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Mass Spectroscopic Fragmentation Reactions, XXI [1] Factors Influencing the Loss of Xylene and Toluene from the Polyene Chain of Carotenoids

B . J o h a n n e s * , H . B r z e z i n k a , a n d H . B u d z i k i e w i c z

Institut für Organische Chemie der Universität K ö l n

Z. Naturforsch. 84b , 300-305 (1979); received N o v e m b e r 7, 1978

MS, Analysis, Carotenoids, Polyene Chain

B y examining suitably substituted analogs o f carotenoids the steric and electronic influences governing the elimination of xylene and toluene f r o m the polyene chain have been clarified.

Introduction t o l u e n e , M - 9 2 , a n d x y l e n e , M - 1 0 6 ) f r o m t h e p o l y e n e T h e m o l e c u l a r i o n s o f c a r o t e n o i d s lose i n a c h a i n [ 2 - 4 ] b y f o r m a l c l e a v a g e o f t h e d o u b l e b o n d s

c h a r a c t e r i s t i c m a n n e r h y d r o c a r b o n m o i e t i e s ( i .a . , ( S c h e m e 1) a s s h o w n b y d e u t e r i u m l a b e l l i n g s [ 5 - 7 ] .

Scheme 1**.

I n a n a l o g y t o t h e t h e r m a l d e g r a d a t i o n [ 8 - 1 2 ] a n d

b y a p p l i c a t i o n o f t h e W o o d w a r d - H o f f m a n n - r u l e s

[10, 12 , 1 3 ] t h e f o l l o w i n g s t e p s h a v e b e e n s u g g e s t e d

( S c h e m e 2 ) :

a) 8 e l e c t r o n c o n r o t a t o r y e l e c t r o c y c l i c r e a c t i o n ,

b) 6 e l e c t r o n d i s r o t a t o r y e l e c t r o c y c l i c r e a c t i o n ,

c) o p e n i n g o f t h e f o u r - m e m b e r e d r i n g .

T h e r e l a t i v e a b u n d a n c e s o f [M-92]+ a n d o f

[ M - 1 0 6 ] t d e p e n d u p o n t h e o v e r a l l s t r u c t u r e o f a

Requests f o r reprints should be sent to Prof . Dr . H . Budzikiewicz , Institut für Organische Chemie der Universität, Lehrstuhl I I , Greinstraße 4, D-5000 Köln 41. * New address: Staatl. Veterinär-Untersuchungsamt,

Eintrachtweg 17, D-3000 Hannover. ** Here and in the fol lowing schemes the analogous

elimination reactions f r o m the right-hand part o f the molecules have been omitted.

0340-5087/79/0200-0300/$ 01.00/0

g i v e n c a r o t e n o i d . T h u s , i t h a s b e e n s h o w n b y

E n z e l l [ 14] t h a t t h e i n t e n s i t y r a t i o R = I ([M-92] + )/

I ( [ M - 1 0 6 ] ! ) d e c r e a s e s w i t h t h e n u m b e r o f c o n j u g a t e d

d o u b l e b o n d s i n t h e c h a i n . E x t e n s i o n o f t h e c h r o m o -

p h o r e [12 , 1 5 ] b y a c a r b o n y l g r o u p or a n a r o m a t i c

r i n g r e s u l t s i n a n a c t i v a t i o n o f t h e a - d o u b l e b o n d

( p r o b a b l y b y e l e c t r o n i c e f f e c t s [12]) a n d i n a d e v i a -

t i o n f r o m t h e c a l c u l a t e d R v a l u e s (e.g., a 6 - k e t o g r o u p h a s a p p r o x i m a t e l y t h e s a m e e f f e c t a s a n

e x t e n s i o n o f t h e u n s a t u r a t e d c h a i n b y t w o C ,C-

d o u b l e b o n d s [ 1 5 ] w h i l e a p h e n y l g r o u p a m o u n t s t o

