Passage 5: Ribose
Ribose (C5H10O5) is biologically important as the pentose component of
ribonucleotides. It primarily exists in its aldose form, making it highly reactive in
oxidative and reductive conditions.
Ribose’s oxidation to ribonic acid and reduction to ribitol are widely known
transformations of this pentose. In bromine water (Br2 and H2O), ribose is converted
to ribonic acid, and in the presence of sodium borohydride (NaBH4), ribose is
converted to ribitol.
Experiment 1:
Researchers performed the following reactions with ribose under controlled conditions:
Reaction 1:
Ribose + Br2 + H2O → Ribonic acid + 2HBr
Reaction 2:
Ribose + NaBH4 + H2O → Ribitol + NaBO2
Observations:
Following these procedures, the researchers used pH indicators and TLC to monitor
reaction progress/success.
Experiment 2:
In order to map cellular processes that synthesize purines and pyrimidines, ribose
was converted to 5-phosphoribosyl-1-pyrophosphate (PRPP), a precursor for these
nitrogenous bases. This intermediate was then used to synthesize a compound that
is structurally similar to xanthine, a derivative of a purine:
Reaction 3:
Ribose-5-Phosphate + ATP → PRPP + AMP
Reaction 4:
PRPP + Amine → Xanthine Derivative
Experiment 3:
In an effort to further explore the reactivity of xanthine derivatives and extrapolate
to biological systems and biochemical reactions, they used their synthesized
xanthine derivative and reacted with alcohol to form an ether. Reaction progress
was monitored via TLC and confirmation of desired product was determined via
NMR. UV-Vis spectroscopy was conducted before NMR to ensure that a chemical
reaction had indeed occurred, because the former analytical technique is
significantly less expensive.
Ribitol, synthesized during Experiment 1 outlined in the passage, was
reacted with thionyl chloride (SOCL2), and the product of this reaction was
subsequently reacted with phenol under basic conditions in dimethyl sulfoxide
(DMSO). What is the major product and how does addition of phenol under
basic conditions in DMSO solvent affect its stereochemistry?
A) Primary ether; the product will be a racemate
B) Secondary ether; the product will be a racemate
C) Primary ether; the product will have inverted stereochemistry
D) Secondary ether; the product will have inverted stereochemistry
Correct answer: C. Addition of SOCl2 to ribitol replaces hydroxyl
groups with chlorides via nucleophilic substitution. The resultant alkyl chloride,
when reacted with phenol under basic conditions in DMSO (a polar aprotic
solvent), leads to another SN2 reaction. This is because deprotonation of phenol
forms phenoxide, a potent nucleophile, and polar aprotic conditions favor SN2.
Nucleophilic attack of the phenoxide ion on a chlorinated alkyl group yields an
ether, and the nature of SN2 (backside nucleophilic attack) causes inversion of
stereochemistry. The ether is primary because although nucleophilic substitution
can occur at any of the chlorine-bearing carbons, steric hindrance at carbons 1 and
5 is lowest (primary carbons), so the major product of this SN2 reaction would be a
primary ether.
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