Fischer’s Proof of the Configuration of D-(+)-Glucose Emil Fischer initiated his research on the stereochemistry of (+)-glucose in 1888, just 12 years after the proposal by van’t Hoff and Le Bel concerning the tetrahedral nature of carbon which led to the concept of stereoisomerism. Structure determination at this time:

1) Melting Points 2) Polarimetry (optical rotation) 3) Chemical Derivatization/Degradation

Known at this time:

1) (+)-glucose is an aldohexose 2) (+)-glucose has 4 stereocenters and thus has 24 = 16 possible

stereoisomers 3) Absolute configuration of organic compounds could not be determined 4) Fischer had assigned D-(+)-gylceraldehyde the following configuration

CHO

CH2OHOHH

D-(+)-glyceraldehyde

Fischer decided to limit his focus to the eight aldohexoses with the D configuration and arbitrarily assigned (+)-glucose to this family. It was not until 1951, when Bijvoet determined the absolute configuration of L-(+)-tartaric acid that Fischer’s assignment of (+)-glucose to the D family was shown to be correct. Fischer’s assignment of the structure shown below to (+)-glucose was based on the following reasoning:

CHO

CH2OH

OHHHHOOHHOHH

D-(+)-glucose

The Eight D-Aldohexoses

How can we deduce the structure of D-glucose??

Hermann Emil Fischer (9 October 1852 – 15 July 1919) was awarded the Nobel Prize in Chemistry in 1902 for his work on sugar and purine synthesis.

Step 1: Oxidation of (+)-glucose Nitric acid oxidation of (+)-glucose furnished an optically active aldaric acid.

CHO

CH2OH

??????OHH

CO2H

CO2H

??????OHH

Optically Active

HNO3

(By convention D-family has OH to the right)

This experiment eliminates 2 of the 8 possible D aldohexoses as the structures shown below would yield meso compounds upon oxidation.

CHO

CH2OH

OHHOHHOHHOHH

CHO

CH2OH

OHHHHOHHOOHH

Step 2: Degradation of (+)-glucose

Wohl degradation of (+)-glucose gives (-)-arabinose, and nitric acid oxidation of (-)-arabinose gives an optically active aldaric acid.

CHO

CH2OH

??????OHH

CHO

CH2OH

????OHH

(-)-arabinose(+)-glucose

1) NH2OH2) acetic anhydride3)NaOCH3

Wohl degradation

HNO3

CO2H

CO2H

????OHH

Optically Active

This experiment eliminates 2 of the 4 possible structures for D-(-)-arabinose as the structures shown below would yield meso compounds upon oxidation. It also tells us that the orientation of the OH group at C2 of (-)-arabinose is left.

CHO

CH2OH

OHHOHHOHH

CHO

CH2OH

OHHHHOOHH

By analogy to the observation above, this experiment also eliminates the following structures for (+)-glucose.

CHO

CH2OH

HHOOHHOHHOHH

CHO

CH2OH

OHHOHHHHOOHH

CHO

CH2OH

HHOOHHHHOOHH

These observations leave the following structures as possibilities for (+)-glucose.

CHO

CH2OH

OHHHHOOHHOHH

CHO

CH2OH

HHOHHOOHHOHH

CHO

CH2OH

HHOHHOHHOOHH

Step 3: Derivatization of (-)-arabinose Kiliani-Fischer synthesis beginning with (-)-arabinose furnishes (+)-glucose and (+)-mannose.

CHO

CH2OH

HHO??OHH

(-)-arabinose

CHO

CH2OH

OHHHHO??OHH

CHO

CH2OH

HHOHHO??OHH

Shared structes of (+)-glucose and (+)-mannose

1) NaCN, HCN2) H2, Pd3) H3O+

Kiliani-Fischersynthesis

Nitric acid oxidation of (+)-mannose gives an optically active aldaric acid. This observation, together with the fact that (+)-glucose gives an optically active aldaric acid upon nitric acid oxidation, tells us the structure of (-)-arabinose.

CHO

CH2OH

HHOOHHOHH

(-)-arabinose

Had (-)-arabinose been the C3 epimer (opposite orientation) of the structure shown above, the Kiliani-Fischer synthesis followed by nitric acid oxidation would have given one optically active aldaric acid and one meso compound. By analogy to the configuration assigned to (-)-arabinose, this experiment also eliminates the structure shown below for (+)-glucose.

CHO

CH2OH

HHOHHOHHOOHH

Step 4: Assignment of configuration to (+)-glucose The structures shown below now remain, one structure represents (+)-glucose and one represents (+)-mannose. Fischer observed that these two structures were epimeric at C2 but assigning the correct structure was most difficult.

CHO

CH2OH

OHHHHOOHHOHH

CHO

CH2OH

HHOHHOOHHOHH

Fischer had previously developed a method for effectively interchanging the two end groups (-CHO and –OH) of an aldose chain. With brilliant logic, Fischer realized that if the first structure was (+)-glucose, an interchange of end groups would yield a different aldohexose.

CHO

CH2OH

OHHHHOOHHOHH

CH2OH

CHO

OHHHHOOHHOHH

CHO

CH2OH

HHOHHOOHHHHO

A new aldohexose

end-group

interchangeby chemical

reactions

180o

If the second structure was (+)-glucose, an interchange of end groups would yield the same aldohexose.

CHO

CH2OH

HHOHHOOHHOHH

CH2OH

CHO

HHOHHOOHHOHH

CHO

CH2OH

HHOHHOOHHOHH

The same aldohexose

end-group

interchangeby chemical

reactions

180o

With this observation, Fischer conducted the end-group interchange starting with (+)-glucose and found that the product was a new aldohexose. This outcome proved that (+)-glucose had the structure shown below and Bijoet proved in 1951 with x-ray diffraction that Fischer’s arbitrary assignment of (+)-glucose to the D-family of aldohexoses was indeed a very good guess!

CHO

CH2OH

OHHHHOOHHOHH

D-(+)-glucose