Lactones, contribution

Of particular interest in terms of atmospheric chemistry are reactions of certain PAHs in VOC-NOx-air atmospheres to form biologically active polycyclic aromatic compounds, PACs. Thus, not only is the fundamental chemistry of the formation and fates of these secondary air pollutants of interest, but it can also have major toxicological implications. For example, in some airsheds certain PACs that are reaction products (e.g., nitro-PAH and nitro-PAH lactones) contribute signifi-... 

The text-book Walden-style cycle which interconverts the stereochemical configurations of chlorosuccinate (3) and malate (5) involves a /3-lactone intermediate (2) in preference to an a-lactone intermediate (4) (Scheme 2) because the OnUc-C-Cl angle in the transition structure for the former (174°) is more favourable than that for the latter (139°), as determined by PCM(e = 78.4)/B3LYP/6-31+G calculations the smaller ring-strain energy of the /3-lactone contributes little to the reactivity difference.2 (V,A-Dimethylethanolamine esterified hexanoic acid about 10-fold faster than did hexanol as a consequence of intermolecular hydrogen bonding a seven-membered transition state (6) was proposed.3... 

Some herbs are standardized for several active constituents, while others are standardized to a single active ingredient. St. John s wort is standardized to contain 0.3% hypericin, whereas ginkgo is standardized to contain 24% flavone glycosides and 6% ter-pene lactones. However, standardizing an herb product to one or more plant component(s) that are identifiable by assay may be incorrect. Many herbalists believe that the whole plant contributes to the efficacy and that there are many unknown active compounds in each plant [6]. 

As always, because of space limitations, only meso- and macrocycles possessing heteroatoms and/or subheterocyclic rings have been reviewed in general, lactones, lactams, and cyclic imides have been excluded. In view of the delayed availability of some articles appearing in previous years, several have been incorporated, where appropriate. I apologize in advance that it is impossible to do justice, to this topic and the numerous researchers that have elegantly contributed to the field, in the allotted twenty pages. 

Simple phenolic compounds include (1) the phenylpropanoids, trans-cinnamic acid, p-coumaric acid and their derivatives (2) the phenylpropanoid lactones called coumarins (Fig. 3.4) and (3) benzoic acid derivatives in which two carbons have been cleaved from the three carbon side chain (Fig. 3.2). More complex molecules are elaborated by additions to these basic carbon skeletons. For example, the addition of quinic acid to caffeic acid produces chlorogenic acid, which accumulates in cut lettuce and contributes to tissue browning (Fig. 3.5). 

The pyran-2-one ring system 174 (Scheme 68) is a potentially aromatic species, due to the contribution of the pyrylium-2-olate structure 174b, but facile cleavage of the ring by nucleophiles makes it most likely a lactone rather than an aromatic system. 

Lactone ring opening can be both enzymatic and nonenzymatic, but it appears that the two mechanistic routes are seldom distinguished in metabolic studies [2], Thus, a number of the reports published on metabolism of lactones assume enzymatic hydration and do not examine what the relative contribution of nonenzymatic processes may be. Like for lactonization reactions, only very few in vitro biochemical studies address the question of relative enzymatic contribution and mechanism. 

The mechanism of the tin(II) bis-(2-ethylhexanoate)-mediated ROP of lactones remained a matter of controversy for many years, and many different mechanisms were proposed. Indeed, tin(II) bis-(2-ethylhexanoate) is not made up of alkoxides but of carboxylates, known as poor initiators for the ROP of lactones. In 1998, Penczek and coworkers made a major contribution in this field. They reported that, if the polymerization is carried out in THF at 80 °C, then tin(II) bis-(2-ethylhexanoate) is converted in situ into a new tin alkoxide by the reaction with either an alcohol, purposely added in the reaction medium, or with any other protic impurity present in the polymerization medium (Fig. 14) [37]. Tin alkoxides formed in situ are the real initiators of the polymerization, which takes place according the usual... 

Table VII lists the important IR peaks given for the alkaloids isolated. The noreugenin moiety contributes peaks at 1550, 1660, and about 3300 cm for the C=C, C=0, and OH groups, respectively. In the pyridine-noreugenin congeners an additional band due to the lactone C=0 is seen at about 1750...

Although the valence bond description of pyran-2-one could include a zwitterionic contribution involving the carbonyl group, thus making the ring aromatic, there is little evidence to support this. For example, the IR carbonyl frequency (1740 cm ) is typical of an unsaturated lactone, and the chemical shifts of the ring protons in the H NMR spectrum indicate that there is no ring current. 

In related contributions, Flirstner disclosed a concise RCM-based approach to a family of potent herbicidal 10-membered lactones with an (E)-double bond, which led to the first total syntheses of herbarium I 227°" and II 228, and allowed also to establish the stereostructure of pinolidoxin 229 (Scheme 42). Again, the stereochemical... 

The lactones are the intramolecular esters of the corresponding hydroxy fatty acids. They contribute to the aroma of butter and various fruits. 15-Pentadeca-nolide is responsible for the musk-like odor of angelica root oil. Of the naturally occurring bicyclic lactones, phthalides are responsible for the odor of celery root oil, and coumarin for woodruff. 

Approximately 75 volatile compounds have been identified in juices prepared from plums Prunus domestica) [35]. Lactones from Ce to C12 are the major class of compound in plums [78]. The distribution of plum lactones differs from that found in peaches in that the C12 y-lactones are found in higher concentrations than the corresponding Cio y-lactones and d-decalactone (Fig. 7.2) [78]. GC sniffing has uncovered benzaldehyde, linalool, ethyl nonanoate, methyl cin-namate, y-decalactone and d-decalactone as volatile compounds contributing to plum juice aroma (Table 7.2, Figs. 7.1, 7.2, 7.4, 7.5) [35]. 

Sugars, acids and aroma compounds contribute to the characteristic strawberry flavour [85]. Over 360 different volatile compounds have been identified in strawberry fruit [35]. Strawberry aroma is composed predominately of esters (25-90% of the total volatile mass in ripe strawberry fruit) with alcohols, ketones, lactones and aldehydes being present in smaller quantities [85]. Esters provide a fruity and floral characteristic to the aroma [35,86], but aldehydes and furanones also contribute to the strawberry aroma [85, 87]. Terpenoids and sulfur compounds may also have a significant impact on the characteristic strawberry fruit aroma although they normally only make up a small portion of the strawberry volatile compounds [88, 89]. Sulfur compounds, e.g. methanethiol. 

Owing to their pleasant odours many y-lactones and d-lactones are known to be Important flavour compounds of fruits and contribute essentially to the characteristic and distinctive notes of strawberries, peaches, apricots and many other fruits [24]. Chiral aroma compounds from fruits and other natural sources are characterised by origin-specific enantiomeric ratios, as their biogenetic pathways normally are catalysed by enzymes. 

However, it should be kept in mind that the origin and natural occurrence of odd-numbered y-lactones is still not understood and their contribution to food flavour impression is rather limited or negligible, when trace amounts—far below their odour thresholds—are detected. 

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