Benzothiopyrylium Salt

The 1-benzothiopyrylium salts are best prepared from the easily accessible thiochroman-4-one [Eq. (35)].6,51,52,302-304 The common dehydrating agents include potassium hydrogen sulfate8 and phosphorus pentoxide51,302-304while sulfuryl chloride/perchloric acid51,302-304 and trityl perchlorate 6 have been useful as hydride extractors. Another method involves the cyclization of /1-ketosulfides (e.g., 81) or j8-arylthio-acroleins (82) with perchloric acid [Eq. (36)].305,306 

Tsujikawa, K. Tsukamura, Y. Nagawa, and Y. Saji, Japan Patent 7,328,472 Chem. Abstr. 78, 159428 (1973). 

Hashimoto, K. Kanai, H. Kitano, and K. Fukui, Nippon Kagaku Zasshi 86, 438 (1965) Chem. Abstr. 63, 11497 (1965). 

Similarly to Eq. (35) for the 1-benzo series, 2-benzothiopyrylium salts are available from isothiochromanone.276,303,304 

Liittringhaus, N. Engelhard and A. Kolb, Justus Liebigs Ann. Chem. 654, 189 (1962). 

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Benzothiopyrylium salts undergo a polar [4+2+] cycloaddition to give l,10a-dihydro-4/f-4a-thioniaphenanthrene salts, which afford 2-substituted 2ff-l-benzothiopyrans on treatment with nucleophiles <96JCS(P1)2227>. 

This behaviour is similar to that reported for the 2-isomers <94JCS(P1)3129>, but it is now shown that 2-benzothiopyrylium salts can behave as electron deficient dienes affording tricyclic molecules such as (31) and (32) by a [4++2] cycloaddition <96CC2185>. 

In the presence of a base, 3-acyl derivatives of 2-benzothiopyrylium salts form a stabilised ylide which undergoes a cycloaddition reaction with unchanged starting material, affording the complex benzothiopyran (33) <96CC1659>. 

Heteroanalogues of spirodibenzopyran, such as spiroquinolinonaph-thopyran 18 and spirobenzothiopyranonaphthopyran 19, are similarly prepared from the corresponding quinolinium salts and benzothiopyrylium salts, respectively (Scheme 12)51,52 Spirobenzothiopyranonaphthopyran 19 has an extended 71-conjugation in 4 -position, and its photomerocyanine... 

A series of ethoxycarbonylbenzo[ >]thiepins (73-76) has also been synthesized by this method 50). When benzothiopyrylium salt (70) was treated with ethyl lithiodiazo-... 

Table 1 Ring openings of l-benzothiopyrylium salts 22 (Equation 4)...

Table 2 Ring transformations of l-benzothiopyrylium salt 22d (Equation 5)...

Thiopyrylium ions have been shown to behave as effective dienophiles. Shimizu and co-workers demonstrated that 2-benzothiopyrylium salts undergo [4+2+]-type cationic polar cycloadditions with various 1,3-dienes yield benzo-fused bicyclic sulfonium salts <1994J(P1)3129> (see Section 8.32.2.1.2 of CHEC-II(1996)). However, in this study the authors observed that cycloaddition of 2-benzothiopyrylium salt 40 with 2,3-dimethylbuta-l,3-diene 41a with prolonged reaction times caused a decrease in yield of the cycloadduct 22a (see entries 1 and 2 in Table 5 and Equation 10). In fact, stirring of the reaction mixture for 30 h did not afford the cycloadduct at all, see entry 3, but gave an undetermined complex mixture. 

Table 5 Polar cycloadditions of 2-benzothiopyrylium salt 40 with dimethylbuta-1,3-diene 41a (Equation 10)...

Benzothiopyrylium salts have also been shown to undergo [4+2+]-polar cycloadditions with 1,3-dienes in dry 1,2-dichloroethane at room temperature <1996J(P1)2227>. Treatment of 43 with 2,3-dimethylbuta-l,3-diene 41a afforded benzo-fused bicyclic sulfonium salts 22, as shown in Equation (12) and Table 6. The 4-cyano-substituted benzothiopyrylium salt 43d proved to be the most reactive of these substrates (see entry 4). 

Table 7 Polar cycloadditions of 1 -benzothiopyrylium salts 43 with isoprene 41 b (Equation 13)...

