Lactones are defined as a group of organic esters that have been drawn from hydroxyl carboxylic acids. Usually, they are formed when a halogen atom or a hydroxyl group reacts chemically with any carboxylic acid group at hand in the same molecule. Intramolecular esterification of the individual hydroxycarboxylic acids result in the formation of lactones. Each lactone comprises a ring with two or additional carbon atoms and a solitary oxygen atom. Lactones comprising three or four rings are extremely reactive mating it especially hard to isolate them. In such cases, it necessitates adopting exceptional means to synthesize the small ring lactones and also those having more than six rings in the laboratory.
The term lactone has its origin in a compound known as lactide, which is formed after lactic acid is dehydrated. They are named by adding a Greek letter as a prefix and the word lactone as the suffix. The prefixes actually suggest the number of rings a particular lactone has. On the other hand, lactones that occur naturally are of two varieties – saturated and unsaturated. Some of the naturally occurring lactones include ascorbic acid, antibiotics, gluconolactone, kavain and neurotransmitters.
Lactones occurring in the nature are mostly saturated, while a number of them are unsaturated γ- and δ-lactones. In addition to these, there are a few instances of finding macrocyclic lactones in the nature. Basically, the γ- and δ-lactones are intermolecular esters belonging to related hydroxyl fatty acids. These lactones are responsible for the scent of various fruits and foods like butter and cheese. For instance, the aroma of angelica root oil, which has a resemblance to musk, is attributed to cyclopentadecanolide. Among the lactones that occur naturally, phthalides and bicyclic lactones impart lovage oils and celery their characteristic aroma, while coumarin is responsible for the odour of woodruff.
The rings of lactone are found extensively and they appear as nature’s building block. This is common in many lactones including ascorbic acid, anticancer drugs (epothilones, vernolepin), enzymes (lactonase), gluconolactone, antibiotics ( for instance macrolides such as amphotericin B, erythromycin) nepetalactobe, kavain and phytoestrogens (cardiac glycosides, resorcylic acid lactones).
There are several common examples of lactones found in the essential oils and other more compound molecules with low volatility. Such lactones include sesquiterpene lactones that are infamous for their propensity to act as skin sensitizers (in fact massoia lactone, alantolactone and dehydrocostus lactone are known to be potentially allergenic). In fact, the names of almost all lactones are somewhat variable. However, -olide and -lactone are very common suffixes.
Basically a benzenoid lactone, coumarin is present in many essential oils and also as derivatives. This lactone comprises two rings that are fused and it may also be regarded as a structural part in addition to being a functional group. The odour or of coumarin is very similar to that of vanilla and the smell of freshly mowed hay is attributed to this lactone. Some coumarin derivates like the 5,7-dimethoxy derivative and citropten are phototoxic.
Lactones with five-member rings (gamma-lactone) and those with six-member rings (delta-lactone) are the most stable. Aside from the naturally occurring lactones, these compounds may also be developed commercially with yeasts by employing biotechnological methods. There are over 100 diverse lactones that have been identified as flavoring elements. It has been found that most of the lactones occurring in the nature are saturated as well as unsaturated gamma and delta-lactones.
Usually, lactones are obtained from hydroxyl fatty acids by a process called β-oxidation. Subsequently, there is another reaction called lactonization. For instance, a brown-rot basidiomycete Piptoporus soloniensis wild strain yields gamma-decalactone in the presence of both 12-hydoxystearic acid and ricinoleic acid together. Nevertheless, when you add oleic acid, stearic acid, palmitic acid and myristic acid to the culture, it results in the formation of gamma-octalactone. On the other hand, addition octanoic acid, linolenic acid, lauric acid, linoleic acid and hexnoic acid to the culture decreases P. solonien growth as well as gamma-octanolactone and gamma-decalactone production.
Many of the naturally occurring lactones that are used as flavoring agents can also be manufactured by employing processes based on using yeast strains that transform ricinoleic acid. For instance, a number of yeasts like Polyporus durus, B. adusta, Phlebia radiate, and C. moniliformis yield commercially valuable decalactones by employing a process called de novo synthesis.
Lactones undergo various types of reactions, such as hydrolysis, reduction and polymerization. These different reactions are nearly similar to those of esters, but yield different products.
Lactones with five rings (γ-lactones) and six rings (δ-lactones) are most stable because, like in all other organic cycles, the five-ringed and six-ringed members diminish the tension of the bond angles. The γ-lactones are so unwavering that when they come in contact with watered down acids at normal room temperature, there is an instant and spontaneous cyclisation and esterification of 4-hydroxy acids to the lactones. Although β-lactones are present, it is only possible to make them undertaking special means. On the other hand, you can detect α-lactones in the form of transitory species when you undertake mass spectrometry experimentations.
It has been found that the different reactions of lactones are almost comparable to those of esters. Below are brief discussions on the three main types of lactone reactions.
When any lactone is heated together with a base (such as sodium hydroxide) it undergoes hydrolysis to form the lactone’s parent compound. Similar to the straight-chained esters, lactones’ hydrolysis-condensation is a reaction that can be reversed with some equilibrium. Nevertheless, the equilibrium that is constant of the lactone hydrolysis reaction is less than that of the esters. In other words, the resultant products of the reaction (hydroxyl acid) are not as favoured in the instance of lactones. The main reason for his is that while the enthalpies of ester hydrolysis as well as lactones are more or less the same, the entropy of lactone hydrolysis is lower compared to that of the straight-chained esters. After undergoing hydrolysis, straight-chained esters yield two products, which makes the entropy change additionally positive compared to the lactones which only yield a solitary product following hydrolysis.
This is another type of lactone reaction, wherein lactones are reduced to various other substances. For instance, lithium aluminum hydride works to condense lactones to diols. This reaction entails severing the ester bond and subsequently the carboxylic acid group is decreased to the alcohol group.
Lactones undergo various other types of reactions including polymerization, aminolysis and Michael reaction. In fact, lactones contribute to the formation of polyesters. Aminolysis involves a chemical reaction between lactones and ethanolic ammonia to yield amides and alcohol. Sesquiterpene lactones occur naturally in many plants and they possess the aptitude to react with various other types of molecules through a natural process known as Michael reaction.
Although we seldom realize this, much of the fruits’ flavours are attributed to the presence of lactones. In addition, they are also responsible for the odour of many fermented as well as unfermented dairy products. Hence, these compounds are widely used to flavour foods and add fragrance to them. Some good examples of such lactones are δ-decalactone (also known as 5-decanolide) and γ-decalactone (also referred to as 4-decanolide). While the flavour of δ-decalactone is peach and creamy coconut, the characteristic flavour of γ-decalactone is peach-like. Similarly, γ-dodecalactone (also called 4-dodecanolide) has a fruity/ coconut flavour, which is somewhat like that of γ-octalactone (called 4-octanolide), whose characteristic flavour is of herbaceous nature. The natural flavour of γ-nonalactone is very intense coconut like, something that does not occur in the coconut itself. Another flavoring natural lactone is γ-undecalactone.
The odour of macrocyclic lactones (such as 15-pentadec-11/12-enolide and cyclopentadecanolide) is somewhat like macrocyclic ketones having animal origin (such as civetone and muscone). However, these lactones can also be made more easily. For instance, they can be prepared by depolymerizing the relating linear polyesters. It is interesting to note that the odour of lactones hardly change even after a methylene unit is replaced by oxygen. Moreover, the reaction results in the production of oxalactones having 15-17-membered rings aside from cyclopentadecanolide (for instance 12-oxa-16-hexadecanolide).