Transketalization

Acyclic monothio acetals and ketals can be prepared directly from a carbonyl compound or by transketalization, a reaction that does not involve a free carbonyl group, from a 1,3-dithiane or 1,3-dithiolane. They are cleaved by acidic hydrolysis or Hg(II) salts. 

Butylethylidene and 1-phenylethylidene ketals were prepared selectively from the C4-C6, 1,3-diol in glucose by an acid-catalyzed transketalization reaction [e.g., Me3CC(OMe)2CH3, TsOH/DMF, 24 h, 79% yield PhC(OMe)2Me, TsOH, DMF, 24 h, 90% yield, respectively]. They are cleaved by acidic hydrolysis AcOH, 20°, 90 min, 100% yield, and AcOH, 20°, 3 days, 100% yield, respectively. Ozonolysis of the /-butylmethylidene ketal affords hydroxy ester, albeit with poor regiocontrol, but a more sterically differentiated derivative may give better selectivity, as was observed with the ethylidene ketal. ... 

Porco s route to (—)-kinamycin C (3) began with 2,5-dihydroxybenzaldehyde (38), which was elaborated to the enone 35 by the sequence shown in Scheme 3.6. Regioselective bromination [25] followed by methylation and reduction of the aldehyde function afforded the primary alcohol 39. The alcohol 39 was dearomatized by treatment with bis(acetoxy)iodobenzene, to afford the quinone monoketal 41. Transketalization with 1,3-propanediol followed by silylation of the primary alcohol generated the silyl ether 42 in 72 % yield over three steps. 

Dioxazoles 146 are readily prepared by transketalization of 2,2-diethoxypropane, where both the NH and OH moieties are protected in a non-protic form (Scheme 68). The dioxazoles 146 are stable to a wide variety of reaction conditions and readily revert back to the hydroxamic acids 145 and isopropyl ester 147 (145/147 50/1) by treatment with Nafion-H in 2-propanol. The method is applicable to primary, secondary, tertiary and aromatic hydroxamic acids, and the acidity of the protons adjacent to the dioxazole allows R-functionalization. 

Shorter reaction times result in incomplete transketalization. 

A variant of the preceding strategy was applied to the total syntheses of ( )-elwesine (439), ( )-epielwesine (449), and ( )-oxocrinine (415) (Scheme 39) 200). This approach featured a new methodology for the construction of substituted tetrahydrobenzazepine ring systems that was based on a two-step Tscher-niac-Einhom-like aromatic amidoalkylation as exemplified by the conversion of 443 to 444. To this end, the unsaturated nitrile 440 was allowed to react with nitromethane in the presence of Triton B, and the intermediate nitro compound was converted to the acetal 441 by reaction with methanolic sodium methoxide followed by concentrated sulfuric acid in dry methanol. Transketalization of 441... 

An improved synthesis of 1,4,2-dioxazolines from hydroxamic acids 119 and 2,2-diethoxypropane in the presence of camphorsulfonic acid as an acidic catalyst via a transketalization reaction has been reported (Scheme 23)... 

The cyclohexane-1,2-di acetal group can be introduced by transketal-isation with 1,1,2,2-tetramethoxycyclohexane. The Dispoke and CDA group can be removed by transketalisation with ethylene glycol in the... 

Butyl- and 1 -phenylethylidene ketals were prepared selectively from the C4-C6, 1,3-diol in glucose by an acid-catalyzed transketalization reaction [e.g.,... 

Lactonization may sometimes be combined with deketalization, as shown by equation (66). ° The y-lactone is formed in preference to the 5-lactone, in analogy to related observations.Transketalization is also shown by (183), which forms the 3-anomer of (184) with >10 1 selectivity (equation 67). ... 

Ester 52 afforded the previously characterized ester 16 via transketalization. 

The 3-(benzyloxy)propyl acetal was developed to be deprotected in two stages hydrogenolysis of the benzyl group followed by mild acid treatment to cleave the acetal by intramolecular transketalization. Prolonged hydrogenolysis over Pd-C also resulted in acetal cleavage, but this is most likely the result of residual acid in the catalyst—a well-known problem. ... 

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