanone (53). 

This is a Wittig reaction. The stable ylide is prepared from 29 prior to the reaction. Only the ketone reacts to form the corresponding olefin via the oxaphosphetane intermediate, as the Weinreb amide is not reactive enough. Owing to the use of a stable phosphonium ylide, only the product with the -configured double bond is obtained in 90 % yield ( /Z 99 1). 

Schlosser Modification. Almost pure tran -olefins are obtained from nonstabilized ylides by the Schlosser modification of the Wittig reaction (Wittig-Schlosser reaction). For example, treatment of the (cij )-oxaphosphetane intermediate A with n-BuLi or PhLi at -78 °C results in lithiation of the acidic proton adjacent to phosphoras to produce the P"Oxido phosphonium ylide B. Protonation of B with f-BuOH leads to the trans-1,2-disubstituted alkene C. 

Synthesis of trifluoromethylated compounds 152 has been achieved via ester-enolate [2,3]-Wittig and [3,3]-lreland-Claisen rearrangements. Perfluorocyclo-butane phosphonium ylides, e.g. 153, have been used as a masked fluoride anion source in their reactions with alcohols and carboxylic acids which lead to alkyl-and acyl-fluorides. Ylides 153 are also reported to cleave Si-C and Si-O bonds, cause dimerisation of fluoro-olefins, and also react with acid chlorides or other activated aromatic compounds under halogen exchange. ... 

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