The reaction between Schiff bases derived from 4-amino-2-methyl-5-methylthio-2H-l,2,4-triazole-3(4H) -thione and phenacyl bromides yields 7-aroyl-6-aryl-6,7-dihydro-l -methyl-3-methylthio-5H-1,2,4-triazolo [3,4-b] [1,3,4]thiadiazinium bromides [( 6)-( 11) I .The structure of one of these compounds, (6) (in which both aryl groups are phenyl), has been determined b y X-ray crystallography. It incorporates dioxane molecules and the bromide ion is sandwiched between t w o units. In the 1,3,4-thiadiazinium ring the consecutive hydrogen atoms -N (5) H-C(G)H-C(7)Hare trans to each other, the ring showing an Ec6 conformation. The crystalline trans derivatives equilibrate to a 60:40 mixture of trans and cis 7-aroyl-6-aryl isomers, which have been analysed by 'H and 13C n.m.r. spectroscopy.
Chiral 1,2-dihydro-(3) and 1,2,3,6-tetrahydro-pyrimidines (4) have been synthesized b y reaction of 3-aminoalk-2-enimines (1) w i t h chiral aldehydes, the structure of (4) being confirmed b y an X-ray crystal structure determination of a reduction product; a new strategy for the asymmetric synthesis of 6-amino ketones (2) and y-amino alcohols (6) with t w o or three chiral centres is described.
The crystal and molecular structure of tetrahydr0-3-methyl-4[ (5-nitrofurfurylidene)amino]-2~-thiazine 1,l -dioxide (Nifurtimox) 1, 1 -[(5-nitrofurfurylidene)amino]pyrazole 2, and 1-[(5-nitrothenylidene)amino] pyrazole 3 have been solved by X-Ray crystallography. Compound 1 presents the E-sE conformation along the N =C-C moiety while 2 and 3 presents an E-sZ one. In spite of their molecular similarity sulfur instead of oxygen in the ring), compounds 2 and 3 pack in quite different ways. In solution (lH and r3C NMR) the only difference with the solid structures concerns the -180" rotation of the furane ring in compound 1 which thus becomes similar to the other two compounds (sZ). A detailed analysis of the seven proton system of the 1,4-thiazine 1,l-dioxide ring established that in solution too, the ring has a chair conformation with the C-methyl group in an equatorial position.
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