Triphenylphosphonium bromide intermediates

Reaction intermediates can be detected by reaction monitoring (i.e. analyses at several reaction times), and their presence may be inferred or even observed more readily at low temperatures. In a Wittig reaction, the ylid 32 in Scheme 2.13 was produced from ethyl-triphenylphosphonium bromide and butyl lithium, and reacted with a small excess of cyclohexanone in THF at —70°C the initial product, the oxaphosphetane 33, was identified by 31P NMR and converted to the alkene product and triphenylphosphine oxide (34) above — 15°C (see also Chapter 9). These results provide relatively direct experimental evidence for the mechanism shown in Scheme 2.13 [23]. 

A more concise route to ( )-cherylline was also devised and commenced with the reductive animation of isovanillin with methylamine followed by reaction of the intermediate benzylamine with vinyl triphenylphosphonium bromide to provide the aminophosphonium salt 619. Sequential treatment of 619 with n-butyllithium and the quinone ketal 615 followed by reaction of the resulting crude allylic amine 620 with boron trifluoride etherate gave the phenolic amine 618 in good overall yield (225). 

Erylus formosus [45], was also reported by Kulkami et al. [53]. As shown in Fig. (19), a common bifunctional nine-carbon aldehyde was used for both syntheses. Both syntheses started with the DHP monoprotection of 1,4-butanediol, followed by subsequent PCC oxidation to the corresponding aldehyde. Reaction of this butanal with (4-carboxybutyl)triphenylphosphonium bromide resulted in the corresponding THP protected olefinic acid that was further deprotected and esterified to the methyl ester. A second oxidization with PCC afforded the desired intermediate methyl 8-formyloct-5-enoate which secured the cis A5 double bond in the final targets, Fig. (19). The... 

The vinyl ether (45) and triphenylphosphine gave the phosphonium salt (46). The reaction is thought to proceed by initial attack at halogen followed by quaternization of the phosphine by the intermediate formed. Hydrolysis of (46) gave (l-formylethyl)triphenylphosphonium bromide (47). When triphenylphosphine is heated with the bromoesters (48) quaterniza-... 

Reactions of Phosphonium Salts.— Asymmetric induction is observed on alkaline hydrolysis of the prochiral phosphonium salts (132) under phase-transfer conditions in the presence of an optically active quaternary ammonium salt, forming the chiral oxides (133) with a 0—8 % enantiomeric excess. Alkaline hydrolysis of monobenzyl quaternary salts of a,co-bis(diphenylphosphino)alkanes gives a route to diphosphine monoxides, e.g., (134). Aqueous hydrolysis of (dibromo-fluoromethyl)triphenylphosphonium bromide gives a high yield of dibromo-fluoromethane and triphenylphosphine oxide. When the reaction is carried out in the presence of radiolabelled Br, the evidence points to the involvement of the dibromofluoromethyl carbanion, and not to a carbene intermediate as was observed in the reaction of the related (bromodifluoromethyl)phos-phonium salt. ... 

A related methodology (ref. 112) was employed in which rather than a C.,5 farnesyl component a compound was reacted with a chroman-2-acetaldehyde derivative. In this scheme the C.,4 aldehyde simultaneously formed at the ozonolysis stage in the previosly described synthesis was used. Thus, 2-carboxymethyl-6-hydroxy-2,5,7,8-tetramethylchroman, [(6-hydroxy-2,5,7,8-tetramethylchroman-2-yl)acetic acid] served as a source by resolution with (S)-a-methylbenzylamine of the two enantiomeric acids. The 2(S)-enantiomer was converted by way of the acid chloride to the aldehyde, a homologue of that employed by the Swiss workers in the foregoing method. The aldehyde was reacted with the triphenylphosphonium salt from the C bromide by the Wittig reaction to afford 2R,4 R,8 R-a-tocopherol. A novel aspect of this approach was that it enabled a synthesis of 2R,3 E,7 E-a-tocotrienol to be achieved from the same carboxymethyl intermediate. 

Redistillation gave a colorless oil, with a bp of 98 deg C at 0.11 mm/Hg. Anal. (Cl 1H1203) C,H. This intermediate ketone could be prepared by the Wittig reaction between piperonal and the derivative of triphenylphosphonium propyl bromide and dibutyidisulfide, followed by hydrolysis in a HCI/acetic acid mixture, but the yields were no better, Efforts to prepare this ketone by the iron and acid reduction of the appropriate nitrostyrene... 

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