Stereochemistry at Tetrahedral Centers

Ribose, an essential part of ribonucleic acid (RNA), has the following structure:

(a) How many chirality centers does ribose have? Identify them.

(b) How many stereoisomers of ribose are there?

(c) Draw the structure of the enantiomer of ribose.

(d) Draw the structure of a diastereomer of ribose.

Ribose, an essential part of ribonucleic acid (RNA), has the following structure:

(a) How many chirality centers does ribose have? Identify them.

(b) How many stereoisomers of ribose are there?

(c) Draw the structure of the enantiomer of ribose.

1. Draw the structure of a diastereomer of ribose.

Ribose, an essential part of ribonucleic acid (RNA), has the following structure:

(a) How many chirality centers does ribose have? Identify them.

(b) How many stereoisomers of ribose are there?

(c) Draw the structure of the enantiomer of ribose.

(d) Draw the structure of a diastereomer of ribose.

On reaction with hydrogen gas by a platinum catalyst, ribose (Problem 5-54) is converted into ribitol. Is ribitol optically active or inactive? Explain.

Identify the indicated faces in the following molecules as Re or Si:

(a)

(b)

Identify the indicated faces in the following molecules as Re or Si:

(a)

(b)

Identify the indicated hydrogens in the following molecules as pro-R or pro-S:

One of the steps in fat metabolism is the hydration of crotonate to yield 3-hydroxybutyrate. This reaction occurs by addition of –OH to the Si face at C3, followed by protonation at C2, also from the Si face. Draw the product of the reaction, showing the stereochemistry of each step.

We’ll see in Chapter 11 that alkyl halides react with hydrosulfide ion () to give a product whose stereochemistry is inverted from that of the reactant. 

Ketones react with sodium acetylide (the sodium salt of acetylene,$\mathit{N}{\mathit{a}}^{\mathbf{+}\mathbf{-}}\mathbf{:}\mathbf{C}\mathbf{\equiv }\mathbf{CH}$) to give alcohols. For example, the reaction of sodium acetylide with 2-butanone yields 3-methyl-1-pentyn-3-ol

(a) Is the product chiral?

(b) Assuming that the reaction takes place with equal likelihood from both Re and Si faces of the carbonyl group, is the product optically active? Explain.