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Paper for the Opened-Experiment of Medicinal Chemistry. ——Structure-Activity Study of β-Adrenergic Blockers and Laboratory Synthesis of Practolol. 68k JiDi Class Gracie L.C.

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paper for the opened experiment of medicinal chemistry

Paper for the Opened-Experiment of Medicinal Chemistry

——Structure-Activity Study of β-Adrenergic Blockers and Laboratory Synthesis of Practolol

68k JiDi Class

Gracie L.C

slide2
Abstract: The main content of the opened-experiment of medicinal chemistry is the design of a certain compound withβ-Adrenergic Blocking activity based on the study of its structure - activity relationship . Practolol, one compound in point, which is reported to have certain degree of β1-receptor selectivity and intrinsic sympathomimetic activity is chosen to be synthesized. This paper describes the decision making process in choosing object compound and the detailed laboratory procedure as well as discussion and analysis of problems arisen in the lab work.

Keywords :β-Adrenergic Blockers Structure-Activity Practolol Laboratory Synthesis

slide3
Structure-Activity Study

A Brief Review of Pharmacology of β-Adrenergic Blocker

Literature Information of Practolol

Route of Synthesis

The Procedure of Laboratory Synthesis

Discussion

Reference

Catalogue

slide4
Structure-Activity Relationship
  • Compounds that act as blockers of a receptor should possess the structural features of an agonist that contribute to affinity an not those that contribute to intrinsic activity. It is therefore not surprising that a structural prerequisite for beta antagonism is thephenethanol-amino structure and a hydrophobic group (isopropyl or larger) on the nitrogen. To eliminate intrinsic activity in a direct agonist, the phenolic hydroxyl groups of norepinephrine should be absent. As in most antagonists, the structures are larger than in the agonists and contain either substituted phenyl groups, a naphthalene ring, or heterocyclic ring systems.
  • As for the second generation of β-blockers, the Aryloxypropanolamines, the substituents in this position maintain the same spatial relationships as are present in the phenyl ethanolamine series, with the only difference of a one more –OCH2- group in the side chain of the substituted phenyl ring. (Figure 1. )
slide5
phenethanol-amino

Aryloxypropanolamines

Figure.1:the similarity in the spatial relationship of the two typical structures

slide6
Most derivatives of this series of the aryloxypropanolamines possess various substituted phenyl rings rather than the naphthyl ring. Substitution of methyl, chloro, methoxy, or nitro groups on the ring was most favored at the 2 and 3 positions and least favored in the 4 position. When dimethyl substitutions were made, the 3,5-disubstituted compound was best and the 2,6- or 2,3,6-substituted compounds show the least activity. Presumably, this was due to steric hindrance to rotation about the side chain.
  • Stereochemistry: Compounds with phenethenolamine structure possess high –receptor blockade when the β–C attached to the OH group is in (R) configuration. The (S)-isomer, however, has much lower activity. In the structure of Aryloxypropanolamines, the stereochemistry is just opposite to that of the former type due to the insert of an O which changes the priorities of the substituents attached to the stereogenic center (β-C). Therefore, the (S)-isomer is more active. In fact, the two types of enantiomer are consistent in the arbitrary spatial configuration. (Figure 2.)
slide7
Selectivity: Compounds with enhanced selectivity of the β1response are characterized chiefly by para substitution rather than ortho substitution in the phenoxypropanolamine series. Practolol (our object compound), for example, is reported to inhibit the β1receptor at lower doses than those required to inhibit the β2receptor.

