Heterocycles thiopyrans

The relative stability of lithiated thiopyrans seems to depend upon the heterocyclic ring substitution. Thus, a-lithiated 2,6-diphenyl-2//-thiopyran 16 rearranges into the y-lithiated derivative 17 (Scheme 5) (82JOC680), while the reverse transformation occurs on lithiation of 2,6-diphenyl-4-diethylphosphonylthiopyran (80JOC2453). 

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). 

The annulation of 4//-thiopyran and cyclohexane rings in 50a results in the planarity of the heterocycle and a half-chair conformation of the carbocycle (81KGS1342). On the other hand, a boat conformation of the 2//-thiopyran ring was found in the crystal of224b [91JCS(P2)2061], Other geometrical parameters were within the limits of the expected values (Fig. 2). 

Crystal and molecular structures of 2,4,4,6-tetraphenyl-4//-thiopyran-1-oxide (143) and the corresponding 1,1-dioxide 141a were investigated to show the boat conformations of the heterocycles as well as an interesting... 

Pyrano- and thiopyranopyridines and their aza analogues fused to a five-membered heterocycle 12.17.3.1,5(i) Pyran or thiopyran as the outer ring... 

HETEROCYCLES CONTAINING ONE SULFUR ATOM 6.4.2.1 Thiopyrans and analogues... 

The presence of an electron-withdrawing substituent can favor reductions of heterocycles. In this way, the electroreduction of an activated thiopyran affords a dihy-drothiopyran with elimination of dimethy-lamine (Scheme 14) [19]. 

Subsequent studies on thioamide pyrans with application of the competitive reactions method contribute to our understanding of the mechanisms of heterocyclizations with cyanothioacetamide 114 (89ZOR1331). 2,6-Diaminothiopyrans 115, on heating in benzene, open their thiopyran ring reversibly forming intermediate 116, which can eliminate cyanothioacetamide 114 or malononitrile 27a with formation of UNs 30 or 117, respectively. The direction of the subsequent reaction with... 

Another method for e preparation of aryl w-dimethoxyethyl sulfides was described by Pandya and Tilak. The procedure consists in allowing aryl lithium derivatives to react with 2,2,2, 2 -tetramethoxy-diethyl disulfide. Subsequent cyclization of the sulfides by polyphos-phoric acid yields 2-35% of condensed thiophenes and thiopyrans. By this method, thienothiophene 1 was prepared from 2-thienyllithium [Eq. (12)] and the previously unknown dithieno[2,3-6 3, 2 -i/]thiophene (5) was obtained from thienyl-2,5-dilithium. The possibility of synthesizing a heterocyclic analog (25) of pentacene from dithieno[2,3-6 3 2 -[Pg.130]

A [4-f2] cycloaddition was the key step in the synthesis of substituted dihydrothiopyrans from 2-alkenyl-l,3-oxathianes and an alkene. The reaction was mediated by a Lewis acid. It is assumed that the Lewis acid attacks the oxygen of the heterocycle which upon ring opening gives the highly reactive cationic heterodiene which reacts with alkenes to the thiopyrans in 31-88% yield (Scheme 75) <2000TL371>. 

HETEROCYCLES CONTAINING ONE SULFUR ATOM 6A2.1 Thiopyrans and anali es... 

Various heterocycles, but particularly thiopyrans, ring-open and reclose when reacted with amines to give a variety of condensed heterocycles, including [2,7]naphthyridines (Scheme 64) <1999CHE799>. 

Limited knowledge exists with respect to detailed molecular structures of simple pyrans and heteropyrans. The semiempirical CNDO/2 calculations, using a full geometry optimization procedure, predict 4 and 5 to exhibit small deviations of the heterocyclic rings from the planar conformations.34 Unfortunately, similar calculated data for thiopyrans 6, 7, and 9 were not presented.45 Accounts of several X-ray diffraction investigations on compounds possessing pyran or thiopyran fragments are available.46 51... 

