Selective acetalation

As already noled, lower order cyanocuprates are more Su2 -selective reagents. On irealmenl widi acetale 163, however, a mixture of tlie iwo regioisomets was oblained fetilry 2) [81]. In addih on, y-alkylalion bad taken place witli ca. 2 596 loss ... 

As these acetals could be converted into the 4,6-O-ethylidene derivatives on treatment with acid, it was reasoned that use of a cyclic vinyl ether, namely, 3,4-dihydro-2H-pyran, might prevent this second process, thus leading to a more useful method of selective acetalation.338 An equimolar reaction with methyl a-D-glu-copyranoside for 4 days in N,N-dimethylformamide led to utilization of 88% of the glycoside, and the 6-(tetrahydropyran-2-yl) ether constituted —85% of the crude reaction-product. In contrast to the steric control apparent in this instance, reaction of 3,4-dihydro-2H-pyran with the axial and equatorial hydroxyl groups in dl-1,4,5,6-tetra-O-acetyl-mi/o-inositol was completely unselective,339 a fact that has been rationalized310 in terms of the probable mechanism of these reactions. 

The approach was carried out on a ketohexo backbone bearing acid-sensitive ketal groups (Scheme 29). l,2 4,5-Di-0-isopropylidene-/ -D-fructo-pyranose readily underwent PDC oxidation of the 3-OH, followed by selective acid-catalyzed hydrolysis of the 4,5-ketal to afford a partially protected ketone in 94% overall yield. For the subsequent HSCN condensation, adapted acidic conditions had to be established to avoid 1,2-isopropylidene cleavage under thermal conditions and the target OZT (R = H) could be isolated in 60% yield. When performed in ethanol, the condensation afforded the acetalic counterpart (R = Et) albeit in lower yield. 

The stereochemical issue of the acetalic carbon was subsequently addressed. Specifically, two diastereomers are possible at the stage of mixed acetal formation. Quite remarkably, the formation of mixed acetal proved to be highly selective to give almost exclusively the S-diastereomer. Mixed acetals with various substitution patterns were synthesized and uniformly stereoseletive formation of S-isomers was observed in each case (Scheme 7.21).73... 

Selective acetalation. The reagent catalyzes the acetalation of 3-ketosteroids for example, 5< -pregnane-3,20-dione (1) is converted into 5a-pregnane-3,20-dione 3-dimethyl acetal in 78% yield. Keto groups at C6, C12, C17, and C20 do not react 2-ketosteroids react to some extent.10... 

Selective periodate oxidation of neamine derivatives can lead to novel aminoglycosides, which are analogues of sorbistin antibiotics, as shown in Scheme 2 likewise cleavage of the C-3,C-4 bond of the neosamine unit led to the acetalated glycoside (10) of 2-deoxy-streptamine. ... 

This year has seen several approaches to the core of the zaragozic acids/squalestatins from carbohydrate starting materials. D-Mannose was converted to the glycal 117, elaborated to precursor 118 by a-selective dioxirane oxidation and Cl epoxide ring opening by allyl alcohol, deallylation, oxidation and Grignard addition. Desilylation of 118 with concomitant intramolecular acetalation afforded the core analogue 119 (Schone 30). ... 

Treatment of erythritol and ( )-threitol with dimethox3miethane -toluene- -sulphonic acid - lithium bromide permits the selective acetalation of these polyols to give 1,3-0-methylene derivatives in methanol or 1,3 2,4-di-O-methylene acetals in dichloromethane. Trimethylaluminium has been found to rearrange tetraol vic-acetals to give alkyl ethers as illustrated in Scheme 3. ... 

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