AN IMPROVED SYSTEM FOR THE ENUMERATION AND DESCRIPTION O F RING SYSTEMS B Y JOHN A . SILK Imperial Chemical Industries Ltd., Jealott's Hill Research Station, Bracknell, Berkshire, England
T h e m e t h o d s which have been developed i n r e c e n t y e a r s f o r t h e s y s t e m a t i c d e s c r i p t i o n and e n u m e r a t i o n of r i n g s t r u c t u r e s f a l l m a i n l y i n t o two c a t e g o r i e s . In one c l a s s a r e those1p2v3 b a s e d e s s e n t i a l l y on t r a c i n g a continuous path through t h e a t o m s f o r m i n g a r i n g s y s t e m , and i n t h e o t h e r a r e those4v5s6 i n which individual r i n g s a r e built up one by o n e . While t h e f o r m e r m e t h o d s a p p e a r , a t l e a s t i n s o m e i n s t a n c e s , t o be s i m p l e r t o apply, t h e r e c o n s t r u c t i o n of a conventional r i n g - d i a g r a m f r o m t h e l i n e a r d e s c r i p t i o n of i t is l e s s s t r a i g h t f o r w a r d , b e c a u s e t h e individual r i n g s c o m p r i s i n g a s y s t e m a p p e a r only by f o r m ing cross-1inka:es i n t h e d e s c r i b e d p a t h . Metho d s i n t h e second c a t e g o r y undoubtedly a r e e a s i e r t o i n t e r p r e t , and t h i s i s b e c a u s e t h e y follow m o r e c l o s e l y t h e u s u a l way of r e g a r d i n g ring s y s t e m s ,i . e . , i n t e r m s of individual, s m a l l r i n g s fused t o g e t h e r i n v a r i o u s w a y s . The m o s t c o m p l e t e l y developed method of t h i s type h a s been t h a t of T a y l o r - P a t t e r s o n Dyson ( T - P - D ) , 6 and t h i s p a p e r p r e s e n t s m o d i f i c a t i o n s of t h e i r s y s t e m which yield i m p r o v e d e n u m e r a t i o n p a t t e r n s , show t h e c h a r a c t e r i s t i c t y p e s of r i n g fusion c l e a r l y , and enable m o l e c u lar f o r m u l a s t o b e c a l c u l a t e d f r o m t h e s y s t e m a t i c e x p r e s s i o n s without any additional i n f o r m a t i o n . It w i l l b e a p p a r e n t t h a t t h e application of t h e s e m e t h o d s l i e s m a i n l y i n t h e f i e l d of c h e m i c a l n o t a t i o n , although they could a l s o provide a b a s i s f o r c l a s s i f y i n g r i n g s y s t e m s , and a p r o cedure for converting ciphers to systematic n a m e s h a s been d e s c r i b e d . I The T - P - D method r e q u i r e s t h a t e n u r n e r a tion shall commence i n the l a r g e s t ring at i t s point of fusion with t h e next s e n i o r r i n g , and p r o c e e d i n t h e m a n n e r which g i v e s t h e l o w e s t sequence of n u m b e r s t o t h e ring-fusion p o i n t s . In s y m m e t r i c a l s y s t e m s w i t h all r i n g s of equal s i z e t h e b a s i c p a t t e r n of e n u m e r a t i o n i s , t h e r e f o r e , a s p i r a l one; e n u m e r a t i o n s t a r t s i n t h e c e n t e r r i n g and m o v e s outward on s u c c e s s i v e o r b i t s u n t i l t h e whole s t r u c t u r e h a s been e m braced (Fig. 1 ) . In many other c a s e s , however, i n which t h e s y s t e m i s a s y m m e t r i c , o r t h e c e n t r a l r i n g i s not t h e l a r g e s t o n e , e n u m e r a t i o n h a s t o c o m m e n c e o f f - c e n t e r , and t h i s r e s u l t s (i) i n
Fig. 1.-The spiral pattern of enumeration in symmetrical systems with like rings.
