Wittig Routes

Tellurophosphoranes, obtained through a transylidation reaction between tellurenyl halides and phosphoranes, react with aldehydes to give the expected vinylic tellurides as an E Z isomeric mixture (method a). One other methodology involves the treatment of equimolar amounts of phenyl tellurenyl bromide and phosphonium salts with t-BuOK followed by an aldehyde (method b). Under these lithium-salt-free conditions, (Z)-vinylic tellurides are the main products.  

Method a. (ji-Styryl) phenyl telluride. To a solution of methyltriphenylphosphonium iodide (0.405 g, 1 mmol) in dry THF (4 mL) at room temperature was added dropwise n-BuLi (2 mmol). After stirring at room temperature for 20 min the solution was cooled to -78°C and a solution of PhTeBr (0.28 g, 1 mmol in 2 mL THF) was added, followed by benzaldehyde (0.16 g, 1.5 mmol). The temperature was raised to room temperature and stirred for 3 h. The solvent was removed under vacuum and the residue incorporated on Si02 and purified by flash column chromatography (Si02/hexane) giving an oil. Yield 0.185 g (60%). E/Z=2. 

Symmetrical divinyl tellurides VI have been prepared via a ylidation reaction involving the treatment of bis-phosphonium halotellurate I with excess base and then with aldehydes in THF at -1S°C (method c). ° 

The disproportionation of III into IV and V can be rationalized on the basis of the known reaction of phosphoranes with halogen sources giving a-haloalkyl-phosphonium salts. 

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Either route is feasible and indeed styrene has been prepared from both combinations of reactants Typically there will be two Wittig routes to an alkene and any choice between them is made on the basis of availability of the particular starting materials... 

Kochetkov and coworkers78,121 synthesized KDO by a Wittig route, starting from 2,3,4,5-tetra-O-acety 1-aZdehydo-D-arabinose (91), as follows. The reaction of 91 with (fert-butoxyoxalyl)methylenetriphen-... 

Various -substituted vinylphosphine oxides (14) have been made by the Wittig route, and found to be /raws-isomers.17 The geometry of these oxides is believed to be... 

In the present review the ring systems containing one heteroatom are considered first, except for P-lactams which are given a special section at the end. Interest in azetidines continues to be stimulated by the discovery of the potentially useful trinitro derivative. The requirements for the stereoselective synthesis of substituted oxetane are being explored and derivatives of aluminium are useful in the stereoselective routes to oxetanones. The preparation and subsequent pyrolysis of oxetanones is suggested as an alternative to the Wittig route to olefins. Stereoselective routes to thietanes and thietane 1 -oxides are mentioned. 

Application of the Wittig route through thermolysis of phosphorane 372 yields the trinem analogue 373 in low to moderate yield (Equation 49) <1996BML525>. 

Reactive 17 -phosphaalkene tungsten complexes such as 66, which are accessible by the phospha-Wittig route, are appropriate candidates for cycloadditions, as illustrated in Scheme 16.20 22... 

To use the Wittig reaction in synthesis, you must be able to determine what carbonyl compound and Wittig reagent are needed to prepare a given compound—that is, you must work backwards, in the retrosynthetic direction. There can be two different Wittig routes to a given alkene, but one is often preferred on steric grounds. 

Stork has recently published a Wittig route to (Z)-iodoalkenes, see G. Stork and K. Zang, Tetrahedron Lett., 1989,30, 2173. 

A study comparing, under a range of conditions, diene synthesis by the two alternative Wittig routes, allylic ylide-saturated aldehyde (route 1) and reactive ylide-a,P-unsaturated aldehyde (route 2), has been reported. 9 por the system chosen (Scheme 2) the reactive ylide-a,P-unsaturated aldehyde route is clearly preferred in that the stereochemistry of the new double bond can be controlled more easily and there is little or no isomerisation of the double bond already present in the aldehyde. A route to symmetrically substituted polyenes containing an odd or even number of double bonds has been reported (Scheme 3).20 The Wittig reactions of ylides derived from... 

The Wittig reaction forms both double bond with a nearly free choice of which end comes from the alkyl halide (7) and which from the carbonyl compound (8). Hence the exo-olefin (9), all but impossible to make by elimination, is easily made by either Wittig route. Route (a) is perhaps easier as cyclohexanone is easier to handle than formaldehyde. 

Total synthesis of asperlicin C and E via condensation route by Bock et al. is well known [76-79,243]. The first total synthesis of asperlicin D via both cyclodehydration and intramolecular aza-Wittig routes has been fulfilled very recently by Al-Said et al. [244], and the iminophosphorane intermediates having secondary amide protons have been shown to provide a one-step entry to quinazolino[l,4]benzodiazepine ring system like asperlicin D (a positional isomer of asperlicin C). A facile route to N-benzylsclerotigenin has been reported also by this group [245]. On the other hand, total synthesis of circumdatin F (19) (and circumdatin C) via dehydrative cyclization (or ben-zoxazine) route has been also fulfilled by Bergman et aL [246]. 

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