0-Isopropylidene protection with

With respect to the nucleophilic addition of organocopper reagents, a sharp contrast between the rigid isopropylidene glyceraldehyde and its open-chained analog, 2,3-bis(benzyloxy)propanal. was observed (compare Tables 15 and 16). With the isopropylidene-protected aldehyde a high syn diastereoselectivity could only be obtained when tetrahydrofuran was used as reaction solvent, and the diastereoselectivity dropped considerably in diethyl ether. In contrast, the latter solvent allows excellent syn selectivities in additions to the dibenzyl-protected glyceraldehyde81. On the other hand, tetrahydrofuran yields better results than diethyl ether in the... 

On the other hand, the a,0-isopropylidene-protected analog 13 gives predominant formation of the anO, am(-diastereomer 14 with nucleophiles such as 2-(trimcthylsilyl)thiazole and organo-... 

Diastereoselective reduction of the aldol 221/ can be achieved using AIH3 in toluene at —78°C. The corresponding ra-diol is preferentially formed. The diol can be protected with isopropylidene acetal to provide tricyclic compound 222. This can be converted to conformationally rigid C-l ketone 223 by deprotection of the PMB group and successive oxidation with PDC (Scheme 7-73). 

D-Glucose was reduced to the D-sorbitol with a hydrogen over Ni Raney, then it was turned into the L-sorbose with the acetobacter suboxydans and the hydroxyl groups of L-sorbose were protected with acetone treatment yielded the diaceton-L-sorbose. Subsequent treatment with NaOCI/Raney Ni produced di-O-isopropylidene-2-oxo-L-gulonic acid. Partial hydrolysis with aqueous HCI gave deprotected 2-oxo-L-gulonic acid, which yielded ascorbinic acid by heating with HCI. 

Indeed, stereospecific 1,4-additon of methyl lithium/copper iodide to the conjugated system of the above enone was initially reported by the authors(3 // We synthesized this convenient synthon and attempted to functionalize it further by removal of isopropylidene protecting group followed by acetylation at C-2. (scheme 13). All attempts failed due to extensive decomposition of the starting material, presumably through the p-elimination with formation of secondary polymerization products. 

Another approach that led to differentiation is the combination of bulky group and isopropylidene ketal. With the ketal strongly favoring formation onto vicinal cis-diols, the 6-O-protected galactoside 47 [169,170] and mannoside 49 [171] provided the 2-alcohol 48 and 4-alcohol 50 in high yields, respectively (Scheme 2.18). 

---