1.2- Thiazinylium salts

Table 10 Ring openings of 1,2-thiazinylium salt cycloadduct 76a (Equation 20)...

Table 11 Ring transformations of 1,2-thiazinylium salt cycloadducts 76 (Equation 21)...

Recently, much attention has been paid to hetero Diels-Alder reactions as powerful tools for the construction of heterocyclic compounds. For example, cycloaddition of 2,3-dimethylbuta-l,3-diene 41a with 1,2-thiazinylium salt 95, in acetonitrile at room temperature, resulted in the exclusive formation of product 76a resulting from cycloaddition across the C—S1 bond (see entry 1 in Table 15 and Equation 26) <1999TL1505>. Similarly, isoprene 41b and... 

Table 15 Polar cycloadditions of 1,2-thiazinylium salt 95 with 1,3-butadienes 41 (Equation 26)...

Recently, Shimizu reported a novel polar cycloaddition of a 1,2-thiazinylium salt <99TL95>. Compound 175 was generated in situ from the reaction of compound 165 with trifluoroacetic anhydride and lithium tetrafluoroborate. It can undergo a cyclization reaction... 

Fully conjugated species are somewhat rare in the 1,2-thiazine class of heterocycles. These molecules are comprised of two separate subclasses, the first of which includes the highly reactive 1,2-thiazinylium salts. Although these salts, such as 27, have in some cases been isolated, they readily react regioselectively at C-6 with a variety of nucleophiles including sodium alkoxides 76, silyl enol ethers 77, sodium malonates 78, and sodium thiophenoxide 79 (Scheme 10) <2001T8965>. 

The polar cycloaddition of the in rr / -prepared 1,2-thiazinylium salt 20 from Woxide 80 vide infra) with 2,3-disubstituted butadienes 81a and 81b affords adducts 82a and 82b (Scheme 11) <1999TL95>. Mono- and disubstituted salts 42 and 83 are more stable than the diphenyl-1,2-thiazinylium salt 20 and can in fact be isolated. These dienophiles undergo [4-1-2] cycloaddition reactions with butadiene 81a affording products 84a and 84b with a different regioselectivity than dibenzo-1,2-thiazinylium salts 82a and 82b <1999TL1505, 2001TL4183>. 

Fully conjugated 1,2-thiazines have been prepared with both S(ll) and S(vi) oxidation states. While the zwitterionic compound 11 has been known for some time <1984CHECI(3)995, 1996CHEC-II(6)349>, thiazinylium salts 12 have only recently been prepared. Both fully unsaturated 1,2-thiazine derivatives are considered to be nonaromatic due to poor p-d Jt-bonding. Eurthermore, the six-membered ring of l-alkyl-l,2-thiazine 1-oxide 11 is not planar, but instead exists in a puckered, half-boat conformation thereby precluding aromaticity <1978CC197>. 

The reaction of l,2-thiazinylium salt 84a with nucleophiles yields ring-opened products 178 by cleavage of the S-C bond (Equation 27) <1999TL1505>. 

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