Thiopyran

No tautomeric interconversions have been observed between 4//-pyrans (1, X = O) and 2//-pyrans (2, X = O), 4//-thiopyrans (1, X = S) and 2H-thiopyrans (2, X = S), or their benzo-fused derivatives. Under normal conditions, all these compounds exist as sole isomers that are initially formed, both in solution and in the solid state. The only tautomeric equilibrium reported was that between the two possible 2H isomers of the unsymmetrical sulfone 3 (77TL1149). 

Thiopyrans formed on irradiation of the corresponding 4H derivatives are less prone to ring opening than 2//-pyrans. This led to the study of the photochemistry of variously substituted 4//-thiopyrans. Irradiation of 3,5-unsubstituted... 

The mechanism suggested earlier for the similar isomerization 34 35 involving the reaction of a 1,2-dihydroquinoline molecule with the starting quinolinium salt 33 (Scheme 11) is analogous to that of 4//-thiopyran isomerization discussed above (cf. Scheme 3) and is supported by deuterated substrate studies (85CJC412). Further support for this mechanism is the absence of such isomerization for 4-substituted derivatives such as 1,4-dimethyl-1,2-dihydroquinoline [94JCS(CC)287]. 

The known but newer, more elaborate approach to 4//-thiopyrans by procedures A, B, and C (Scheme 1) resembles the Hantzsch-like syntheses of 4iT-pyrans (83AHC145, Section III,D). The results are summarized in Table I for 3,5-dicyano products 21. The chemistry of cyanothioacetamide as the key starting component has also been reviewed (86H2023 87H205). Cyclocondensations A to C usually proceed under mild conditions and afford 21 in satisfactory yields. 

Some modifications of the procedures B and C have been reported for the preparation of 5-(benzothiazol-2-yl)- or 5-acetyl derivatives 22 (88AP509) and 23 [87ZN(B)107], The use of the sulfur precursor 24 (85S432) or nonsulfur starting components 26 [87ZN(B) 107] led to biheter-ocyclic 4//-thiopyrans 25 or 27, respectively. 

Eq. (3), with lithium diisopropylamide (LDA) to a lithiospecies and in its subsequent reaction with C02 affording via the corresponding 4-carboxylic acid its ethyl ester 59. In the alternative version perchlorate 48e is electro-chemically reduced in acetonitrile to an anionic species that was converted either to a 3 1 mixture of isomers 56 (R = f-Bu) and 60 or to 4//-thiopyran 56 (R = PhCH2) with f-BuI or PhCH2Br, respectively (90ACS524). The kinetics of the benzylation procedure was followed by cyclic voltammetry [88ACS(B)269]. 

The addition of diphenylmethoxyphosphane to 2,6-diphenylthiopyry-lium perchlorate (48a) in the presence of sodium iodide in acetonitrile was found to lead to isolable 4//-thiopyran 70a. The transformation 71a —> 72a proceeded analogously [90ZOB(L)1012],... 

Thiopyrans 50a-c were also obtained from the corresponding 2-methoxy-2,3-dihydropyrans by treatment with a H2S-HC1-Ac20-Ac0H mixture (86KGS28). 

Thiopyran 134a (R = 2-thienyl) was prepared from perchlorate 135 (R = H) with 2-thienylmagnesium bromide (91KGS900). 

A number of 4//-thiopyrans were converted to their corresponding sulfones with hydrogen peroxide in acetic acid 132... 

Diphenyl-4//-thiopyran (9) was oxidized to its S,S-dioxide 141g with Ru04 (82CJC574). 2,4,4,6-Tetraphenyl-4//-thiopyran-S-oxide (143) was obtained from 56 (R = Ph) with H202-Ac0H under mild conditions [92JCS(P2) 1301 ]. 

Further 4//-thiopyrans as well as 2//-analogs were reduced mainly to their corresponding tetrahydro derivatives. In addition, the hydrogenation of 4//-thiopyran-5,5-dioxides have been described. 

Thiopyran-S,5-dioxide 140c underwent hydrogenation to its tetrahy-dro derivative 159 under the same conditions as the annulated substrates 160b and 161 (83KGS1058). 

R3 = Me) - 151 (R = H, R = Ph). The reductions proceed via a 3,5-protonation of the 4//-thiopyran ring as follows from experiments with 132(R, R2,R3 = H, Ph, H Me, Ph, H) using the deuterium-labeled TESH or TFAA (83KGS200, 83MI2). In some cases HC104 instead of TFAA could be used (83KGS200). 

Thiopyrans 169 have been reported to isomerize to thiophenes 170 on heating in ethanol (90TL115). The sulfoxide 47b was found to afford 2//-thiopyran isomer 171 on heating in AcOH but another 4//-thiopyran,... 

Two examples involving as key intermediates the corresponding thio- or selenopyran anions are mentioned in Section IV,A,2, e.g., 48e — 60 + 56 (R = /-Bu) and 54d + 55d -> 59 (85CL1119 90ACS524). A deuteriode-protonation of 2,6-diphenyl-2//-thiopyran (87) to 4-deuterio-2,6-diphenyl-4//-thiopyran (51) took place after the initial reaction with BuLi-THF at... 

The thermolysis of azido-4//-thiopyrans 69 (R = H, Ph, r-Bu) has been considered to proceed according to Scheme 9, giving mixtures of thiazep-ines 200 and pyridines 199 and 201 in different ratios depending on reaction conditions [89PS(43)243]. 

Dimeric heterocyclic compounds 206 (R, R = Ph, Ph Ph, MeO MeO MeO) were prepared by treatment of diazo-4//-thiopyrans 203a, 203b, and 203e with 2,4,6-triphenylthiopyrylium salt 48e in an Et3N-CHCl3 solution at 20°C. A mechanism is discussed in detail (85T811). 

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