Masked carbonyl functionality

Similar carbohydrate-like chiral synthons are obtained by yeast reduction of the -acetoxyketones 58 and 59 ( ), bearing in 1,6 and 1,5 relationship, respectively, two masked carbonyl functions accessible chemoselectively with different reagents. The mode of reduction of 58 and 59 is, however, opposite. Whereas yeast treatment of 58 affords... 

Having served their purpose as masked carbonyl functionalities in the preceding 4 steps of the synthesis, the benzylidene groups in fused terpyridyl 11 are cleaved by ozonolysis in Protocol 7. As shown in Scheme 6.10, the reaction solvent is a mixture of methanol and dichloromethane. Reactant 11 is insoluble in methanol, so dichloromethane is used as a cosolvent to enhance its concentration in the cold (-78°C) reaction mixture. Methanol is a convenient cosolvent for conducting ozonolysis reactions safely because it cleaves ozonides in situ to carbonyl compounds and peroxyhemiacetals or... 

Dioxolans 146 can be considered as masked carbonyl functionalities, and are cleaved under acidic conditions. In solution, the olefination of a dioxolan-protected ketone would therefore be a two-step transformation consisting of deprotection and olefination. Using polymer-supported adds, e.g., strongly addic Amberlyst resin 147 and polymer-supported phosphonates 135 (Scheme 25), the two-step transformation involving the carbonyl compound 137 as intermediate could be performed simultaneously in one pot (Scheme 27) [113]. The product 136 was isolated by filtration. This procedure would not work as a one-pot sequence in solution because the acidic catalyst would immediately quench the basic phospho-nate resin. 

The use of an alkene as a masked carbonyl function was employed by Sharpless and Crispino [88] to prepare optically active 7,7-dimethyl-6,8-dioxabicyclo-... 

Alkylation of ethyl tnfluoroacetoacetate enolate occurs at the carbon atom provided that the carbonyl function is masked as a dioxolane or an N,N dimeth-ylhydrazone [IIS] (equation 102)... 

The essential feature of enamines is that they are nitrogen analogues of enols and behave as enolate anions. They effectively mask a carbonyl function while activating the compound towards nucleophilic substitution. 

Another electrophile that can interact with the isocyanide-derived amide is an aldehyde. However, an additional carbonyl function cannot be introduced as such into one of the components without interfering with the Ugi process. Thus it is better used in masked form as an acetal (Fig. 9). Starting from amine 49 a series of oxopiperazines 51 has been prepared, also on solid phase [66],... 

Compound (119) is a trioxadamantane derivative, which was prepared from the methanol adduct (6) of secologanin dimethylacetal (4a), in which all masked carbonyl groups were in acetal form. Aqueous acidic hydrolysis, under termodynamic control, transformed them groups of (6) into totally cyclized acetal structure. The backbone of the molecule has a high symmetry, which is broken by the functional groups [15]. 

Due to the stabilizing effect of two sulfur atoms, 1,3-dithianes are easily lithiated at the a-position on treatment with n-BuLi (see 2-Lithio-l,3-dithiane) The 2-lithio-1,3-dithianes constitute an important class of acyl anion equivalents, permitting electrophilic substitution to occur at the masked carbonyl carbon. Hydrolysis of the 1,3-dithiane functionality into a carbonyl group is effected in the presence of mercury(II) ion (eq 4). ... 

Acetals are widely used as protection groups in synthetic chemistry since they are stable to base. This allows chemical transformations to be conducted in the presence of carbonyls and diols that could undergo competitive reaction. Likewise, a carbonyl function can be masked during... 

Fig. 38. Several solvents used in the Meiri-field process are masked carbonyl compounds suitable to irreversibly block polymer bound amino functions...

In many instances, ketals can serve as masked carbonyls in the Passerini reaction. The product a-alkoxyamides, exemplified here by 21 and 22, can be obtained in high yield from the appropriate ethyl or methyl ketals mediated by either Bronsted or Lewis acids. Both cyclic and acyclic acetals can function as suitable substrates. 

The carbonyl function, as present in aldehydes and ketones, is probably the most versatile functional group in organic chemistry and not surprisingly a great deal of work has been done on the protection and masking of aldehyde and ketone groups. 

In the conversion of an aldehyde or ketone to an acetal, the reactive carbonyl function is transformed into a relatively unreactive ether-tike moiety. Because acetal formation is reversible, this process amounts to one of masking, or protecting, the carbonyl group. Such protection is necessary when selective reactions (for example, with nucleophiles) are required at one functional site of a molecule and the presence of an unprotected carbonyl group elsewhere in the same molecule might interfere. This section describes the execution of such protecting strategies. 

The transamination reaction is important biologically in amino acid metabolism. Simple aldehydes are rare in biological systems and are mostly masked as imines. Biochemists often refer to them as Schiff bases, which are a special class of aldehyde imine where the nitrogen atom is substituted by an alkyl or aryl group. The transamination reaction interconverts amino and carbonyl functionalities (Figure 14.32). The enzymes involved in the process are called transaminases, and they require pyridoxal phosphate as a cofactor. 

Silyl enol ethers are a class of electron-rich, nonaromatic compounds that easily form reactive radical cations on one electron oxidation. The silyl enol ether functional group is closely related to the carbonyl function and consequently, syntheses of silyl enol ethers generally make use of enolates. In addition, silyl enol ethers can be described as masked enols or enolates since their reactions often yield ketones. A number of oxidation reactions of silyl enol ethers making use of oxygen or oxygen-containing reagents such as peroxides, peracids (known as Rubottom oxidation), dioxirane, osmium tetraoxide, or triphenyl phosphite ozonide have been described in the literature. In all cases either a-hydroxy-ketones or the silyl enol ether epoxides are formed. 

Azomethine ylides are also frequently obtained by ring opening of aziridines, and the analogous carbonyl ylides from oxiranes. These aspects are dealt with in Section 3.03.5.1. A variety of five-membered heterocycles can also function as masked 1,3-dipoles and this aspect is considered in Section 3.03.5.2. 

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