Organic synthesis hydroxy groups protection

Transition metal complexes have been widely investigated as catalysts for the synthesis of alkoxysilanes via alcoholysis of hydrosilanes. The system provides a convenient method for the protection of hydroxy groups in organic synthesis and the synthesis of silyl ethers. The general reaction is shown in Eq. (59). 

Electroenzymatic reactions are not only important in the development of ampero-metric biosensors. They can also be very valuable for organic synthesis. The enantio- and diasteroselectivity of the redox enzymes can be used effectively for the synthesis of enantiomerically pure compounds, as, for example, in the enantioselective reduction of prochiral carbonyl compounds, or in the enantio-selective, distereoselective, or enantiomer differentiating oxidation of chiral, achiral, or mes< -polyols. The introduction of hydroxy groups into aliphatic and aromatic compounds can be just as interesting. In addition, the regioselectivity of the oxidation of a certain hydroxy function in a polyol by an enzymatic oxidation can be extremely valuable, thus avoiding a sometimes complicated protection-deprotection strategy. 

Tcrt-butyldimethylsilyl ethers have been used extensively for the protection of hydroxy groups. They are more stable to hydrolysis than trimethylsilyl ethers by a rate factor of 104 and they are compatible with a much wider range of reagents used in organic synthesis. 

The simultaneous protection of the 3 and S hydroxy groups of nucleosides and the 4,6- or 3,4-hydroxyls of hexopyranoses is a common problem in organic synthesis. In the case of hexopyranoses, we have already seen that benzylidene acetals and, in certain circumstances, isopropyiidene acetals can be used to good effect. An alternative silicon-based group would offer a wider repertoire of conditions for mild deprotection and such a group was devised by Markiewicz the 1,1,3,3-tetraisopropyldisiloxanylidene group (abbreviated TIPDS),22 229 TIPDS groups are stable to water, 0.3 M p-toluenesulfonic acid in dioxane, 10% tri-fluo mace tic acid in chloroform, 5 M ammonium hydroxide in dioxane-H20 (4 1), and tertiary amines in pyridine,... 

Before all these acetal-based protecting groups were introduced, the tetrahydropyranyl (THP) ether had found extensive use in organic synthesis. It can easily be synthesized from a variety of hydroxy-containing compounds like carbohydrates, amino acids, steroids and nucleotides by the acid-catalyzed reaction with dihydropyran. It is stable to bases, but the protection is removed through acidic hydrolysis with hydrochloric acid, toluenesulfonic acid or acidic ion-exchange resin (Scheme 27). In the case of acid sensitive substrates, e.g. containing an epoxide or a further acetal, pyridinium p-toluenesulfonate should be applied for particularly mild deprotection conditions. ... 

Allylstannanes work as allylating reagents under free-radical reaction conditions. Free-radical reactions have several advantageous features in organic synthesis -neutral reaction conditions, compatibility with Lewis acids, and no need to protect reactive functional groups such as hydroxy and amino groups. In these days, therefore, free radical allylation procedures have been widely used in organic synthesis and several reviews on free radical reactions are available [94]. 

Arenesulfonyl derivatives are frequently employed in organic synthesis to activate hydroxy groups for nucleophilic substitution reactions or to protect primary and secondary amines. The PET cleavage of tosyl groups is closely related to reductions... 

Protecting groups, as described in the text, are essential parts of many organic syntheses. An example is the synthesis of the sex hormone testosterone (Section 4-7) from a cholesterol-derived starting material. Natural sources of steroid hormones are far too limited to meet the needs of medicine and research these molecules must be synthesized. In our case, the hydroxy function at C3 and carbonyl function at C17 of the starting mat al have to trade places to furnish the desired testosterone precursor. In other words, selective reduction of the carbonyl group at C17 and oxidation of the hydroxy group at C3 are required. In the scheme that was... 

This chapter covers preparations of organosilicon reagents and their use in organic synthesis. Because many books, book sections,and reviews are already available on these topics and only a limited space is available here, this chapter focuses on the reactions that involve cleavage of C-Si bonds for C-C bond formation, aldol reactions of silyl enol ethers, and oxidation of C-Si bonds. Therefore, topics concerning carbonyl reduction and the protection of functional groups, such as hydroxy with organosilanes, are not included in this chapter. 

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