Classify the following monosaccharides. (Examples: D-aldohexose, L-ketotetrose.)

(a) Give the products expected when (+)-glyceraldehyde reacts with HCN.

(b) What is the relationship between the products? How might they be separated?

(c) Are the products optically active? Explain.

The relative configurations of the stereoisomers of tartaric acid were established by the following syntheses:

(1) D-(+)-glyceraldehyde $\stackrel{HCN}{\to }$ diastereomers A and B (separated)

(2) Hydrolysis of A and B using aqueous Ba(OH)₂ gave C and D, respectively.

(3) HNO₃ oxidation of C and D gave (-)-tartaric acid and meso-tartaric acid, respectively.

(a) You know the absolute configuration of D-(+)-glyceraldehyde. Use Fischer projections to show the absolute configurations of products A, B, C, and D.

(b) Show the absolute configurations of the three stereoisomers of tartaric acid: (+)-tartaric acid, (-)-tartaric acid, and meso-tartaric acid.

Predict the products obtained when d-galactose reacts with each reagent.

(b) NaOH, H₂O

Predict the products obtained when d-galactose reacts with each reagent.

(c) CH₃OH, H⁺

Predict the products obtained when D-galactose reacts with each reagent.

(d) Ag(NH₃)₂^{+ –}OH

Predict the products obtained when D-galactose reacts with each reagent.

(e) H₂, Ni

Predict the products obtained when d-galactose reacts with each reagent.

(f) excess Ac₂O and pyridine

Predict the products obtained when d-galactose reacts with each reagent.

(g) excess CH₃I, Ag₂O

Predict the products obtained when D-galactose reacts with each reagent.

(h) NaBH₄

Predict the products obtained when D-galactose reacts with each reagent.

(i) Br₂, H₂O, then H₂O₂ and Fe₂(SO₄)₃

Predict the products obtained when D-galactose reacts with each reagent.

(j) (1) KCN/HCN; (2) H₂, Pd/BaSO₄; (3) H₃O⁺

Predict the products obtained when D-galactose reacts with each reagent.

(k) excess HIO₄

Draw the following sugar derivatives.

(a) methyl β-D-glucopyranoside

(b) 2,3,4,6-tetra-O-methyl-D-mannopyranose

(c) 1,3,6-tri-O-methyl-D-fructofuranose

(d) methyl 2,3,4,6-tetra-O-methyl-β-D-galactopyranoside

Draw the structures (using chair conformations of pyranoses) of the following disaccharides.

(a) 4-O-(α-D-glucopyranosyl)-D-galactopyranose

(b) α-D-fructofuranosyl-β-D-mannopyranoside

(c) 6-O-(β-D-galactopyranosyl)-D-glucopyranose

Erwin Chargaff’s discovery that DNA contains equimolar amounts of guanine and cytosine and also equimolar amounts of adenine and thymine has come to be known as Chargaff’s rule:

G = C and A = T

(a) Does Chargaff’s rule imply that equal amounts of guanine and adenine are present in DNA? That is, does G = A?

(b) Does Chargaff’s rule imply that the sum of the purine residues equals the sum of the pyrimidine residues? That is, does A + G = C + T?

(c) Does Chargaff’s rule apply only to double-stranded DNA, or would it also apply to each individual strand if the double helical strand were separated into its two complementary strands?

Which of the following sugars are reducing sugars? Which ones would undergo mutarotation?

(a) methyl β-D-glucopyranoside

(b) 2,3,4,6-tetra-O-methyl-D-mannopyranose

(c) 1,3,6-tri-O-methyl-D-fructofuranose

(d) methyl 2,3,4,6-tetra-O-methyl-β-D-galactopyranoside

Which of the following sugars are reducing sugars? Which ones would undergo mutarotation?

(a) 4-O-(α-D-glucopyranosyl)-D-galactopyranose

Which of the following sugars are reducing sugars? Which ones would undergo mutarotation?

(b) α-D-fructofuranosyl-β-D-mannopyranoside

Which of the following sugars are reducing sugars? Which ones would undergo mutarotation?

(c) 6-O-(β-D-galactopyranosyl)-D-glucopyranose

An unknown reducing disaccharide is found to be unaffected by invertase enzymes. Treatment with an α-galactosidase cleaves the disaccharide to give one molecule of D-fructose and one molecule of D-galactose. When the disaccharide is treated with excess iodomethane and silver oxide and then hydrolyzed in dilute acid, the products are 2,3,4,6-tetra-O-methylgalactose and 1,3,4-tri-O-methylfructose. Propose a structure for this disaccharide, and give its complete systematic name.

Which of the D-aldopentoses will give optically active aldaric acids on oxidation with HNO₃?

Which of the D-aldotetroses will give optically active aldaric acids on oxidation with HNO₃?

Sugar X is known to be a D-aldohexose. On oxidation with HNO₃, X gives an optically inactive aldaric acid. When X is degraded to an aldopentose, oxidation of the aldopentose gives an optically active aldaric acid. Determine the structure of X.

Even though sugar X gives an optically inactive aldaric acid, the pentose formed by degradation gives an optically active aldaric acid. Does this finding contradict the principle that optically inactive reagents cannot form optically active products?

Show what product results if the aldopentose formed from degradation of X is further degraded to an aldotetrose. Does HNO₃ oxidize this aldotetrose to an optically active aldaric acid?

When the gum of the shrub Sterculia setigera is subjected to acidic hydrolysis, one of the water-soluble components of the hydrolysate is found to be tagatose. The following information is known about tagatose:

(1) Molecular formula C₆H₁₂O₆

(2) Undergoes mutarotation.

(3) Does not react with bromine water.

(4) Reduces Tollens reagent to give D-galactonic acid and D-talonic acid.

(5) Methylation of tagatose (using excess CH₃I and Ag₂O) followed by acidic hydrolysis gives 1,3,4,5-tetra-O-methyltagatose.

(a) Draw a Fischer projection structure for the open-chain form of tagatose.

Some protecting groups can block two OH groups of a carbohydrate at the same time. One such group is shown here, protecting the 4-OH and 6-OH groups of β-d-glucose.

(a) What type of functional group is involved in this blocking group?

(b) What did glucose react with to form this protected compound?

(c) When this blocking group is added to glucose, a new chiral center is formed. Where is it? Draw the stereoisomer that has the other configuration at this chiral center. What is the relationship between these two stereoisomers of the protected compound?

(d) Which of the two stereoisomers in part (c) do you expect to be the major product? Why?

An important protecting group developed specifically for polyhydroxy compounds like nucleosides is the tetraisopropyldisiloxanyl group, abbreviated TIPDS, that can protect two alcohol groups in a molecule.

(a) The TIPDS group is somewhat hindered around the Si atoms by the isopropyl groups. Which OH is more likely to react first with TIPDS chloride? Show the product with the TIPDS group on one oxygen.

(b) Once the TIPDS group is attached at the first oxygen, it reaches around to the next closest oxygen. Show the final product with two oxygens protected.

(c) The unprotected hydroxy group can now undergo reactions without affecting the protected oxygens. Show the product after the protected nucleoside from (b) is treated with tosyl chloride and pyridine, followed by NaBr, ending with deprotection with Bu₄NF.

Draw the structures of the following nucleotides.

(a) guanosine triphosphate (GTP)