(S)-isomer

(R) -isomer

Figure2.the consistence in spatial configuration of the two structures

slide8
Structure-Activity Study

A Brief Review of Pharmacology of β-Adrenergic Blocker

Literature Information of Practolol

Route of Synthesis

The Procedure of Laboratory Synthesis

Discussion

Reference

Catalogue

slide9
Pharmcologic Study
  • Effects on the Cardiovascular System: Beta-blocking drugs lower blood pressure. This effect is the result of several factors, including effects on the heart and blood vessels, the renin-angiotensin system, and possibly the central nervous system. Beta-receptor antagonists have prominent effects on the heart. The negative inotropic and chronotropic effects are predictable from the role of adrenergic receptors in regulating these functions. In the vascular system, beta-receptor blockade opposes β2-mediated effects. Beta-blocking drugs antagonize the release of renin caused by the sympathetic nervous system.
  • Effects on the Respiratory Tract :Blockade of the β2 receptors bronchial smooth muscle may lead to an increase in airway resistance, particularly in patients with asthma. β1 receptor-selective antagonists when blockade of β1 receptors in the heart is desired and β2 –receptor blockade is undesirable.
slide10
Effects on the Eye :Several nonselective beta-blocking agents reduce intraocular pressure, especially in glaucomatous eyes.
  • Effects Not Related to Beta Blockade:Partial beta-agonist activity was significant in the first beta-blocking drug synthesized. It has been suggested that retention of some intrinsic sympathomimetic activity is desirable to prevent untoward effects such as precipitation of asthma. Local anesthetic action, also known as “membrane-stabilizing” action, is a prominent effect of several beta-blockers. This action is the result of typical local anesthetic blockade of sodium channels and can be demonstrated in neurons, heart muscle, and skeletal muscle membrane.
slide11
Decision making in choosing object compound

acebutolol

diacetolol

practolol

slide12
Structure-Activity Study

A Brief Review of Pharmacology of β-Adrenergic Blocker

Literature Information of Practolol

Route of Synthesis

The Procedure of Laboratory Synthesis

Discussion

Reference

Catalogue

literature information of practolol
Literature Information of Practolol
  • Structure:
  • CA Name:N-[4-[2-Hydroxy-3-[(1-methylethyl)amino]propoxy]pheyl]acetamide
  • Formula and Molecular Weight:
  • Physical Property:fine,white or almost white, ordourless powder. soluble in alcohol (1:40), slightly soluble in acetone and acetic acid

Aqueous solution is most stable at PH6(protected from light)

slide14
Structure-Activity Study

A Brief Review of Pharmacology of β-Adrenergic Blocker

Literature Information of Practolol

Route of Synthesis

The Procedure of Laboratory Synthesis

Discussion

Reference

Catalogue

slide15
Route of Synthesis

(Ⅰ)condensation

(Ⅱ)amination

reagents and apparatus
Reagents and Apparatus

Reagents

Raw Materials: 4-acetamidophenol (impure), epichlorohydrin, isopropylamine

Other Reagent: glacial acetate acid, alcohol absolute, activated charcoal

Apparatus

Apparatus for reflux: three-necked boiling flask(250ml,500ml), mechanical stirrer, iron rings, clamps, reflux condenser,

Apparatus for vacuum filtration: Buchner funnel, suction flask, water aspirator

Apparatus for distillation: distilling flask, condenser, distillation adapter, water aspirator

Others: beakers (several ), stirring rod, drying tube, infrared light, filter paper, boiling stones

slide17
Structure-Activity Study

A Brief Review of Pharmacology of β-Adrenergic Blocker

Literature Information of Practolol

Route of Synthesis

The Procedure of Laboratory Synthesis

Discussion

Reference

Catalogue

slide18
the Laboratory Synthesis of Practolol
  • Condensation (the first day)
  • The sodium hydroxide solution(40%,w/w) was added with stirring to a mixture of 4-acetamidophenol (30g) and H2O (42.9ml) at a temperature below 25 ℃. Stirring was continued for a further 30min and there is thus obtained a clear solution with its color changing from dark blue to purple .
  • Epichlorohydrin was added in drops at a stable temperature slightly changing from 38℃ to 40℃. Then the reaction mixture was cooled to 35℃. A further stirring for 4h is required until milky white emulsus solid could be seen separated out from the reaction solution.
  • Remove the milky white emulsus solid to a flask and place it for 8 hours. The crude product was filtrated under reduced pressure. Wash it by water to PH 7 and get it dried under infrared light. There was thus obtained 1-(4-acetamidopheoxy)-2,3-epoxypropane. (31g) M.P. 110℃
slide19
Amination(the second day)
  • The 1-(4-acetamidopheoxy)-2,3-epoxypropane (15g) and isopropylamine (42g, 62ml) were heated under reflux for 5 hours. In the initiation of the reaction the mixture appeared to be dark brown solution. After stirring for 2 hours yellow white emulsus solid was seen separate out in great quantities, with only little liquid left. An addition of about 20ml extra isopropylamine was given in order that the reaction could be thoroughly completed.
  • After the reaction was completed, the mixture was evaporated under reduced pressure to thoroughly recover isopropylamine.
  • The residue got cooled, and added in glacial acetic acid (15ml) together with 135ml water. Keep stirring for 1 hour until a solution was obtained. Add active carbon as decolorant with stirring for a further hour.
slide20
The whole reaction system was ice-cooled to a temperature below 10℃ and underwent the vacuum filtration. The filtrate obtained was green and clear.
  • The filtrate was brought to PH between 8 and 9 by the addition of NaOH aqueous (35%) at the temperature between 10℃ to 20℃. Keep stirring during the process and white solid was seen separate out with NaOH added, which dissolve again once stirred. Then add the same NaOH aqueous slowly to regulate PH to 11~12 in order that crystals could separate out totally. Place it for several days to complete the aging process. The final product was obtained after further purification.
slide21
Problem Discussion

