Esters and Lactones

FIGURE 2.24. Benzoic acid, ammonium salt. A. N—H and C—H stretch, 3600-2500 cm. B. Ring C=0 stretch, 1600 cm. C. Asymmetric carboxylate anion C(—0)2 stretch, 1550 cm-1. D. Symmetric carboxylate C(=0)2 stretch, 1385 cm1. 

C—O stretching band in the region where a weaker band occurs for ketones. There is overlapping in the C=0 frequency of esters or lactones and acids, but the OH stretching and bending vibrations and the possibility of salt formation distinguish the acids. 

C=0 absorption band of saturated aliphatic esters (except formates) is in the 1750-1735 cm-1 region. 

The C=0 absorption bands of formates, a,/3-unsaturated, and benzoate esters are in the region of 1730-1715 cm-1. Further conjugation has little or no additional effect upon the frequency of the carbonyl absorption. 

In the spectra of vinyl or phenyl esters, with unsaturation adjacent to the C—O— group, a marked rise in the carbonyl frequency is observed along with a lowering of the C—O frequency. Vinyl acetate has a carbonyl band at 1776 cm-1 phenyl acetate absorbs at 1771 cm 

Sarma and R.P.Sharma, Synthesis of o-Methylene- -butyrolactones, Heterocycles, 1986, 23 441. 

Petragnani, H.M.C.Ferraz and G.V.J.Silva, Advances in the Synthesis of a-Methylenelactones, Synthesis, 1986, 157. 

Pavlova, Y.A.Davidovich and S.V.Rogozhin, Alkyl Orthoesters and their Applications in Organic Synthesis, Russ.Chem.Rev., 

See page 309, Section 56, for elimination of 6-hydroxy acids via -lactones. 

Pashkevich and V.I. Salontin, Fluorine Containing 3-Ketoesters, Russ.Chem. Rev., 1985, 1185. 

Hoffmann and J. Rabe, Synthesis and Biological Activity of a-Methylene-TT-Butyrolactones, Angew. Chim. Int. Eng. Edn., 1985, 2 , 94.  

The ester carbonyl absorption follows closely in behaviour the ketonic carbonyl band insofar as frequency shifts arising from environmental changes are concerned, and, as will be seen, the shifts 

As in the case of ketones, no discussion of carbonyl frequencies of esters with nitrogen attached to the carbonyl group is included in this section, as such materials will be dealt with in Chapter 12. 

The single bond C—O absorption falls in the same general region as other C—O stretching vibrations of unsaturated ethers and alcohols, and is not particularly significant as such, especially as the frequency shows less stability than is the case with the carbonyl absorption. However, sufficient work has been done in some cases to enable the smaller and more specific frequency ranges of individual classes of 

In the case of the acetates, a very extensive study of the structure of the C—O band of a large number of sterol acetates has enabled Jones et al [ 1 ] to obtain evidence as to the stereochemical structure of C3 acetoxy-sterols in relation to the hydrogen atom at C5. 

Sugar lactones are hydrogenated to the aldoses over platinum at room temperature and atmospheric pressure. 9 50 Other 8-lactones have been hydrogenated to the cyclic ethers in very good yields at room temperature and atmospheric pressure over platinum oxide in acetic acid containing a small amount of perchloric acid (Eqn. 18.36). 5L 52 

In oxalates and a-keto esters, as in a-diketones, there appears to be little or no interaction between the two carbonyl groups so that normal absorption occurs 

BENZOIC ACID. AMMONIUM SALT [TjT C H9N02 M.W 139.15 M.P 196-200°C KBr Wafer 

BENZOIC ACID. AMMONIUM SALT 0NH4 C7H9NOa MW t3915 MP 198-200°C KBr Wifar 

It is relevant to refer to a recent review of crystal structure of esters by Schweizer and Dunitz, which may be useful also in the present context. 

Shift with respect to C-(C-CH3) -6 ppm Shift with respect to C-(OH) +2 to +10 ppm 

For C-aromatics, shift with respect to CH-(C-H) ortho =+0.7 ppm, meta =+0.1 ppm, para =+0.2 ppm For C-aromatics, shift with respect to CH-(C-H) ortho =-0.2 ppm, meta =0 ppm, para =-0.1 ppm 

Aliphatic esters weak, tendency to protonate Aliphatic lactones medium to weak, tendency to protonate 

Lactones loss of a-suhstituents (attached to ether carbon), decatbonylation, for aromatic lactones also double decarbonylation 

Elimination of the alcohol side chain with double hydrogen transfer (for C2 alcohols) 

NaBH3CN, BF3OEt2 (R=benzylic) EtjSiH, CFjCOjH (R—benzylic) EtjSiH. UC104 (R=2° allylic) PhjSiH, (t-BuO)i p-PbjSiHC HtSiHPhj, (/-BuO)2 

NaBH3CN, BF3 OEt2 (R = benzylic) EfeSiH, CF3CO2H (R = benzylic) EtaSiH, UCIO4 (R=2° aUylic) Ph3SiH, (r-BuO 2 / -Ph2SiHC6H4SiHPh2, (r-BuO)2 

1 alCH-COOR 2.0-2.5 ppm CH3COOR 2.0 ppm CH2COOR== 2.3ppm CHCOOR== 2.5ppm 

IR c=0st 1745-1730 cm-i Strong range for ahphatic estCTs Higher wavenumbers for hal-C-COOR, 

Rearrange- Alkene elimination from the alcohol moiety  

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Baeyer-Villiger Oxidation- oxidation of ketones to esters and lactones via oxygen insertion... 