r e l a t e d s y s t e m s , which d i f f e r only i n s i z e s a n d / o r s e q u e n c e s of r i n g s , having quite d i f f e r e n t p a t t e r n s of e n u m e r a t i o n , and (ii) i n a g r e a t e r n u m b e r of o r b i t s of t h e g e n e r a t i n g r i n g being required to complete the enumeration. (In Fig. 2 , left-hand column, c o m p a r e A , B and C; D and E ; F and G.) It h a s now been found t h a t , i f a l l s y s t e m s a r e t o be d e s c r i b e d with m a x i m u m i n t e r m s of t h e f e w e s t and s m a l l economy, k, e s t o r b i t s , and i f r e l a t e d s y s t e m s a r e t o be enumerated similarly, the enumeration must always s t a r t i n the m o s t c e n t r a l ring ( r e g a r d l e s s of s i z e ) , and f u r t h e r , t h a t i t m u s t p r o c e e d i n t h e s a m e d i r e c t i o n throughout and i n t h e m a n n e r which gives not t h e lowest s e q u e n c e of l o The validity of c a n t s , but t h e l o w e s t =. t h e s e findings as g e n e r a l p r i n c i p l e s h a s been r e i n f o r c e d s t r o n g l y by t h e exploration of t h e new method explained below f o r d e s c r i b i n g and enumerating peri-fused systems; the satisfact o r y application of t h i s method m a k e s i t e s s e n t i a l f o r a l l t h e component r i n g s of a s y s t e m t o be e n u m e r a t e d i n t h e s a m e d i r e c t i o n ( c l o c k w i s e o r anticlockwise) and i n t h e m a n n e r which g i v e s t h e l o w e s t t o t a l of l o c a n t s . In connection with t h e s e modifications i t m a y be noted t h a t i n t h e W i s w e s s e r s y s t e m , 2 which u s e s a continuous-path m e t h o d , t h e c o r responding r u l e i s a l s o a "lowest t o t a l " one, and m a t h e m a t i c a l s u p p o r t h a s been c l a i m e d f o r t h i s . F u r t h e r m o r e , i f one d r a w s a n analogy with e l e c t r o n o r b i t s i n a t o m s , i t m a y be s e e n t h a t t h e d e s c r i p t i o n of a s y s t e m i n t e r m s r e q u i r i n g m o r e
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A SYSTEM F O R ENUMERATION O F RING SYSTEMS
W f i e d mes
T-P-D Riles
!?:ants
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(Unchanged) Cipher:
1 4
C3(4)
1 4
17
59
The notation accompanying the e x a m p l e s a l s o d i f f e r s slightly f r o m the T - P - D o n e . C n d e n o t e s a s y s t e m of g f u s e d r i n g s ( c y c l e s ) , and the fusion-point l o c a n t s a r e c i t e d a f t e r t h i s i n p a r e n t h e s e s . Since the f i r s t locant i s 1 i n a l l c a s e s , t h i s n o r m a l l y m a y be o m i t t e d and only t h e subsequent o n e s quoted. In addition, s i n c e s i x - m e m b e r e c i , a r o m a t i c r i n g s a r e by- f a r t h e c o m m o n e s t , i t m a y be u n d e r s t o o d that all a r e of t h i s t y p e i n t h e a b s e n c e of o t h e r i n d i c a t i o n s . S u c c e s s i v e l o c a n t s a r e s e p a r a t e d by s t o p s . O t h e r s i z e s of r i n g m a y be shown by s u b s c r i p t s to t h e a p p r o p r i a t e l o c a n t s , or by :n i f i t i s d e s i r e d t o avoid t h e u s e of s u b s c r i p t s ( a s o n punched c a r d s ) . When t h e f i r s t r i n g i s not s i x m e m b e r e d , l o c a n t 0 i s i n t r o d u c e d to c a r r y the r i n g - s i z e n u m b e r , and f o r t h e second r i n g l o c a n t 1 m a y be u s e d t o c a r r y a s u b s c r i p t ( a s in t h e s e e x a m p l e s ) , o r :n m a y be u s e d on i t s own.
