The 4^+ 2 -electron prerequisite for cyclic stabilization is shown to be a necessary consequence whenever orbitals of symmetry are constrained to interact within a pericyclic topology. Corresponding symmetryimposed rules are then derived for three other topologies in a way that permits still further extension. Finally, a topological definition of aromaticity is provided as a stimulus and guide to further experiments.

CQ dication stabilizes the DNA structure by electrostatic interaction between the positive charges on CQ and the phosphate groups on DNA. In this complex the side chain of CQ is visualized as forming a bridge across the minor groove of the double helix. Since the distance across the minor groove, -10.5 A, is larger than the maximumo possible extended distance between N-2 and N-3, 7.5 A, and much larger than the distance observed in the crystal, 5.5 A, a N-2,N-3 complex across the minor groove would appear to be a quite unfavorable possibility. Moreover, the pmr protonation data33 point to a dication in which positive charges are located at N-1 and N-3 rather than N-2 and N-3. The N-1-N-3 distance in this case is appreciably larger in the crystal, 14.1 A, and in solution, 6.7 (20") and 8.1 8, (48"), than N-2-N-3, and a bridging complex would appear to be more favorable. However, the present solution data, though determined for the free base form of CQ, do suggest that the side chain has considerable conformational flexibility and is influenced by a variety of factors including solvent medium, temperature, and pH.Acknowledgments. The authors wish to acknowledge Dr. Fred E. Hahn for helpful discussions on the properties of antimalarial drugs. We also wish t o thank Dr. J. M. Stewart for kindly providing the crystal coordinate data.former was determined for the diprotonated form of CQ (N-1 and N-3 protonated) while the latter relates to the free base. Electrostatic repulsion between the positive charges would tend to force the CQ molecule into a more open structure with an extended side chain. Interaction with phosphate groups further acts to stabilize the extended form in the cation. On the other hand the unusual buckling of the side chain in solution may arise in part from an intramolecular dipole-induced dipole type of interaction between the N-3 ethyl groups and the r-electron system of the quinoline ring. Such an interaction has its counterpart in the wellknown intermolecular dipole-induced dipole interactions between polar aliphatic amides, amines, etc., and various aromatic ring systems. The key factors favoring this type of interaction, namely a polar acceptor and a well-defined r-electron donorr ing, are both present in CQ.3. Chloroquine Interaction with DNA. Several attempts have been made to relate the conformational features of CQ to its binding interaction with doublestranded DNA. Hahn and O'Brien34 have recently made the interesting suggestion, based on studies of the effect of CQ on the thermal stability of DNA, that the (34) R. L. O'Brien and F. E. Hahn, Antimicrob. Ag. Chemother., 315 (1966). Communications to the Editor Boat and Chair Transition States of 1,5-Hexadiene1 Sir:The stereospecific Cope rearrangement of acyclic 1,5dienes continues to attract theoretical i n t e r e~t~-~ of increasingly more quantitative ambitions. 4,5 Supplementing the previous stereochemical and kinetic6 investigation, we here report discovery of a second degenerate rearrangement of 1,5-hexadiene-one that requires an add...

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