Thiochromones reactions

Thianaphtheneqiiinone - (109) and diazomethane give a different reaction from that found with isatin, A -methylisatin and coumarandione. In as far as crystalline products could be isolated, the ring expansion occurs here between the sulfur and the carbonyl group in the 2-position. Depending on the solvent, there are formed 3-hydroxy-thiochromone (110), its 0-methyl derivative (111), or (presumably by attack on the 3-keto group of the tautomeric 3,4-diketo form of 110), 3,3 -epoxy-3-methylthiochromone (108). 

Unusual reactions occur between diazomethane and heterocyclic thiocarbonyl compounds. For example, pyran-4-thiones give methylene ethers of 1,2-dimercaptans formed by dimerization (cf. 115 —>116). 4-Thioflavones and 4-thiochromones react similarly. 

Treatment of dibromides 2 with sodium azide in N,N-dimethylformamide (DMF) at room temperature resulted in the formation of two products, 3-(a-azidobenzyOchromones 2a-c,g or -1-thiochromones 2d-f and the 3-arylidenechromanones la-c,g,h or -1-thiochromanones Id-f, respectively (eqn. 2). As shown by yield data given in Table 2, the substituent at position 2 plays decisive role in the product ratio. Dibromides unsubstituted at position 2 tended to give almost exclusively azides 3a-f and only a small amount of 1 was obtained. On the contrary, the reaction of flavanone derivatives 2gjh gave 3-arylideneflavanones... 

Ethoxy-2/7-l-benzothiopyran with dichlorocarbene yields the dichlorocyclopropane (67),279 280 which in the presence of hot quinoline279 produces the thiochromone (68), or with methoxide280 forms the ring-expanded benzo[6]thiepin (69) and 2//-l-benzothiopyran (70), as in Scheme 13. Reaction of dichlorocarbene with the simple 2//-l-benzo-thiopyran (4) produces281 the dichloromethyl derivatives (71) and (72) of Eq. (33). 

These reactions are virtually limited to molecules possessing an ylidene functionality at the 4-carbon, formally derived from thiochromone condensations with active methylene compounds (e.g., see Section... 

Oxidation of thiochromones does not lead to the sulfoxide or sulfone derivatives these are better prepared from the corresponding thiochromanones by oxidation followed by bromination and dehydrobromination. The 1-oxide is an extremely reactive species and gives a Diels-Alder adduct with butadiene. The 1,1-dioxide is also very reactive. Photolysis in benzene affords a 2 1 adduct in 30% yield (116) (79TL1097) via an initial [2 + 2] addition followed by a Diels-Alder reaction. Simple [2 + 2] addition is found with cyclohexene. 

Silyloxybenzopyrylium salts, readily derived from thiochromones, undergo a diastereoselective annulation on reaction... 

Conversion of thiochromones into 4-silyloxybenzothiopyrylium triflates 352 facilitates the 1,2-addition of nucleophiles across the S=C bond. Of course, the process corresponds to an overall 1,4-addition to the thiochromone. For example, reaction with silylenol ethers, readily derived from an enolizable ketone, affords the thiochroman-based 1,5-diketone or a silylenol ether derivative <1996SL182>. In like manner, allyltri- -butyltin yields the 2-(2-propenyl)-4-silyloxy-2//-l-benzothiopyran <2001T1005> and 1-morpholinocyclopentene affords the 2-(2-oxocyclopentyl) derivative (Scheme 78) <1997J(P1)2807>. 

Acylation of the thiol moiety of thiosalicylic acid is the starting point for a synthesis of 2-substituted thiochromones in which the key step is an intramolecular Wittig reaction (Scheme 203) <2001T9755>. 

Trifluoromethyl derivatives of thiochromone are also available from thiosalicylic acid by conversion to alkyl 2-mercaptophenyl ketones followed by a Baker-Venkataraman reaction with trifluoroacetic anhydride in triethyl-amine (Scheme 205) <2005PS(180)1315>. 

The direct conversion of 2-phenylthiochroman-4-ones to thiochromones (thioflavones) 525 can be achieved with DDQ, iodobenzene diacetate (IBDA) or iodine (Equation 185) <1999SC1857>. Thioflavanones afford 3-substituted thioflavones 526 by isomerization of the initially formed aldol product from reaction with a substituted benzaldehyde (Equation 186) <1991BSF976>. 

Thiochroman-4-ones are converted to thiochromones by conversion to their trifropropylsilyl enol ethers and dehydrogenation with ceric ammonium nitrate. It is proposed that the reaction proceeds via a radical cation which undergoes desilylation. (l-Elimination from the cation generated by single-electron transfer completes the sequence (Scheme 206) <1995TL3985>. 

Furo[3,4-/ ][l]benzopyran-9-ones are formed directly by deprotection of the acetal 18. The corresponding thiochromones behave similarly <02TL4507>. Incorporation of a trifluoromethyl group into chroman-4-ones can be effected through reaction of 2-trifluoromethyl-4ff-chromen-4-imines with malonic acid the acid serves as a methylating agent <02S2341>. 

The reaction of thiosalicylic acids with A,A-dialkylacetamides with an electron withdrawing group at the 2-position leads to 2-dialkylamino-3-substituted thiochromones. The use of A-alkylated piperidones produces the benzothiopyrano[2,3-Z)]pyridine ring system <05AJC864>. Cyclisation of alkyl 2-mercaptophenyl ketones, prepared from thiosalicylic... 

Thiochromone reacts with 2-piperidinobuta-1,3-diene to give a single diastereoisomeric thioxanthene-3,9-dione. Reaction of 4-silyloxy-l-benzothiopyrylium triflate, derived from thiochromone, with enamines involves regioselective 1,2-addition leading to 2-substituted thiochromanones <97JCS(P1)2807>. 

Apparently, the cyclization of 5-phenyl-3-oxobutanethioates (181) to 2H-1-benzothiopyran-2-ones (182) in polyphosphoric acid is successful only with the m-methoxy-derivative (181 R = m-MeO). In all other cases the isomeric thiochromones, e.g. (183), are formed. Spectroscopic methods of distinguishing between these hitherto difficultly distinguishable isomers are discussed. In the H n.m.r. spectrum the thiochromones show characteristic deshielding of the 5-proton by the peri carbonyl group. Equally diagnostic are their mass spectra, which show fragmentation of the parent ions to the ketens (184) by a retro-Diels-Alder reaction. 

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