Wittig Olefmation Reactions

The lithium etiolate of acetaldehyde DMH has recently been utilized in the opening reaction of the ot-epoxide obtained by DM DO oxidation ofenol ether 142, to provide hemiacetal 143 after mild oxidative acid hydrolysis. The protected carbonyl functionality was subsequently used for the introduction of the trans enyne chain through a Wittig olefmation reaction to provide alcohol 144, which was then transformed into (+)-laurenyne (Scheme 8.37) [71]. 

Besides nucleophilic attack, cyclative cleavage can be eaffected for example by Stille reactions [120], Wittig olefmation reactions [177], Wittig-Horner [178, 179] or metathesis reactions [180-183]. For more details of C-C-bond formation, see Section 3.3.2. 

Maryanoff BE, Reitz AB. The Wittig olefmation reaction and modifications involving phosphoryl-stabilized carbanions. Stereochemistry, mechanism, and selected synthetic aspects. Chem Rev 1989 89 863-927. 

Vedejs, E. Peterson, M. J. The Wittig Reaction Stereoselectivity and a History of Mechanistic Ideas (1953-1995) Advances in Carbanion Chemistry, 1996, 2, 1-85. Vedejs, E. Peterson, M. J. Stereochemistry and Mechanism in the Wittig Reaction Topics in Stereochemistry 1994,21,1-157. Maryanoff, B. E. Reitz, A. B. The Wittig Olefmation Reaction and Modifications Involving Phosphoryl-Stabilized Carbanions. Stereochemistry, Mechanism, and Selected Synthetic Aspects Chemical Reviews 1989, 89, 863-927. 

In a similar way, carbocycles having a quaternary center could be obtained from acyclic unsaturated 1,3-dicarbonyl compounds [206]. Other combinations are the domino hydroformylation/Wittig olefmation/hydrogenation described by Breit and coworkers [207]. The same group also developed the useful domino hydroformyla-tion/Knoevenagel/hydrogenation/decarboxylation process (Scheme 6/2.14) [208] a typical example is the reaction of 6/2-66 in the presence of a monoester of malonic acid to give 6/2-67 in 41 % yield in a syn anti-ratio of 96 4. Compounds 6/2-68 and 6/2-69 can be assumed as intermediates. 

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... 

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. 

Barbas developed this procedure further by introducing an asymmetric three-component Michael reaction that should be applicable to many other conjugate addition reactions. He used a Wittig olefmation to prepare, in situ, an a,P-unsatu-rated ketone that subsequently underwent a conjugate addition with malonates (Scheme 21) [94]. The rate of the conjugate addition process was observed to be considerably faster than the analogous reaction reported by Jprgensen which was attributed to the presence of triphenylphosphine oxide within the reaction mixture. 

Trans P-sinensal 441 is obtained from the aldehyde 438 prepared by selective ozonolysis of trans-famesene 437. Wittig olefmation of 438 with 1-formylethylidene-phosphorane 440 gives trans P-sinensal 441. Reaction of 438 with 1-ethoxycarbonyl-ethylidene-triphenylphosphorane 439 yields the ethyl ester 442, from which 441 can also be obtained 237 (Scheme 77). Furthermore, a working group of the BASF synthesized a-sinensal 443 and p-sinensal 441 using Wittig reactions 238). 

The reaction of the C20 ylide (45 a) with the unsymmetrical C10 dialdehyde, 2,6-dimethyl-2,4,6-octatriene-1,8-dial (49, R = H), shows a remarkable regioselectivity. Only one of the two aldehyde groups participates in a Wittig olefmation, leading uniformly to p-apo-8 -carotenal (48) 4 ). The Wittig reaction between the C20 ylide (45a) and ethyl-2,6-dimethyl-8-al-octatrienoate (49, R = OQHj) correspondingly yields an ethyl-p-apo-8 -carotenate (48, R = OC2H5)s8). 

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

Triphenylphosphine, the indispensable auxiliary for Wittig olefmations, is mass-produced by direct condensation of phosphorus trichloride with phenylsodium. The latter is generated in situ from chlorobenzene and sodium in a kind of Wurtz-Fittig reaction. Congeners such as the methoxymethoxy-armed phosphine 49 or tri-2-furylphosphine (50) can be conveniently prepared by metalation of the (het)aromatic precursor and its subsequent condensation with phosphorus trichloride or a trialkyl phosphite (Scheme 1 0). " ... 

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