R E TIC U LAR SYSTEMS " R e t i c u l a r " is u s e d h e r e in the l i m i t e d s e n s e i n which i t w a s defined by T a y l o r 4 and w a s u s e d i n m y e a r l i e r s y s t e m of n ~ t a t i o n . ~ B a s i c a l l y , i t d e n o t e s t h e p e r i - f u s e d type of s y s t e m , t h e c h a r a c t e r i s t i c unit of which is
W(3.7)
1
7 11
In t h i s t h e e n c i r c l e d c e n t r a l a t o m , which m a y conveniently be r e f e r r e d t o a s t h e atom, i s c o m m o n t o a g r o u p of t h r e e r i n g s , and t h e t h r e e a t o m s joined to t h i s a r e e a c h c o m m o n to one of t h e t h r e e p a i r s of r i n g s . With t h e T - P - D r u l e s s y s t e m s of t h i s type often r e q u i r e a hyphenated l o c a n t t o show a link between n o n - c o n s e c u t i v e p o s i t i o n s , and a r e v e r s a l i n t h e d i r e c t i o n of e n u m e r a t i o n p a t t e r n i n s o m e r i n g s , which d e s t r o y s t h e s y m m e t r y of the enumeration pattern (3A,3B). F r o m the r e sulting e x p r e s s i o n s t h i s c h a r a c t e r i s t i c and f u n d a m e n t a l type of fusion cannot be r e c o g n i z e d readily, n o r molecular f o r m u l a s calculated directly.
2-
14812
C5(4.8.12)
Fig. 2.--Comparison of enumerations of other typical systems.
t h a n t h e m i n i m u m n u m b e r of o r b i t s about t h e s t a r t i n g r i n g c o r r e s p o n d s t o t h e d e s c r i p t i o n of a s t a t e of h i g h e r e n e r g y . Only t h e l o w e s t t o t a l r u l e e n a b l e s a l l s y s t e m s t o be d e s c r i b e d i n t h e i r ground s t a t e s . T h e m o d i f i c a t i o n s to which t h e s e r u l e s give r i s e a r e illustrated i n Fig. 2 , right-hand column. It c a n be s e e n t h a t the e n u m e r a t i o n of C now c l o s e l y r e s e m b l e s t h a t of A and B and t h a t f e w e r o r b i t s of t h e g e n e r a t i n g r i n g have t o be m a d e . S i m i l a r c o n s i d e r a t i o n s apply t o E and G, i n r e l a t i o n t o D and F, r e s p e c t i v e l y .
@J +S 7
(A)
b
1,3-10 Fig. 3.-T-P-D
(B)
1,3,5,7-%10,14
enumeration of reticular systems.
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JOHN A. SILK
The nucleus of a g r o u p of r i n g s i n r e t i c u l a r fusion i s the _Y-atom, anti t h e s t a g e s by which t h e s i m p l e s t r e t i c u l a r s t r u c t u r e should be built up s y s t e m a t i c a l l y a r e shown i n (4A). To t h e s t a r t i n g ring i s a d d e d , f i r s t l y , a t h r e e - a t o m , r e t i c u l a r adduct; t h i s d o e s not i n i t s e l f c o m p l e t e the second r i n g , but i t f o r m s t h e b r i d g e between t h e s t a r t i n g r i n g and the next r i n g - a d d u c t , which s t a r t s f r o m the 2-position. This second adduct, which c o m p l e t e s t h e s t r u c t u r e , contains t h e n o r m a l n u m b e r of f o u r a t o m s , b e c a u s e i t j o i n s d i r e c t l y onto i t s p a r e n t ring ( a t the 3 - p o s i t i o n ) .
the n u m b e r s . F u r t h e r e x a m p l e s of r e t i c u l a r s y s t e m s a r e a r e e n c i r c l e d in shcwn i n F i g . 5. The Y--atoms t h e s e , and i t c a n be s e e n that they a r e a l w a y s a t a position one higher than t h e locant with which they a r e u s e d . Since a r e t i c u l a r adduct cont a i n s one a t o m l e s s than a n o r m a l adduct, e a c h Y-symbol r e q u i r e s CH t o be s u b t r a c t e d f r o m the g e n e r a l f o r m u l a C 4 n t 2 H ~ n t 4 f o r a s y s t e m of six-membered, edge-fused, a r o m a t i c rings.