Dehydration procedures have led to 2//-thiopyrans. Thus unsubstituted parent heterocycle 6 was prepared by the dehydration of hydroxy derivative 287 with potassium hydrogen sulfate.20 The same procedure applied to a mixture of isomeric hydroxy 2-methyl-5,6-dihydro-2//-thiopyrans and 3,5-dihydroxy-2,4,5,6-tetrahydrothiopyran gives all the corresponding thio-pyrans.91 3-Phenylthiopyran dioxide 289 was obtained after the dehydration of hydroxy derivative 288a with phosphoric acid at elevated temperatures.300,301 The same approach was explored for the preparation of furano-2//-thiopyran 14.42... 

Substitutions may be electrophilic or nucleophilic according to the reagent used. Electrophilic chlorination or bromination usually takes place at position 3 and /or 5 where a relatively high 7t-electron density is located in all pyran-like heterocycles. In a second process, electrophilic attack occurs only after deprotonation of a starting substrate with a strong base substitution in the 2-, 4-, or 6-positions is typical. Nucleophilic substitution usually takes place if a pyran or thiopyran has in the 2-, 4-, or 6-position a chlorine, methoxy, or an amine leaving group. 

Benzo-2//-pyrans and their thia analogs, e.g., heterocycles not treated in this chapter, are photochemically labile and form reversible photochromic systems with open-chain and colored photoisomers. This behavior was observed for 2-benzyl-2,4,6-triphenyl-2//-species 467a,b (Y = H)399,409 and naturally occurring 2//-pyran 92a452a but only at about 77K. At room temperature, these as well as other 2//-pyrans are photochemically stable. Only 2,2,4,6-tetramethyl-2//-pyran (176) was photochemically changed to its exocyclic double bond isomer 308a.404 An irreversible photolysis of 2-azido-2//-thiopyran derivative 212b (R = H) has been discussed in reference 254. 

TetramethyI-2//-pyran (176) and (MeCN)3W(CO)3 gave zero-valence complex 623. 6-MethyI-2//-thiopyran with (MeCN)3Cr(CO)3 afforded the 7i-complex of the parent heterocycle 624.454a 3,5-Diformyl-4-ethynyl-4//-pyran (85d) replaced an azido ligand in some cobalt complexes to give 625. ... 

Hydroxy-2//-thiopyran as a pseudobase was determined by UV spectrophotometry and discussed with HMO calculations together with other heterocycles.287 Ionization constants of 2//-thiopyran 1,1-dioxides 484 (R = H R1 = Ph R2 = H, Me) were determined spectrophotometrically in H20 and MeOH at 25°C.4S8a... 

Pyrans, thiopyrans, and selenopyrans are comprehensively reviewed by Kuthan in the first unified treatment of this subject. The triazolopyridines were last reviewed in 1961 the chemistry of these five heterocyclic systems is covered by Jones and Sliskovic. 

Hydrazones of thiochroman-4-ones are converted into thiochromans under Wolff-Kishner-Huang conditions65 and into the azo dimers by silver oxide.66 Fisher indolization of the phenylhydrazones (33) gives 6,ll-dihydrobenz[6]indolo[2,3-d]thiopyrans (34), which by hydride loss form the thiopyrylium salts (35), or on dehydrogenation produce the pseudoazulenes (36) (heterocyclic analogs of the carcinogen, benz[a]-carbazole), as shown in Scheme 6.87-73... 

Whereas the chemistry of pyrans is an immense body of knowledge, that of thiopyrans has been less extensively investigated this is probably a reflection of the widespread availability of diverse six-membered oxygen heterocycles and their congeners in Nature vis-a-vis the almost total non-existence of naturally occurring sulfur-containing analogues. Apart from a small number of compounds found in crude oil, all thiopyrans are only... 

Thiopyran preparation by conversion of other heterocycles is known, but is rather restricted in scope. 

These adducts easily undergo cycloreversion reactive unstable thioaldehydes can thus be conveniently stored and recovered, and their chemistry has been greatly developed over the last 10 years, mainly by Vedejs and Kirby [126, 253]. Vedejs elegantly used the thiopyranic heterocycles, via a [2,3] sigmatropic shift of a sulfur ylide [508], as precursors for medium sulfide rings, applied to the synthesis of complex natural products in the cytochalasin-zygosporin family [522, 523] for instance. 

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