Phosphonate ylids, reaction with aldehydes

Adducts from various quaternary salts have been isolated, in reactions with aldehydes, a-ketoaldehydes, dialkylacylphosphonates and dialkyl-phosphonates, isocyanates, isothiocyanates, and so forth (Scheme 15) (36). The ylid (11) resulting from removal of a Cj proton from 3.4-dimethyl-S-p-hydroxyethylthiazolium iodide by NEtj in DMF gives with phenylisothiocyanate the stable dipolar adduct (12) that has been identified by its NMR spectrum and reactional product, such as acid addition and thiazolidine obtention via NaBH4 reduction (Scheme 16) (35). It must be mentioned that the adduct issued from di-p-tolylcarbodiimide is separated in its halohydrogenated form. An alkaline treatment occasions an easy ring expansion into a 1,4-thiazine derivative (Scheme 17) (35). 

More general methods depend on Wittig reactions with functionalised ylids. The ylid24 from 116 and the lithium derivatives of 117-120 all react with aldehydes and ketones to give enol ethers that can be hydrolysed to chain-extended aldehydes. Yields with the ylid from 116 are not always wonderful and the phosphonate ester 120a with a chelating substituent generally does better.25... 

A.iu. Phosphine Oxides and Phosphonate Esters. Many extensions of the Wittig reaction have been introduced that improve or modify the reactivity and/or stereoselectivity of the ylid. Horner et al. showed that a-lithiophosphine oxides such as that derived from 552 react with aldehydes or ketones to give a p-hydroxy phosphine oxide (553) as an isolable species. Subsequent treatment with base liberates the alkene, (554). Wadsworth and Emmons modified the Horner reaction to use phosphonate ester derivatives such as... 

The phosphonate XXIV was converted to the respective ylid with sodium hydride and yielded on reaction with ketones or aldehydes the corresponding Schiff bases XXV. These products, in contrast to the aldehyde or ketimine adducts with their intramolecular hydrogen bonds, can easily be converted to the unsaturated carbonyl compound in good yield. 

Ylids are also convenient sources of carbanions and they react with aldehydes and ketones to give alkenes. With a suitable reactant, alkenyl amino acids can be prepared using this approach. One example used a phosphonate ester ylid in a Homer-Wadsworth-Emmons reaction 12 with amino aldehyde 1.189. The product... 

Other functionality can be incorporated into the ylid prior to reaction with an aldehyde or ketone. Aldehyde 1.195, for example was converted 1.196 by reaction with a fluorine-bearing phosphonate ylid. 5 The ester group was reduced to an aldehyde moiety (see 1.197) with diisobutylaluminum hydride. This allowed final conversion to 5-(N-Boc amino)-4-fluoro-6-phenylhex-3E-enoic acid (1.198), in four steps (3% overall yield the first and second steps gave a combined yield of 14% and step five proceeded in 28% yield). 

The extra stabilisation makes the ylid rather unreactive and phosphonate esters 91 are often used instead of phosphonium salts in these reactions. Treatment with a base (NaH or RO is often used, BuLi will certainly not do) gives an inherently more reactive enolate anion 92 rather than an ylid. These Horner-Wadsworth-Emmons reagents (H WE as we shall call them, though they go under many other names) react with ketones as well as aldehydes and the product is normally the E-alkcnc16 93. 

FIGURE 10.16 Synthesis of higher sugar enones hy reaction of sngar aldehydes with stabilized sugar ylids or sugar phosphonates. 

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