Ketals alkaline conditions

As was observed in 1888 by Meunier,24 during a study of the properties of tribenzylidene-D-mannitol, acetals and ketals are readily hydrolyzed by aqueous acids, but are usually stable under alkaline conditions. In this... 

The free optically active cyanohydrine was prepared after chromatographic separation or fractionated crystallization of the ketal 300 with 1-R-cis-caronaldehyde hemiketal 155 [634,635,636] and subsequent Uberation by acidic treatment (Reaction scheme 212) preferentially with BCI3 at —30 °C [637]. It is not stable under weakly alkaline conditions. The (S)-4-fluoro-analogue 301 was similarly prepared [638]. 

Formation of Acetals and Ketals under Alkaline Conditions... 

A case in point is cyclohexane-1,3-dione. This may be converted into a mono-dioxolan [227, 228], or into a mono-dithiolem or -oxathiolan [228]. The dioxolan is partially cleaved to the hydroxyethyl enol ether (with which it is in thermal equilibrium) by very dilute base, and it reverts to the diketone under more strongly alkaline conditions [227]. The free ketone group in these ketals can be made to react vtith various nucleophilic reagents such as phenyl lithium [228] and cyanide ion [227] and can also be subjected to the Reformatski reaction [227], but the yields in these reactions appear to be rather low. 

On Birch reduction under mild conditions, the ethylene ketal (193) obtained from the ketone (140) gave predominantly the anti-trans-dihydro-ketal (194) contaminated with a small amount of the anti-cis- and syn-cis-isomers. Further Birch reduction led to a mixture of the ketones (196) and (197) with the former predominating. Hydrogenation under alkaline conditions enabled the D-homo ketone (195) with a saturated 13,14-bond but with retention of the A -bond to be obtained. The same product can also be obtained directly from the ketal (193) without isolating the ketones (196) and (197) formed as intermediates. 

The side chain in spirostan derivatives (44) can undergo certain exchange reactions which are characteristic for this skeleton. In general, this spiro-ketal moiety is quite stable under alkaline or neutral conditions, but in acidic media it undergoes a reversible opening of ring F (45 and 46). In... 

Many functional groups are stable to alkaline hydrogen peroxide. Acetate esters are usually hydrolyzed under the reaction conditions although methods have been developed to prevent hydrolysis.For the preparation of the 4,5-oxiranes of desoxycorticosterone, hydrocortisone, and cortisone, the alkali-sensitive ketol side chains must be protected with a base-resistant group, e.g., the tetrahydropyranyl ether or the ethylene ketal derivative. Sodium carbonate has been used successfully as a base with unprotected ketol side chains, but it should be noted that some ketols are sensitive to sodium carbonate in the absence of hydrogen peroxide. The spiroketal side chain of the sapogenins is stable to the basic reaction conditions. 

Cyclic acetal and ketal systems constitute useful protecting groups in that they are stable in most neutral and alkaline media [30, 126, 127]. Thus they are unaffected by the basic conditions required for alkylation and acylation they are also stable to oxidation by CrOa-pyridine, periodate, Pb(OAc)4, AgjO, alkaline permanganate and under Oppenauer conditions, and to reduction by NaBH4, LLA1H4, Na-Hg and, with the exception of benzylidene acetals, to catalytic hydrogenation. 

The stability of dialkyl-acetals and -ketals to neutral and alkaline reaction conditions generally parallels that of dioxo-lans. Unlike the latter, however, they may decompose to enol ethers at higher temperatures, e.g., during Wolff-Kischner reduction [31]. 

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