Problem arose in the first step of the synthesis of Acebutolol :

Changes made:

1)

2)

Reflux 12h

(Ⅰ)

(Ⅱ)(a kind of phenyl ester )

slide22
Problem: There was something unexpected occurred in reaction !

Descriptions in literature: The starting material were heated together under reflux until a solution formed. This solution was cooled and treated with water. The benzene layer was separated and the aqueous layer was again extracted with benzene. The extracts were dried and evaporated to dryness under reduced pressure to give (Ⅱ) as an off-white solid.

The actual phenomenon: After heating under reflux for 1 hour, the reaction mixture separated into two layers with the lower phase as kind of oil. The situation continued during the whole reflux process.

my handling approaches
My handling Approaches:
  • Poured the upper layer (methyl phenyl phase) into a beaker, then white crystals separate out in large quantities. After vacuum filtration, the melting point of the crystal was measured. The result turned to be much higher than that of theoretical product (lower than100 ℃~105 ℃). However, It was near the M.P of acetaminophen. So I guess that it was the 4-acetamidophenol that hadn’t totally took part in the reaction.
  • I wanted to make sure if there was some substance soluble in the reaction solvent (methyl phenyl phase), which might be exactly the product I wanted. So I drew off the filtrate gained from last step by reduced pressure distillation. Only a few off-white solid, the M.P of which was 116 ℃~120 ℃, was obtained. Undoubtedly, it was not the theoretical product.
  • The oil-like component in the reaction mixture was insoluble in either methyl phenyl or water, but dissolved in alcohol. I tried to get the mixture heated with water and it was found that the oil turned less and softened. After heating, three layers formed: the methyl phenyl phase, the water phase and the oil. (from upper to lower) Solid separated out between the upper two layers when cooled. Extract the aqueous layer with methyl phenyl, combined the organic phase and filtrate the crystals. Repeat such operation several times to accumulate the solid. Take measure of its M.P and the value was 156 ℃, even higher than acetaminophen ! Thus I was forced to stop the synthesis of Acebutolol due to all the uncertainties above.
analysis
Analysis:
  • I looked up some reference and organic textbooks about phenyl esters and their reactions. It is common that compounds like phenyl ester appear to be oil-like substances, however, few exists as crystal as described in the literature of acebutolol synthesis. So now I think it’s very possible that the oil-like substance is just the product I need. One proof that supports my idea is the solid I got after heating the oil., of which the M.P is near that of pure acetaminophen, may be the hydrolyte of phenyl ester. Because heating with water is just the proper condition to generate the hydrolysis reaction. And the extraction and filtration operation made its hydrolyte (acetaminophen) greatly purified, resulting in a much higher M.P value than raw material (4- acetaminophenl) .To turn the oil-like crude product to crystal form may involve some special purification procedure that wasn’t mentioned in detail in my literature.
slide25
Reference
  • William O.Foye. : Adrenergic Drugs : Principles of Medicinal Chemistry(3rd Edition) Philadelphia Lea & Febiger,1989
  • Bertram G.Katzung: Adrenergic Receptor-Blocking Drugs : Basic & Clinical Pharmacology(8th Edition),Los Altos, California, LANGE Publications,1982
  • B.Basil, J. R. Clark, E. C. J. Coffee, R. Jordan, A. H. Loveless, D. L. Pain, and K. R. H. Wooldridge.1976,Journal of Medecinal Chemistry 19(3):399 ~ 402
  • Merck Index(11th Edition)
  • British Pharmaceutical Codex,1973,398
  • 上海医药产品工艺汇编
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