Wittig-Type Olefmation

The use of organotitanium compounds in the synthesis of allenes involves mainly Wittig-type olefmation reactions of carbonyl compounds [86] with titanium ylides. The formation of allenes according to the scheme Q + Q + Q was described by... 

In the presence of dibutyl telluride, iodomethyl triphenylphosphonium iodide reacts with aldehydes, in accordance with a Wittig-type olefmation, giving methylenation products. ... 

Stabilized telluronium ylides such as dibutyltelluronium carbethoxy, phenacyl/ cyano- and carbamoylmethylide (easily prepared by the reaction of dibutyl teUurides with the appropriate substituted methyl hahdes, followed by treatment with a base), undergo Wittig-type olefmation reactions with a variety of carbonyl compounds, giving the expected olefins in satisfactory yields (method A). This behaviour is in sharp contrast to that of stabilized sulphonium yhdes, which are inert towards carbonyl compounds. 

Dibutyltelluronium benzylide, generated by treatment of the telluronium salt with potassium f-butoxide, behaves similarly to the above-stabihzed ylides, undergoing Wittig-type olefmations with aromatic aldehydes. 

HORNER WADSWORTH - EMMONS Olefmation Wittig type reaction ot phosphonate stabilized carbamons with aldehydes or ketones to form... 

Because the geometry of the 9-double bond was not clear at that time, Corey et al. 75> tried to prepare the (Z)-9-isomer as well as the ( )-9-isomer of leukotriene-A (78 and 86). In the synthesis of the former isomer the tribenzoyl derivative of D-(—)-ribose (79) was converted in 8 stepy into the optically active epoxyaldehyde 71 and the latter to 72. 72 was olefmated with ylide 82, generated by treatment of the corresponding phosphonium mesylate with lithium diisopropylamide in THF/ HMPA75) (Scheme 15). In the first olefination step 72+82- 78, however, similar to the first method, a A9-isomer mixture was formed. The loss of (Z)-selectivity of the Wittig reaction is due to the use of conjugated unsaturated, i.e. moderate ylides of type 82, and had to be expected because of the mechanism of the Wittig reaction (see Sect. 2). 

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