Lactone

Source: Wikipedia, the free encyclopedia.

Lactones are cyclic

carboxylic esters are intramolecular esters derived from hydroxycarboxylic acids. They can be saturated or unsaturated. Some contain heteroatoms replacing one or more carbon atoms of the ring.[1]

Lactones are formed by intramolecular

esterification of the corresponding hydroxycarboxylic acids, which takes place spontaneously when the ring that is formed is five- or six-membered. Lactones with three- or four-membered rings (α-lactones and β-lactones) are very reactive, making their isolation difficult. Special methods are normally required for the laboratory synthesis of small-ring lactones as well as those that contain rings larger than six-membered.[2]

Nomenclature

Greek prefixes in alphabetical order indicate ring size.

Ring size
(number of atoms in the ring) 		Systematic name
IUPAC name
 Common name(s) Structure
3 α-lactone Oxiran-2-one Acetolactone
4 β-lactone Oxetan-2-one
  • β-Propiolactone
  • Propiolactone
5
γ-lactone
 Oxolan-2-one γ-Butyrolactone
6 δ-lactone Oxan-2-one
7 ε-lactone Oxepan-2-one
  • ε-Caprolactone
  • Caprolactone
  • Hexanolide

Lactones are usually named according to the precursor acid molecule (aceto = 2 carbon atoms, propio = 3, butyro = 4, valero = 5, capro = 6, etc.), with a -lactone suffix and a Greek letter prefix that specifies the number of carbon atoms in the heterocycle — that is, the distance between the relevant -OH and the -COOH groups along said backbone. The first carbon atom after the carbon in the -COOH group on the parent compound is labelled α, the second will be labeled β, and so forth. Therefore, the prefixes also indicate the size of the lactone ring: α-lactone = 3-membered ring, β-lactone = 4-membered, γ-lactone = 5-membered, δ-lactone = 6-membered, etc.

Macrocyclic lactones are known as macrolactones.[3]

Look up macrolactone in Wiktionary, the free dictionary.

The other suffix used to denote a lactone is -olide, used in substance class names like butenolide, macrolide, cardenolide or bufadienolide.

To obtain the preferred IUPAC names, lactones are named as heterocyclic pseudoketones by adding the suffix 'one', 'dione', 'thione', etc. and the appropriate multiplicative prefixes to the name of the heterocyclic parent hydride.[4]

Etymology

The name lactone derives from the ring compound called

alpha-propiolactone
, a lactone with a 3-membered ring.

In 1880 the German chemist Wilhelm Rudolph Fittig extended the name "lactone" to all intramolecular carboxylic esters.[6]

Occurrence

D-glucono-δ-lactone (E575)

The most stable lactones are the 5-membered γ-lactones and 6-membered δ-lactones because, as in most organic cycles, 5 and 6 membered rings minimize the strain of

bond angles. β-lactones appear in a number of natural products, but are only stable in artificial conditions (i.e. a test tube).[7] α‑Lactones can be detected as transient species in mass spectrometry experiments.[8]
The reactions of lactones are similar to those of esters.

Many naturally-occurring lactones are γ- and δ-lactones, both saturated and unsaturated. They contribute to the aroma of fruits, butter, cheese, and other foods.

Macrocyclic lactones are also important natural products.

phthalides are responsible for the odors of celery and lovage oils, and coumarin for woodruff.[9] Lactones are present in oak wood, and they contribute to the flavour profile of barrel-aged beers.[10]

Lactone rings occur widely as building blocks in nature, such as in

epothilones), phytoestrogens (resorcylic acid lactones, cardiac glycosides
).

Synthesis

Oxandrolone synthesis

Many methods in ester synthesis can also be applied to that of lactones. Lactonization competes with polymerization for longer hydroxy acids, or the strained β‑lactones. γ‑Lactones, on the other hand, are so stable that 4-hydroxy acids (R-CH(OH)-(CH2)2-CO2H) spontaneously cyclise.

In one industrial synthesis of

esterification.[11][12]

iodolactonization

In

halolactonization, an alkene is attacked by a halogen via electrophilic addition with the cationic intermediate captured intramolecularly by an adjacent carboxylic acid.[13]

Specific methods include

Corey-Nicolaou macrolactonization, Baeyer–Villiger oxidation and nucleophilic abstraction
.

γ-Lactone synthesis from fatty alcohols and acrylic acid

The γ-lactones

fatty alcohols to acrylic acid, using di-tert-butyl peroxide as a catalyst.[9]

An alternative radical reaction yielding γ-lactones is the manganese-mediated coupling.

Reactions

Lactones exhibit the reactions characteristic of esters.

Hydrolysis and aminolysis

Heating a lactone with a base (

hydrolyse the lactone to its parent compound, the straight chained bifunctional compound. Like straight-chained esters, the hydrolysis-condensation reaction of lactones is a reversible reaction, with an equilibrium. However, the equilibrium constant of the hydrolysis reaction of the lactone is lower than that of the straight-chained ester i.e. the products (hydroxyacids) are less favored in the case of the lactones. This is because although the enthalpies of the hydrolysis of esters and lactones are about the same, the entropy
of the hydrolysis of lactones is less than the entropy of straight-chained esters. Straight-chained esters give two products upon hydrolysis, making the entropy change more favorable than in the case of lactones which gives only a single product.

