Triphenylphosphine hydrobromide

The reaction of triphenylphosphine hydrobromide with phenylpropiolic acid gives a mixture of the )- and (Z)-isomers of 2-carboxy-l-phenyl-vinyltriphenylphosphonium bromide (67), not just the (Z)-isomer as previously reported. ( )-2-Ethoxycarbonyl-l-phenylvinyltriphenylphos-phonium bromide (68) can be prepared in a similar reaction from ethyl phenylpropiolate. 

The stereochemistry of the addition of phosphorus pentachloride to isolated acetylenes in non-polar solvents has been shown by n.m.r. to be CIS, as illustrated for the adduct (46) from propyne. This observation has been explained in terms of a four-centre process. Contrary to a previous report, the reaction of triphenylphosphine hydrobromide with phenyl-acetylene carboxylic acid (47) yields both the trans- and the known c/5-adducts. 

Phosphonium salts, e.g. (124), derived from polynuclear hydrocarbons and used as intermediates in helicine synthesis, have been prepared by the reaction of the appropriate benzylic alcohol with triphenylphosphine hydrobromide.120 Similarly, the salt (125), of value as an intermediate in carotenoid synthesis, has been prepared by the reaction of a precursor allylic alcohol with triphenylphosphine hydrobromide.121 Bromomethyl a-diketones react with triphenylphosphine to form... 

The synthesis of vitamin A was certainly a pioneering work in the industrial application of the Wittig reaction 6). The decisive step in this synthesis performed by the BASF, which had already established a plant for the production of vitamin A in 1971 2S4), is the Wittig olefination of vinyl-P-ionol 503 with y-formylcrotyl acetate 507 to vitamin A acetate 508. The phosphonium salt 505 is obtained by reaction of the alcohol 503 with triphenylphosphine hydrobromide 504 2S5) (Scheme 85). 

The C10-dialdehyde 539 serves as the central olefination synthon in the preparation of the symmetrical carotenoid zeaxanthin 560 270). In this synthesis the phospho-nium salt 559 which can easily be prepared from the vinyl hydroxyionol 558 and triphenylphosphine hydrobromide, 504, is reacted with 539 in 1,2-epoxybutane, a solvent which seems to be especially suitable for Wittig reactions with polyene dialdehydes 270). The same phosphonium salt 559 was used in the synthesis of P-eryptoxanthin and zeinoxanthin. 

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). 

OL-Alkoxyphosphonium bromides enol ethers. These salts (2) are prepared by reaction of an enol ether (1) with triphenylphosphine hydrobromide at 0 —> 20° in CH2CI2 in 85-90% yield. The ylides formed on deprotonation undergo Wittig reactions to give enol ethers (3), which are suitable for further transformations. An example is the synthesis of a C-glycoside (4). 

Tertiary alcohols can also be protected in this way if triphenylphosphine hydrobromide is used as a catalyst Bolitt, V. Mioskowski, C. Shin, D. Falck, J.R. Tetrahedron Lett. 1988, 29, 4583. 

The addition of hydrogen to C-2 (instead of the heteroatoms discussed so far) leads directly to 2-deoxyglycosides. Various sulfonic acids have been used as activators for this purpose, including MsOH [601], CSA [602-604], pTsOH [605], and dehydrated AG50 WX2 resin [606]. In addition to these, triphenylphosphine hydrobromide [607], BCI3 or BBr3 [608], and ceric ammonium nitrate (CAN) [609] are effective for this purpose. 

Under strongly acidic conditions, glycals add alcohols. The acid of choice is triphenylphosphine hydrobromide. The stereochemistry at Cl appears to be dictated by the position of anomeric equilibrium, with axial products predominating, except in the case of tribenzylallal, where they would be disfavoured by 1,3-diaxial interactions. Running the reaction in deuterated solvents gave a mixture of axial and equatorial deuterium at C2, with between 5 1 and 2 -l predominance of equatorial protonation, except for dibenzyl-3-deoxyglucal. ... 