-
CJ(Y.Z).H
& 1
I9
'(
Fig. 5.-Simple reticular systems,
C5(Y. 2Y.jY.4)
Fig. 4.-Analysis
of reticular systems.
(4B) shows t h a t s e v e r a l r e t i c u l a r a d d u c t s , e a c h of t h r e e a t o m s , c a n be added i n s u c c e s s i o n , and, a s i n (4A), t h e s t r u c t u r e i s c o m p l e t e d by a n o r m a l , f o u r - a t o m adduct. ( 4 C ) shows t h e addit i o n of a c o m p l e t e c y c l e of r e t i c u l a r a d d u c t s , giving c o r o n e n e . In t h i s c a s e the s i x t h r e t i c u l a r adduct c o m p l e t e s t h e s t r u c t u r e by bridging t h e s t a r t i n g r i n g and t h e f i r s t adduct, s o t h a t t h e r e is no f o u r - a t o m adduct. T h i s a n a l y s i s c l e a r l y d i s p l a y s t h e p e r f e c t s y m m e t r y of the c o r o n e n e s t r u c t u r e , and i t e n a b l e s t h e p e r i m e t e r of t h i s s y s t e m to be e n u m e r a t e d i n one r e g u l a r and continuous s e q u e n c e . By t h e T - P - D m e t h o d , on t h e o t h e r h a n d , c o r o n e n e i s built up i n two unequal s t a g e s v i a t r i p h e n y l e n e , and, b e c a u s e t h e d i r e c t i o n of e n u m e r a t i o n i s r e v e r s e d i n one r i n g , t h e s y m m e t r y i s d e s t r o y e d (3B). R e t i c u l a r fusion c a n be shown s i m p l y by i n s e r t i n g _Y a f t e r t h e a p p r o p r i a t e l o c a n t s . T h u s , f o r (4A) w e m a y w r i t e C3(Y.2).H t o denote t h a t t h e adduct s t a r t i n g a t t h e 1-position i s i n r e t i c u l a r fusion with t h e adduct s t a r t i n g a t the 2 position (H s p e c i f i e s t h e e x t r a h y d r o g e n ) . E n u m e r a t i o n continues smoothly f r o m the r e t i c u l a r adduct t o t h e n o r m a l one. P o s i t i o n 10 m i g h t be r e g a r d e d as belonging equally to t h e second or t o t h e t h i r d r i n g , but s y s t e m a t i c a l l y i t is c o r r e c t to a s s o c i a t e i t with t h e l a t t e r . In t h e e x a m p l e s t h i s a n a l y s i s i s shown by the positioning of
Fig. &--Complex retfcular systems,
E] Y
CZ(U2)H
CP(U)H
CP(X)H
Y
W(U2.5.7)H
C5(U2. Z5?5.7)H
Fig. 1.--Bridged and spiro systems.
A SYSTEM F O R ENUMERATION O F RING SYSTEMS
C o m p a r a t i v e l y few known r i n g s y s t e m s exhibit a g r e a t e r d e g r e e of condensation t h a n c o r o n e n e , but a n a l y s i s i n t e r m s of t h e i r Y - a t o m s e n a b l e s t h e m a l l t o be e n u m e r a t e d and d e s c r i b e d s y s t e m a t i c a l l y with r e m a r k a b l e s i m p l i c i t y . T h e r e a r e two d i f f e r e n t s i t e s a t which f u r t h e r r i n g s c a n be added to c o r o n e n e . At o n e , f o r e x a m p l e , position 8 , a n o r m a l adduct i s r e q u i r e d , and a t t h e o t h e r , for e x a m p l e , position 9 , a r e t i c u l a r one i s n e e d e d , while when two a r e added i n s u c c e s s i o n both a r e r e t i c u l a r . T h i s i s shown i n F i g . 8 ( A , B , C ) , and i t m a y be noted that, because rings a r e enumerated i n the s a m e d i r e c t i o n , i t would be i n c o r r e c t in t h e second c a s e f o r t h e adduct t o s p a n p o s i t i o n s 8 and 24, i n s t e a d of 9 and 11.
