Reactions aldol

Aldol reaction of the campholenic aldehyde with 2-butanone gives the intermediate ketones from condensation at both the methyl group and methylene group of 2-butanone (Fig. 6). Hydrogenation results in only one of the two products formed as having a typical sandalwood odor (160). 

Aldol reaction of campholenic aldehyde with propionic aldehyde yields the intermediate conjugated aldehyde, which can be selectively reduced to the saturated alcohol with a sandalwood odor. If the double bond in the cyclopentene ring is also reduced, the resulting product does not have a sandalwood odor (161). Reaction of campholenic aldehyde with -butyraldehyde followed by reduction of the aldehyde group gives the aHyUc alcohol known commercially by one manufacturer as Bacdanol [28219-61 -6] (82). 

Aldol reaction is the condensation of an aldehyde to produce longer-chain hydroxy aldehydes 

The aldol reaction is an equilibrium which can be "driven" to completion. 

When the aldol reaction is carried Wt under thermodynamic conditions, the product selectivity is often not as high as under kinetic conditions. All the regioisomeric and stereoisomeric enolates may participate as nucleophiles. The adducts can return to reactants, and so the difference in stability of the stereoisomeric anti and syn products will determine the product composition. 

Aldol Additions. These reactions catalyzed by lyases are perhaps the most synthetically useful enzymatic reactions for carbon—carbon bond formation. Because of the broad synthetic utiUty of this method, the enzymatic aldol reactions have received considerable attention in recent years and have been extensively covered in a number of books and reviews (10,138—140). 

This cleavage is a retro aldol reaction It is the reverse of the process by which d fruc tose 1 6 diphosphate would be formed by aldol addition of the enolate of dihydroxy acetone phosphate to d glyceraldehyde 3 phosphate The enzyme aldolase catalyzes both the aldol addition of the two components and m glycolysis the retro aldol cleavage of D fructose 1 6 diphosphate 

Non-chelation aldol reactions proceed via an "open" transition state to give syn aldols regardless of enolate geometry. 

Some of the earliest studies of the aldol reaction were carried out by Aleksander Borodin Though a physician by training and a chemist by profession Borodin is re membered as the composer of some familiar works in Russian music See pp 326-327 in the April 1987 issue of the Journal of Chem ical Education for a biogra phical sketch of Borodin 

The synthesis of 3-phenyl-]-(2-pyridyl)-2-propen-]-one (2.4c) via an aldol reaction of 2-acetylpyridine with benzaldehyde has been described in the literature ". O jmpound 2.4a-e have been prepared in high yields, using slightly modified versions of these literature procedures. 

A probable mechanism of these base-catalysed aldol reactions may be written in gena-al terms as follows  

Bernard A, A M CapeUi, A Comotti, C Gannari, J M Goodman and I Paterson 1990. Transltion-St Modeling of the Aldol Reaction of Boron Enolates A Force Field Approach. Journal of Orga Chemistry 55 3576-3581. 

Other multifunctional hydroxycarboxylic acids are mevalonic and aldonic acids which can be prepared for specialized uses as aldol reaction products (mevalonic acid [150-97-0] (13)) and mild oxidation of aldoses (aldonic acids). 

Thiazolecarboxaldehydes exhibit many reactions typical of aldehydes. However, they give no aldolization reaction (no a-hydrogen), but they do react with different compounds such as acetic anhydride, hippuric acid, acetylglycine, and so for (37, 101, 102). Thus 2-phenyl-4-fonnylthiazole (31) mixed with hippuric acid and treated with AcOa and anhydrous NaOAc gives the azalactone (32) (Scheme 32). 

Stereoselectivities of 99% are also obtained by Mukaiyama type aldol reactions (cf. p. 58) of the titanium enolate of Masamune s chired a-silyloxy ketone with aldehydes. An excess of titanium reagent (s 2 mol) must be used to prevent interference by the lithium salt formed, when the titanium enolate is generated via the lithium enolate (C. Siegel, 1989). The mechanism and the stereochemistry are the same as with the boron enolate. 

Neopentyl glycol (2,2-dimethyl-1-propanol [126-30-7]) another important iadustrial derivative of isobutyraldehyde, is obtained from the aldol reaction product of isobutyraldehyde with formaldehyde followed by hydrogenation. 

Cleavage reactions of carbohydrates also occur on treatment with aqueous base for prolonged periods as a consequence of base catalyzed retro aldol reactions As pointed out m Section 18 9 aldol addition is a reversible process and (3 hydroxy carbonyl com pounds can be cleaved to an enolate and either an aldehyde or a ketone 

The use of indium in acpieous solution has been reported by Li and co-workers as a new tool in org nometallic chemistry. Recently Loh reported catalysis of the Mukaiyama-aldol reaction by indium trichloride in aqueous solution". Fie attributed the beneficial effect of water to a eg tion phenomena in connection with the high internal pressure of this solvenf This woric has been severely criticised by 

Li+, Mg 2+. AP+= enolates give comparable levels of diastereoselection for kinetic aldol reactions. 

