Carbonyl compounds functional group transformations

At oxidation level 2 we will first examine the carbonyl compounds and alkynes, which are readily formed either as a result of reactions utilized for creating C-C bonds or with the help of functional groups transformations. 

Beside the Grignard and other C-C bond-forming reactions, a number of functional group transformations may also serve as an entry into allylic systems. Some of them, namely the reduction of a, -unsaturated carbonyl compounds (products of crotonic condensation), halogenation of alkenes at the allylic position with Af-bromosuccinimide (NBS) and epoxide isomerization, are shown in Scheme 2.56. 

Functional group transformations classical and chemoselective methods for oxidation and reduction of organic substrates, and the availability and utilization of regio-, chemo-, and stereoselective agents for reducing carbonyl compounds... 

In the previous section one of the two major aspects of silicon syntheses was described. A major problem for the organosilicon chemist is producing a silicon-carbon bond. A second synthetic area involves functional-group transformations. Most useful reactions at silicon centers are substitution processes. This is an important difference between the chemistry of silicon compounds and that of carbon compounds. The synthetic or reaction processes in organic compounds are increased enormously by the availability of double bonds present in olefins and carbonyl derivatives. In fact, substitution at sp3-carbon centers may represent the least interesting of organic reactions. 

The conjugate addition using silylcuprate reagents enables )3-hydroxy carbonyl compounds to be synthesized. Functional group transformation is relatively simple when the silicon atom of such reagents bears an electron-withdrawing substituent (e.g., Ph). 

Let s look at construction of a 1,2-dicarbonyl compound within the context of ketoaldehyde 29. There is no simple way to construct the bond between the carbonyls without use of an A-fimction. An analysis of this problem reveals the need for acyl anion equivalents of type 32 or 33. Several possibilities are shown here. We have already seen that a monofunctional compound (27) can be converted to a 1,2-difimctional compoimd (23) using diazomethane (A-fiinction chemistry). The problem now simply becomes one of oxidation states. The chloride has to be oxidized to the aldehyde oxidation state. One method that could be used is the Kornblum oxidation (carbon is oxidized and sulfur is reduced). This is an example of a functional group transformation (FGT). It does wo involve construction of a difiinctional relationship. That work was done in the first step of the sequence 27 23). It... 

The required nitrite esters 1 can easily be obtained by reaction of an appropriate alcohol with nitrosyl chloride (NOCl). The 3-nitroso alcohols 2 formed by the Barton reaction are useful intermediates for further synthetic transformations, and might for example be converted into carbonyl compounds or amines. The most important application for the Barton reaction is its use for the transformation of a non-activated C-H group into a functional group. This has for example been applied for the functionalisation of the non-activated methyl groups C-18 and C-19 in the synthesis of certain steroids. ... 

The removal of functional groups from the a-carbon of carbonyl compounds is an important transformation in organic synthesis. Anionic tellurium reagents offer additional useful methods to attain this. 

Despite the rich chemistry of 288 that may be anticipated, " " synthetic methods for this type of compounds are limited to the one involving oxidation of the corresponding alcohols. In contrast, 288 is readily derived from 287 by a simple and one-pot operation. Since propargylic alcohols are readily accessible from ketones or aldehydes, the straightforward transformation from 70 to 288f provides a novel method for carbonyl olefination of ketones and aldehydes. For example, ethisterone, 301, is tolerable to this transformation (route 2, Scheme 16) without any protection of the functional groups to give 302 (Equation (51))." ... 

The ring-opening of oxiranes, leading to the formation of isomeric carbonyl compounds by the action of acid catalysts as a result of rearrangement, is of continuing interest (refs. 1-4). However, most of these studies focus mainly on the transformations of terpene oxides or oxiranes with other functional groups in the liquid phase, under homogeneous reaction conditions. 

Aldol reactions provide a valuable synthetic method for forming carbon-carbon bonds. They can be adapted to extend the length of a carbon chain, to form cyclic compounds, and to provide intermediates that can be transformed into more useful materials. An important feature of these intermediates is that functional groups useful for later reactions are located close to or on the carbons of the newly formed C-C bond. There is an almost bewildering number of variations on the aldol reaction and we shall not mention all of them. The main thing to recognize in all of these reactions is that the acceptor molecule always is a carbonyl compound, best an aldehyde, sometimes a ketone, even an ester (see Section 18-8E). The donor molecule is some type of carbanion usually, but not always, an enolate anion. However, any substance that has a... 

For this reason reactive groups are usually introduced before quinuclidine ring closure and various transformations are effected afterward. The carboxyl and carbonyl derivatives, e.g., quinuclidine-2-carboxylic acid, quinuclidin-3-one, and so on, are useful compounds containing such functional groups. Nearly all substituted quinuclidines were obtained from their carboxylic acids and carbonyl derivatives by common synthetic methods. 

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