Thiapyrans, synthesis

Besides being useful precursors to pyrroles pyridine-2-ones -4-ones, -4-thiones. and -4-imines 4-alkylidene-dihydropyridines thiophenes 1,2,4-triazoles thiapyrane-2-thiones, isoquinoline-3-ones isoben-zothiophenes and 4-mercaptoimidazolium hydroxide inner salts, mesoionic thiazoles are potentially useful in the construction of molecules with herbicidic (39). central nerve stimulating, and antiinflammatory properties (40,41). Application in dye synthesis has likewise been reported (42). 

UV spectra, 3, 955 X-ray crystallography, 3, 948 Thianthrenium salts, 5-phenyl-chloroplatinate salts, 3, 967 Thianylium salts chlorination, 3, 901 1,2A -Thiaphosphiranes synthesis, 7, 196 Thiapyrans — see Thiins l,3A -Thiarsoles nomenclature, 1, 32 Thiaselenafulvenes synthesis, 6, 965 Thiaselenins synthesis, 3, 914... 

Obviously, the advantages of this new synthetic method are i) valence isomerization can be performed under very mild conditions and ii) due to the fairly strong acidity of the bridgehead hydrogens of a bicyclobutane moiety, some substituents can be introduced regioselectively. Nevertheless, one serious disadvantage of this synthesis is the limitation of its applicability Thus, the formation of the precursor bicyclobutane derivatives is limited only to 46 and 56, and many attempts to prepare bicyclobutane precursors from other thiapyran derivatives have been unsuccessful so far. 

Thiapyran derivatives can be prepared from preformed organoiron and -cobalt complexes as in Scheme 160 the organocobalt complex (136) can be used as an intermediate in the synthesis of 1,2-dithia cyclopent-4-en-3-thione (See Scheme 122 in Section IV,H.)... 

The thiapyran ring does, like thiophene, provide a heterocyclic system on which one can perform standard sorts of reactions and then through a reductive desulfurization step expose a functionalized carbon chain. Such chemistry was first employed in a very elegant synthesis of the Cecropia juvenile hormones (Scheme 166) (73JA4444). 

A different route to pyrones is the preparative electrochemical oxidation of enamines in acetonitrile in the presence of tetraethylammonium perchlorate (88MI2) (Scheme 46). The synthesis of 2-pyrone derivatives has been carried out by reaction of /3-dicarbonyl compounds with methyl-a-benzoylamino-/3-dimethylaminoacrylate (96JHC751). Thiapyran derivatives can be obtained by interaction of enamines based on (/3-amino-a-cyanoacryloylmethyl)pyridinium chloride derivatives with carbon disulfide (95M711).The synthesis of pyridine derivatives based on analogous enamines has been described as well (95M711). 

One of the pathways to synthesis of a,a -dehydronaphtho[c f)-thiapyrans 383a-d includes step-by-step heterocyclization of peri-bis-bromomethyl, peri-bis-hydroxymethyl, or peri-bis-hydroxybenzyl-... 

Thioxomalonates and a-thioxopropanoates have been prepared and subsequently trapped [220] with various dienes [221] (Table 3, entries 6 and 7). The resulting dihydro-2H-thiapyrans have been exposed to a strong base to effect a ring contraction providing a new synthesis of cyclopentenes [220]. Other adducts led to the formation of vinyl cyclopropanes, which are possible intermediates in cyclopentene synthesis. 

A cyanosulfine [242] and an oxosulfine [162] were trapped with 2,3-dimethyl-1,3-butadiene (Table 6, entries 2 and 3). Capozzi and his group have extended their phthalimido-sulfenyl chemistry to the synthesis of a-oxosulfines, and have observed a dichotomic behaviour towards cycloaddition. With 1,3-dienes, these sulfines act [243, 244] as dienophiles through their C=S bond (Table 6, entry 4) to afford dihydro-2H-thiapyran S-oxides. With alkenes (Table 7), such as 2,3-dimethyl-2-butene (entry 1) or vinyl-ethers (entry 2), they behave as dienes to give dihydro-1,4-oxathiin S-oxides [243-245]. 

Kondo et al. (1972) used 4-methyl-dMihydro-thiapyrane as structural unit in the stereospecific synthesis of juvenile hormone, the sulfur atom being removed by reductive desulfuration (Scheme 1.22). 

The 4 -thiapyran-4-thione route to thiathiophthens has been modified so as to yield 6a-selenahthiopthens (35). The intermediate selenoketones (38) are unstable, but react satisfactorily with sodium sulphide in aqueous dimethyl sulphoxide to give dianions which are oxidized, as in the original synthesis, with ferricyanide. N.m.r. data on 6a-selenathiophthen, and on its 3,4-dimethyl and 2,5-diphenyl derivatives, indicate real or time-averaged... 

Thiapyrans.—As part of a general study of the synthesis of pheromones, Lip-kowitz and his coIIeagues have studied the [4 + 2]cycloaddition of enones to yield dihydropyrans, which in turn are easily converted into the corresponding thia-analogues (Scheme 65). 

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