61
OTHER TYPES O F RING FUSION The d e s c r i p t i o n of b r i d g e d and s p i r o s y s t e m s a l s o benefits f r o m t h e i n t r o d u c t i o n of s p e c i a l s y m b o l s t o signify t h e s e c h a r a c t e r i s t i c t y p e s of fusion. When fusion of two r i n g s t a k e s p l a c e along two f a c e s i n s t e a d of only one f a c e , U i s p l a c e d a f t e r t h e l o c a n t t o denote t h e o c c u r r e n c e of a o n e - a t o m b r i d g e . S i m i l a r l y , U 2 , U3 and U4 a r e u s e d f o r b r i d g e s of two, t h r e e and four a t o m s , r e s p e c t i v e l y . F o r s p i r o f u s i o n , which involves only one a t o m i n s t e a d of two, X i s p l a c e d a f t e r t h e l o c a n t . It i s c l e a r that U' r e q u i r e s t h e s u b t r a c t i o n of 2 c a r b o n s i n c a l c u l a t i o n s of m o l e c u l a r f o r m u l a e , and X t h e a d d i t i o n of one c a r b o n i n r e l a t i o n to t h e f o r m u l a f o r o r t h o - f u s e d s y s t e m s . S y s t e m s of t h e s e t y p e s ( F i g . 7 ) a r e m a i n l y s a t u r a t e d o n e s , and i n t h e c i p h e r s t h e addition of H i m m e d i a t e l y a f t e r the c l o s i n g p a r enthe s i s d e n o t e s c o m p l e t e hydro genation. Combined r e t i c u l a r and b r i d g e d fusion i s e n c o u n t e r e d o c c a s i o n a l l y , a s in h e x a m i n e 8 B ) . Since r e t i c u l a r fusion i n v o l v e s two o t h e r r i n g s , U p r e c e d e s Y when i t r e f e r s to t h e f i r s t of t h e s e r i n g s , and U follows Y when i t r e f e r s t o the s e c o n d . In o r d e r t o r e c o g n i z e r e t i c u l a r fusion, i t i s helpful t o d r a w out s u c h s y s t e m s i n a plane f o r m , avoiding c r o s s e d bonds a s f a r a s p o s s i b l e . In t h e r a r e c a s e s w h e r e a b r i d g e s p a n s two o r m o r e r i n g s and c a n n o t , t h e r e f o r e , be t r e a t e d a s a r e t i c u l a r s y s t e m , i t s t e r m i n a t i o n m u s t be shown by citing t h e a p p r o p r i a t e l o c a n t , and t h e s e a r e now t h e only s i t u a t i o n s which r e q u i r e a p a i r of l o c a n t s to be joined by a hyphen. When t h e a p p l i c a t i o n of the r u l e s of t h e l o w e s t t o t a l , and then t h e lowest s e q u e n c e , of l o c a n t s s t i l l l e a v e s a c h o i c e , t h e p r e f e r r e d exp r e s s i o n i s t h e one giving t h e l o w e s t locant to the l a r g e s t r i n g , and then t o the s e n i o r type of f u s i o n (i) r e t i c u l a r , (ii) n o r m a l ( s i m p l e o r t h o t y p e ) , (iii) s p i r o , and ( i v ) b r i d g e .
(cf.
Fig. 8.-Corrbined
reticular and bridge fusion.
