Association of Names and Formulas of the Aldose Sugars EARL D. STEWART Schwarr; Laboratories, Inc., New York City
T .
HE nomenclature of the aldose sugars with its arbitrary names, having no key to the configuration of the corresponding sugar, is a difficult one to remember even for chemists working in the field of carbohydrate chemistry. The systematic arrangement of the carbohydrates according to their formation by the Kiliani synthesis' affords a rational basis for the configurations assigned to the sugars, but is not of much help in associating the names with the configurations. However, the aldose sugars can be grouped in a way that makes remembering the configurations easy and when coupled with a few simple mnemonic devices will enable any chemist to associate names and configurations with certainty. Since in the d- and 1-system of sugar nomenclature, there are 4 tetroses, 8 pentoses, and 16 hexoses, but only half these numbers of names, i t is only necessary to remember that the d- and 1-configurationsare mirror images having the same names with d- or 1-prefixes. Next, the position of the hydroxyl groups attached to the carbon atoms between the terminal aldehyde and primary alcohol group determines the configuration and the name of the sugar. These carbon atoms are designated by a , p, y, 6, etc., beginning with the one next to the terminal aldehyde group. The d-sugars always have the hydroxyl group on the carbon atom next to the primary alcohol on the right of the carbon chain when it is written vertically with the aldehyde group a t the top and the primary alcohol group a t the bottom. By selecting the d-sugars having the hydroxyl group in the a-position on the right of the carbon chain, the number of sugars is reduced to one tetrose, two pentoses, and four hexoses. There are the same number of epimers or sugars with the hydroxyl in the a-position on the left of the carbon chain. It is now necessary to formulate a logical arrangement of the right-handed d-epimers and a way of associating their names with the names of the lefthanded d-epimers. For the tetroses this is very simple. (Since there is only one triose, glycerose in d- and 1-form, i t is not considered in this system.) In terms of the hydroxyl groups the d-tetroses are d-erythrose a, 8, d-threose -, 8,
the hydroxyl groups are ALL on the RIght the names and corresponding configurations are readily associated and remembered. The 1-sugars are, of course, the mirror images and have the same names preceded by "I-," i. e., 1-erythroseand 1-theose. The d-pentoses grouped by epimeric pairs are, d-ribose a, 8, y, d-arabinose -, 8,y, d-xylose a, -, 7 d-lyxose -, -, y. The d's, a's, and y's are the same for all these sugars, so only the p's have to be placed correctly. In RIbose ALL the hydroxyls are on the RIght. In XYlose the "X" and "Y" are in the RIght order while in lYXose the order is reversed. The similarity of the names RIBose and aRABinose, and XYlose and lYXose, makes remembering them easy. The hexoses become a little more complicated, but exactly the same principles can be followed. In epimeric pairs the hexose arrangement is, d-allose a, 8, y, 6, d-glucose a,-, y, 6, d-gulose a. 8.-, 6, d-galactose a,-, -, 6,
d-altrose , 8,y , 6, d-mannose-, -, y , 6, d-idose -, 8, 6. d-talose-, -, -, 6,
-.
Again the d's, a's, and 6's are the same for all the sugars. Only the p's and y's must be fixed in the memory. In ALLose ALL of the hydroxyls are on the RIght side of the column of carbon atoms. After that the p hydroxyl only is shifted to the left in glucose and mannose, then the y hydroxyl in gulose and idose, then both p and y in galactose and talose. This is a natural order and easy to remember. To associate names with these configurations a mnemonic device is resorted to, the only feat ~f sheer memory required by the system. By taking the first two letters of each member of the epimeric pairs (three letters in the case of glucose), and combining them, the following four nonsense words are obtained, al-nl
glu-ma
gu-id
gn-la
It is believed that these are distinctive enough to be remembered readily, since the names each syllable suggests are already part of a chemist's vocabulary. By recalling that for ALLose ALL the hydroxyls are on the RIGHT of the carbon chain, the correct orientation and association of formulas and names is obtained. The 1where the "-" indicates a hydroxyl group on the left formulas are again the mirror images, of course. side of the carbon chain. By noting that in eRYthrose To summarize the method of classification: ' DEGEIUNG, ED. F., "An Outline of the Chemistry of the 1. Place the d-sugars having ALL the hydroxyls on Carbohydrates," John S. Swift Co., Cincinnati, 1943, p 118; PORTER, C. W.,AND T. D. STEWART, "Organic Chem~stry," the RIGHT of the vertical carbon chain in the first place of the arrangement. Ginn and Company, Boston, 1943, p. 262.
2. Place the d-sugar having only the hydroxyl in the 6 position on the left, second in the arrangement. 3. Place the d-sugar having only the hydroxyl in the y position on the left, third in the arrangement. 4. Place the d-sugar having the hydroxyl groups in both 6 and y positions on the left, fourth in the arrangement. 5. The epimer of each sugar in the preceding arrangements has the hydroxyl group in the or-position on the left of the carbon chain. 6. The 1-sugars are the mirror images of the dsugars (but not necessarily of opposite rotation.) Obviously, the second, third, and fourth statements do not apply to the tetroses, and the fourth does not apply to the pentoses.
Some may prefer to use the numbered carbon system rather than the Greek letters, in which case the a; 6, y, &positions become 2, 3, 4, 5, respectively. The natural order is still preserved in the shiftings of the hydroxyl groups. By using the Greek-letter designations the system ties in with the formation of the ring structures of the sugars. Since the ring formulas as written bear a conventional relation to the chain formulas, this system can be used to aid in recalling them 86 well. The key words to the system are "ALL RIGHT." Anyone who can remember this and the four nonsense words necessary for recalling the hexoses, can associate quickly the name of a sugar with its epimer and its structural formula.
