Wittig reagents described

The /3-lactam carbonyl group transformations have been utilized successfully in the synthesis of azetidine derivatives. The reduction of the carbonyl group in azetidin-2-ones yielding azetidine derivatives is described in Section 2.01.2.8.10. Treatment of jV-BOC-protected 4-(trifluoromethyl)azetidin-2-one 229 with a stabilized Wittig reagent yielded the azetidines 28 and 230 (Equation 61) <20030L4101>. 

Taylor and Martin (1019, 1020) have described a new procedure for the direct introduction of alkenyl substituents into the pyrazine nucleus 2-chloropyrazine (63) with methylenetriphenylphosphorane (a Wittig reagent) (64, R = H) (from methyltriphenylphosphonium bromide and butyllithium) in 1,2-dimethoxyethane and subsequent treatment with benzaldehyde gave 2-styrylpyrazine (67, R = H). A similar reaction with propionaldehyde gave 2-(but-l -enyl)pyrazine (1020). 

It has been shown that oxiranes can replace 1,2-dihaloalkanes in the synthesis of phosphiranes, provided that appropriate anionic RP reagents are used. The first example was described by Yoshifuji et al. (Equation (54)) <85CL44l>. Subsequently, it has been shown by Marinetti et al. that the so-called phospha-Wittig reagents transform oxiranes into phosphiranes with an inversion of configuration at carbon (Scheme 19) <92Sl57,93OM1207). Decomplexation by 1,2-bis(diphenylphos-phino)ethane is easily achieved in the molybdenum series. Optically active phosphiranes are readily accessible by this route. 

Bestmann and Schulz describe a method for the synthesis of a,B-unsaturated y-keto esters illustrated by the reaction of phenacyl bromide with two moles of the Wittig reagent (2), triphenylphosphineearbomethoxymethylene. Presumably the... 

Wittig and Pohmer prepared this reagent by shaking a solution of o-fluorobromo-benzene in furane with lithium amalgam for four days they found that it is cleaved by mineral acid to a-naphthol. In a simpler route to the reagent described by Fieser... 

The second Gerlach synthesis of 90 involves an interesting permutation of the synthesis just described. f-Butyl 9-hydroxydecanoate (102) was acylated with bromoacetyl bromide and then converted to the stabilized Wittig reagent... 

A reaction of tungsten Wittig reagents to give enamines has been described . 

Further examples have appeared of the use of epoxides as the source of the base in olefin synthesis, among them the synthesis of crocetin dialdehyde (1) shown in Scheme 2, Additional polymeric Wittig reagents have been described and used in olefin synthesis. ... 

Various ways of functionalization of the lactol 205 leading to C-nucleoside analogs were described.Compound 205 readily reacted with carboethoxymethylenetriphenylphosphorane with formation of trans ester 212, whereas the Wittig reagent prepared from pyruvate ester gave a product of an intramolecular Michael reaction (221). Acrylate ester 212 was converted on addition of diazomethane into pyrazoline 213, and further, by successive brom-ination-dehydrobromination followed by acid hydrolysis, was transformed into DL-3-(carboxamido)-4-P-ribofuranosylpyrazole 214. Another precursor of heterocyclic C-ribofuranosides (215) was obtained in the reaction of lactol 205 with... 

Ylides can be described as carbanions that are stabilized by attached positive groups, and hence they are dipolar. Most commonly, the phos-phonium ion is used as the stabilizer and the usual expression of a Wittig reagent is shown as structure 4.47 in Scheme 4.48. The literature sometimes shows this stmcture as a member of a resonance hybrid... 

Other examples have been given of the preparation of branched-chain and extended-chain alkenes by application of Wittig reagents to sugar aldehydes and ketones,and the mass spectra of alkenes derived from 1,2 5,6-di-O-iso-propylidene-D-r/Z o-hexos-3-ulose have been described.Compound (25) was... 

However, this resonance structure (with a C=P double bond) does not contribute much character to the overall resonance hybrid, because thep orbitals on C and P are vastly different in size and do not effectively overlap. A similar argument was used in describing S=0 bonds in the previous chapter (Section 19.3). Despite this fact, the phosphorus ylide above, also called a Wittig reagent, is often drawn using either of the resonance structures shown above. 

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