2,3,6,7‐Tetramethoxythianthrene Dication: An “Aromatic” π System “Gives Up the Ghost”

Abstract

Table 2. Preparation of w-ureido D-amino acids Hydantoin o-amino acid Content of ~[ h ] 0 0 n = l '0 n = 2 n = 3 D-alhizziin (101 D-4-ureido-2-aminohutyric acid[ll] 0-ci trulline I1 21 . . n = 4 D-homocitrulline 1131 H 0 H H,N N d S + C O O H I, NH, 0 V 69 73 79 70 > 99 98-99.8 96-99.8 60 m = 1 0 o-w-ureido-AEC [c] 71 98-99.8 m = 2 D-w-carhamoylthialysine 1141 78 98-99.8 [a] Yield of isolated product [YO]. [h] Amount Of D enantiomer [%I, determined by comparison with literature data or by chromatography with the racemic compounds for comparison[l0-14]; the ranges refer to different runs. [c] AEC = S-aminoethylcysteine.Furthermore, we desired to determine the spectrum of substrates accepted by this biocatalyst. Readily obtainable racemic or L-n,w-diamino carboxylic acids were converted into hydantoins analogous to 5['' and subjected to biotransformation (Table 2).This approach provides a series of interesting w-ureido 6-amino acids that are chemically synthesized only with difficulty and are not obtainable at all through fermentation. These examples show the advantages of combining chemical and enzymatic steps in the preparation of enantiomerically pure amino acids unknown in nature. Experimental ProcedureSummary of the synthesis ofo-citrulline (1, C,H,,N,O,, M = 175.19)[15]: A solution of 5 (4 g, 22.8 mmol), prepared according to 191, in water (55 mL) was adjusted to pH 8.4 by addition of concentrated aqueous sodium hydroxide. The solution was then degassed by agitating in a mechanical shaker at 40 "C for 10 min, treated with the bacterial culture (Agrobaclerium radiobacter, 0.5 g) under N,, degassed once again, and then sparged with N,. The vessel was shaken for 24 h at 40 "C under N, at 6 bar and then for 24 h at 40 "C under normal pressure before cooling it and adjusting the pH of the solution to pH9. The solid portion of the culture was separated by centrifugation and the remaining liquid was treated with charcoal to clear the solution. Yield in solution 95%. Isolation was accomplished by filtering the solution through a strongly acidic ion exchanger (Merck, IR 120) and eluting the product with 5 % aqueous NH,. The solution thereby obtained was evaporated and the residue was dissolved in a small amount of water. Addition of ethanol precipitated the product. 3.4g (85%), m.p. 215-218 "C (dec.), [a]:': +22 (c = 2 in 1 N HCI) 'H NMR (250 MHz, D,O): 6 = 3.73 (m, 1 H, Ha), 3.1 (m. 2H, Hy); 1.4-2.0 (2 x m, each 2H, Hfi + H;?.Well over half a century ago, E. Hiickel proposed the (4n + 2)-electron rule for cyclic IT systems. Since then, this simple and fascinating concept"' has stimulated the syntheses of a great variety of novel molecules and molecular ions, which in lab jargon are frequently termed aromatic (Greek: aromatikos = herbal smelling) and which, according to numerous experimental data as well as quantum-chemical results, are distinguished by an energetically favorable electron distribution.['] Deviations from the optimum number of electrons, therefore, should inflict severe perturbations and, as is known f...

Electronic structure of zwitterionic diamino-meta-quinonoid molecules: identity of UV absorption bands

“…Following different substitutions at the remaining ring positions, a large variety of derivatives have been prepared showing typical properties. For example, aminoquinones involve in an interesting structure-color relationship [2]. Most of the known stable quinonoids were however derived from the parent para-quinone (I) and ortho-quinone (II).…”

Dynamic Redox Systems as Electrochromic Materials: Bistability and Advanced Response

“…Changes in the p-electron density ( 13 C NMR) as well as reduced bond-order changes (X-ray) clearly show that each positive charge in 4c 2+ is delocalized over the dithienylmethylium unit as in the case of newly prepared reference monocation 5 + ( Figure 5). 20 Thus, dication 4c 2+ is stabilized due to polymethine conjugation, which is more effective than the aromatization effect in some cases 28 The nonplanar structures in both the neutral and the cationic state make 4 quite unsuitable for use as a component of a conducting organic solid. However, the beautiful deep-blue color as well as the reversible change in geometry upon electron transfer were so impressive that the principle author determined to change his research project.…”

Generation and structure determination of 5,6‐bis(methylthio)‐4,7‐diethylbenzo[1,2,3]trithiole dication MBT(2+)

“…The dication seems to be soluble in acetonitrile. In order to facilitate precipitation of the product, MBT was then treated with two equivalents of SbCl 5 in dichloromethane, instead of dichloromethane/acetonitrile, for 1 h (Scheme 5) [17]. A dark-brown precipitate produced by the oxidation was filtered under an argon atmosphere and washed with dichloromethane.…”

“…The reaction mixture was extracted with CH 2 Cl 2 and the solution was dried with MgSO 4 . The solvent was evaporated and the residue was purified by column chromatography (silica gel; CHCl 3 :hexane = 5:1, and then hexane:AcOEt = 2:1) to give MBT (32 mg, 20%), MBTMO (26 mg, 16%), 17%), and MBT 2-O (13 mg, 8%), together with a mixture of bissulfoxide MBTO2 (5 mg, 3%).…”