Paper for the Opened-Experiment of Medicinal Chemistry

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Paper for the Opened-Experiment of Medicinal
Chemistry

• Structure-Activity Study of ß-Adrenergic
  Blockers and Laboratory Synthesis of Practolol

68k JiDi Class Gracie L.C
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Abstract The main content of the
opened-experiment of medicinal chemistry is the
design of a certain compound withß-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 ß1-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 ß-Adrenergic Blockers
Structure-Activity Practolol Laboratory
Synthesis
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Catalogue

• Structure-Activity Study
• A Brief Review of Pharmacology of ß-Adrenergic
  Blocker
• Literature Information of Practolol
• Route of Synthesis
• The Procedure of Laboratory Synthesis
• Discussion
• Reference

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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 the
  phenethanol-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 ß-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. )

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phenethanol-amino
Aryloxypropanolamines
Figure.1the similarity in the spatial
relationship of the two typical structures
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• 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 ßC 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 (ß-C). Therefore, the (S)-isomer is more
  active. In fact, the two types of enantiomer are
  consistent in the arbitrary spatial
  configuration. (Figure 2.)

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(S)-isomer
(R) -isomer
Figure2. the consistence in spatial configuration
of the two structures

• Selectivity Compounds with enhanced selectivity
  of the ß1 response 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 ß1 receptor at lower doses than
  those required to inhibit the ß2 receptor.

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Catalogue

• Structure-Activity Study
• A Brief Review of Pharmacology of ß-Adrenergic
  Blocker
• Literature Information of Practolol
• Route of Synthesis
• The Procedure of Laboratory Synthesis
• Discussion
• Reference

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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
  ß2-mediated effects. Beta-blocking drugs
  antagonize the release of renin caused by the
  sympathetic nervous system.
• Effects on the Respiratory Tract Blockade of
  the ß2 receptors bronchial smooth muscle may
  lead to an increase in airway resistance,
  particularly in patients with asthma. ß1
  receptor-selective antagonists when blockade of
  ß1 receptors in the heart is desired and ß2
  receptor blockade is undesirable.

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• 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.

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Decision making in choosing object compound
acebutolol
diacetolol
practolol
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Catalogue

• Structure-Activity Study
• A Brief Review of Pharmacology of ß-Adrenergic
  Blocker
• Literature Information of Practolol
• Route of Synthesis
• The Procedure of Laboratory Synthesis
• Discussion
• Reference

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Literature Information of Practolol

• Structure
• CA NameN-4-2-Hydroxy-3-(1-methylethyl)aminop
  ropoxypheylacetamide
• Formula and Molecular Weight
• Physical Propertyfine,white or almost white,
  ordourless powder.
  soluble in alcohol (140), slightly
  soluble in acetone and acetic acid
• Aqueous solution is most stable at
  PH6(protected from light)

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Catalogue

• Structure-Activity Study
• A Brief Review of Pharmacology of ß-Adrenergic
  Blocker
• Literature Information of Practolol
• Route of Synthesis
• The Procedure of Laboratory Synthesis
• Discussion
• Reference

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Route of Synthesis
(?)condensation
(?)amination
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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

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Catalogue

• Structure-Activity Study
• A Brief Review of Pharmacology of ß-Adrenergic
  Blocker
• Literature Information of Practolol
• Route of Synthesis
• The Procedure of Laboratory Synthesis
• Discussion
• Reference

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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-acetamidopheox
  y)-2,3-epoxypropane. (31g) M.P. 110?

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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.

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• 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 1112 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.

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Problem Discussion
Problem arose in the first step of the synthesis
of Acebutolol
Reflux 12h
(?)
(?)(a kind of phenyl ester )

• Changes made
• 1)
• 2)

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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 (?) 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.
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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
  hadnt 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.

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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 its 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 wasnt mentioned in detail in my literature.

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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
• ??????????

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