3.5- Dimethyl-substituted thiopyrans

Selected i I NMR chemical shift data for several dimethyl substituted benzo- and naphthothiopyrans and some oxidized analogues are depicted in Figure 22. The fusion of a benzene ring across the 7-face of a 2//-thiopyran to afford the 2//-l-benzothiopyrans results in downfield shifts of both H-4 and H-3 by ca. 0.4 and 0.2 ppm, respectively, and also... 

The cycloaddition reaction of chiral thiocarbonyl 5-oxides derived from terpene alcohols such as (—)-menthol, ( —)-borneol, (+)-a-fenchol and (—)-3-tWo-hydroxy-2-CTitfo-phenylbornane affords 2-substituted 2-alkoxysulfonyl-3,6-dihydro-4,5-dimethyl-2//-thiopyran 1-oxides IIA and 11B as a mixture of diastereomers85. 

Moriata and co-workers performed thermolysis and reactions with amines of diethyl 1-sulfinylmethylphosphonates (174) and (175), substituted by tetrazyl (Tet) and benzothiazolyl (Btz) moieties at the sulfinyl sulfur atom. ° Thermolysis in the presence of 2,3-dimethyl-1,3-butadiene afforded 2-phosphoryl substituted thiopyrans (176) and (177), while novel ot-thioketophosphonate derivatives (178) were obtained in the reaction with amines (Scheme 61). 

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]. 

Highly substituted 4-dimethylamino-4/7-thiopyrans result from a cycloaddition-cycloreversion-cycloaddition sequence commencing with the heterodiyne 417 and dimethyl acetylenedicarboxylate (DMAD) <1996J(P2)2623, 1997MI2185>. Activated alkynes cycloadd to 3-aryl-2-cyanothioacrylamides yielding 4-aryl-477-thiopyrans 418 (Scheme 127) <1994J(P1)989>. 

Thiocoumarin (127) underwent [2 + 2]-photocycloaddition reactions in better yields than 126. In contrast to coumarin, cis- and trans-fused products are being found, however, for example, in the reaction with 2,3-dimethyl-2-butene, possibly because the thiopyran ring is more flexible than the pyran ring due to the longer C—S bonds. HTproduct is favored with electron donor-substituted olefins [121]. Electron acceptor substitution in 3-position, as in 3-cyano-l-thiocoumarin (128), leads to an improved performance in [2 + 2]-photocycloaddition reactions [122]. 

When chiral thiocarbonyl S-oxides derived from (S)-proline, prepared from the appropriate l-(trimethylsilylalkylsulfonyl)-2-(alkoxymethyl)pyrrolidine, are subjected to Diels-Alder reaction with 2,3-dimcthyl-l,3-butadiene, the 2-substituted 2-[(2-alkoxymethyl-l-pyrrolidinyl)sul-fonyl]-3,6-dihydro-4.5-dimethyl-2/f-thiopyran 1-oxides 10A and 10B are formed as a mixture of diastereomers. Their ratio depends on the nature of the substituents R1 and R2 as well as on the reaction temperature. The diastereomeric excess values, determined by H NMR in the presence of Yb(tfc)3 or by HPLC, vary from 0 to 41 % 84. 

---