Benzo thiopyran

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). 

The synthetic approach reported in Scheme 50 has also been applied to the synthesis of 6a, 7-dihydro-6//[l]benzothiopyrano[3,4-c][l,5] benzothiazepine (162) starting from 5 and 3-phenylmethylene-4//-1 -benzo-thiopyran-4-one (161) (Scheme 52) (81MI1 820MR133 83MI1). >H-NMR investigations proved that the phenyl group at C-7 exists in a quasi-equatorial position, analogous to that in the isosteric benzopyrano derivatives. 

SYNS S-(6-CHLORO-3,4-DIHYDRO-2H-l-BENZO-THIOPYRAN-4-YL) o,o-DIMETHYL PHOSPHORO-DITHIOATE PHOSPHORODITHIOIC ACID, S-(6-CHLORO-3,4-DIHYDRO-2H-l-BENZOTHIOPYRAN-4-YL) 0,0-DIMETHYL ESTER... 

Thiosalicylaldehyde reacts with various hex-5-en-2-ols to give pyrano[2,3-c]benzo-thiopyrans an intermediate o-thioquinonemethide is postulated which undergoes an intramolecular cycloaddition. The process occurs with complete /roK5-stereoselectivity... 

SCHEME 16.48. Enantioselective thia-Michael/Michael organocascade leading to benzo-thiopyran derivatives. 

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). 

By Acid-Catalyzed Ring Enlargement of Benzo A cyck>propa e]thiopyrans... 

By Elimination with Concerted Ring Expansion from Benzo[6]cyclopropa[r/]thiopyrans... 

The photocyclodimerization of coumarin (2//-l-benzo[b]pyran-2-one) in direct and sensitized experiments is known to proceed rather inefficiently in the absence of Lewis acids and in better yields in their presence.42,43 In contrast, 2-quinolone (22, X = NH)44 or 1-thiocoumarin (2//-l-benzo[b]thiopyran-2-one 22, X = S)45 afford anti head-to-head dimers 23 on direct irradiation in very good yields. Psoralens (7//-furo[3,2-g][l]benzopyran-7-ones) usually exhibit a behavior similar to that of coumarins.46... 

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. 

Table 2 Benzo Fused Thiopyrans Structure and Nomenclature...

Thiopyrans are readily available by manipulation of dihydrothiopyrans, either by elimination of water, amines or halogens, or via Pummerer reaction of the S-oxide (79LA784). Elimination reactions are of course particularly facile in the benzo fused systems and may be used to specifically make 2H- or AH-1- benzothiopyrans. On the other hand, extrusion reactions are relatively unknown, though an example has been reported which produces 1H-2-benzothiopyran 2,2-dioxides (equation 110) (72US(A)(2)287), but sadly its scope appears to be rather restricted. 

The cycloaddition of a variety of sulfur heterocycles to alkenes has also been reported. Dibromomaleic anhydride undergoes [ 2 + 2] cycloaddition to thiophen295 and to benzo[b]thiophen.296 The photoaddition of benzo-[(>]thiophen 1,1-dioxides to alkenes also affords cyclobutane derivatives.297 Of particular interest is the addition of 3-acetoxybenzo[6]thiophen (350) to cyclohexene to give the adduct (351), which on treatment with base, undergoes a retro-aldol ring opening to benzo[6]thiepinone (352).298 A mixture of stereoisomeric cyclobutane derivatives were obtained on irradiation of 2,6-diphenyl-4//-thiopyran-4-one 1,1-dioxide in cyclohexene.299 The [ 2 + 2] cycloaddition of sulfolen to maleic anhydride and to maleimide has been reported.300... 

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