With three chiral centres how many isomers
will there be? The answer is eight. Click
to see the structures.
The answer is eight. Click to see the structures
The Isomers of Menthol
What is menthol used
It is one of the most important flavouring
chemicals, with over 6300 tonnes produced per year
isomer would you choose to make?
look at this chart, which shows
the flavour profiles of two of the the three isomers selected on
the basis of their cooling effect. A good choice would be high in
pleasant flavours and low in unpleasant ones.
The desired isomer
will give the greatest cooling effect and have the most pleasant
flavour. These two bar charts show the result of research into cooling
effect and flavour.
Noyori's catalyst gives a 100
% yield of the desired isomer in a synthesis from myrcene, obtained
The Takasago synthesis of menthol
The company has produced
1,000 metric tons per year of synthetic menthol by asymmetric hydrogenation
If you are interested in how the catalyst efficiency was developed,
The table shows how
the catalyst was developed over time. The "turnover number"
here is defined as the number of moles of product produced by 1
mole of catalyst Because of the expense of the catalyst the turnover
number should be more than 50 000
This chart shows the minimum quantity
of each isomer required for the cooling effect to be perceived.
Which three isomers would you select on
the basis of cooling effect?
The three requiring the smallest amount to produce the cooling effect:
1R, 3S, 4S
1S, 3S, 4R
1R, 3R, 4S
Which of the two isomers would you select
on the basis of both cooling effect and flavour?
1S, 3S, 4R, though scoring reasonably high in positive flavours, also
includes moderate scores in several negative ones. The isomer 1R, 3R,
4S scores very high in postive flavours and low in negative flavours,
and has a low cooling threshold. This is the best choice.