Wittig conditions

Although of little general use under normal Wittig conditions, tetra-n-butyl-ammonium iodide aids the reaction between polymer-bound benzylphosphonium salts and aromatic aldehydes with yields generally >90% [17]. 

In one report the Tebbe reagent has been found to be advantageous to ordinary Wittig conditions.342... 

Scheme 2.6 shows the synthetic strategy for the constmction of BTX A (35) (Scheme 2.6a), in bond disconnection (Scheme 2.6b), and retro-synthetic (Scheme 2.6c) formats. This strategy led to the expectation of using D-glucose 38 (Scheme 2.6c) and D-mannose 10 (Scheme 2.6c) as starting materials for the constmction of the requisite BCDE 36 (Scheme 2.6b) and GHIJ 37 (Scheme 2.6b) advanced intermediates, respectively. The union of 36 and 37 under Homer-Wittig conditions was... 

During the stereospecific total synthesis of (7S,15S) and (7R, 15S)-dolatrienoic acid by G.R. Pettit et al., the C7-C10 and C11-C16 subunits were coupled using the highly ( )-selective Wittig-Schlosser reaction The traditional Wittig conditions resulted in a mixture of alkenes in which the (Z)-stereoisomer was predominant. When the Schlosser conditions were applied, the stereoselectivity was reversed in favor of the ( )-alkene. 

The basic nature of methylenetriphenylphosphorane caused D-homoannulation of 17a-acetyloestr-4-en-17/3-ol (294) before effecting a Wittig condensation the mixed 17-methylene-D-homoandrostanes (295) were formed. When the 17/3-OH group was protected by acetylation, an intramolecular Claisen condensation occurred in polar solvents under Wittig conditions, giving the spiro-lactone (296) and a... 

Miscellaneous Reactions. So-called Oxidation-Wittig conditions , where a primary alcohol is oxidised using manganese dioxide in the presence of a stabilised ylide, have been applied for the first time to unactivated alcohols, producing a, 3-unsaturated esters in high yields (Scheme 18). Fluorenyl-... 

Triphenylphosphonium ylides attached a to a cyclopropane react with aldehydes and ketones under standard Wittig conditions to give 1-cyclopropylalk-l-enes in good to excellent... 

A more recent synthesis by our group utilized Mukaiyama s trimethylsilyl cyanide addition to aldehydes as the key step to introduce the carboxy functionality [33,36], This approach also required the preparation of (Z)-5-pentadecenal as the key intermediate, Fig. (12). In this case, commercially available decyl aldehyde was coupled with 4-carboxybutyltriphenylphosphonium bromide under Wittig conditions, resulting in a 10 1 mixture of the known (Z)- and ( )-5-pentadecenoic acids. The acids were then reduced to the desired (Z)-5-pentadecenal via (Z)-5-pentadecen-l-ol, a known pheromone. Addition of trimethylsilyl cyanide to (Z)-5-pentadecenal, under triethylamine catalysis, yielded... 

The basic hydrolysis of the bisphosphonium salt (80) gave a phosphonium salt (81) and a phosphine oxide (82) in a ratio which depended upon the acidity of the solvent used. The reaction is presumed to proceed via a carbanion from which the phosphonium salt arises by protonation by the solvent and the phosphine oxide by phenyl migration from phosphorus. The mechanism of the unexpected formation of 2-methyl-2 T-benzopyran from the phosphonium salt (83), under normal Wittig conditions, has been discussed in some detail. ... 

Lithium Halide-catalyzed Reversal via Betaine Lithium Halide Adducts. This is the most common mechanism for loss of stereospecificity under Wittig conditions and often contributes to the stereochemical outcome of bezaldehyde reactions in the presence of lithium ion. The process is most facile for ylides containing anionic (alkoxide, carboxylate, or amido) substituents. 

Z)-Allylic selenides 179 are formed in 58—69% yield under Wittig conditions using salt-free alkylidene triphenylphosphorane. A-Chlorosuccinimide/carbamate-promoted [2,3]-sig-matropic rearrangement affords allylic amines 180 in 45—64% yield. The olefin is transformed to an acid by conversion to an aldehyde followed by Jones oxidation. The resulting D-amino acids 181 are produced in 58—72% yield with enantiomeric excess values of 78—84%. 

The preparation of a,j5-unsaturated ester 565 is easily accomplished by treatment of 464 with either trimethyl phosphonoacetate under Homer—Emmons conditions [178] or with the corresponding phosphorane under Wittig conditions [179]. Reduction of the ester to alcohol, conversion to chloride, and displacement with tributyltin lithium gives the allylstannane 566. Reaction of this stannane with aldehydes under Lewis acid catalysis furnishes 2-vinyl-l,3-diol derivatives 567 or 568, the stereochemistry of which depends on the nature of the Lewis acid [179]. The best examples are shown in Scheme 80. 

Trialkylsilyl groups, on the other hand, migrate very readily under Wittig conditions. Benzylthiotrimethylsilane (2) on treatment with tert-butyllithium is rapidly converted to a-trimethylsilyl toluene-a-thiol (3) in almost quantitative yield. 

Isopropylldene-D-mannose and 2,3,, 6-tetra-O-benzyl-D-glucose have been converted to the octonic acid derivatives ( 7) and ( 8), respectively. Under Wittig conditions the former cyclized partially to give a furanoid C-glycoside (see Chapter 3) and the latter, when made by the Knoevenagel-Doebner conditions, isomerlzed... 

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