Reaction of esters and lactones with dimethylaluminium amide TL 1979,4907... 

The Tebbe reaction, which converts the oxo groups of esters and lactones to methylene groups to give enol ethers, is described in section 2.1.2. 

The conversion of carboxylic acid derivatives (halides, esters and lactones, tertiary amides and lactams, nitriles) into aldehydes can be achieved with bulky aluminum hydrides (e.g. DIBAL = diisobutylaluminum hydride, lithium trialkoxyalanates). Simple addition of three equivalents of an alcohol to LiAlH, in THF solution produces those deactivated and selective reagents, e.g. lithium triisopropoxyalanate, LiAlH(OPr )j (J. Malek, 1972). 

The reaction can be applied to the synthesis of q, /3-unsaturated esters and lactones by treatment of the ketene silyl acetal 551 with an allyl carbonate in boiling MeCN[356]. The preparation of the q,, 3-unsaturated lactone 552 by this method has been used in the total synthesis of lauthisan[357]. 

Although deprotonation of simple 1,3-dithiolanes at the 2 position is usually accompanied by cycloreversion to the alkene and dithiocarboxylate, this does not occur for the 2-ethoxycarbonyl compound 55. The anion of this is readily generated with LDA and undergoes conjugate addition to a,(3-unsaturated ketones, esters, and lactones to give, after deprotection, the a,8-diketoester products 56 (73TL2599). In this transformation 55 therefore acts as an equivalent of Et02C-C(0) . 

The mechanism of ester (and lactone) reduction is similar to that of acid chloride reduction in that a hydride ion first adds to the carbonyl group, followed by elimination of alkoxide ion to yield an aldehyde. Further reduction of the aldehyde gives the primary alcohol. 

Conversion of Esters into Alcohols Grignard Reaction Esters and lactones react with 2 equivalents of a Grignard reagent to yield a tertiary alcohol in which two of the substituents are identical (Section 17.5). The reaction occurs by the usual nucleophilic substitution mechanism to give an intermediate ketone, which reacts further with the Grignard reagent to yield a tertiary alcohol. 

Reactions of esters and lactones (Section 21.6) (a) Hydrolysis to yield acids... 

Silyl enol ethers and ketene acetals derived from ketones, aldehydes, esters and lactones are converted into the corresponding o/i-unsaturated derivatives on treatment with allyl carbonates in high yields in the catalytic presence of the palladium-bis(diphenylphosphino)ethane complex (32). A phosphinc-free catalyst gives higher selectivity in certain cases, such as those involving ketene acetals. Nitrile solvents, such as acetonitrile, are essential for success. 

Carboxylic esters undergo the conversion C=0— C=CHR (R = primary or secondary alkyl) when treated with RCHBr2, Zn, and TiCl4 in the presence of A,A,A, iV -tetramethylethylenediamine. Metal carbene complexes R2C=ML (L = ligand), where M is a transition metal such as Zr, W, or Ta, have also been used to convert the C=0 of carboxylic esters and lactones to CR2. It is likely that the complex Cp2Ti=CH2 is an intermediate in the reaction with Tebbe s reagent. 

Lewis acid strength and hardness of the lithium cation. Both LiBH4 and Ca(BH4)2 are more reactive than sodium borohydride. This enhanced reactivity is due to the greater Lewis acid strength of Li+ and Ca2+, compared with Na+. Both of these reagents can reduce esters and lactones efficiently. 

The most widely used reagent for partial reduction of esters and lactones at the present time is diisobutylaluminum hydride (DiBAlH).83 By use of a controlled amount of the reagent at low temperature, partial reduction can be reliably achieved. The selectivity results from the relative stability of the hemiacetal intermediate that is formed. The aldehyde is not liberated until the hydrolytic workup and is therefore not... 

E. Tertiary alcohols from ketones, esters, and lactones... 

Calein A (24) which contains one a, 0-unsaturated ketone, ester and lactone moiety has only minor effects on most seed germinations but strongly inhibits Palmer amaranth (75%) at the 0.05 mM level. 

In some cases it is possible to differentiate between the various alkyl substituents. Primary, secondary and tertiary nitrates and nitrites all show clearly different infrared absorptions. The spectra of acid fluorides can be used to differentiate chain-end groups from pendant acid groups. Furthermore, the loss of all -OH species upon sulfur tetrafluoride exposure allows the reliable estimation of ketones, esters and lactones without the complication of hydrogen-bonding induced shifts in the spectra. Preliminary results from the use of these reactions to characterize y-ray oxidized polyethylene and polypropylene are used to illustrate the scope of the methods. 

Hope, D. B., and P. W. Kent Ester and Lactone Linkages in Acidic Poly-... 

The photoelimination of carbon dioxide from esters and lactones is a process that has been the subject of detailed investigations. Discussion here is limited to nitrogen containing systems. 3,4-Diphenylsydnone (464), on irradiation in benzene, is converted via the nitrile imine 465 into 2,4,5-triphenyl-1,2,3-triazole (466)388 initial bond formation between N-2 and C-4 followed by loss of carbon dioxide to give the diazirine 467 is proposed to account for the formation of the nitrile imine. Nitrile imines generated in this way have been trapped with alkenes and alkynes to give pyrazoles389... 

Scheme 6.47 Palladium-catalyzed carbonylation reactions yielding acids, esters, and lactones using molybdenum hexacarbonyl as a solid source of carbon monoxide.

Table 10. organosilane reduction of Esters and Lactones (continued)... 

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