Thiopyran synthesis

Thiopurinols applications, 5, 343 synthesis, 5, 319 tautomerism, 5, 308 Thiopyrans — see Thiins... 

Thiopyran and thiopyrylium salts synthesis and properties 98MI18. 

Halo-l-benzothiepins 4 can be synthesized by the treatment of 7a-halobenzo[fe]cyclopropa-[e]thiopyran-7-ols 2 with hydrogen bromide and subsequent hydrogen bromide elimination from the 2,4-dihalo-2,3-dihydro-l-benzothiepins 3 by l,5-diazabicyclo[4.3.0]non-5-ene (DBN).9 The alcohols 2 are prepared by Grignard reaction of the corresponding 7a-halobenzo[ft]cyclopropa[c]thiopyran-7-ones l18 and are used for the synthesis without purification. The intermediate dihalodihydro-l-benzothiepins 3 are not isolated due to their thermal lability related and more stable compounds are described in Section 2.1.2.1. 

Normal Diels-Alder Reactions. Synthesis of Pyrones and Thiopyrans... 

Radical promoted reactions feature in a synthesis of 3-substituted derivatives of 2,3-dihydro- and tetrahydro- thiopyran-4-ones from the 3-methylene compounds <96SL261> and in the formation of 2-methyltetrahydroselenopyran from a selenoalkyl (phenyltelluro)formate <96JOC5754>. 

For the synthesis ofthiepins and oxepins, [(ri3-C3Hs)PdCl]2-catalyzed decomposition of 4-diazomethyl-4-methyl-4/f-thiopyrans 387) or -pyrans38] ) is the method of choice. Purely thermal decomposition of the former diazo compounds would require higher temperatures and thus would cause extrusion of sulfur from the primarily formed thiepin, yielding a benzene derivative. 

When studying the carcinogenic activity of polycyclic hydrocarbons and their antdogs containing thiophene rings (cf. Tilak "), Tilak et al synthesized both thienothiophene 1 and its isomer 2 in low yield from 2-thienyl dimethoxyethyl sulfide (23) and 3-thienyl diethoxyethyl sulfide (24), respectively, by the method developed for synthesis of thiophenes and thiopyrans - [Eqs. (10) and (11)]. The compounds 23 and 24 were prepared from 2- and 3-mercaptothiophenes. ... 

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>. 

Compound 99 reacts with alkynes to generate thieno[2,3-f]thiopyranthiones and thieno[3,2-f]thiopyranthiones. The ratio of isomers is typically 1 1. The reaction proceeds via a cycloaddition reaction and an unprecedented molecular rearrangement (Equation 35) <20050L791>. Thieno[2,3-f]- and [3,2-f]thiopyrans can be prepared from thiophene-2-carbaldehyde in a multistep synthesis that leads to the thienothiopyrans in excellent yields <1999J(P1)2639>. 

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. 

Sec. III.A] PYRANS, THIOPYRANS, AND SELENOPYRANS III. Synthesis from Acyclic Precursors... 

The synthesis of 4//-thiopyrans from 1,5-dicarbonyl compounds consists of the cyclization of 1,5-dioxo precursors with hydrogen sulfide in the presence of protic acids or with various phosphorus polysulfides. 

The first synthesis starting with glutaric dialdehydes and involving intermediates 42 succeeded in preparing unsubstituted 4//-thiopyran (7)19,24 as well as alkylated derivatives.7,90,91... 

The first approach to 2//-thiopyrans from acetylenes resembles the synthesis of oxa analogs 92a,b. In contrast to the latter, intermediates like 100 have been directly used as starting compounds. 

The preparation of unsubstituted 2//-thiopyran (6, up to 82%) from 3-propynyl vinyl sulfide seems to be the simplest example.21,23 Similarly, homologs 101a,21 cyanides 101b,146 ketones 101c,141 and ethyl esters lOld148 were prepared in satisfactory yields. The same approach was subsequently explored for the synthesis of 15 and similar condensed systems21,43,137 (see also 5,5-dioxides in Section V,G,3). 

The standard elimination procedure for mesylates was successfully applied to the synthesis of 2,6-diisopropyl-2//-thiopyran (281), prepared in 82% yield according to Eq. (13).285... 

The elimination of the RS02 group may also be used in the synthesis of 6-arylthio-2//-thiopyrans 299a-c via intermediates 298a-c elimination takes place by means of bases.304... 

The desulfurization of 2-substituted 4-methyl-2//-thiopyrans, accompanied by dimerization of primary intermediates, is caused by ZnHg. This approach was explored for the synthesis of polyenes,183-186 including / -carotene (Eq. 24a). 

---