The addition of t h r e e s u c c e s s i v e a d d u c t s l e a d s t o a new, h i g h e r d e g r e e of c o n d e n s a t i o n , i n which t h e m i d d l e r i n g i s f o r m e d by t h e a d d i tion of only two a t o m s ( F i g . 8 D , E ) . A n a l y s i s of t h e g e n e r a l s i t u a t i o n shows t h a t w h e r e a s t h e c o n v e r s i o n of benzene t o c o r o n e n e r e q u i r e s s i x s i n g l e r e t i c u l a r a d d u c t s , e a c h of t h r e e a t o m s , the c o n v e r s i o n of c o r o n e n e t o " c o r o n o c o r o n e n e " r e q u i r e s s i x p a i r s of a d d u c t s , e a c h p a i r cons i s t i n g of five ( 3 t 2 ) a t o m s . T h e s y m b o l Y Y m a y be u s e d to signify addition of t h i s t w o - r i n g , f i v e - a t o m a d d u c t . S i m i l a r l y , Y Y Y would denote a t h r e e r i n g - a d d u c t of 3 t 2 t 2 a t o m s , i f s y s t e m s of t h i s c o m p l e x i t y b e c a m e known. When l e s s t h a n a c o m p l e t e c y c l e of t h e s e a d d u c t s is r e q u i r e d , t h e l a s t one i n a s e r i e s n e c e s s a r i l y c o n t a i n s one o r two additional a t o m s , and i t w i l l , t h e r e f o r e , be c i p h e r e d i n a s e p a r a t e o p e r a t i o n . Y Y r e q u i r e s t h e s u b t r a c t i o n of C3H3 f r o m t h e f o r m u l a calculated f o r a simple ortho-fused system.
REVERSAL O F ENUMERATION The obvious d i s a d v a n t a g e of both t h e T - P - D m e t h o d s and t h e p r e s e n t d e v e l o p m e n t s of t h e m is t h a t they a s s i g n t h e low n u m b e r s t o t h e f u s e d p o s i t i o n s , s o t h a t even t h e s i m p l e s t s y s t e m s , l i k e naphthalene and i n d e n e , a r e e n u m e r a t e d d i f f e r e n t l y f r o m e s t a b l i s h e d p r a c t i c e . The f u s e d p o s i t i o n s a r e , m o r e o v e r , the o n e s l e a s t l i k e l y t o be s u b s t i t u t e d . I t is p r o p o s e d , t h e r e f o r e , t h a t f o r t h e p u r p o s e s of c i t i n g t h e p o s i t i o n s of h e t e r o a t o m s and s u b s t i t u e n t s t h e e n u m e r a t i o n of a l l s y s t e m s , as obtained by t h e m e t h o d s d e s c r i b e d , should b e r e v e r s e d f r o m beginning to end. T h e l o w e r - n u m b e r e d p o s i t i o n s t h e n bec o m e predominantly the unshared a t o m s , a s i s c u s t o m a r y , and t h e l o w e s t - n u m b e r e d r i n g s a r e a l w a y s t h e o u t e r m o s t o n e s . Consequently, f o r
JOHN A . SILK
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t h e c o m m o n e s t type of s y s t e m , n a m e l y , n o r m a l fusion with s i x - m e m b e r e d r i n g s ( r e c o g n i z a b l e by t h e c o m p l e t e a b s e n c e of modifying s y m b o l s ) , we know without even working out t h e r i n g s t r u c t u r e t h a t positions 1, 2, 3 and 4 a r e i n one e n d - r i n g and p o s i t i o n s 5, 6 , 7 and 8 i n a n o t h e r end-ring. This r e s u l t s i n the unshared a t o m s i n naphthalene, p h e n a n t h r e n e , indene and fluor e n e retaining t h e i r a c c e p t e d e n u m e r a t i o n s , and a n t h r a c e n e differing only i n t h e 10-position being r e n u m b e r e d 12--a c h a n g e which is h a r d l y l i k e l y t o be m i s u n d e r s t o o d . H e t e r o c y c l i c analogs of t h e two-ring s y s t e m s a r e a l s o unchanged, but f o r t h r e e - r i n g h e t e r o s y s t e m s no g e n e r a l s t a t e m e n t c a n be m a d e ; a n t h r a c e n e , phenanthrene and f l u o r e n e a r e exceptions t o t h e Ring Index r u l e s and so a r e a c r i d i n e and c a r b a z o l e , which a r e n u m b e r e d s i m i l a r l y . Some o t h e r h e t e r o s y s t e m s of f l u o r e n e s t r u c t u r e a r e n u m b e r e d l i k e t h e h y d r o c a r b o n , but t h o s e r e s e m b l i n g a n t h r a c e n e and p h e n a n t h r e n e a r e g e n e r a l l y not.
