Abstract
Dimeric phenolic compounds - lignans and dilignols - form
in the so-called oxidative coupling reaction of phenols.
Enzymes such as peroxidases and laccases catalyze the
reaction using hydrogen peroxide or oxygen, respectively,
as oxidant generating phenoxy radicals which couple
together according to certain rules. In this thesis, the
effects of the structures of starting materials -
monolignols - and the effects of reaction conditions such
as pH and solvent system on this coupling mechanism and
on its regio- and stereoselectivity have been studied.
After the primary coupling of two phenoxy radicals a very
reactive quinone methide intermediate is formed. This
intermediate reacts quickly with a suitable nucleophile
which can be, for example, an intramolecular hydroxyl
group or another nucleophile such as water, methanol, or
a phenolic compound in the reaction system. This reaction
is catalyzed by acids. After the nucleophilic addition to
the quinone methide, other hydrolytic reactions,
rearrangements, and elimination reactions occur, leading
finally to stable dimeric structures called lignans or
dilignols. Similar reactions occur also in the so-called
lignification process when monolignol (or dilignol)
reacts with the growing lignin polymer. New kinds of
structures have been observed in this thesis. The dimeric
compounds with a so-called spirodienone structure have
been observed to form both in the dehydrodimerization of
methyl sinapate and in the -1-type cross-coupling
reaction of two different monolignols. This -1-type
dilignol with a spirodienone structure was the first
synthesized and published dilignol model compound, and at
present, it has been observed to exist as a fundamental
construction unit in lignins. The enantioselectivity of
the oxidative coupling reaction was also studied for
obtaining enantiopure lignans and dilignols. A rather
good enantioselectivity was obtained in the oxidative
coupling reaction of two monolignols with chiral
auxiliary substituents using peroxidase/H2O2 as an
oxidation system. This observation was published as one
of the first enantioselective oxidative coupling reaction
of phenols. Pure enantiomers of lignans were also
obtained by using chiral cryogenic chromatography as a
chiral resolution technique. This technique was shown to
be an alternative route to obtain enantiopure lignans or
lignin model compounds in a preparative scale.
Original language | English |
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Qualification | Doctor Degree |
Awarding Institution |
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Supervisors/Advisors |
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Award date | 27 Sept 2008 |
Place of Publication | Espoo |
Publisher | |
Print ISBNs | 978-951-38-7110-9 |
Electronic ISBNs | 978-951-38-7111-6 |
Publication status | Published - 2008 |
MoE publication type | G5 Doctoral dissertation (article) |
Keywords
- regioselectivity
- stereoselectivity
- oxidative coupling reactions
- phenols
- spirodienones
- lignans
- dilignols
- dehydrodimerization
- peroxidases
- chirality
- pH
- catalysis