S heterocycle

Prereactive dihalogen complexes with O- and S-heterocycles as Lewis bases in the gas phase 99AG(E)2686. 

Eormation of N-, 0-, and S-heterocycles in reactions with participation of hy-pervalent iodine (25 years of development at University of Thessaloniki) 98SL221. 

Amfidinolides (unique macrolides from marine dinoflagellates) 97H(44)543. Antitumor N-, N,S-, and S-heterocycles and macrocyclic lactams from ascidi-ans 97M122. 

Conformational flexibility of six-member dihydro N-, O- and S-heterocycles 97IZV2095. 

Synthesis and stereochemistry of seven- and eight-membered S-heterocycles 99MI7. 

The N,0- and N,S-heterocyclic fused ring products 47 were also synthesized under radical chain conditions (Reaction 53). Ketene acetals 46 readily underwent stereocontrolled aryl radical cyclizations on treatment with (TMSlsSiH under standard conditions to afford the central six-membered rings.The tertiary N,0- and N,S-radicals formed on aryl radical reaction at the ketene-N,X(X = O, S)-acetal double bond appear to have reasonable stability. The stereoselectivity in hydrogen abstractions by these intermediate radicals from (TMSlsSiH was investigated and found to provide higher selectivities than BusSnH. 

The [2 + 2] photodimerization of a, j8-unsaturated sulfones is correctly viewed as a photoreaction of alkenes, rather than the sulfone group, and this aspect has been reviewed recently by Reid, as part of a wider survey of the photoreaction of O- and S-heterocycles. The topic continues to attract considerable interest and a few recent examples, as well as some synthetic applications, will be discussed here. Much of the photodimerization work has been carried out on the benzo[fc]thiophene (thianaphthene) 1,1-dioxide system. For example. Porter and coworkers have shown that both 3-carboxybenzo[i]thiophene 1,1-dioxide (65) and its methyl ester give only the head-to-head (hth), anti dimer (66) on irradiation in ethanol. In a rather unusual finding for such systems, the same dimer was obtained on thermal dimerization of 65. Similar findings for a much wider variety of 3-substituted benzo[fi]thiophene 1,1-dioxides have been reported more recently by Geneste and coworkers . In the 2-substituted analogs, the hth dimer is accompanied by some of the head-to-tail (htt), anti dimer. The formation of the major dimer appears to proceed by way of an excited triplet and the regiochemistry observed is in accord with frontier MO theory. 

Wittig reactions have also been employed in domino processes. For example, Schobert and coworkers developed an effective addition/Wittig reaction protocol which provides access to a, 3-disubstituted tetronic acids, tetronates, as well as to five-, six- and seven-membered O-, N-, and S-heterocycles [149]. 

Scheme 6.45 MW-assisted synthesis of S-heterocycle-containing liquid crystalline compounds.

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