l o c a n t s a r e then c i t e d i n t h e s a m e o r d e r . The c i p h e r s f o r h e t e r o - r i n g s a r e placed i n f r o n t of those for the ring s y s t e m , because this i m p r o v e s t h e c l a s s i f i c a t i o n produced i n c i p h e r i n d e x e s ; i t b r i n g s t o g e t h e r , f o r e x a m p l e , all s y s t e m s containing a thiazine r i n g . A few s i m p l i fications a r e p e r m i s s i b l e , ( a ) i n one- and twor i n g s y s t e m s i n which t h e h e t e r o - r i n g i s not s i x - m e m b e r e d , t h e specification of ring- s i z e i n d e s c r i b i n g t h e h e t e r o - r i n g with Dn - m a k e s i t unnecessary to repeat this in describing the ring s y s t e m with C;, and ( b ) when a h e t e r o - a t o m i s c o m m o n t o two o r m o r e r i n g s t h e l o c a n t s i n t h e e x p r e s s i o n s f o r r i n g s i n which a l l a t o m s h a v e p r e v i o u s l y been c i t e d a r e o m i t t e d . The e x a m p l e s i n F i g . 9 i l l u s t r a t e t h e s e m e t h o d s and t h e r e v e r s a l of e n u m e r a t i o n i n specifying h e t e r o a t o m s and s u b s t i t u e n t s . S
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HETEROCYCLIC SYSTEMS I n s t r u c t u r a l s e a r c h e s upon collections of o r g a n i c compounds, i t i s often d e s i r e d t o identify a l l s y s t e m s which contain within t h e m a s p e c i f i c h e t e r o - r i n g , s u c h a s p y r i d i n e o r t h i a z o l e . The a s s o c i a t i o n of a h e t e r o a t o m with a r i n g of a s p e c i f i c s i z e , and, f u r t h e r , of two o r m o r e h e t e r o a t o m s with t h e s a m e r i n g i s not always r e a d i l y deducible f r o m c i p h e r s which specify s i m p l y t h e positions of h e t e r o a t o m s . It i s s u g g e s t e d , although i t i s not a n e s s e n t i a l p a r t of t h e p r e s e n t p r o p o s a l s , t h a t e a c h individual h e t e r o - r i n g should be d e s c r i b e d s e p a r a t e l y , i n o r d e r t o f a c i l i t a t e s e a r c h e s of t h i s type i n both p r i n t e d i n d e x e s and p u n c h e d - c a r d o r t a p e r e c o r d s . I n t h e notation' i n which t h e m e t h o d s of t h i s p a p e r a r e u t i l i z e d , D denotes a s i x - m e m b e r e d h e t e r o - r i n g and DE a n ;-membered h e t e r o - r i n g , w h e r e ; i s not s i x . T h e s e a r e followed by t h e s y m b o l s f o r t h e hetero-atoms in the standard o r d e r , the symbols f o r l i k e a t o m s being r e p e a t e d a s n e c e s s a r y ; t h e
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systems and reversal of enumeration.
REFERENCES M. Gordon, C. E. Kendall and W. H. T. Davison, "Chemical Ciphering, " Royal Institute of Chemistry, London, 1948. W. J. Wiswesser, " A Line-Formula Chemical Notation," T. Y. Crowell Co., New York, N. Y., 1954. A. P. Terent'ev. A. N. Kost. A. M. Zukerman and V. M. Potauov, Chem. Tech. (Berlin), 7, 579, (1955). F. L. Taylor, Ind. Eng. Chem., 2, 734 (1948). J. A. Silk, " A New System of Organic Notation," 1951, distributed privately.
(6) International Union of Pure and Applied Chemistry, " A Proposed International Chemical Notation, " Longmans, Green and Co., 1958. (7) G. M. Dyson, " A New Notation and Enumeration System for Organic Compounds," Longmans, Green and Co., 2nd edition, 1949, mapte; VIII. (E) J. A. Silk, " A Linear Notation for Organic Compounds," 1961, (copies available on application to the author).
