2H-Thiopyrane

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). 

Diels-Alder cycloadditions of enaminothiones with electrophilic die-nophiles (83AHC145, section III,G) have been widely used for the preparation of various thiopyran derivatives. In addition to expected 2H-thiopyrans, 4//-isomers were trapped as intermediates or even as final products. Nitroolefines (90T1951), olefinic carbonyl derivatives [80JH405 85JOC1545 92JCS(P1)2603] allenedicarboxylic esters... 

Intramolecular Cyclizations via Valence-Bond Tautomerism of 2H-Thiopyrans... 

Six-membered rings. Thioketones react as dienophiles with conjugated dienes81,87 92 in Diels Alder [4+2] cycloadditions to form 3,6-dihydro-2H-thiopyrans regio- and stereo-selectivitely. 4 and 6 reacting with differently substituted 1,3-butadienes, Scheme 12, have shown a superdienophile activity.87 89... 

Substituted 5,6-dihydro-2H-thiopyrans 204 and 205 were obtained98,99 by condensation of 1-methoxy-1,3-butadiene or 1,4-diace-toxy- 1,3-butadiene with methyl cyanodithiofonnate this reaction opened an approach to the total synthesis of sulfur-containing monosaccharides. 

Thiopyran derivatives may be formed when a sulfide participates in an aldol transformation. An excellent example is the formation of 2H-thiopyran-5-carbaldehyde (19a) from benzaldehyde and acetaldehyde in the presence of sodium disulfide (Scheme 11 A).196... 

Dihydrothiopyran dioxide 284 disproportionates to a mixture of 2H-thiopyran 285 and its tetrahydro derivative 286 in the presence of piperidine,300 as shown in Eq. (14). 

Enamines derived from thiopyran-3-one, although tautomeric, tend to exist predominantly in conjugation with the sulfur atom the Fischer indole reaction, when applied to that ketone, affords solely the systems fused 2,3 on to the thiopyran (Scheme 10) (76CL5). 4-Amino-3,4-dihydro-2Ff-thiopyrans readily eliminate ammonia or amines on heating or treatment with acid, with formation of 2H- thiopyrans <78CR(C)(286)553). 

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