Configurations of the Aldoses

Problem 25.6 Assign R 01 S configuration to cacli chirality center in the following nioiiosaccha- 

Louis F. Fieser (1899-1977) was born in Columbus, Ohio, and received his Ph.D. at Harvard University in 1924 with James B. Conant. He was professor of chemistry at Bryn Mawr College and then at Harvard University from 1930 to 1968. While at Bryn Mawr, he met his future wife, Mary, then a student. In collaboration, the two Fiesers wrote numerous chemistry texts and monographs. Among his scientific contributions, Fieser was known for his work in steroid chemistry and in carrying out the first synthesis of vitamin K. He was also the inventor of jellied gasoline, or napalm, which was developed at Harvard during World War II. 

Aldotetroses are four-carbon sugars with two chirality centers and an aldehyde carbonyl group. Thus, there are 2 = 4 possible stereoisomeric aldotetroses, or two D,L pairs of enantiomers named erythrosc and threose. 

Aldopentoscs have three chirality centers and a total of 2 = 8 possible stereoisomers, or four d,l pairs of enantiomers. These four pairs are called rilwse, (irabinose, xylose, and lyxose. All except lyxosc occur widely. i)-Ribose is an important constituent of RNA (ribonucleic acid), L-arabinose is found in many plants, and D-xylosc is found in wood. 

Louis Fieser of Harvaid Universitv suggested tfie following procedure for remembering the names and structures of the eight n aldohexoses  

Problem 25.6 j Assign R or S configuration to each chirality center in the following monosaccha- rides, and tell whether each is a ir sugar or an i. sugar  

Problem 25.7 (-i-)-Arabinose, an aldopentose that is widely distributed in plants, is systematically named (2/, 35,45)-2,3,4,5-tetrahydroxypentanal. Draw a Fischer projection of i (-i-)-arabinose, and identify it as a o sugar or an i. sugar. 

Set up eight Fischer projections with the -CHO group on top and the -CH2OH group at the bottom. 

alternate four -OH groups to the right and four to the left. 

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If steric strain due to repulsions between substituents in the chair ring is the chief factor deciding the stability of 1,6-anhydroaldopyranoses, it is possible to calculate that, as the configuration of the aldose is varied, the amount of anhydride present at equilibrium in dilute acidic solutions should decrease in the following orcjpr of configurations,... 

RELATIVE CONFIGURATIONS OF THE ALDOSES AN EXERCISE IN STRUCTURE DETERMINATION... 

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