Phosphoranes orthoester

Although Z-alkene isosteres have been obtained from Wittig alkenation reactions of a-amino aldehydes using triphenyl[3-(trimethylsilyl)prop-2-ynylidene]phosphorane (Section 10.5.2.1.2.1), this stereoisomer was always obtained in minor amounts. Z-Selective alkena-tions were obtained using ylides containing dioxolane-protected aldehydes 42 or orthoester-protected carboxylic acid functions. 58 No experimental data were published, however. 

Ethoxy- and diethoxy-vinylidenetriphenylphosphorane add alcohols to the 0=C bond according to the general scheme described above yielding orthoester phosphoranes (equation... 

Because of its enhanced reactivity, diethoxyvinylidenetriphenylphosphorane adds a variety of CH acids, even those which do not react with 0x0-, thioxo- and imino-vinylidenetriphenylphosphorane. Irreversible loss of ethanol from the initially formed Michael adduct gives phosphoranes which are vinylogs of 3-keto ylides if the starting material is an acidic carbonyl compound (equation lOS). CH-acidic carbonyl compounds also may add to diethoxyvinylidenetriphenylphosphorane via the enolate oxygen, thus leading to orthoester phosphoranes which can undergo intramolecular Wittig reaction. ... 

Mixed substituted orthoesters, e.g. (383 equation 180), can be obtained by addition of alcohols to ketene 0,0-acetals. With the aid of phosphoranes containing a ketene O,(7-acetal structure, various orthoesters, e.g. (384) and (385) (Scheme 70) were prepared. " Cyclic orthoesters (386)-(390) (Scheme 71) are formed in cycloaddition reactions of ketene 0,0-acetals with aldehydes, ketones, 7.848 gj.yj cyanides, oxiranes, a-keto esters, o-diketones or ketones (with irradiation), a,3-unsaturated aldehydes and ketones (under pressure or catalyzed by ZnCh), diazoaceto-phenone, 7 diazoacetone and azodicarboxylate. ... 

Orthoesters and pentaalkoxyphosphoranes are the alkylated analogues of tetrahedral and TBP intermediates respectively. Numerous examples of stable orthoesters exist in the literature, and some examples of stable (but reactive) pentaalkoxyphosphoranes are known (Hamilton et ai, 1965 Ramirez, 1968). The true analogues of tetrahedral and TBP intermediates are hemi-orthoesters and hydroxy phosphoranes. Again, a number of examples of stable hemi-orthoesters have been isolated (e.g. [3]) or observed... 

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