4 Analysis of Chiral Aroma Components in Trace Amounts R. Tressl, K.-H. Engel, W. Albrecht, and H. Bille-Abdullah
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Forschungsinstitut für Chemisch-technische Analyse, Technische Universität Berlin, Seestrasse 13, D-1000 Berlin 65, Federal Republic of Germany
Methods for the capillary gas chromatographic separation of optical isomers of chiral compounds after formation of diastereoisomeric derivatives were developed. Analytical aspects of the GC-separation of diastereoisomeric esters and urethanes derived from chiral secondary alcohols, 2-, 3-, 4- and 5-hydroxyacid esters, and the corresponding and lactones were investigated. The methods were used to follow the formation of optically active compounds during microbiological processes, such as reduction of keto-precursors and asymmetric hydrolysis of racemic acetates on a micro-scale. The enantiomeric composition of chiral aroma constituents in tropical fruits, such as passion fruit, mango and pineapple, was determined and possible pathways for their biosynthesis were formulated. C a p i l l a r y column g a s c h r o m a t o g r a p h y h a s p r o v e d t o b e a s u i t a b l e t e c h n i q u e f o r t h e d e t e r m i n a t i o n o f t h e enant i o m e r i c c o m p o s i t i o n o f c h i r a l compounds i n t r a c e amounts The g a s c h r o m a t o g r a p h i c r e s o l u t i o n o f c h i r a l s u b s t a n c e s can be a c h i e v e d e i t h e r b y s e p a r a t i o n o f e n a n t i o m e r s o n a n o p t i c a l l y a c t i v e s t a t i o n a r y phase o r by formation o f d i a s t e r e o i s o m e r i c d e r i v a t i v e s and a n a l y s i s on a n o n - c h i r a l p h a s e (2). S e p a r a t i o n o f enantiomers on c h i r a l phases w i t h o u t d e r i v a t i z a t i o n s have been d e s c r i b e d o n l y f o r a few comp o u n d s , s u c h a s m o n o t e r p e n e s (3)· M o s t s e p a r a t i o n s require d e r i v a t i z a t i o n o f t h e enantiomers, e.g. c o n v e r s i o n i n t o N - c o n t a i n i n g d e r i v a t i v e s (A) , a n d a t p r e s e n t t h e number o f a v a i l a b l e t h e r m a l l y - s t a b l e c h i r a l s t a t i o n a r y phases i s l i m i t e d . D u r i n g o u r s t u d i e s o f t h e b i o g e n e s i s o f c h i r a l comp o u n d s , we d e v e l o p e d m i c r o - m e t h o d s f o r t h e a n a l y s i s o f 0097-6156/85/ 0289-0043S06.00/ 0 © 1985 American Chemical Society
Bills and Mussinan; Characterization and Measurement of Flavor Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
44
CHARACTERIZATION AND MEASUREMENT OF FLAVOR COMPOUNDS
n a t u r a l l y o c c u r i n g trace-components u s i n g n o n - c h i r a l phas e s . I n t h i s p a p e r we d e s c r i b e two methods f o r f o r m i n g d i a s t e r e o i s o m e r i c d e r i v a t i v e s s u i t a b l e f o r gas c h r o m a t o graphic i n v e s t i g a t i o n s of a broad spectrum of c h i r a l h y d r o x y compounds, t h e f o r m a t i o n o f d i a s t e r e o i s o m e r i c e s t e r s o f R-(+)-oC-methoxy- °C - t r i f l u o r o m e t h y l p h e n y l a c e t i c a c i d (R-(+)-MTPA) , and o f d i s t e r e o i s o m e r i c u r e t h a n e s o f R- ( + ) - p h e n y l e t h y l i s o c y a n a t e (R-(+)-PEIC). By u s i n g h i g h r e s o l u t i o n c a p i l l a r y gas c h r o m a t o g r a phy, t h e s c o p e o f use o f t h e s e r e a g e n t s , w h i c h w e r e i n i t i a l l y i n t r o d u c e d f o r N M R - a n a l y s i s (5)and f o r GC s e p a r a t i o n o f s e c o n d a r y a l c o h o l s (6), was e n l a r g e d c o n s i d e r a b l y (Z'Q) · e a p p l i c a t i o n o f t h e s e methods t o t h e a n a l y s i s o f o p t i c a l l y a c t i v e compounds f o r m e d d u r i n g m i c r o b i o l o g i c a l p r o c e s s e s and t o t h e d e t e r m i n a t i o n o f t h e e n a n t i o m e r i c c o m p o s i t i o n o f c h i r a l aroma c o n s t i t u e n t s i n some t r o p i c a l f r u i t s i s d e s c r i b e d i n t h i s paper.
