Optically Active Di- and Tetra-methyl-spiro-5 : 5-Dihydantoins

In a previous paper* we described the resolution into optically active components of thespiro -5: 5-dihydantoin 7 6 1 2 NH—CO—C—NH—CO ∣ ∣ CO—NH—C— CO-NH 8 9 4 3 prepared by Biltz, Heyn, and Bergius; these compounds are probably the simplest optically active spiranes which can be obtained and may therefore be regarded as the type case of optical activity associated with one particular kind of molecular configuration. We also adduced evidence, based on the different rotatory dispersions shown by the optically active dihydantoins in absence and presence of alkali, indicating that these substances could assume three tautomeric forms. This work has now been extended to the study of the optically active 3: 7-dimethyl-spiro -5: 5-dihydantoins and their acidyl derivatives and tetramethyl-spiro -5: 5-dihydantoins. Tetramethyldihydantoin, which can only exist in the ketonic state, gives, in methyl alcohol, a rotatory dis-persion curve of the simple form, with λ0 = 2478 A, nearly congruent with that of the unsubstituted dihydantoin in alcoholic solution with λ0 = 2453 A. This is a further indication that in neutral solvents and pyridine the dihydantoin has the ketonic form. The 3: 7-dimethyl- dihydantoin also shows normal rotatory dispersion in methyl alcohol and the curve, with λ0 = 2381 A, is nearly congruent with those just quoted; it should therefore be of the ketonic form in neutral solvents. The initial rotatory power of this compound in dilute soda solution is only about one-seventh of that in alcohol; passage to a tautomeric enolic form is thus indicated. The specific rotatory power in soda solution rapidly falls, however, to —1-6° for λ = 5461, a value which persists for some time; the falling off in rotatory power is due to hydrolysis with formation of 3: 8-dimethylallantoin, CH3 . NH . CO . NH . CH—NH . CO ∣ CO . N . CH3 Although this compound was only isolated from the soda solution in an optically inactive form, it would seem to exist in the optically active form in the cold soda solution. This result is of interest in connexion with recent work on the optical activity of allantoin itself.*