B. Johannes et al. • Mass Spectroscopic Fragmentation Reactions 301

o n e d o u b l e b o n d ) . S t e r i c i n f l u e n c e s m a y a l s o a l t e r

t h e v a l u e o f R : 2 ( = 1 , 2 2 - d i m e t h y l - l ) , e.g., s h o w s a b o u t t h e s a m e R a s 3. T h e p r e s e n c e o f t h e t w o

m e t h y l g r o u p s c o m p e n s a t e s f o r t h e a c t i v a t i n g in-

fluence o f t h e t w o p h e n y l r i n g s [12] . A s i m i l a r e f f e c t

h a s b e e n o b s e r v e d w i t h p r e n y l g r o u p s [16] .

Scheme 3. 1: R = H, 2: R — CH3.

T o o b t a i n s y s t e m a t i c i n f o r m a t i o n o n t h e c h a i n

f r a g m e n t a t i o n c o m p o u n d s 5 - 2 4 h a v e b e e n s y n t h e -

s i z e d . D e u t e r i u m l a b e l l i n g o f t h e c e n t r a l d o u b l e

b o n d a l l o w s t o d e t e r m i n e t h e o r i g i n e o f t h e e x p e l l e d

n e u t r a l s ince D s c r a m b l i n g h a s b e e n r e p o r t e d

[5, 6, 13] n o t t o o c c u r t o a n y m a j o r e x t e n t .

Results and Discussion

1. Xylene vs. toluene elimination

F r a n c i s s u g g e s t s [ 1 2 ] t h a t t h e l o s s o f x y l e n e is

f a v o r e d o v e r t h a t o f t o l u e n e b y a b o u t o n e o r d e r o f

m a g n i t u d e b e c a u s e i n t h e r a t e d e t e r m i n i n g first

c y c l i s a t i o n s t e p ( f o r m a t i o n o f c y c l o o c t a t r i e n e , see

S c h e m e 2) t h e r e is n o i n t e r f e r e n c e b y a m e t h y l

g r o u p . F o r t h e loss o f t o l u e n e , h o w e v e r , C - 9

( r e a c t i o n (2)) or C - 1 3 ' ( r e a c t i o n (3)) b e c o m e s q u a t e r -

n a r y . I f t h i s a s s u m p t i o n is c o r r e c t E 1 a n d 3 s h o u l d

b e h i g h l y f a v o r e d f o r 5 (no m e t h y l a t e i t h e r t e r -

m i n a l C o f t h e o c t a t e t r a e n e p o r t i o n s i n v o l v e d ) .

T h i s is c l e a r l y n o t t r u e f o r E l ( E 1 + 2 t o g e t h e r

y i e l d o n l y 9 % ) .

Compound

elimination (E) Me interference fragment M-rel. amount [ % ]

92

2 3 4 + — + 92 93 94

68 23

Scheme 4

T h e s e r e s u l t s l e a d t o t h e c o n c l u s i o n t h a t (a) a

m e t h y l g r o u p a t a t e r m i n a l C o f t h e o c t a t e t r a e n e

p o r t i o n d o e s r e d u c e t h e e l i m i n a t i o n r a t e (cf. S c h e m e 4, E 2 a n d 4) , (b) t h e b u l k y c y c l o h e x e n y l

r i n g d r a s t i c a l l y i n t e r f e r e s w i t h t h e e l i m i n a t i o n (see

S c h e m e 4, E 1), a n d (c) s ince " n o r m a l " c a r o t e n o i d s

(see S c h e m e 1) d o l o s e x y l e n e i n p r o c e s s 1 , x y l e n e

m u s t b e l o s t m o r e r e a d i l y t h a n t o l u e n e f o r e n e r g e t i c

r e a s o n s (cf. re f . [ 12] , p . 1449) . T h e s e c o n c l u s i o n s a r e

c o n f i r m e d b y t h e f o l l o w i n g r e s u l t s .

