1.3- Dienes thiopyrans, dihydro

Dihydro-2iy-thiopyrans, derived from dimethylbuta-1,3-dienes, Na2S203-5H20 and various activated alkyl h des, ring contract on treatment with a strong base leading to vinyl cyclopropanes and cyclopentenes <96JOC4725>. 

Diels-Alder cycloaddition reactions of enaminothiones153 155 or enamino thioesters154,156,157 as nucleophilic dienes with electrophilic dienophiles R4CH = CHX lead to appropriate 4-amino-2,3-dihydro-2//-thiopyran intermediates 110, which easily eliminate amines R2R3NH to give the corresponding 2//-thiopyrans 111. 

Cycloaddition strategies have also featured widely in the synthesis of variously substituted 3,6-dihydro-2//-thiopyrans from thiocarbonyl-containing precursors and dienes. Selected H NMR data for 3,6-dihydro-2//-thiopyr-ans feature in Figure 20. 

The use of phosphonodifluorodithioacetate as a 2ji component in cycloadditions with a variety of dienes provides simultaneous direct access to fluorine and phosphorus containing 3,6-dihydro-2/7-thiopyrans, for example, 217 and their 3-ones 218. Complex signals result from the heteronuclear couplings <2002TL2033>. 

Hydrolysis of the acyclic adducts formed when metallated 1-methylthiobuta-l,3-diene reacts with isothiocyanates using a stoichiometric amount of dilute acid results in electrocyclization to a 6-amino-2/7-thiopyran which tauto-merizes to the 2-imino-5,6-dihydro-2/7-thiopyran (Scheme 135) <1998TL2631>. 

Both the sulfone-tethered enyne 426 and diene 427 undergo RCM to give 5,6-dihydro-2//-thiopyran 1,1-dioxides when treated with a second generation Grubbs mthenium catalyst 425 tagged with an ionic liquid (Scheme 136) <2005JOM(690)3577>. 

Both thiobenzophenones and thiofluorenones undergo [4+2] cycloadditions with acyclic and cyclic 1,3-dienes to give 3,6-dihydro-2//-thiopyrans (Scheme 141). The thiofluorenones are more reactive, even yielding adducts with electron-deficient cyclopentadienes at room temperature. In all cases, the progress of the reaction can be followed by the loss of color of the thione. Unsymmetrically substituted dienes show good to excellent regioselectivity. 

Activated alkyl halides react with sodium thiosulfate to form the Bunte salts 438 which, on treatment with base, generate thiocarbonyl compounds <1984CC922>. Trapping with 1,3-dienes affords 3,6-dihydro-2-//-thiopyrans in satisfactory yields and in a one-pot reaction, although with unsymmetrical dienes the regio and diastereo selectivities are not good (Scheme 143) <1996JOC4725>. 

Doubly activated methylene compounds react with thionyl chloride in the presence of triethylamine to generate sulfines which can be trapped by 1,3-dienes to give 3,6-dihydro-27/-thiopyran 1-oxides 446. Facile deoxygenation makes this an attractive route to 2,2-disubstituted 3,6-dihydrothiopyrans (Scheme 151) <1999PS(153)119, 2004EJ074>. 

The benzothiazepinone 448 is a source of 2-substituted 3,6-dihydro-2-//-thiopyrans through its conversion to the bridgehead salt 449 by a [4+2+] cycloaddition with dienes and subsequent reduction (Scheme 154)... 

The phosphonodithioformate (336) has been used as an interesting hetero-dienophile in the Diels-Alder reactions with different dienes. A selective radical desulfanylation of the cycloadducts (337) using BusSnH leads to new (3,6-dihydro-2H-thiopyran-2-yl) phosphonates (338)(Scheme 90). ... 

More complex cases can also be handled correctly by A/(r) indeed the regio-selectivity of the DA reaction involving a diene with a hetereoatom such as sulphur, can also be predicted.47 The reaction of (E)-4-aminobut-3-ene-2-thione with acroleine produces 4-amino-3,4-dihydro-6methyl-2H-thiopyran-3-carbaldehyde (ADMTC)47. This is the ortho adduct, as can be seen in Figure 11. 

Six-membered rings. Thioketones react as dienophiles with conjugated dienes in Diels-Alder [4-1-2] 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. ... 

Cycloadditions of thiocarbonyl compounds to dienes, or of vinyl thiocarbonyl derivatives to dienophiles, have received particular attention. Thus, A -dihydro-thiopyrans, e.g. (28), have been obtained by the addition of diphenyl thioketone to 1,3-dienes. Oxidation of (28), followed by a vinylogous Pummerer reaction, gave the 2i/-thiopyran (29). ... 

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