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Unformatted text preview: t necessarily the correct forms. Consult other sources of information to make a final decision. The ACD/Tautomers algorithm does not proceed with the following classes of chemical structures:
• Structures containing metal atoms; • Structures containing charged atoms, other than the non-ionic derivatives of IV-valent nitrogen (+) bonded to oxygen (–); • Structures containing elements in their non-typical valence; • Structures with coordinating bonds; and • Structures containing more than 255 atoms. Now we are going to try this option on the illustrative example: 1. Using the technique described in the previous chapters draw the following structure:
N O HN N O ACD/ChemSketch Tutorial 67 Special Function Keys 2. Select the structure (use the Select/Move tool), and, on the Structure toolbar, click Check for Tautomeric Forms . The program starts generating and checking tautomeric forms of the drawn structure and when the process is finished, the following dialog box is displayed: 3. Select the required tautomeric form using the Next >> or << Prev buttons. 4. Click Replace to change the drawn structure for the currently selected one. 7.2.1 Examples
Here are a few “classic” types of tautomerism which you can try out with the Check Tautomeric Forms feature described above.
Keto-Enol Example H3C H3C OH CH3 H3C H3C O CH3 Phenol-keto O OH Heterocyclic ring O OH N H N ACD/ChemSketch Tutorial 68 Special Function Keys Tautomerism
Nitro-acid form Example CH3 HO N
+ - CH3 CH3 O N
+ - CH3 O
Nitroso-oxime O H3C N CH3 O H3C H3C N CH3 OH Imine-Enamine H3C NH H3C CH3 CH3 N H3C CH3 CH3 7.2.2 Mistakes in the Chemical Literature
Chemical structures are often drawn as one tautomeric form, and other existing forms are simply ignored. Unfortunately, you can never be sure that a particular compound will be drawn the same way by another chemist. Therefore, the consideration of possible tautomeric forms is very important for structure search, data prediction, and interpretation. All of the structures below have been taken from respected publications. Many have incorrect or ambiguous assignment of tautomeric forms under the experimental conditions described. With ACD/Tautomers, you no longer run the risk of overlooking a common tautomeric form for the compound you are trying to publish, interpret observed data for, or run ACD/Labs predictions on.
Questionable Structures from Various Publications
S H N SH H N OH HN N N N N H H2N N N N N N Thioguanine (antineoplastic) 12 possible forms
HO N N N N OH Cl NH2 6-Mercaptopurine (antineoplastic) 6 possible forms
O OH N (antiurolithic) 6 possible forms
NH2 O N HO O Leucopterin >10 possible forms Chlorzoxazone (skeletal muscle relaxant) 2 possible forms Pemoline (CNS stimulant) 3 possible forms ACD/ChemSketch Tutorial 69 Special Function Keys Questionable Structures from Various Publications
H2N H2N N N HS N N SH NH2 NH N N CH3 H N N Purpald 4 possible forms
H N O Methimazole (antihyperthyroid) 2 possible forms
O HO N
- Amitrole (herbicide) 5 possible forms + HO N SH N F NH2 CH3 I N Flucytosine (antifungal) 6 possible forms
HO N OH
Ciclopirox (antifungal) 2 possible forms
H3C N OH Iodothiouracil (thyroid inhibitor) 6 possible forms
Cl NH N NH N N S Cl NH HO HO O H2N Citrazinic acid 2 possible forms Methisazone (antiviral) 2 possible forms
O O S N CH3 NH Guanoxabenz (antihypertensive) 3 possible forms CH3 HN NH N NH2 N NH NH NH2 OH O OH S OH O N Mitoguazone (antineoplastic) >10 possible forms Tenoxicam (anti-inflammatory, analgesic) 4 possible forms Anthralin (antipsoriatic) 2 possible forms ACD/ChemSketch Tutorial 70 Special Function Keys 7.3 Dictionary—Commercial version only!
ACD/Dictionary is an add-in module that is supplied with all the commercial copies of ACD/ChemSketch. It is immensely useful at finding “chemicals” by their common names. ACD/Dictionary finds chemical structures according to their chemical name. It contains more than 165,000 systematic and non-systematic names and their corresponding molecular structures. The dictionary is searchable by both the full chemical name and name fragments. ACD/Dictionary is covered in full in the ACD/Dictionary User’s Guide, located in the ACD/Labs documentation folder, \\DOCS\DICT.PDF.
Note If you have purchased ACD/ChemSketch, but do not see the Dictionary button in the ChemSketch window when you are in the Structure mode, please verify that you have correctly entered TWO registration numbers at the time of installation—the one for ACD/ChemSketch (Main), and another for the ACD/Dictionary (Add-in). 7.4 ACD/Name Freeware Add-on
Now it’s possible to use ACD/Name Freeware Add-on right in the ACD/ChemSketch. This extra functionality is available as a button on the General toolbar: This tool is easy to use: draw the structure or structures to be named, click this button and the name for one structure or a mixture is inserted as a text string into the workspace. Let’s try naming some examples. 1. Using ChemSketch tools, draw the following structure: N O HO 2. If there is more than one struc...
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This note was uploaded on 06/22/2010 for the course AS 1 taught by Professor Chem during the Spring '10 term at University of Alaska Southeast.
- Spring '10