A s e x p e c t e d , a c o m p a r i s o n o f 6 a n d 7 s h o w s t h a t 6

l o o s e s x y l e n e ( a b o u t 1 0 % o f t h e s u m o f t h e t o l u e n e

e l i m i n a t i o n s ) w h i l e 7 d o e s n o t s h o w a n y M - 9 2 . I t

s h o u l d b e n o t e d t h a t t h e r a t i o f o r E 2 a n d 3 i s t h e

s a m e f o r 6 a n d f o r 7 t h o u g h E 2 s h o u l d b e less

f a v o r e d f o r 6 ( R = C H 3 ! ) . T h i s c a l l s f o r a n o v e r -

w h e l m i n g e f f e c t o f t h e b u l k y c y c l o h e x e n e r i n g e v e n

i f r e m o t e b y o n e e t h y l e n e u n i t .

Compound 6

eliminatiom (E) 1 2 3 1 2 3 Me interference — + + — — + fragment M- 106 93 94 92 93 94 rel. amount [ % ] 5 65 30 0 66 34

Scheme 5. 6: R = CH3, 7: R = H.

T h e i n f l u e n c e o f t h e t e r m i n a l c y c l o h e x e n e r i n g is

d e m o n s t r a t e d b y a c o m p a r i s o n o f 8 a n d 9. W h i l e 8

d o e s n o t g i v e a n y M - 9 3 (r ings a d j a c e n t a n d 1 e t h y -

l e n e u n i t r e m o t e ) , M - 9 4 ( E 2) is g r e a t l y f a v o r e d

f o r 9. N e v e r t h e l e s s , t h e s u m o f t h e t o l u e n e e l i m i n a -

t i o n s f r o m 9 a m o u n t s t o a b o u t 2 % o f t h e i n t e n s i t y

o f M+ a s c o m p a r e d w i t h a b o u t 2 5 % f o r 6.

302 B. Johannes et al. • Mass Spectroscopic Fragmentat ion React ions 302

9 : R = H , 10 : R = CH 3 .

Scheme 6.

T h e p r e f e r e d l o s s o f x y l e n e a g a i n b e c o m e s o b -

v i o u s f r o m t h e m a s s s p e c t r u m o f 10, t h e d i m e t h y l

Compound 8 9 10

elimination (E) 1 1 2 1 2 Me interference + — + — + + fragment M- 93 93 94 107 94 rel. amount [ % ] 0 21 79 80 20

h o m o l o g o f 9 ( c o m p a r e M - 9 3 vs. M - 1 0 7 ) . T h e s u m o f x y l e n e a n d t o l u e n e l o s s e s a m o u n t s t o a b o u t 1 0 % o f

t h e i n t e n s i t y o f M+.

I t is, t h e r e f o r e , n o t s u r p r i s i n g t h a t loss o f b e n z e n e

is n o t o b s e r v e d a t a l l a s c a n b e s e e n f r o m t h e m a s s

s p e c t r u m o f 11. A g a i n , i n s p i t e o f t h e n e i g h b o r i n g c y c l o h e x e n e r i n g l o s s o f x y l e n e is t h e d o m i n a n t

p r o c e s s f o r 1 2 .

Scheme 7. 11 : R = H , 12 : R = CH 3 .

C o m p o u n d 11 12

elimination (E) 1 2 3 4 5 1 2 3 4 5 Me interference — + + — + — + + f ragment M- 92 78 92 93 94 106 92 106 93 94 rel. amount [ % ] (33) 0 ( 3 3 ) 5 1 16 (76) 4 (76) 9 11

