Phosphonium tribromides, reactions

This reaction gives fair-to-good yields of monoorganotin tribromides and trichlorides when quaternary ammonium or phosphonium catalysts are used (149). Better yields are obtained with organic bromides and staimous bromide than with the chlorides. This reaction is also catalyzed by tri alkyl antimony compounds at 100—160°C, bromides are more reactive than chlorides in this preparation (150,151). a,C0-Dihaloalkanes also react in good yield giving CO-haloalkyltin trihaHdes when catalyzed by organoantimony compounds (152). 

General Reactions.—Various products were obtained when phosphorous acid was heated with halogens in a sealed tube. Iodine gave phosphoric and hydriodie acids, phosphonium iodide and an iodide of phosphorus, whilst bromine gave phosphoric acid, phosphorus tribromide and hydrobromie acid.6 A dry ether solution of the acid was not oxidised by bromine or dry palladium black, but oxidation proceeded readily in the presence of moisture.7... 

The first synthesis of lycopene (1) was reported by the school of Karrer [82] in 1950. By analogy with his pioneering synthesis of p,p-carotene (2) the Ci6 + C8 + Ci6 = C4o route was used. The first synthesis of lycopene (1) via the Wittig reaction used the strategy Cio + C2o + Cio = C4o [83]. Linalool (148) was converted, with phosphorus tribromide and triphenylphosphine, into the phosphonium salt 149. The Wittig reaction with crocetindialdehyde (27) and BuLi as base gave lycopene (1) in an overall yield of 36% referred to 148 Scheme 33). 

The synthesis of symmetrical i -carotene (152), which has 7,8 and 7, 8 single bonds, was carried out efficiently according to the C15 + C10 + C15 = C40 strategy by use of the Wittig reaction to couple the building blocks [84]. frans-Nerolidol (154) was converted, with phosphorus tribromide and triphenylphosphine, into the Ci5-phosphonium salt 155. The Wittig reaction with the Cio-dialdehyde 45 and BuLi gave -carotene (152) in an overall yield of 45% referred to 154 (Scheme 34). 

In this synthesis famesyl bromide (156) was converted, with 2-nitropropane and KOH, into famesal (157). Homer-Emmons reaction with the Cs-phosphonate 73 and reduction with UAIH4 gave the alcohol 158 (32% yield referred to 156) which was transformed, with phosphorus tribromide and triphenylphosphine, to the C2o-phosphonium salt 159 (Scheme 35). 

For the synthesis of phytoene (150) and phytofluene (151) the C20 + C20 = C40 strategy has been applied [85,86], For the preparation of phytoene (150), trans-geranyllinalool (165) was treated with phosphorus tribromide to give geranylgeranyl bromide (166) which was reacted with triphenylphosphine to give the C2o-phosphonium salt 167. The C2o-aldehyde 168 was prepared by treatment of 166 with KOH and 2-nitropropane. The Wittig reaction of 167 and 168 with PrLi gave a mixture of (E/Z)-isomers of phytoene (150) in an overall yield of 6% referred to 165 (Scheme 38). 

S-methylhept3n-2-one (178) with methyl diethylphosphonoacetate (179) and sodium methoxide followed by reduction with LiAIH4 resulted In the alcohol 180. Treatment of 180 with phosphorus tribromide and triphenylphosphine gave the Cio-phosphonium salt 181. The Wittig reaction with apo-8 -lycopenal (182) and sodium methoxide gave rhodopin (177) in an overall yield of 20% referred to 178 (Scheme 42). 

Discotic phenanthrene derivatives have been prepared by Scherowsky and Chen [39]. Their synthesis is illustrated in Scheme 14. The phosphonium salt prepared by bromination of trimethoxybenzyl alcohol and reaction of the benzyl bromide with triphenyl phosphine was deprotonated and a Wittig reaction with 2,4-dimethoxy-or 2,3,4-trimethoxybenzaldehyde produced the stilbene. Photocyclization gives the phenanthrene, which can be demethylated with boron tribromide and acylated using the Steglich method. 

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