Ketal Wittig reaction

EthytidenationThe ketone (2) undergoes Wittig reaction with ethylidene-triphenylphosphorane in yields of only about 40%, with recovery of 2. The difficulty may be enolization promoted by the basic ylide. Use of the Grignard reagent from a-chloroethyltrimethylsilane results only in reduction of the carbonyl group. The problem is solved by use of 1, which reacts with 2 to give the internal ketal 3. Hydro-... 

Wittig reaction, 151 —, cyanohydrin, 149, 324 —, ketal, bromination, 168 —, oxime, Beckmann rearrangement, 343... 

Wittig reactions of a-alkoxy aldehydes and sugar lactols, such as pentose ketal (48), with stabilised ylides usually proceed with low ( )-selectivity. However, Harcken and Martin have discovered that treatment of these aldehydes with (methoxycarbonylmethylene)tributyl phosphorane (49) and a catalytic quantity of benzoic acid produces the heptenonate (50) with a E Z ratio of 95 5. The stereoselectivity of the reactions between aldehydes and spirophosphoranes (51) has been examined and the phosphoranes found to favour the formation of (Z)-a,p-unsaturated aldehydes and amides. ... 

Strain can exert other influences on molecules. l-Aza-2-adamantanone (126) is an extreme case of a twisted amide." " The overlap of the lone pair electrons on nitrogen with the 7i-system of the carbonyl is prevented. In chemical reactions, 126 reacts more or less like a ketone, giving a Wittig reaction (16-44) and it can form a ketal (16-7). A twisted biadamantylidene compound has been reported." ... 

The C-10 vinyl steroid (362) obtained by Wittig reaction on the 19-aldehyde has been converted to the 5a-bromo-6) -hydroxy-derivative which, on oxidation with lead tetra-acetate, produced the 19-methylene-6, 19-oxide which was then reduced quantitatively by zinc-acetic acid to the 10j8-acetyl compound (363). An analogous 10) -acetyl compound (364), whose formation involves a 1,5-hydride shift, was obtained along with some of the desired 19-hydroxy-19-methyl steroid on attempted base hydrolysis of the 19/ -19-acetoxy-19-methyl-androstanedione (343). The main product of the reaction however, was the ketal (365), or, if the reaction mixture was neutralised before work up, the hemi-acetal... 

I- ol, and elaborated by conjugate addition of nitrile, ketal formation, conversion with diisobutylaluminium hydride to the hydroxyaldehyde (129) and then the sequence Wittig reaction, alcohol oxidation with chromium trioxide—sulphuric acid in dimelhylformamide, ketone reduction and ketal hydrolysis to give (1)-... 

Ketals are converted to allyl vinyl ethers after Wittig olefination [14]. Wittig reactions permit the incorporation of various substituents into the allyhc terminal of the ether. Enol ethers have been conveniently prepared by cleavage of acetals with various Lewis acids. Cleavage of the ketals with the Lewis acid, for example, triethylsilyl triflate, in the presence of diisopropylethylamine in refluxing 1,2-di-chloroethane afforded the enol ethers (Eq. 3.1.9). The resulting enol ethers were then heated to effect the Claisen rearrangement without isolation. 

Despite many attempts 12 failed to undergo the Wittig reaction necessary for the introduction of the double bond between C10 and Cl 1. As this may have been due to the acidic hydrogen at C2, reduction of the ester functionality to an alcohol was carried out, as shown in Scheme 3. The methyl ketone 13 was treated with ethylene glycol and then reduced with lithium aluminum hydride, at -15 0 to give the alcohol 14. Hydrochloric acid removed the ketal and protection under the usual conditions with t-butyidimethylsilylchloride gave 15 in an... 

Nicolaou et al. have prepared a library of 22 Sarcodictyin-analogs by attaching core structure 2 onto solid phase using polymer-bound Wittig-ylide 1 (Scheme 1) [74]. The OH-groups of the core scaffold were decorated by alkylation or converted to carboxylic acid or amino functionalities which could be derivatized as esters, carbamates, or secondary amines. Cleavage off the soUd phase was performed via trans-ketalization, a reaction step which was exploited to increase the diversity by using different kind of alcohols. 

Asymmetric synthesis of a chroman. Solladie and Moine have effected an en-antiospecific synthesis of the chroman-2-carboxaldehyde 7, a key intermediate in the synthesis of a-tocopherol, from (R)-( +)-l. The phosphonate 2, derived from 1, undergoes a Wittig-Horner reaction with the dimethyl ketal of pyruvaldehyde to afford the optically active vinyl sulfoxide 3. Condensation of the aldehyde 4 with the lithio derivative of 3 affords, after silyl deprotection, the allylic alcohol 5 as the only diastereoisomer. This... 

Ketene diethyl ketals. These useful intermediates can be obtained by a Wittig-Homer reaction of the anion of 1 with aldehydes and ketones. Higher yields are obtained when the adducts are isolated and then treated with potassium f-butoxide.2... 

The last example illustrates one advantage of this method the enol ether function of 1 can be hydrolyzed without effect on a ketal grouping. Another advantage is that the intramolecular Wittig-Homer reaction proceeds without isomerization of the initially formed cyclopentenone. 

The Wittig reagent presumably underwent reaction with the ring-opened form of ketal 103 to provide the intermediate enoate 104. Dipolar cycloaddition of the pendant azide across the n-bond then produces triazole 105 that undergoes a subsequent dipolar cycloreversion reaction to give diazoamine 106. 

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