2. The terminal group

I t h a s b e n p o i n t e d o u t t h a t a t e r m i n a l c y c l o -

h e x e n e r i n g r e d u c e s t h e p r o b a b i l i t y o f t o l u e n e

e l i m i n a t i o n f r o m t h e e n d o f t h e p o l y e n e c h a i n . F o r

c o m p a r i s o n p u r p o s e s a n u m b e r o f a c y c l i c c o m p o u n d s

h a s b e e n s y n t h e s i z e d . A g a i n losses f r o m t h e e n d o f

t h e p o l y e n e c h a i n a r e l e a s t f a v o r a b l e ; t h e i n h i b i t o r y

i n f l u e n c e o f t h e b u l k y e n d g r o u p s e e m s t o d e c r e a s e

w i t h t h e d i s t a n c e f r o m t h e c h a i n e n d (14, t h o u g h t h e b a l a n c i n g e f f e c t s o f t h e d i s t a n c e a n d t h e p r e s e n c e o f

m e t h y l g r o u p s s e e m s t o b e r a t h e r s u b t l e , 14 a n d 15). F r o m 4 t h e o v e r r i d i n g e f f e c t o f t h e n u m b e r o f m e t h y l g r o u p s i n t h e e x p e l l e d m o i e t y c a n b e seen.

Compound 13 14

elimination (E) 1 2 1 2 Me interference + + + — f ragment M- 93 94 92 93 rel. amount [ % ] 8 92 7 30

el imination (E) Me interference f ragment M-rel. a m o u n t [ % ]

C o m p o u n d 15

Scheme 8. 1 5

B. Johannes et al. • Mass Spectroscopic Fragmentation Reactions 303

17, 20, 23

R 18, 21, 24

3 : R = H , 4 : R = D .

Scheme 9.

Compound 4

elimination (E) 1 Me interference + fragment M- 93 rel. amount [ % ] 9 ( f rom refs. [5] and [14])

2 3 4 — + +

106 92 92 75 16

T h e e x a m p l e s d i s c u s s e d s h o w c l e a r l y t h a t x y l e n e

is l o s t m u c h m o r e r e a d i l y t h a n t o l u e n e w h i l e

b e n z e n e is n o t e l i m i n a t e d a t a l l . B u l k y e n d g r o u p s

i n t e r f e r e s t r o n g l y w i t h t h e r i n g c l o s u r e ( S c h e m e 2,

s t e p 1 ) ; t h e i r i n f l u e n c e d e c r e a s e s w i t h t h e d i s t a n c e

f r o m t h e b o n d s i n v o l v e d in t h e c y c l i s a t i o n s t e p .

M e t h y l g r o u p s a t t h e e n d o f a t e t r a e n e m o i e t y

i n t e r f e r e w i t h s t e p 1 o f t h e c y c l i s a t i o n a s w e l l , b u t

t h e i r i n f l u e n c e is less p r o n o u n c e d .

16, 19, 22

3. Electronic effects

T h e p r e s e n c e o f a p h e n y l g r o u p c o n j u g a t e d w i t h

t h e p o l y e n e c h a i n c l e a r l y f o s t e r s c y c l i s a t i o n in-

v o l v i n g t h e n e a r e s t d o u b l e b o n d a s s h o w n b y a

c o m p a r i s o n o f 8 a n d 16 , 9 a n d 1 7 , 7 a n d 1 8 ( T a b l e I ,

cf. re f . [12]). Prima facie t h e i n f l u e n c e o f t h e e l e c t r o n d o n a t i n g (73-OCH3) a n d w i t h d r a w i n g (7J-NO2) g r o u p s

is less o b v i o u s . T h e r a t e o f t h e t e r m i n a l c l e a v a g e

s e e m s t o b e i n f l u e n c e d a s e x p e c t e d ( e n h a n c e m e n t b y

OCH3, l o w e r i n g b y N 0 2 ) i n t h e ser ies 18, 2 1 a n d 24 ( e l i m i n a t i o n 1) w h i l e t h e e f f e c t is r e v e r s e d f o r

e l i m i n a t i o n 2. T h i s c o u l d e x p l a i n w h y a p p a r e n t l y n o

d i f f e r e n c e s a r e o b s e r v e d f o r 1 7 , 20 a n d 2 3 : T h e

i n f l u e n c e s o f t h e t w o p h e n y l g r o u p s s h o u l d b e

o p p o s i n g e a c h o t h e r a n d w i t h t h e s h o r t e r c h a i n t h e

i n f l u e n c e o f t h e s e c o n d e n d g r o u p s h o u l d b e b i g g e r .