Lactones also react with amines to give the ring-opened alcohol and amide.

Reduction

Lactones can be reduced to diols using lithium aluminium hydride. For instance, gamma-lactones is reduced to butane-1,4-diol, (CH2(OH)-(CH2)2-CH2(OH).

Polymerization

Lactones readily form polyesters according to the formula, and have been shown to oligomerize without catalyst as well:[14][15]

The double lactone called lactide polymerizes to polylactic acid (polylactide). The resulting materials, polylactic acid, have many attractive properties.[16][17]

Uses

Flavors and fragrances

Lactones contribute significantly to the flavor of fruit, and of unfermented and fermented dairy products,

γ-nonalactone, which has an intense coconut flavor of this series, despite not occurring in coconut,[20] and γ-undecalactone
.

Macrocyclic lactones (cyclopentadecanolide, 15-pentadec-11/12-enolide) have odors similar to macrocyclic ketones of animal origin (muscone, civetone), but they can be prepared more easily, for example, by depolymerization of the corresponding linear polyesters. Replacement of a methylene unit by oxygen barely affects the odor of these compounds, and oxalactones with 15 – 17-membered rings are produced in addition to cyclopentadecanolide (e. g., 12-oxa-16-hexadecanolide).[9]

Prebiotic chemistry

Prebiotically plausible lactones, such as ε-caprolactone and δ-valerolactone, have been shown to oligomerize without the usage of catalysts forming oligomers that may have been relevant during the origin of life.[21]

Plastics

Polycaprolactone is an important plastic.

Dilactones

See also

Look up lactone in Wiktionary, the free dictionary.

References and notes

  1. Compendium of Chemical Terminology, 2.3.3, International Union of Pure and Applied Chemistry
    , 2014-02-24, p. 817
  2. ^ Francis A. Carey; Robert M. Giuliano (2011), Organic Chemistry (8th ed.), McGraw-Hill, pp. 798–799
  3. ^ Steven A. Hardinger. "Illustrated Glossary of Organic Chemistry". Department of Chemistry & Biochemistry, UCLA.
  4. ISBN 978-0-85404-182-4
    .
  5. ^ Pelouze, J. (9 December 1844). "Mémoire sur l'acide lactique" [Memoir on lactic acid]. Comptes rendus (in French). 19: 1219–1227. From p. 1223: "Indépendamment de la lactide dont je viens de rappeler l'existence dans les produits de la distllation de l'acide lactique, celui-ci donne encore, par sa décomposition, une autre substance, que je propose d'appeler lactone, parce qu'elle me paraît être à l'acide lactique ce que l'acétone est à l'acide acétique." (Independently of the lactide of which I have just recalled the existence in the products of the distillation of lactic acid, this [i.e., lactic acid] gives further, by its decomposition, another substance, which I propose to call lactone, because it seems to me to be to lactic acid what acetone is to acetic acid.)
  6. doi:10.1002/jlac.18802000102
    . From p. 62: "Es ist wünschenswerth, für diese Gruppe von Verbindungen, deren bis jetzt einfachster Repräsentant der im Vorstehenden beschriebene Körper ist, eine allgemeine Bezeichnungsweise zu haben, und da der Name "Lactide" nicht anwendbar ist, weil dann das Lactid κατ εξοχην kein Lactid sein wurde, so schlagen wir als Gruppenbezeichnung den Namen "Lactone" vor". (It's desirable for this group of compounds — whose simplest representative until now has been the substance that's described in the preceding — to have a general designation, and since the name "lactide" isn't applicable because then the archetypal lactide would not be a lactide, we therefore suggest the name "lactone" as the designation of this group [of compounds].)
  7. doi:10.1021/jo00003a047
    .
  8. doi:10.1016/S0168-1176(97)00150-X
  9. ^ a b c d Karl-Georg Fahlbusch; et al. (2007), "Flavors and Fragrances", Ullmann's Encyclopedia of Industrial Chemistry (7th ed.), Wiley, pp. 74‒78
  10. ^ Oliver, Garrett. "The Oxford Companion to Beer definition of barrel-aging". Craft Beer and Brewing.
  11. doi:10.1021/op060231b
  12. lead tetraacetate with ring-opening and finally reduction of the aldehyde to the alcohol with sodium borohydride
    and intramolecular lactone formation
  13. ^ Organic Syntheses, Coll. Vol. 7, p.164 (1990); Vol. 64, p.175 (1986) Article link
  14. ^ Wilhelm Riemenschneider; Hermann M. Bolt (2007), "Esters, Organic", Ullmann's Encyclopedia of Industrial Chemistry (7th ed.), Wiley
  15. PMID 33067516
    .
  16. ISBN 978-0-470-29366-9
    .
  17. PMID 15584698
    .
  18. ^
    ISBN 9783540493396
    . Retrieved 2 July 2015.
  19. ISBN 9781439853351
    . Retrieved 2 July 2015.
  20. ISBN 9781420017045
    . Retrieved 2 July 2015.
  21. PMID 31963928
    .
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