Starting from tetrahydrothiopyran-4-one (4d) a rational NMR spectra, redox behavior, and crystal structure (H 4 -3a and 3a). The very low solubility of 3a and 3b prevented eight-step synthesis of 3a has been developed with 25 % overall yield. This route passes H 4 -3a and H 2 -3a which are further reactions. compared with 3a and its dithione 3b by IR, UV/Vis, 13 C-In a group of scientists looking for new organic materials in the starting dihydrothiopyrone 2 for most ring-closing reactions (Scheme 1). with conducting or even magnetic properties we proposed the then unknown system 3. The specific combination of Scheme 1 two thiopyrone moieties promised both reversal oxidation and reduction, backed by MO calculations (vide infra). System 3, especially 3b, carries two opposite chelating units which possibly could form polymeric metal chelates.
Exploiting the chemistry of [1,2]dithiol[4,3-c][1,2]dithioldithione (4) in an unusual reaction sequence E. Fanghänel recently arrived at 3 albeit with ca. 7% overall yield starting from 5. [4] [5] The crucial step of this synthesis includes the decarboxylation of a 3-dicarboxylic acid under rather harsh conditions. [5] Therefore, substituted systems of 3 may be accessible with difficulties by that route.
We now present a different protocol with 25% overall yield and the principal possibility for incorporating definite substituent patterns into 3. Besides some closely related in-Synthesis of System 3 termediates, calculations and cyclovoltammograms will be As crucial intermediate in the reaction sequence to 3a we reported.
headed for 10 which was expected to form the basic skel-A rational approach to 3 is primarily offered by two eton of 3 by reaction with hydrogen sulfide. For that purroutes:
pose we wanted to exploit the stabilization of carbanions 1. Route A starts with precursors of type 1, carrying alby sulfur [6] and the higher acidity of sp 2 -compared to sp 3ready the central backbone of 3 from which both rings have CϪH bonds. Therefore, we started from the commercially to be constructed at the same time. Despite of easily accesavailable tetrahydrothiopyranone 5 which is smoothly sible precursors 1 and promising analogies in the literature, transformed by N-chlorosuccinimide and pyridine to diwe were unable to approach 3 by this seemingly simple hydrothiopyranone 6. [8] In 6 the oxo group had to be proroute. tected by acetalization. This standard procedure is known to be unreliable with cyclohexenone. [9] Correspondingly, the 2. We therefore turned to route B which requires only the addition of one ring onto a preformed (hydro)thiopyranone formation of the unknown acetal 8 proceeded unsatisfactorily with proved catalysts like p-toluenesulfonic acid unit. After some dead ends, this approach finally led to a successful eight-step synthesis of 3. The general problem and strong acidic ion exchangers but afforded 8 in nearly quantitative yield with the distannoxane 7 as a catalyst. consists in an unfavorable polarization of the double bond