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A n a l y t i c a l aspects of the GC-separation of d i a s t e r e o i s o m e r i c R-(+)-MTPA and R - ( + ) - P E I C d e r i v a t i v e s Alcohols In Table I the data f o r the c a p i l l a r y GC-separation o f d i a s t e r e o i s o m e r i c R-(+)-MTPA e s t e r s and R - ( + ) - P E I C d e r i v a t i v e s o f some c h i r a l a l i p h a t i c a l c o h o l s a r e summar i z e d . For comparison, the data f o r the s e p a r a t i o n o f i s o p r o p y l u r e t h a n e d e r i v a t i v e s o f a l c o h o l e n a n t i o m e r s on an o p t i c a l l y a c t i v e s t a t i o n a r y p h a s e (9) a r e a l s o l i s t e d . I t i s o b v i o u s t h a t t h e s e p a r a t i o n s a r e s t r o n g l y dep e n d e n t on t h e s t r u c t u r e s o f t h e a l c o h o l s . The h i g h e s t s e p a r a t i o n f a c t o r s w i t h i n the s h o r t e s t time of a n a l y s i s were o b t a i n e d w i t h d i a s t e r e o i s o m e r i c u r e t h a n e d e r i v a t i v e s . The methods d e s c r i b e d a r e a l s o u s e f u l f o r t h e i n v e s t i g a t i o n of c h i r a l terpene a l c o h o l s . Figure 1 presents the s e p a r a t i o n of the e i g h t menthol stereoisomers. Neom e n t h o l ( 1 ) , n e o i s o m e n t h o l ( 2 ) , m e n t h o l (3) and i s o m e n t h o l (4) c a n be s e p a r a t e d and i s o l a t e d by p r e p a r a t i v e GC. By f o r m a t i o n o f R-(+)-MTPA e s t e r s , o n l y n e o m e n t h o l and m e n t h o l c o u l d be r e s o l v e d . I n c o n t r a s t , m e n t h o l s t e r e o i s o m e r s c o u l d be s e p a r a t e d a f t e r d e r i v a t i z a t i o n w i t h R - ( + ) - P E I C S c h u r i g and Weber (3) s e p a r a t e d t h e e n a n t i o mers o f m e n t h o l by c o m p l e x a t i o n c a p i l l a r y gas c h r o m a t o g r a p h y . B e n e c k e and Kônig (K>) i n v e s t i g a t e d i s o p r o p y l urethane d e r i v a t i v e s u s i n g a c h i r a l s t a t i o n a r y phase.
Bills and Mussinan; Characterization and Measurement of Flavor Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
Bills and Mussinan; Characterization and Measurement of Flavor Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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Downloaded by EAST CAROLINA UNIV on March 8, 2017 | http://pubs.acs.org Publication Date: November 8, 1985 | doi: 10.1021/bk-1985-0289.ch004
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C H A R A C T E R I Z A T I O N A N D M E A S U R E M E N T O F FLAVOR C O M P O U N D S
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F i g u r e 1. C a p i l l a r y GC s e p a r a t i o n o f m e n t h o l i s o m e r s as R-(+)-MTPA d e r i v a t i v e s (CP Wax 57 CB, 50 m/0.32 mm i . d . , 180 °C) and R - ( + ) - P E I C d e r i v a t i v e s (F I : CP Wax 57 CB, 50 m/0.32 mm i . d . , 230 °C; F I I : DB 210, 30 m/0.33 mm i . d . , 180 ° C ) .
Bills and Mussinan; Characterization and Measurement of Flavor Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
4. TRESSL ET A L .
2- a n d 3 - H y d r o x y a c i d
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47
Chiral Aroma Components in Trace Amounts
esters
Table I I presents the data f o r the GC-separation o f R-(+)-MTPA a n d R - ( + ) - P E I C d e r i v a t i v e s o f 2- a n d 3 - h y d r o xyacid esters. Whereas R-(+)-MTPA e s t e r s o f 2 - h y d r o x y a c i d s c a n b e r e s o l v e d o n p a c k e d columns (1J_) , t h e s e p a r a t i o n o f R - ( + ) MTPA d e r i v a t i v e s o f c h i r a l 3 - h y d r o x y a c i d s r e q u i r e s a n e f f i c i e n t c a p i l l a r y c o l u m n . The R - ( + ) - M T P A - e s t e r s p o s s e s s h i g h e r s e p a r a t i o n f a c t o r s t h a n t h e P E I C d e r i v a t i v e s , which depend o n t h e c h a i n l e n g t h o f t h e a c i d . The s e p a r a t i o n o f R-(+)-MTPA d e r i v a t i v e s o f 3 - h y d r o x y a c i d e s t e r s i s i n f l u e n c e d b y t h e s t a t i o n a r y p h a s e . The e f f i c i e n c y o f s e p a r a t i o n improves w i t h i n c r e a s i n g p o l a r i t y o f t h e phase, t h e h i g h e s t QC-value was o b t a i n e d w i t h a bonded t r i f l u o r o p r o p y l m e t h y l s i l i c o n e phase (1^2) .
T a b l e I I . S e p a r a t i o n D a t a f o r t h e C a p i l l a r y GCS e p a r a t i o n o f R-(+)-MTPA a n d R-(+)-PEIC D e r i v a t i v e s o f Some 2- a n d 3 - H y d r o x y a c i d E s t e r s R-(+)-MTPA Compound T(°C)
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