P1250095225JOBQb

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D. PAVIA
NATURAL PRODUCTS CHEMISTRY
CHEM 425a/552
THE GOLD PROBLEM
To answer this problem, you need to give a
structure for each compound that is represented
by a letter in a circle. This problem amounts to
a structure proof of compound A (what is it ?).
CH3NO2
H2 / Pd-BaSO4
G
F
E
Rosenmund
NaOEt / EtOH
C11H13NO5
C10H12O4
C10H11O4Cl
1) Zn/HOAc 2) Na/Hg
SOCl2
D
CrO3 / H2SO4
B
A
Jones
C10H12O5
xs CH3I NaOH
C11H17NO3
derivative
1) SOCl2
an acid mp 240oC decomp.
1)NaOH/D
C
D
mp 242oC
Tannins
2) H3O
(note 7)
2) NH3
from oak galls or tea leaves
C7H6O5
C7H7NO4
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PART TWO
1) Postulate a reasonable biosynthetic sequence
that the plant might use to synthesize
compound A.
2) Mechanisms are not required, but you should
have a step-by-step sequence of
intermediates. At each step, place the identity
of the coenzyme or enzyme that could bring
about the desired transformation.
3) Here are some of the coenzymes and
intermediates you could choose from
any amino acids or a-ketoacids formaldehyde or
acetaldehyde NAD/NADH CoenzymeA (CoASH)
ATP oxygen/hydroxylase combination Acetyl
Coenzyme A SAM (SAdM) pteridines FAD/FADH2
thiamine (B1) pyridoxyl-5-phosphate (B6)
4) How could you prove (by experiment) that your
sequence of steps is correct?
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NOTES
1. You will have to look up the structure of
tannins. They are found in oak galls, big
knobby (gnarly) growths that grow on oak trees
due to insect damage, and in tea leaves. When
they are hydrolyzed you get compound C and
glucose. 2. You will have to look up compounds C
and D in tables of unknowns and
derivatives. 3. If you dont know the Rosenmund
Reduction you will have to look it up. 4.
The step F to G is like an aldol condensation,
the nitro compound has acidic hydrogens. The
NO2 group stabilizes the negative charge by
resonance. The anion is a good nucleophile. 5.
The reagents given in G to A are really strong
reducing reagents. They are not subtle. We
would probably use LiAlH4 today. 6. You should
also be able to give me the actual name for
compound A. HINT Its in your lecture
notes! 7. In some reference books, compound D is
listed with mp 189o, but as mp 242o in others!
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8. Derivatives Old-time chemists (those who
worked before spectroscopy was invented) used
derivatives to identify compounds. A derivative
is a second compound made from the first one in
order to allow a confirmation of its identity.
For instance, a common way of identifying an acid
would be to convert it to an amide using SOCl2
followed by NH3. The original compounds identity
would be confirmed if the melting point of the
amide had the expected value (one already
reported in the literature) for the amide
derivative of that compound. Most textbooks of
qualitative organic analysis have tables of
derivatives listed in the back of the book
organized by the starting functional group. A
table of acids, for instance, would list a series
of carboxylic acids by increasing boiling point
and then by melting point. In a column
opposite each acid in the table the melting
points of commonly used derivatives would be
listed for acids the melting points of the
corresponding amides and anilides (a derivative
made with aniline instead of ammonia) are given.
Useful derivatives are usually solid compounds
because solids are easier to purify
(crystallization) and melting points are easier
to determine than boiling points. Over the years
chemists developed suitable derivatives for every
major functional group and they became
standardized. Therefore, when you look in the
tables only certain derivatives (the standard
ones) are listed. Most chemists would know what
these standard derivatives are.
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EXAMPLE Suppose you had a ketone with a boiling
point from 231-234o C.
A portion of what you will see in the tables of
unknowns follows
semicarbazone
2,4-dinitrophenyl- hydrazone
bp mp
ketones
2-undecanone 231 12 122 63 4-chloroacetophenon
e 232 12 204 236 4-phenyl-2-butanone 235
- 142 127
There are three possible ketones in this range to
consider. But if you make the 2,4-dinitrophenylhy
drazone derivative and the melting point is 236o,
you know immediately that you have
4-chloroacetophenone. If you wanted additional
confirmation you could also make the
semicarbazone (204o).
There are a number of books in the organic lab
(under or on the melting point stands) that have
tables of unknowns. The most complete book
is Zvi Rappoport, CRC Handbook for Tables for
Organic Compound Identi- fication, CRC
Press. Also useful are the book by Shiner, Fuson
and Curtin and the 354-355 lab book by the
Organic Three Musketeers P,L and K.