Table I. Toluene Elimination f r om compounds 16-24 . Compound R Fragment

M-92 M-93 M-94

16 H 0 0 100 (E 1) 19 N 0 2 0 0 100 22 OCHG 0 0 100 17 H 0 63 (E 1) 37 (E 2) 20 N 0 2 0 71 29 23 OCH 3 0 70 30 18 H 45 (E 1) 34 (E 2) 21 (E 3) 21 N 0 2 29 43 28 24 OCH3 58 21 21

T h e c o r o l l a r y o f o u r i n v e s t i g a t i o n is t h a t t h e

c o m p e t i n g r e a c t i o n s r e s u l t i n g i n t h e e l i m i n a t i o n o f

t o l u e n e a n d x y l e n e f r o m t h e p o l y e n e c h a i n o f

c a r o t e n o i d s are g o v e r n e d (a) b y t h e p r e f e r e d loss o f

x y l e n e as c o m p a r e d w i t h t o l u e n e , (b) t h e d i s t a n c e

304 B. Johannes et al. • Mass Spectroscopic Fragmentat ion React ions 304

f r o m t h e b u l k y e n d g r o u p , (c) t h e i n t e r f e r e n c e o f

m e t h y l s u b s t i t u e n t s , a n d (d) f o r a r o m a t i c c a r o t e n o i d s

t h e a c t i v a t i n g i n f l u e n c e o f t h e c o n j u g a t e d b e n z e n e

r i n g s . C o m p a r i s o n o f R v a l u e s s h o u l d , t h e r e f o r e , b e

r e s t r i c t e d t o c o m p o u n d s o f s i m i l a r s t r u c t u r e s ince

a c t i v a t i n g a n d d e s a c t i v a t i n g i n f l u e n c e s g i v e a r a t h e r

c o m p l e x o v e r a l l p i c t u r e .

CHO

Experimental

M a s s s p e c t r o m e t e r M A T 7 3 1 ( V a r i a n M A T , B r e -m e n ) ; 8 k V , 1 3 - 2 0 o r 100 e V , p r o b e 1 4 0 - 2 6 0 °C, s o u r c e 250 °C.

T h e c o m p o u n d s w e r e s y n t h e s i z e d b y W i t t i g r e a c t i o n s ( T a b l e I I ) , t h e s t a r t i n g m a t e r i a l s w e r e p r e p a r e d b y p r o c e d u r e s d e s c r i b e d i n t h e l i t e r a t u r e

Table I I . Synthesis o f Compounds 4—24.

6 A + 2 C 15 B + 2 J 5 B + 2 C 16 A + 2 K

11 B + 2 F 17 A + 2 M 12 A + 2 G 18 B + 2 M

8 D + E ( C H 3 O D / N a O D ) 19 R + 2 0 ( + D a ) 9 A + 2 H 20 R + 2P ( + D 2 ) 7 B + 2 H 21 R + 2 Q ( + D 2 )

10 A + 2 D 22 A + 2 L 13 A + 2 I 23 A + 2 N 14 A + 2 J 24 B + 2 N ( X = P+(C 6 H 5 ) 3 Br- )

K: R = H

L: R = OCH„

M: R = H

N: R = OCH„

W e w i s h t o t h a n k D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t f o r t h e m a s s s p e c t r o m e t e r a n d F o n d s d e r C h e -m i s c h e n I n d u s t r i e f o r f i n a n c i a l a s s i s t e n c e .

B. Johannes et al. • Mass Spectroscopic Fragmentation Reactions 305

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