The starting material 6-methylhept-5-en-2-one (170) was converted, with methanol, into the methoxy compound 171 which was condensed with ethyl bromoacetate (172) in a Reformatsky reaction to give the ester 173. Reaction of 173 with NBS and then dehydrobromination led to the ester 174 which was reduced with LiAIH4 and transformed with triphenylphosphine hydrobromide to the Cio-phosphonium salt 175 and then coupled with crocetindialdehyde (27) in the presence of sodium methoxide as base in a Wittig reaction to give spirilloxanthin (169) in an overall yield of 2% referred to 170 [87] (Scheme 40). 

The desired building block 204 was obtained by partial reduction of the triple bond in 218, followed by successive protection of the primary alcohol, oxidation of the secondary hydroxy group to the ketone, deprotection of the alcohol, Grignard reaction with vinylmagnesium bromide (65) and acetylation. The C2o-building block 204 was converted, with triphenylphosphine hydrobromide, into the phosphonium salt 219, reaction of which with the Cio-dialdehyde 45 gave C.p. 450 (203) Scheme 48). 

The end group of (2S,2 S)-bisanhydrobacterioruberin (249) [94,106] was obtained from 251 by treatment with vinylmagnesium bromide (65) followed by conversion into the phosphonium salt 266 with triphenylphosphine hydrobromide. Wittig reaction of 266 with crocetindialdehyde (27) and NaOH afforded (2S,2 S)-bisanhydrobacterioruberin (249) Scheme 54). 

The Wittig reaction was also employed to synthesize lycopene (31) by the C 5 + Cio + Ci5 = C4o strategy [4]. j/-Ionone (1) was reacted with lithium acetylide (16) to give the alcohol 17 which was reduced with H2 and Lindlar catalyst to vinyl- /-ionol (18), which was converted with triphenylphosphine hydrobromide into the Cis-phosphonium salt 19. Coupling with the C 10-dialdehyde 20, with sodium methoxide as base, gave lycopene (31) in an overall yield of 20% referred to 1 (Scheme 4). 

Tetradehydrolycopene (29) has been synthesized starting from 2-methylbut-3-yn-2-ol (43) [9]. Condensation of this with diketene (44) and pyrolysis gave ketone 45 which was reacted with lithium acetylide (16) to give the alcohol 46. Hydrogenation with Lindlar catalyst gave the triene 47 which was reacted with triphenylphosphine hydrobromide to give the... 

For the synthesis of 36, 6-methylhept-5-en-2-one (49) was selected as starting material. Hydrogenation of 49 in the presence of palladium gave the ketone 50 which, in a Horner-Emmons reaction, was condensed with ethyl diethylphosphonoacetate (51) to give the a,P-unsaturated ester 52. Reduction with LiAlHa and treatment of the resultant alcohol 53 with triphenylphosphine hydrobromide gave the phosphonium salt 54. Wittig reaction with crocetindialdehyde (536) and BuLi resulted in 1,2, r,2 -tetrahydrolycopene (36) in an overall yield of 10% referred to 49 and the C3o-aldehyde 55 as a byproduct [10,11] (Scheme 14). 

In another synthesis of spirilloxanthin (166) [13] the ketone 62 was ethynylated with sodium acetylide (68) to give 69 which was partly hydrogenated to 70. Treatment with triphenylphosphine hydrobromide led to the Cio-phosphonium salt 77. The Wittig reaction with crocetindialdehyde (536) and PhLi gave l,r-dimethoxy-l,2,r,2 -tetrahydrolycopene (170)... 

For the synthesis of the symmetrical dihydroxylycopene (169), linalool (14) was reacted with triphenylphosphine hydrobromide to give the phosphonium salt 75, which was hydrated by heating under reflux in water to give 73. The Wittig reaction of two moles of 73 with crocetindialdehyde (536) and sodium methoxide gave dihydroxylycopene (169) in an overall yield of 55% referred to 14 [9] (Scheme 22). 

In another synthesis methylenephosphorane (137) was reacted with a-methoxyisobutyric acid chloride (138) to give the phosphorane 139 which was elongated to the alcohol 140 in a Wittig reaction with the aldehyde 141. Treatment with triphenylphosphine hydrobromide gave the Cis-phosphonium salt 142 and a Wittig reaction with the Cio-dialdehyde 20 provided 2,2 -diketospirilloxanthin (416) [16,17] (Scheme 32). 

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