Page 181 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN007F-314 July 6, 2001 16:59

Heterocyclic Chemistry 337

O binationofsugarswithanthocyanidins,ofwhichflavylium
is the parent cation.
HN NH
H H
H S
S COOH S COOH
O
biotin alpha-lipoic acid +
flavylium cation
C. Six-Membered Rings with One Heteroatom Pyrones, pyrans, and their benzannelated derivatives
Benzene, pyridine, pyridinium, and pyrylium cations form can be obtained from the corresponding pyrylium salts.
an isoelectronic series with decreasing electron densities Diels–Alder reactions are also useful routes to pyrans.
at ring carbon atoms and have increasing susceptibility to Coumarins (benzo[b]pyrones) are often prepared by the
nucleophilic attack. von

O H 2 SO 4
+
N
OH EtO O O O
benzene pyridine
synthesis of 4-methylcoumarin
Pechmann reaction, in which phenols are condensed with
+
N O
+ beta-ketoesters in the presence of a catalyst. Coumarin
R itself is a flavor additive found in sweet clover. Vitamin
pyridinium pyrylium
E (alpha-tocopherol) possesses properties of antisterility
and is a fused pyran. In the body, vitamin E acts as an
Pyrylium salts are usually synthesized by condensation antioxidant which after oxidation can be regenerated by
methods. vitamin C (ascorbic acid).
Since they are very reactive, pyrylium salts undergo
many trasnformations, giving pyridines with ammonia, OH
HO
pyridinium salts with primary amines, and even benzenes O
HO
with certain carbon nucleophiles, all by S N ANRORC (CH 2 CH 2 CH 2 CHCH 3 ) 3 CH 3 HO
O OH
mechanisms (nucleophilic substitution with addition of HO
nucleophile, ring opening, and ring closure). vitamin E alpha-D-glucose
The hydroxypyran alpha-D-glucose is a main source
of energy for living organisms. Cellulose and starch are
MeNO 2
polymers of glucose.
t
Bu OK
O
+ _ Saturated or partially saturated pyran rings are con-
X NO 2
stituents of many pheromones such as serricornin, a prin-
cipal pheromone attractant secreted by the cigarette beetle,
a major pest of cured tobacco leaves, and (−)-alpha-
HX
+ (MeCO) 2 O multistriatin, an important component of the pheromone
OH bouquet of the elm bark beetle. One route that al-
O
+ _
X lows highly stereocontrolled placement of substitutents
involves a mercury(II)-catalyzed rearrangement of an
acetylenic epoxy alcohol.
CHO
Cl O O
+
O
CF 3 SO 3 H
+ O O
OH
+
Cl O O serricornin multistriatin
_
CHO 2CF 3 SO 3 O
OH
Benzannelated pyrylium salts are naturally abundant; H 2 SO 4
O
anthocyanin pigments of red and blue flowers are a com- Ph HgSO 4 O Ph