Neo acids are prepared from selected olefins using carbon monoxide and acid catalyst (4) (see Carboxylic Acids, trialkylacetic acids). 2-EthyIhexanoic acid is manufactured by an aldol condensation of butyraldehyde followed by an oxidation of the resulting aldehyde (5). Isopalmitic acid [4669-02-7] is probably made by an aldol reaction of octanal. 

Many commercially important isobutyraldehyde derivatives are prepared through aldol and/or Tischenko condensation reactions. For example, isobutyraldehyde undergoes the aldol reaction to form isobutyraldol (2,2,4-trimethyl-3-hydroxypentanal [918-79-6]) which, when hydrogenated, gives 2,2,4-trimethyl-1,3-pentanediol (TMPD) [144-19-4], 

A useful catalyst for asymmetric aldol additions is prepared in situ from mono-0> 2,6-diisopropoxybenzoyl)tartaric acid and BH3 -THF complex in propionitrile solution at 0 C. Aldol reactions of ketone enol silyl ethers with aldehydes were promoted by 20 mol % of this catalyst solution. The relative stereochemistry of the major adducts was assigned as Fischer- /ir o, and predominant /i -face attack of enol ethers at the aldehyde carbonyl carbon atom was found with the (/ ,/ ) nantiomer of the tartaric acid catalyst (K. Furuta, 1991). 

Aldehydes fiad the most widespread use as chemical iatermediates. The production of acetaldehyde, propionaldehyde, and butyraldehyde as precursors of the corresponding alcohols and acids are examples. The aldehydes of low molecular weight are also condensed in an aldol reaction to form derivatives which are important intermediates for the plasticizer industry (see Plasticizers). As mentioned earlier, 2-ethylhexanol, produced from butyraldehyde, is used in the manufacture of di(2-ethylhexyl) phthalate [117-87-7]. Aldehydes are also used as intermediates for the manufacture of solvents (alcohols and ethers), resins, and dyes. Isobutyraldehyde is used as an intermediate for production of primary solvents and mbber antioxidants (see Antioxidaisits). Fatty aldehydes Cg—used in nearly all perfume types and aromas (see Perfumes). Polymers and copolymers of aldehydes exist and are of commercial significance. 

Butyraldehyde undergoes stereoselective crossed aldol addition with diethyl ketone [96-22-0] ia the presence of a staimous triflate catalyst (14) to give a predominantiy erythro product (3). Other stereoselective crossed aldol reactions of //-butyraldehyde have been reported (15). 

Ba.se Catalyzed. Depending on the nature of the hydrocarbon groups attached to the carbonyl, ketones can either undergo self-condensation, or condense with other activated reagents, in the presence of base. Name reactions which describe these conditions include the aldol reaction, the Darzens-Claisen condensation, the Claisen-Schmidt condensation, and the Michael reaction. 

Protonated pyridazine is attacked by nucleophilic acyl radicals at positions 4 and 5 to give 4,5-diacylpyridazines. When acyl radicals with a hydrogen atom at the a-position to the carbonyl group are used, the diacylpyridazines are mainly converted into cyclo-penta[ f]pyridazines by intramolecular aldol reactions (Scheme 43). 

Some unsaturated ketones derived from acetone can undergo base- or acid-catalyzed exothermic thermal decomposition at temperatures under 200°C. Experiments conducted under adiabatic conditions (2) indicate that mesityl oxide decomposes at 96°C in the presence of 5 wt % of aqueous sodium hydroxide (20%), and that phorone undergoes decomposition at 180°C in the presence of 1000 ppm iron. The decomposition products from these reactions are endothermic hydrolysis and cleavage back to acetone, and exothermic aldol reactions to heavy residues. 

Ketones, in which one alkyl group R is sterically demanding, only give the trans-enolate on deprotonation with LDA at —12°C (W.A. Kleschick, 1977, see p. 60f.). Ketones also enolize regioseiectively towards the less substituted carbon, and stereoselectively to the trans-enolate, if the enolates are formed by a bulky base and trapped with dialkyl boron triflates, R2BOSO2CF3, at low temperatures (D A. Evans, 1979). Both types of trans-enolates can be applied in stereoselective aldol reactions (see p. 60f.). 

In the last fifteen years macrolides have been the major target molecules for complex stereoselective total syntheses. This choice has been made independently by R.B. Woodward and E.J. Corey in Harvard, and has been followed by many famous fellow Americans, e.g., G. Stork, K.C. Nicolaou, S. Masamune, C.H. Heathcock, and S.L. Schreiber, to name only a few. There is also no other class of compounds which is so suitable for retrosynthetic analysis and for the application of modem synthetic reactions, such as Sharpless epoxidation, Noyori hydrogenation, and stereoselective alkylation and aldol reactions. We have chosen a classical synthesis by E.J. Corey and two recent syntheses by A.R. Chamberlin and S.L. Schreiber as examples. 

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