Lecture 10 Heterocyclic Aromatic Compounds: Nomenclature PDF

Lecture 10 Heterocyclic Aromatic Compounds: Nomenclature College of Pharmacy Department of Pharmaceutical Chemistry Sections -Nomenclature of monocycles -Nomencla...

Lecture 10 Heterocyclic Aromatic Compounds: Nomenclature College of Pharmacy Department of Pharmaceutical Chemistry Sections -Nomenclature of monocycles -Nomenclature of fused heterocycles -Class Activity -References Intended Learning Outcomes (ILO’s) At the end of this unit students will be able to:  Identify different heterocyclic ring system.  Write nomenclature for monocyclic and fused heterocyclic rings.  Utilize these knowledge in writing name for any heterocyclic medicinal compounds. 22-Dec-24 3 A. Nomenclature of Monocycles The name consists of three parts: (1) prefix (2) stem (3) suffix 1. The prefix indicates the type of the heteroatom, their numbers and their positions. When two or more heteroatom of same type are present, the prefix di, tri,...etc. is used. If the heteroatoms are different the order of naming will be: O > S > Se >N >P> Si > B > Hg ….etc 22-Dec-24 4  Continued… Table I: Prefix name of heteroatom and their order of priority: Heteroatom Prefix Priority order Oxygen Oxa 1 Sulphur Thia 2 Selenium Selena 3 Nitrogen Aza 4 Phosphorus Phospha 5 Silicon Sila 6 22-Dec-24 5 Continued… 2. The stem: It determines the ring-size from 3 membered ring to 10 membered ring as shown. 22-Dec-24 6  Continued… 3. The Suffix: It determines the state of saturation or unsaturation which depends upon: (i) The presence or absence of N. (ii) The ring size. (a) The complete saturation is indicated by prefixing "perhydro" added before the total name of the unsaturated (b) no suffix (c) stem 22-Dec-24 7 Continued… Numbering of the monocycles depends upon the following rules: (i) Monocycles containing one heteroatom, are numbered beginning at the heteroatom and the substituents takes the least possible number. (ii) Monocycles with two or more similar atoms contained in a ring are indicated by the prefixes ‘di-’, ‘tri’, etc. (iii) Monocycles containing more than one different heteroatoms the numbering begins at the atoms of the higher order of priority and proceeds round the ring in order to give other heteroatoms the least possible numbers without regards to the substituents. (iv) If more than one hetero atom occur in the ring, then the heterocycle is named by combining the appropriate prefixes with the ending in the table in order of their preference, O > S > N. 22-Dec-24 8  Continued… Examples 22-Dec-24 9 Continued… Examples 22-Dec-24 10  Continued… Examples 22-Dec-24 11 Continued… Examples 22-Dec-24 12  Continued… Examples 22-Dec-24 13 Continued… Examples 22-Dec-24 14 B. Nomenclature of Fused Heterocycles (I) Carbocycle fused with heterocyclic system. (II) Heterocycle fused with another heterocyclic system. Many fused heterocycles have trivial names which are retained for the compound and the IUPAC organization still permit the use of such trivial name besides the usual IUPAC name. 22-Dec-24 15 I. Nomenclature of Carbocycle fused with Heterocycles: 1.The Parent name (written at the end) is the name of the heterocyclic ring. 2. The fused name (written at first) is the name of the fused benzene called(Benzo). 3. The Side fusion of the parent ring with the fused benzene ring is indicated by alphabetical numbering and put such letter (a, b, c,... etc) between square brakets in between the prefix and parent name. In this case the parent ring is numbered as usual and the sides 1,2. take letter (a), side 2,3 take letter (b), side 3, 4 take letter (c),... etc. 4.The total numbering of the complete fused system is done to determine the positions of saturations or substitutions according to the following rules: a) The fused system is oriented so that: The greatest number of ring are in horizontal row, and the remaining rings above the right of this horizontal row. b) The numbering starts from the atom next to the fusion giving the heteroatom the least possible numbering and continue numbering in clockwise direction (whenever possible) and the fused carbons are given the same previous number adding to it letters (a, b, c, etc). 22-Dec-24 16  Continued… c) In case of polycycles, the parent name is given to the largest heterocycles containing many rings (provided it has a famous trivial name). Examples 22-Dec-24 17 Continued… The following are the agreed trivial and IUPAC names of the some of the famous carbocycles fused with heterocycles. 22-Dec-24 18  II-Nomenclature of Heterocycle fused with another heterocycles The IUPAC rules for naming such systems are also composed of 4 points: 1-The Parent name given to the more prior heterocycle is used as suffix. 2- The fused ring name is the less prior rings and is used as prefix. 3- The side of fusion for both rings. 4- The numbering of the total system. 22-Dec-24 19 Continued… The above points are discussed as follows: 1. The selection of the parent ring(s) (as suffix) should be according to the following order of preferences: i) The Nitrogen containing ring must be taken as a parent ring: ii) If no Nitrogen, the ring contain the more prior heteroatom (according to table I), is considered as a parent ring: 22-Dec-24 20  Continued… iii) The largest ring size is taken as a parent ring if the two rings contain Nitrogen or does not contain Nitrogen. iv) The largest number of rings with famous trivial name is always used as a parent name: 22-Dec-24 21 Continued… 3. Determination of the sides of fusion for both sides of the two fused heterocycles as such: (a)The side of fusion with the parent ring is numbered alphabetically (as mentioned Before and the letter of fusion is placed between square brackets at its end. (b) The side of fusion of the prefix ring is indicated by two numbers denoting the two positions of fusion with the parent ring these two numbers are also placed at first in the square brackets. The order of writing these two numbers conforms the direction of lettering of the parent ring 22-Dec-24 22 Class Activity Q. Identify the heterocyclic rings in the given drugs. Sulfapyridine Sulfisoxazole Buspirone Phenmetrazine, 22-Dec-24 23 Q. Write the name of following heterocyclic rings. 22-Dec-24 24 References CRC Handbook of Chemistry and Physics by Robert. C. Wand Melvin. A. 60th edition. 1979- 1980 p. ,(C33-C44). 22-Dec-24 25