Wittig products

Reactions of these compounds with phenybsothiocyanate and a number of aromatic and aUphatic aldehydes have been investigated (169,170). Only where R is or CgH CH=CH are the normal Wittig products obtained ... 

Such ylides are unstable and react with carbonyl compounds to give both the Wittig product (p. 545) as well as AsPh3 and an epoxide. However, this very reactivity is sometimes an advantage since As ylides often react with carbonyl compounds that are unresponsive to P ylides. Substituted quaternary arsonium compounds are also a useful source of heterocyclic organoarsanes, e.g. thermolysis of 4-(1,7-dibromoheptyl)trimethylarsonium bromide to l-arsabicyclo[3.3.0]octane ... 

In the following procedure, use is made of the basic character of epoxides. In the presence of phosphonium salts, ethylene oxide removes hydrogen halide and the alkylidenetriphenylphosphorane is produced. If a suitable carbonyl compound is present in the reaction mixture, its reaction with the in situ generated phosphorane proceeds readily to give the Wittig product. 

Wittig product (57). The methylenecyclobutanes (60) were formed from the same phosphoranes and the methylenecyclopropene (59). The formation of the pyran-2-ones may involve the intermediate cyclobutenones (56) as shown. 

Wittig products. However, reactions with aldehydes carrying... 

The Wittig reaction, or its phosphonate modification, can be a very useful reaction in the synthesis of anomerically functionalized precursors to C-nucleosides, particularly when the ring-closure reaction leads to five-membered, anhydro derivatives, and occurs with a high degree of stereocontrol. Zhdanov and coworkers,130 showed that the following five-membered, Wittig products (179-182) were formed from various free sugars and (p-methoxybenzoyl)- and (acetyl-methylene)-triphenylphosphoranes. 

Stabilized ylides react with aldehydes in water to give Wittig products, sometimes with remarkable acceleration.260 For example, pentafluorobenzaldehyde reacts with ester-stabilized ylide, Ph3P=CHC02Me, at 20 °C in 5 min in 86% yield, with 99 1 E Z-selectivity. Water s ability to stabilize the polar transition state of the reaction, and its participation in the reaction (as determined by deuterium exchange), are discussed. 

Table 1. Influence of lithium salts on the (Z/fJ-ratio of Wittig products (solvent benzene)...

Five-membered Wittig Products (188) from D-Glucose and its Derivatives... 

The only normal Wittig products [61 and 63 (R = OMe)] from the reaction of glyceraldehyde with acetyl- and (p-methoxybenzoyl)-methylenephospborane, respectively, were identified during these investigations [Refs. 7(a) and 7(f), respectively]. 

With a stabilized ylide, 2-acetyl-5-bromothiophene yields about 25% of the normal Wittig products as a mixture of (E)- and (Z)-isomers. The phosphonium salt 182 is the other product (32% yield) of the reaction (Scheme 47). 

Generally, ester enolates of structure (202 R = M, R = Oalkyl) rearrange via a 3,3-shift, whereas the corresponding amide enolates (202 R = M, R = N(alkyl)2) and acid dianions (202 R = M, R = OM) prefer the 2,3-pathway (equation 20). Both pathways have been observed with ketone enolates (202 R = M, R = alkyl). With substrate (179), Koreeda and Luengo observed only traces of Wittig rearrangement product (205), except for the lithium enolate, where (205) accounted for up to 20% of the reaction mixture (equation 21). ° Thomas and Dubini, however, reported predominant formation of 2,3 Wittig products (207) and (209) under base treatment of ketones (206) and (208) (equation 22). ... 

The readily available aldehyde (118) has served as a suitable precursor for a number of 6-endo-( al-kenyl)bicyclo[3.1,0]hex-2-enes. For example, treatment of (118) with PhsP —CHC02Me gives a mixture of die bicyclic diene esters (121)-(123) (Scheme 18). In view of the stereospecific nature of the Cope rearrangement process (vide supra), it is highly likely that (121) and (122) are derived by bond reorganization of the initially formed Wittig products (119) and (120), respectively. The ratio of (121) (122) is, therefore, a reflection of the (expected) fact that the Wittig reaction produces primarily the rrans-a,P-un-saturated ester (119). The diene ester (123) is, presumably, formed by partial isomerization of (121) and (or) (122). 

N-acylaziridines occurs on treatment with triphenylphosphine with the formation of the ylides (67), thereby providing a route to a-substituted primary allylic amines via their in situ reactions with aldehydes.The reactions of triphenylphosphine with epoxides in phenolic media provide a new access to the vinyl-phosphonium salts (68). A route to 1-hydroxyalkylphosphonium salts is provided by the reactions of trimethyl- or triethyl-phosphine with aldehydes or ketones in the presence of anhydrous acids. The related 1-trimethylsiloxyalkylphosphonium salts can be isolated in the presence of trimethylsilyl chloride.Further Wittig-products have been isolated from the reaction of 1,2,3-indanetrione with triphenylphosphine, and, in related work, it has been shown that furoin is deoxygenated on heating with triphenylphosphine to form 2-furfuryl(2-furyl)ketone and triphenylphosphine oxide.The structure of the blue product from the reaction of triphenylphosphine with 2,3,5,6-tetracyano-7-oxabicyclo(2,2,1]hepta-2,5-diene has now been shown by X-ray techniques to be the stable ylide (69). ... 

SCHEME 7.96 Selenium-mediated cyclization of Wittig products. 

Another example of the use of Wittig products is shown in Scheme 11.35. Two total syntheses by Chida s group made use of the Overman rearrangement to introduce nitrogen functionality on a tertiary carbon atom [109,110]. This was illustrated by the construction of lactacystin 143, a potent neurotrophic factor found in Streptomyces culture broth. 

There are many other rearrangements utilizing Wittig products as substrates. These, and others, will continue to be addressed in later sections of this chapter because of their diversity and general applicability to carbohydrate chemistry. 

---