The focus will be directed to five ring systems thiochromans (1), thiochromones (2), isothiochromans (3), benzothiopyrans (4—6), and, benzothiopyrylium salts (7, 8). It is difficult completely to segregate their chemistry since they are so interrelated cross-references will be employed as often as possible. Thiocoumarins and thioxanthones are excluded since these molecules differ considerably from those reviewed here. 

Thiochromans (1 R = H2) result from disproportionation of 2H- -benzothiopyrans (4) and also from thio-Claisen rearrangements of phenyl allyl sulfides. The first pathway was discovered by Tilak and Vaidya,24 who attempted to prepare one benzothiopyran (15) by cyclizing the /S-ketosulfide (14) in polyphosphoric acid (PPA), but obtained the thiochroman (16) and a benzothiopyrylium salt as in Eq. (5). This reaction involves25,26 an acid-promoted disproportionation... 

Aryl Grignard reagents with 1-benzothiopyrylium salts yield 4/7-1-benzothiopyrans [Eq. (30)].271 As mentioned in Section II,D, Pummerer... 

Z/-l-Benzothiopyrans with catalytic amounts of acid undergo disproportionation to thiochromans and benzothiopyrylium salts. In the case of 3,4-dimethyl-2//-l-benzothiopyran, intermolecular hydride transfer yields an 85 15 mixture of cis- and intermediate bridged sulfonium ion has been suggested to be responsible for the stereochemical control of this reaction.276 Bromination of 2H-1-... 

Hydrogenation of 1,4-dimethyl-l//-2-benzothiopyran produces, as expected, cis-1,4-dimethylisothiochroman, the S, 5-dioxide of which exists in the boat conformation.260 As with 2H-l-benzothiopyran, the anion formed from the 5,5-dioxide of 6 shows considerable delocalization and potential aromaticity.286 Treatment of 1-substituted lH- 2-benzothiopyrans with thionyl chloride followed by perchloric acid yields 1-substituted 2-benzothiopyrylium salts,287 and Raney nickel desulfurization produces indane structures.288,289 Finally, the enamine... 

Phenyllithium also converts 2-benzothiopyrylium salts to stable 2-thianaphthalenes271 (isomers of 83) which possess NMR spectra resembling that of an aromatic system. Formation of l,l -dihydro-2,2 -dithia-l,l -binaphthyl (85) is possible from the one-electron reduction of 2-benzothiopyrylium salts.314... 

Benzannelated thiopyrylium species are also efficiently generated in this manner, but alternatively thioxanthylium salts (and 1-benzothiopyrylium salts) may be prepared by acid-catalyzed dehydration of the alcohols (145) and (146). The dehydration approach is obviously limited in scope. 

Only a single example of the formation of a 2-benzothiopyrylium salt 223 from its oxygen analog 222 is known (76KGS858 77TH1). The 2-benzopyrylium salt 222, having an electron-withdrawing substituent in the cation, takes part in the recyclization without dimerization. 

Treatment of the 2H-1 -benzothiopyran 1 with trityl fluoroborate in nitromethane at room temperature gave the corresponding benzothiopyrylium salt in 90% yield as an unstable green solid which reacted smoothly with 2,3-dimethylbuta-1,3-diene at room temperature to give the expected cycloadduct in 94% yield. Addition of the cycloadduct to an ice-cold solution of... 

For a series of 3-halogeno-2-benzothiopyrylium salts 185, H-l resonates in the range 10.09-10.66 ppm. In keeping with the observed trend of a downfield shift in hetero-ring protons upon benzo-fusion, the comparable proton in the naphtho[l,2-f] analogue resonates at 6 10.93 <1998JOC4626>. 

Benzothiopyrylium salts react with activated methylene compounds even in the absence of base to give the 1-substituted isothiochromene (Equation 37) <1994J(P1)3129>. 

Hydride ion abstraction from 177-2-benzothiopyrans by triphenylcarbenium fluoroborate produces the highly colored 2-benzothiopyrylium salts 303 (Equation 43) <1998JOC4626>. 

Thiopyran, derived from ethyl vinyl sulfide, is converted into thiopyrylium fluoroborate by reaction with triphenylcarbenium fluoroborate in 54% overall yield (Scheme 235) <2001EJ02477>. 3-Benzoylisothiochromene is oxidized to the 2-benzothiopyrylium salt in a similar manner <1994J(P1)3129>. 

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