Enantiomers, of limonene

The two enantiomers of limonene have completely different tastes. One has the taste of lemon (as the name implies) and the other of orange. Assign R or S forms tc the two enantiomers of limonene. 

Although the different enantiomers of a chiral molecule have the same physical properties, they usually have different biological properties. For example, the (+) enantiomer of limonene has the odor of oranges, but the (-) enantiomer has the odor of pine trees. 

Numerous examples of the different biological effects of enantiomers are available. One of the enantiomers of limonene smells of lemons, the other of oranges one of carvone smells of caraway, the other of spearmint These differences obviously have important... 

Both the (+)- and (-)- enantiomers of limonene were transformed by larvae of the cutworm Spodop-tera litura (Miyazawa et al. 1998). For both of them the reactions involved are (a) dihydroxylation between C-8 and C-9 and (b) oxidation of the C-1 methyl group to carboxyl. These transformations were not dependent on the intestinal microflora in contrast to the transformation of a-terpinene to p-mentha-l,3-dien-7-ol and p-cymene whose formation could be attributed to the intestinal flora. 

Limonene is a monoterpene that occurs in citrus fruits. Two enantiomers of limonene produce two distinct flavours (—)-limonene is responsible for the flavour of lemons and (+)-limonene for orange. Similarly, one enantiomeric form of carvone is the cause of caraway flavour, while the other enantiomer has the essence of spearmint. 

Despite successes, many goals remain as yet unrealized, such as a widely applicable chiral solid catalyst for performing asymmetric synthesis of enantiomerically pure flavors, pharmaceuticals, etc. Two examples to elucidate the importance of the separate synthesis of enantiomers are limonene and asparagine while the (S)-enantiomer of limonene has citrus odour the (R)-enantiomer smells like orange while the (S)-enantiomer of asparagine has a bitter taste the (R)-enantiomer is sweet. 

Both enantiomers of limonene are available naturally. The product obtained from cnantiomeri-cally pure limonene is an epimeric mixture since the hydroboration of the terminal double bond... 

For example, the enantiomers of limonene smell and taste like oranges or lemons, the enantiomers of phenylalanine taste bitter or sweet, the enantiomers of carvone taste like spearmint or caraway, all depending on the absolute configuration. 

Figure 15. Energy-minimized tube (top) and space-filling molecular models of the enantiomers of limonene, (R)-(+)- on the left. (S)-(-)- on the right.

Figure 20.71 The enantiomers of limonene. The isomer found in the peel of citrus fruit is (+)-limonene. The chiral centre is shown by the asterisk... 

The human body can distinguish between enantiomers. For example, consider the enantiomers of limonene. The R-(+) enantiomer has the odor of orange and the S-(-) enantiomer the odor of lemon. The designation (+) indicates that plane polarized light is rotated in the clockwise direction or that the compound is dextrorotatory and (-) indicates counterclockwise rotation, also known as levorotatory. R and S indicate the positioning of the substituents on the chiral carbon. Some R compounds are dextrorotatory and other R compounds are levorotatory. R-(-) carvone has a spearmint odor and S-(+) carvone has a caraway odor. R-(-) amphetamine has a musty odor and S-(+) a fecal odor. There are examples of other compounds where enantiomers do not have different odors, but there are several examples that prove that enantiomers can have different odors [30]. 

Abdolahi A, Hassani A, Ghosta Y et al (2010) Essential oils as control agents of postharvest Altemaria and Penicillium rots on tomato fruits. J Food Safety 30 341-352 Aggarwal KK, Khanuja SPS, Ateeque A et al (2002) Antimicrobial activity profiles of the two enantiomers of limonene and carvone isolated from the oils of Mentha spicata and Anethum sowa. Flavour Fragr J 17 59-63... 

Figure 2.11 The molecuiar structure of the enantiomers of limonene (a, c) the chemical structure and ball-and-stick three-dimensional model of S-limonene (b, d) corresponding illustrations for R-limonene.

The two enantiomers of limonene (shown in the margin) smell quite differently. The S-isomer is present in the cones of spruce trees and has a turpentine-like odor the P-isomer gives oranges their characteristic fragrance. P-Limonene is a by-product of the juice industry and is the major constituent of citrus peel oil. Every year, more than 110 million pounds of the oil are made in the United States alone. Draw the respective products of the hydrogenation of both double bonds in (R)- and (S)-limonene. Are these products isomers, identical, chiral, achiral, or optically active/ inactive ... 

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