Cyamelurates are salts of cyameluric acid, a derivative of tri-s-triazine (1,3,4,6,7,9-hexaazacyclo[3.3.3]azine or s-heptazine). These compounds are thermally very stable and possess interesting structural and optical properties. Only very few tri-s-triazine derivatives have been reported in the literature. The water-soluble alkali cyamelurates were extensively characterized using NMR, FTIR, Raman, UV, luminescence spectroscopy and elemental analysis. In addition, the single crystal X-ray structure analyses of the four hydrates of lithium, sodium, potassium and rubidium cyamelurates (Li(3)[C(6)N(7)O(3)].6H(2)O; Na(3)[C(6)N(7)O(3)].4.5H(2)O; K(3)[C(6)N(7)O(3)].3H(2)O; Rb(3)[C(6)N(7)O(3)].3H(2)O) are presented. Thermogravimetric analysis shows that the dehydrated salts start to decompose at temperatures above 500 degrees C. The thermal stability does not depend on the cations which is in contrast to the analogous s-triazine salts, i.e. the alkali cyanurates M(3)[C(3)N(3)O(3)]. The photoluminescence spectra indicate a very strong solid state UV-emission with maxima between 280 and 400 nm.

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N‐(Hydroxy)thiazole‐2(3H)‐thiones 6–10 have been prepared in a short and efficient synthesis from p‐substituted acetophenones. Alkylation of heterocycles 6–10 in the form of their potassium or tetraalkylammonium salts 11–15 affords N‐alkoxy‐4‐arylthiazole‐2(3H)‐thiones 16–20 in good to satisfactory yields. The hitherto unknown thiones 16–20 have been subjected to a detailed structural investigation (NMR spectroscopy and X‐ray crystallography) and furthermore to a mechanistic study in order to explore their utility as sources of oxygen‐centered radicals in solution. From the results of these studies, the following conclusions can be drawn: (i) X‐ray analyses of the p‐chlorophenyl‐substituted acid 9, of the O‐alkyl derivatives 19c, 19f, and of the O‐mixed anhydride 19k indicate short C–S bonds [C2–S2 = 1.637(5)–1.684(2) Å] and long N–O connectivities [N3–O1 = 1.369(3)–1.379(2) Å] in the thiohydroxamate functionalities. Furthermore, O‐alkyl‐ or O‐acyl substituents at O1 are twisted out of the thiazolethione plane by ca. 90°, which points to lone‐pair repulsion between nitrogen and oxygen atom as the underlying structural motif of the cyclic thiohydroxamate derivatives. (ii) Alkylation of ambidentate thiohydroxamate anions (salts 11–15; oxygen and sulfur nucleophiles) affords almost exclusively O‐esters 16–20 (alkylation at the oxygen atom). (iii) Based on the results of X‐ray diffraction studies and on the 1H‐ and 13C‐NMR spectra, guidelines for the characterization of N‐(alkoxy)thiazolethiones 16–20 and 2‐(alkylsulfanyl)thiazole N‐oxides 21–25, i.e. the products of S‐alkylation of thiohydroxamate salts 11–15, could be derived. (iv) Photolyses of substituted N‐(4‐pentenoxy)‐4‐arylthiazolethiones 16–20 in general and in particular of p‐chloro derivatives 19 were carried out in the presence of the hydrogen donor Bu3SnH, and afforded substituted tetrahydrofurans 31 or tetrahydropyrans 32 as major products in good yields. The observed stereo‐ and regioselectivities of ethers 31 and 32 point to alkoxyl radicals 30 as reactive intermediates, which add intramolecularly by selective 5‐exo‐trig or 6‐endo‐trig pathways to the olefinic double bonds. In terms of synthetic access and ease of handling of the radical precursors, the p‐chlorophenyl‐substituted thiazolethiones 9 and 19 exhibit significant advantages over all the other thiones used in this study and are considered as excellent substitutes for the pyridinethiones as efficient sources of free alkoxyl radicals. Consequently, the present compounds may be of use in both mechanistic and synthetic studies.

Keywords: Oxidation / Tetrahydrofuran / Catalysis / Asymmetric synthesis / Vanadium Vanadium(V) complexes 4 have been prepared from tridentate Schiff-base ligands 3 and VO(OEt) 3 . All vanadium(V) compounds were characterized (IR, UV/Vis, and 51 V NMR spectroscopy, and in selected examples by X-ray diffraction analysis) and were applied as oxidation catalysts for the stereoselective synthesis of functionalized tetrahydrofurans 2 starting from substituted bis(homoallylic) alcohols 1 (mono-or trisubstituted C−C double bonds). Oxidation of secondary or tertiary 1-alkyl-, 1-vinyl-, or 1-phenyl-substituted 5,5-dimethyl-4-penten-1-ols under optimized conditions [TBHP as primary oxidant and 1,2-(amino)indanol-derived vanadium(V) reagent 4g as catalyst] provided 2,5-cis-configured tetrahydrofurans in synthetically useful yields and diastereoselectivities (22−96% de). On the other hand, trans-disubstituted oxolanes (62%−96 de) were obtained from oxidations of 2-or 3-alkyl-and 2-or 3-phenyl-substituted 5,5-dimethyl-4-penten-1-ols bis(homoallylic) alcohols. Treatment of 4-penten-1-ols (i.e. substrates with monosubstituted olefinic π-bonds) with TBHP and catalytic amounts of vanadium(V) complex 4g furnished trans-disubstituted tetrahydrofurans as major products (20−96% de), no matter whether an alkyl or a phenyl substituent was located in position 1, 2, or 3 of the alkenol chain. The mechanism of this reaction has been investigated in detail. Based on re-

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