Natural aroma chemicals for use in foods and beverages - Natural Food Additives, Ingredients and Flavourings - page 223

the structures below are shown in the trans form for the sole reason that the author
is too lazy to draw the cis -isomer as well and fi nds the trans -isomers to be more
aesthetically pleasing. 2-Methyl-4-propyloxathiane (tropathiane) [10.49] is an
example of cis-trans geometry across a ring.
[10.17] [10.28] [10.22] [10.49]
'Chirality', or optical isomerism, is a feature of many molecules used as
aroma chemicals. As our sensory system is based on biomolecules that are
themselves chiral, we can often differentiate enantiomers by their odour. The best
known, and perhaps most extreme, case is the enantiomers of carvone; the
R -isomer [10.50] is spearmint and the S -isomer [10.51] caraway. However, these
extremes are rare and more commonly both enantiomers have some shared
character, with one of the enantiomers being more powerful and more 'typical'
than the other.
In addition, and most importantly, most aroma chemicals are found as mixtures
of both isomers, i.e. even if one enantiomer dominates, the other is usually found
at a lower level. Very few, if any, aroma chemicals are found in nature in
'homochiral' form (corresponding to an enantiomeric excess of 100%). Similarly,
they are rarely found to be truly 'racemic', i.e. an enantiomeric excess of zero. We
can illustrate this with the enantiomers of hazeltone (5-methyl-2-hepten-4-one)
[10.52] and [10.53]. Both enantiomers have the nutty, buttery, metallic character
implied by the name, with the d-isomer [10.53] (which has the S -confi guration)
having the lower odour threshold, and both are found in nature: ca. 80:20 ( S:R ) in
raw hazelnuts and 70:30 in roasted hazelnuts.
￿ ￿ ￿ ￿ ￿
[10.50] [10.51] [10.52] [10.53]
With this in mind, we strongly wish to dispel the concept that 'only chiral (or
more strictly, homochiral) natural aroma chemicals are truly natural'. The reality
is both more complex and more interesting! An area where this has become
something of a cause celebre is alpha -ionone [10.54] and [10.55].