In the Classroom
A Mnemonic for the Inositols The Mnemonic Recalls the Names of the Nine Inositols and Serves as a Code, Allowing the Derivation of a Conformational Formula for Each Terence J. Painter University of Trondheim, Norway
Inositol derivatives (1) are becoming increasingly recognized as important building units of dozens of new aminocyclitol antibiotics (2) and as intracellular messengers (3). Historically, they have featured so prominently in the development of modern ideas about conformational energies and symmetry properties that it is hard to give a course in stereochemistry without discussing them (4). Unfortunately, it is hard to remember the configurational prefixes that form the basis of the modern nomenclature (5), and with the exception of cis-inositol, it is even harder to remember the configuration that each refers to. The mnemonic proposed here originated as a teaching aid at this university about 15 years ago. Former students report that they still have 100% recall after 10 years or more. It replaces the drudgery of rote learning with a simple exercise in logic, and it allows teachers to set examination questions of a higher standard than would otherwise be considered fair. The mnemonic derives from the mythical tale of Scylla and Charybdis in Homer’s Odyssey (chapter 12). It takes the form of an imaginary headline in a newspaper: SCYLLA MEETS CHARYBDIS—EPIC NEWS MUCH ALARMS SICILY. The first two or three letters in each of these eight words remind the user that the nine configurational prefixes are scyllo-, meso- (or myo-), chiro- [(+) and (2)], epi-, neo-, muco-, allo-, and cis-, respectively (5). The mnemonic also arranges the prefixes in an order that allows the configurations to be derived in a logical manner by performing a defined sequence of imaginary configurational inversions (epimerizations). scyllo-Inositol (1) is selected as the starting-point because it is relatively easy to remember the HO-groups are arranged alternately above and below the plane of the ring, and are therefore all equatorial in the most stable chair conformation. Comparison of a molecular model with the six-headed monster Scylla provides an entertaining interlude in the classroom, which can have significant mnemonic value for younger students. Older students may be more interested to note that Homer imagined Scylla as having three rows of teeth in its jaws, and that this is a characteristic anatomical feature of some fish of the shark family. scyllo-Inositol was first isolated in 1858 from a shark which, at that time, was named Scyllium canicula (6), obviously in cognizance of Homer’s tale. This name has now been changed to Scyliorhinus cuniculus, but the original prefix has been retained. meso-Inositol (myo-inositol) (2) is obtained from scyllo-
inositol by moving any one of the six equatorial HO-groups to an epimeric, axial position on the same carbon atom. This carbon atom then becomes C-1 in a provisional ringnumbering system. With the axial HO-1 pointing towards the observer, the remaining five carbon atoms are numbered around the ring. With clockwise numbering, (+)chiro-inositol (3) is obtained by moving HO-2 to an axial position, whereas with counterclockwise numbering, (2)chiro-inositol (4) is generated by this operation. epi-Inositol (5) and neo-inositol (6) are obtained from meso-inositol by epimerization at positions 3 and 4, respectively; the direction of the numbering is immaterial in these cases, because the products are achiral. muco-Inositol (7) is obtained from meso-inositol by moving both HO-2
Figure 1. Conformational formulas for the nine inositols.
Vol. 73 No. 10 October 1996 • Journal of Chemical Education
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In the Classroom
and HO-3 to axial positions, and again the product is achiral. allo-Inositol (8) is obtained from meso-inositol by moving both HO-2 and HO-4 to axial positions. Formally, the direction of the numbering does matter here, the (+)enantiomer being obtained by clockwise numbering and the (2)-enantiomer by counterclockwise numbering. The two enantiomers are, however, isoenergetic chair conformers in rapid equilibrium, and the distinction is significant only in cases of partial or multiple substitution. The configuration of cis-inositol (9) is already given by its name, so no mnemonic is needed for this. This systematic sequence of permutations (1; 1 and 2; 1 and 3; 1 and 4; 1, 2 and 3; and 1, 2 and 4) for meso- to allo-inositol inclusive is easy to remember, as also is the rule for obtaining the (+) and (2) enantiomers of chiroand allo-inositols. It should be noted, however, that the provisional ring-numbering system that arises spontaneously from this mnemonic differs from the one approved by IUPAC for purposes of nomenclature (5). Hence the numbers may not be used as locants for substituents or
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other sites of modification. Briefly, C-1 in the provisional system becomes C-2 in the official one, except in the case of allo-inositol, when it becomes C-6. However, the original rules (5) should be consulted for details. Acknowledgment The author thanks Laurens Anderson (Madison, Wisconsin) and Stephen J. Angyal (Sydney, Australia) for advice. Literature Cited 1. Hudlicky, T.; Cebulak, M. Cyclitols and Their Derivatives; Verlag Chemie: Weinheim, 1993. 2. Collins, P. M., Ed. Carbohydrates; Chapman and Hall: London, 1987. 3. Potter, B. V. L.; Gigg, R. Carbohydr. Res. 1992, 234, xi–xxi. 4. Stoddard, J. F. Stereochemistry of Carbohydrates; Wiley–Interscience: New York, 1971. 5. IUPAC. Report of Committee on Nomenclature of Cyclitols: Pure Appl. Chem. 1974, 37, 285–297. Reprinted in Eur. J. Biochem. 1975, 57, 1–7 and Biochem. J. 1976, 153, 23–31. 6. Staedeler, G.; Frerichs, F. T. J. prakt. Chem. 1858, 73, 48–55.
Journal of Chemical Education • Vol. 73 No. 10 October 1996
