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Introduction to Synthesis Applications of Carboxylic Derivatives

Synthesis is the construction of complex molecules by a series of suitable reactions carried out from simple starting compounds. Synthesis is not only of immense practical significance (aspirin and nylon are two instances of commercially valuable synthetic compounds), but it also permits us to prepare novel molecules with which to test our understanding of reactivity and structure. Three challenges must be met in devising a synthesis for a particular compound:

1. The carbon atom skeleton or framework that is found in the required compound (the target) must be assembled.
2. The functional groups that characterize the required compound must be introduced or transformed from other groups at suitable locations.
3. If centers of stereoisomerism are exists, they must be fixed in a correct manner.

1. Carbon-Carbon Bond Formation

A helpful assortment of carbon-carbon bond creating reactions have been explained in this and earlier chapters. These includes:
  (1)   Friedel-Crafts alkylation and acylation.
  (2)   Diels-Alder cycloaddition.
  (3)   addition of organometallic reagents to aldehydes, ketones & carboxylic acid derivatives.
  (4)   alkylation of acetylide anions.
  (5)   alkylation of enolate anions.
  (6)   Claisen and aldol condensations.

With the exception of Friedel-Crafts alkylation these reactions all give results having one or more functional groups at or adjacent to the bonding sites. Result, consequent functional group introduction or alteration may be carried out in a comparatively straightforward manner. This will be demonstrated for Claisen and aldol condensations in the section.

Modification of Condensation Products

A. Reactions of Aldol Products

The aldol reaction makes beta-hydroxyaldehydes or ketones and some subsequent reactions may be accomplished with these products. As displayed in the following diagram, they might be (i) reduced to 1,3-diols, (ii) a 2º-hydroxyl group might be oxidized to a carbonyl group, (iii) the acid or base catalyzed beta-dehydration may produce an unsaturated aldehyde or ketone (iv) organometallic reagents might be added to the carbonyl group (assuming the hydroxyl group is protected as an ether or a second equivalent of reagent is used).

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Reactions of Claisen Products

The Claisen condensation makes beta-ketoesters. These results may then be changed or improved by further reactions. Between these, the diagram demonstrates (i) partial reduction of the ketone with NaBH4, (ii) the complete reduction to a 1,3-diol by LiAlH4, (iii) enolate anion alkylation (iv) ester hydrolysis is followed by thermal decarboxylation of the resulting beta-ketoacid.

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Synthesis Examples

To demonstrate how the reaction sequences explained above may be employed to prepare a range of dissimilar compounds, five instances are provided here. The first is a usual aldol reaction followed by reduction to a 1,3-diol (2-ethyl-1,3-hexanediol). In second instance, the nonexistence of alpha-hydrogens on the aldehyde favors the mixed condensation and conjugation of the double bond makes easy dehydration. The doubly-activated methylene group of malonic and acetoacetic acids or esters make them fine donors in any condensation, as is illustrated by the third aldol-like reaction. The  concerted dehydrative-decarboxylation (displayed by the magenta arrows) leads to the unsaturated carboxylic acid result. Amine bases are frequently employed as catalysts for aldol reactions, like in equations no.2 & 3. The fourth reaction illustrates that the conjugate base of the beta-ketoester results from Claisen or Dieckmann condensation might be alkylated directly. Thermal decarboxylation of the resultant beta-ketoacid gives a mono-alkylated cyclic ketone. At last, both acidic methylene hydrogens in malonic ester or ethyl acetoacetate might be substituted and the irreversible behavior of such type of alkylations permits strained rings to be created. In this case thermal decarboxylation of a substituted malonic acid produces a carboxylic acid. In all these instances the remaining functional groups could be employed for additional synthetic operations.

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