From: Paul Handley (p.handley$##$chemistry.uq.edu.au)
Date: Sun Feb 27 2000 - 23:26:39 EST
>Dear Friends:
>I need a simple procedure for the regio-selective allylic oxydationof
>Limonene (at the acyclic end). I am aware of Crawfords work (JACS, 1972,
>4298). HOwever, I am always getting about10% impurity, which is difficult
>to be seperated. I need pure alcohol.
>Since we cannot purchace n-BuLi used in this procedure, I normally make
>and titrate it against, a standard (say, diphenyl acetic acid).
>I would like to know, any other, cleaner and high yielding method for this
>oxydation.
>Krish M. Chary
I use this reaction in my own work, and I find it to be very reproducible,
yield is low, but is as stated by the authors. I always get the 10%
impurity which the previous authors did not notice in the days before
capillary gc, but I find it to be easily separated by careful column
chromatography on silica. Use a rough column first, elute the residual
limonene with hexane, then do a second column with plenty of silica (at
least 100:1) and 20% ether/hexane and the 9-hydroxy elutes just before the
impurity. This gives material pure by gc and nmr.
There was another way I thought of, ie epoxidise limonene with
perbenzimidic acid, separate the 8,9-epoxides from the 1,2-epoxides and
diepoxides by spinning band distillation (yuck) should give you under 50%
yield (Aust J Chem 1986 39 441), then theres ways to open epoxides to
allylic alcohols (see Larock, Comprehensive Organic Transformations p116
for some references) to give the 9-hydroxy, but this way will not be
simpler or cleaner.
Unless someone else can come up with a better way, I say stick with
Crawfords reaction, its not that bad really. I might change my tune if I
had to make my own BuLi though.
Paul Handley
Dept. of Chemistry
University of Queensland
Brisbane, Australia.
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