Phosphoranes with double bonds

With the aid of a Knoevenagel condensation and a Staudinger reaction, 2-azidocyclopent-l-ene 1-carbaldehydes (195) can be converted into suitable products for a cyclopenta[b]pyridine synthesis [89JCS(P1)1369], as shown in Scheme 72. In order to bring the ester carbonyl function and the imino-phosphorane group into close proximity suitable for cyclization reactions, the 1,3-diene system 196 should possess the s-cis conformation. Furthermore, the exocyclic double bond should show a cis conformation. To achieve... 

Involvement of 625 in a Wittig reaction with 4-carboxybutyl(triphenyl)phosphorane dissolved in dimethyl sulfoxide generates a mixture of double bond isomers rich in 623. 

In contrast to the P=C bond of phosphaalkenes (Section 8.2.2), double bonds between phosphorus and a heteroatom have not been used much as dipolarophiles. Most of the studies reported so far were devoted to the reactivity of the (X )P=N bond of iminophosphanes. Amino(iminophosphanes) react with diazoalkanes to form 4,5-dihydro-3//-l,2,3,4-triazaphospholes or, by N2 loss from the latter, to imino(alkylidene)-X -phosphoranes (5,238). With P-halogeno-(arylimino)phos-phanes 174 and the appropriate diazo compounds, 3//-l,2,3,4-triazaphospholes 175 (167) and 176 (239) (Scheme 8.40) were obtained as the major products after cycloaddition and eliminative aromatization. 

Selective epoxidation of the isolated double bond (Equation 35) in the ester 79, prepared from citronellal and triphenyl(ethoxycarbonylmethylene)phosphorane, followed by treatment with Na2PdCl4 and v/-butyl hydroperoxide gives the bis-ether 80 <1994CC903>. 

However, the reactivity of g-ketophosphoranes 8 with R =Ph or Me is different from the alkylidene phosphoranes 5 owing to the double bond character of the C-g bond due to an important... 

Hydroxybenzofuran-5-carbaldehydes 682 can react with carbethoxymethylenetriphenyl-phosphorane 683 to afford biologically active furanocoumarins 684 in high yield. The reaction proceeds via Wittig reaction of the phosphorane 683 with the aromatic aldehyde to form the intermediate cinnamate, which then undergoes isomerization of the double bond and lactonization to furnish the desired furanocoumarins 684 (Scheme 165) <1995T3087>. 

The possibility of synthesizing pure (Z)-olefins by means of reactive salt-free ylides predestinates the (Z)-double bond at C-5 in e.g. 106 and 107 to be introduced into the corresponding aldehyde via the Wittig reaction. The ( )-configurated double bond at C-13 (105,106 and 107) with its vicinal hydroxy group was frequently formed by the phosphonate method (cf. Chapter 2). In some cases, however, it could also be obtained by ( )-selective Wittig olefination using resonance-stabilized phosphoranes. 

The ( )-double bond at C-6 of a racemic JH1 is formed by condensation of ethyl ketone 178 with phosphorane 194 in the New Brunswick synthesis. In this synthesis about 40 % (Z)- and 60 % ( )-isomer were formed which were separated by preparative gas chromatography. Compound 182 was converted into ( )-JHl by use of the phosphonate method 148,149) (Scheme 36). Applying two Wittig reactions and a phosphonate condensation the same authors could prepare racemic JH2 171148,149) (Scheme 36). 

A synthesis elaborated by the BASF affords directly pear ester 232 with about 85 % of the isomer with (Z)-geometry of the C-4 double bond. This synthesis involves reaction of phosphorane 93 with fumaric ester aldehyde 233 (sodium amide technique). The latter is formal by ozonolysis of sorbic ester163) (Scheme 45). Principally, the same method was published by a Belgian research group, leading to 70% of (2 ,4Z)-2,4-isomer 164>. 

A synthesis of the pear ester worked out in our laboratory uses the (Z)- and the ( )-stereoselective Wittig olefination for the generation of the two double bonds in the molecule 232165). Reaction of 2-ethoxyvinyl-triphenylphosphonium bromide 234 with one equivalent sodium ethanolate yields 2,2-ethoxyethylene-triphenyl-phosphorane 235166,167). (Z)-Stereoselective olefination of 235 with hexanal 236... 

Treatment of 2-acetyl-5-bromothiophene with excess methylidenetriphenylphosphorane has yielded some unexpected results <2000T7573>. The major product (46%) was the thienofuran 180 along with the phosphorane 181 (22%). The suggested mechanism for the formation of these two products involves initial nucleophilic displacement of the bromine, followed by addition of a second molecule of the ylide to the conjugated double bond of the 2-acetylthiophene unit (Scheme 46). 

Enantioselective Hydrosilylation of C=0 Double Bonds in Ketones. The use of Rh-phosphorane catalyst systems to promote asymmetric hydrosilylation of prochiral ketones with silanes of the type RsSiH has met with only limited success. Thus, hydrosilylation of acetophenone with Ph2SiH2 promoted by [Rh(COD)Cl]2-(S,S)-DIOP catalyst afforded the (S)-(-)-phenylmethylcarbonyl with an optical yield of 32% ee. Similarly, the use of a Rh-NORPHOS catalyst in this reaction proceeded with an optical induction of only 16% ee. S... 

A synthetic approach toward tunicamycin 267, based on this principle, has been reported. Tunicamycin shows a direct carbon link between C6 of a galactosamine residue and C5 of a uridine moiety. The formation of this link has been carried out by Wittig reactions on model compounds using Secrist s phosphorane [181]. As shown in Scheme 11.58, the phosphonium salt 263 was treated with lithium hexamethyldisilazane to generate the phosphorane, which was reacted with aldehyde 264. Reduction of the double bond and benzyl hydrogenolysis of 265 was followed by acetylation to provide the model compound 266. 

Examples of substrates [23] with heteroatom double bonds (C = X) are presented in the rosette of Scheme 5. Here we feature the efficient denitrogenation of diazoalkanes, diazoquinones, the hydrazone of benzil, and benzophenone oxime into the respective carbonyl products [23]. On the other hand, the bis-oxime afforded furoxane, while the phosphorane led to methyl maleate on elimination of triphenylphosphine oxide [23], Thiones are oxidized to the corresponding S-oxides and the iodonium ylide yields the labile w c-trione [23],... 

It is sometimes desirable to avoid the use of alkoxide during the formation of C—C double bonds in the synthesis of / -lactams [3324]. An iminophosphorane (formed in situ from an azide and triphenylphosphine) reacts with a carboxylic ester group in a neighbouring malonylidene side-chain to form this ring [3792]. The classic Wittig reaction has been extended to the synthesis of 2-vinylindoles [3603], and other indoles have been obtained from isothiocyanato phosphoranes [3297]. 

Witting s cw-olefination could readily be used for the preparation of derivatives containing several double bonds of Z- and -configuration. Thus, for example, the reaction of the unsaturated phosphoran (9) with 9-acetoxynonanal yields (Z, )-9,12-tetradecadienyl acetate (10) (Vostrowsky et al., 1973). This compound is the sex attractant of several storehouse pests. 

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