Selective Protection Strategies

4-OH free Here, for all the three monosaccharides, a derivative with the 4-OH free can be obtained from the 4,6-O-benzylidene derivative through regioselective reductive opening. Formation of the 4,6-acetal with consecutive 2,3-protection (acetylation, benzoylation, benzylation) and reductive opening using various reagents yields the desired compounds. In mannosides, the selective formation of the 4,6-6 -acetal is not trivial, but can be accomplished with a 50 to 70% yield. 

SCHEME 3.25 Further examples of regioselective protection of D-galactose. 

Routes to 2 -OH and 3-OH acceptors, however, are not directly apparent. Two ways to 2 -OH lactose derivatives are polyisopropylidenation [53] or consecutive use of lipases [56]. The first strategy exploited the finding that isopropylidenation of lactose under forced conditions yields the 

SCHEME 3.29 Strategies to 6-, 4 - or 6 -OH methyl lactoside derivatives from a 4, 6 -0-benzylidene 

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Scheme 4.17 Immobilized lanthanide-based Lewis acid (31) as tool for selective protection strategies of 1,3-diols.

An obvious outcome of the Hantzsch synthesis is the symmetrical nature of the dihydropyridines produced. A double protection strategy has been developed to address this issue. The protected chalcone 103 was reacted with an orthogonally protected ketoester to generate dihydropyridine 104. Selective deprotection of the ester at C3 could be accomplished and the resultant acid coupled with the appropriate amine. Iteration of this sequence with the C5 ester substituent ultimately gave rise to the unsymmetrical 1,4-dihydropyridine 105. 

Fig. 15.14 Illustration of selective deposition strategies for catalyst nanoparticles (left) on the inner and (right) on the outer surface of CNTs according to Ref. [Ill], For inside deposition the CNTs are (a) impregnated with an ethanolic solution of the metal precursor, followed by washing with distilled water to protect the outer surface, and (c) subsequent drying and final treatment to form the catalyst nanoparticies. For outside deposition the CNTs are (d) impregnated with an organic solvent to block the inner tubule, followed by (e) impregnation with an aqueous solution of the metal precursor and (f) subsequent drying and final treatment.

In the early synthesis of deamino-dicarba-oxytocin, the intermediate Z-Asu(OMe)-OH was used which requires a saponification step prior to cyclizationJ1-2 Subsequently, a synthesis more consistent with the general protection strategies in peptide synthesis was developed with the intermediate Z-Asu(OtBu)-OH.12,24 As outlined in Scheme 9, upon selective deprotection of the side-chain carboxy group of the Asu residue by exposure to TFA, the octapeptide derivative 26 is converted into the 2,4,5-trichlorophenyl ester 27 using the tri-fluoroacetate method.129,20 Hydrogenolytic Na-deprotection of 27 in dilute solution leads to... 

Two new methods have been discovered for the selective protection of mmr-diequatorial vicinal diols [44,45]. One of them uses 3,3, 4,4 -tettahydro-6,6 -bi-2W-pyran (bis-DHP) 40 to transform diols into dispiroacetals and the second uses 1,1,2,2-tetrahydromethoxycyclo-hexane 41 to obtain cyclohexane-1,2-diacetal-protected sugars. These two methods have been compared [44,45] and we give here, as representative examples, one procedure for each strategy. 

The wide availability of various polysaccharides provides an important source of some of the monosaccharides. Such monosaccharides are now used in organic reactions as low-cost starting materials in the synthesis of a range of simpler optically pure compounds (e.g. Expt 5.77). These synthetic strategies have been made possible from earlier work on the development of numerous selective protection methods, on the application of new selective reagents for functional group modification within the monosaccharide molecule, and on the realisation of the role of conformation in the interpretation of a reaction course. The preparative examples in this section are illustrative of these developments. 

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