Designation of Fusion Points1 ROBERT 6. KRUPP and MICHAEL KONDAS Research Division, American Cyanamid Company, Bound Brook, New Jersey
IT
WOULD indeed be an understatement to say that a t times the nomenclature of fused ring systems is confusing to the practicing chemist. Explanations of the rules governing this topic are available in two common sources. The Chemical Abstracts "Blue Book" (I) gives a sketchy review while "The Ring Index" (2) goes into a very detailed and often confusing discussion. Fortunately, there are a number of rather easily understood basic rules t o govern the more simple ring systems and it is some of these rules which will be demonstrated. I n particular, there has been evidence of the need for a somewhat oversimplified explanation of the rules involved in designating the fusion points in heterocyclic ring systems. One frequently misunderstood portion of this type of nomenclature is the symbolism needed to indicate the points of ring fusion. This is the part of the name which appears in brackets and contains a series of numbers and/or letters such as [3.4-c]. The numbers, which always appear first, refer to that part of the ring system name which precedes the brackets and is designated by the prefix name while the letters always refer to that part of the ring system name which follows the brackets and is called the parent ring or base component. Thus, in 1H-pyrazolo[3.4-c]pyridine (3) (I),
H I
(1)
the "3.4" refers to the "pyrazolo" (111) portion (the prefix) and the "c" to the "pyridine" (11) portion (the base component). Usually the name of a simple fused ring structure can be broken down readily into two major sections. Thus in (I) we have "pyrazolo" and "pyridine." Now in order t o understand the designation for the points of fusion, the two sections are pictured as being unfused and numbered as shown in (11) and (111).
Thus in (11), positions 1 and 2 create side a, 2 and 3 create side b, etc. Normally, numbers are not assigned to rings which take lettered-side designations. So when (11) and (111) are fused t o give (I), it is seen that side c of (11) and positions 3 and 4 of (111) are the points of contact or fusion. The numbering and lettering are directed so as to give the lowest numbers and letters possible a t the points of fusion consistent with the fixed numbering of the component rings. The rules invoked when more than one heterocyclic atom is present in a particular ring take precedence (as in (111)) over the rules concerning points of fusion. I t should now be noted that the numbering of (11) is in a clockwise manner while (111) has been numbered in a counter-clockwise direction. An important rule t o keep in mind here is that the order of the numbers involved in fusion designation is dictated by the direction (clockwise or counter-clockwise) in which the rings are numbered. If the rings are numbered in opposite directions, then the fusion position numbers are stated in a consecutive manner [3.4]. If both rings are numbered in the same direction, then the numbers are listed in reverse order [4.3]. Since in the cases of (11) and (111) the numbering directions are opposite (see arrows), 3 and 4 are listed consecutively. This designation is then put immediately inside the left bracket. A dash follows and then comes the letteredside symbol and the bracket is closed. Thus [3.4-c] is used for structure (I). In the case of (IV) below, the numbering of both rings is counter-clockwise, and so the fusion numbers used are reversed to give 1H-pyrazolo[4.3-clpyridine (4) (IW. H
I n cases where the numbers on a ring can be assigned arbitrarily in either direction, that direction is chosen which gives consecutive numbering within the fusion position brackets. Such a case is (V). Note the numbering directions of its parts, (VI) and (VII). Letters are assigned t o the sides of the base component which terminates the name, in this case "pyridine." VOLUME 35, NO. 8, AUGUST, 1958
'Originally this article was prepared as a. guide for American Cyenamid ohemists and was not meant to be a thorough tre& ment of the subject.
When two or more different hetero-atoms appear in the same component ring, the starting point for numbering of that ring is the hetero-atom of as high a group in the periodic table of the elements, and as low an atomic number in that group, as possible. The order of the more commonly occurring hetero-atoms is: oxygen, sulfur, nitrogen, phosphorus, and silicon. Thus, in oxazolo[5.4-dlpyrimidine (9) (XI), the
I n a structure where a hetero-atom is common t o both rings, that atom is included in both rings when numbers and letters are assigned. Thus in 2H-pyrido[1.2-alpyrimidine (6) (7111)
yyi N
(XI)
numbering of the oxazole begins with the oxygen atom rather than with nitrogen as shown in (XIb).
note how the sections are numbered: Another example of this is 2H-isothiasolo[2.3-alpyridine ( 1 0) (XII)
I n cases where no ambiguity can result, unnecessary numbers or letters may be omitted. For example, in pyrido[3.4]pyrazine (7) (IX), the pa~a-position of the nitrogen
atoms in the right-hand ring indicates that this is a "pyrazine" and no other arrangement of these atoms would give the same fused ring system. If these were shifted one position making one nitrogen common t o both rings but still keeping %heother nitrogen para to it (IXa or IXb), the left-hand ring would no longer
be "pyrido" and thus the name itself would be changed. The "pyrido" ring in (IX) is therefore numbered as
is the numbering used. I t should be remembered that numbers assigned t o indicate fusion points are different from the numbering of the fused system, and should not be used to indicate substituents which may appear on the ring system. For structure (IX) the final numbering would actually be as shown below.
This discussion has touched only briefly on some of the fundamentals involved in a very complex nomenclature problem. Most of these procedures carry over into the larger multi-ringed compounds where the basic principles mentioned here still apply. As a slightly more complex fusion designation, the following is a good example:
@? CH2
and no letters need be assigned t o the "pyrazine" ring. The appearance of oxygen, sulfur, phosphorus, etc. in these heterocyclic ring systems does not change the principle of designating the point of fusion. Thus thieno(3.2-clpyridine (8)is the name for (X).
(XIII)
Here the assigned name is 5H-furoquinoline and for determining the fusion point designation the structure is divided into "furo" (XV) and 'Lquinoline" (XIV). Note how the letters are assigned in (XIV) and the numerals in (XV). Note too that the numerals are arranged consecutively [2.3.4] since the direction of numbering (counter-clockwise) is opposed t o the direction of lettering in (XIV). The competed name for the structure therefore becomes 5H-furo[2.3.4delquinoline (11). JOURNAL OF CHEMICAL EDUCATION
LITERATURE CITED (1) AMERICAN CHEMICAL SOCIETY. "The Naming and Indexing
Considerable assistance in understanding the fusion positions of complex ring systems is available if a study is made of examples shown in "The Ring Index." This reference work of almost 4000 organic ring structures can be a very useful tool if a measure of patience is applied in its use.
VOLUME 35, NO. 8, AUGUST, 1958
of Chemical Compounds by Chemical Abstracts," Columbus, Ohio, American Chemical Society, 1945 (also includes supplements for 1947 and 1952). (2) P a n e ~ s o n A. , M., AND I.. T. CAPELL, "The Ring Index," New York, Reinhold, 1940. 661 pp. See also: P A ~ R SON, A. M., "Nomenclature of Parent Ring Systems," J . Am. Chern. Soe., 50, 307487 (1928). (3) Ring Index 760. (4) Ibid., 758. (5) Ibid, 961. (6) Ibid., 970. (7) lbid.. 969. isj zaid.; 849. (9) Ibzd., 723. (10) Ibid., 735. (11) Ibid,, 1703.
