Heterocyclic Flavor Compounds

1
Heterocyclic Flavor Compounds
2
Important Heterocyclic Flavor Compounds from
Nonenzymatic Reaction
R
C
H2
N
O
S
C
H
3
Thiophene
Furan
Pyrrole
N
C
H
CH3
3
N
N
Pyridine
Pyrazine
3
Furan

39
O

.
68 (M .
)
29
40
42
m/z

• Furan is a 6-electron heteroaromatic system and
  produces with an intense molecular ion. The
  energetically favored bond-cleavage is a
  carbon-oxygen bond and produces the ring-opened
  molecular ion
• Homolytic cleavage produces cyclopropenyl ion
  
• Heterolytic cleavage would form formyl ion

4
Furan
Formyl ion
4
3

-
C3H3


(
H
C
O
)
(
(
)
)
2
5
O
O
O
m/z 29
1

m/z 68
(1b)
(1a)
(1d)
- CHO
- CO
H
H
H
H
- H



Cyclopropenyl ion
m/z 39
m/z 40
(1c)
5
2-Methyfuran
C
H
3


-

CHO
)
(
(
)
(
)
C
H
O
O
C
H
O
3
C
H
3
3
(2b)
m/z 53
(2a)
(15.9 S)
S
- C3H3
(21.6
)
(2)
- C
H
O
2
3
(
base peak)
Methylcyclopropenyl ion

H
C
C
O
3
Cyclopropenyl ion
m/z 43
m/z 39
(4.4 S)
(20 S)

• The cleavage of the O-C2 or the O-C5 bond of
  2-methylfuran may result in two different
  ring-opened molecular ions (2a and 2b,
  respectively). These fragments produce the
  intense cyclopropenyl and methylcyclopropenyl
  ions.

6
2-Alkylfuran
-
C
H
5
2
g

b
C
H
C
H
C
H
O
b
2
2
3
O

m/z 81
m/z 110 (11.9
)
S
(43.1
)
S
Pyrilium ion
. b-fission becomes the dominant fragmentation
process.
7
2-Alkylfuran
a

or
R
O
O
O

• Cleavage to the furan ring with loss of the
  alkyl group is insignificant
• as it leads to an unfavored vinyl or
  diradical ion.

8
2-Alkylfuran
- CH
CHR
2


O
C
H
C
H
2
C
H
O
O

2
2
H
H
H
C
H
2
C
H
R
n-Butylfuran
n-Pentylfuran

n-Propylfuran
11.9 S
10.6 S
m/z 82 3.4 S
41 S
m/z 81 43.1 S
46.1 S
Pyrilium ion

• The C2 with n-propyl or longer causes a
  McLafferty rearrangement. 2-n-Butyl- and
  2-n-pentylfuran the lose propene and butene,
  respectively, results in m/z 82.

9
2-Alkenylfuran
-
H
C
H
C
H
C
H
C
H
O
C
H
C
H
H
O
2
2


m/z 107 (3.5 S)

Conjugated oxonium ion
M
m/z 108
(16.7 S)
CO
-
Benzonium ion
H
-


2
C
H
6
5
(8.1 S)
m/z 77
H
H
Phenyl ion
(15.1 S)
m/z 79

• 2-n-Propenylfuran lose H and produce the fully
  conjugated oxonium ion. It does not favor
  ring-opening fragmentation.
• Loss of CO forms the intense benzonium ion which
  further loses a molecule of hydrogen to give the
  phenyl ion.

10
2-(1-Pentenyl)furan
C
H
C
H
C
H
C
H
C
H
2
2
3
O
m/z 136
107
Cyclopentadienyl ion
94
135
Pyrilium ion
81
77
Phenyl ion
Benzonium ion
50
Cyclopentadiene radical
39
136
m/z
11
2-(1-Pentenyl)furan
H
H
CH2CH2CH3
H
H
H
m/z 108
- CO
Cyclopentadiene radical
CHCHCH2CH2CH3
- H
O
m/z 136
- CHO
H
H
- H2CCHCH3
H
CH2CH2CH3

H
Vinylfuran
m/z 107
CHCH2
Cyclopentadienyl ion
O
m/z 94
12
2-(1-Pentenyl)furan
Pyrilium ion
O

m/z 81

- CHCHCH2CH3
(55)
- CHCCH2CH3
(54)
CHCHCHCH2CH3
CHCHCH2CH2CH3
O
O

-H


m/z 136
-CO

H
CH2CH3
m/z 107
- CH2CH2
Benzonium ion
-
H


2
Phenyl ion
C6H5
H
H
m/z 79 m/z 77
13
2-Furanaldehyde

_
H
)
(


O
CHO
O
C
O
O
C
Furoyl ion

O

Furoyl ion
M
m/z 95 (21.2 S)
m/z 96 (21.8 S)
_
CO

_
CO
Cyclopropenyl ion.

O
m/z 39
m/z 67
(27.6 S)
(1.6 S)

• The mass spectra of 2-furanaldehydes have strong
  parent ion and M-1 ion, the resonance-stabilized
  furoyl cation. This fragment lose carbon
  monoxide, forming a cyclopropenyl ion.

14
2-Furyl alkyl ketones
furoyl ion
- C3H6
- C4H9
a

CH2
CH2
O
C
O
O
O
C
C
CH2
O
Furoyl ion
CH

O
H
H

CH3

• An intense furoyl ion is shown in 2-furyl alkyl
  ketones. If the side chain is n-butyryl or
  longer, the McLafferty rearrangement involving
  the carbonyl group becomes an important process.
  Thus, The base peak of the spectrum of
  2-n-valerylfuran is the formation of the furoyl
  ion.