1.3- Dithienium fluoroborate

Evans s oxazolidinone O-silylaminals 5.64 react highly selectively with 1,3-dithienium fluoroborate or PhSCl, provided that the silicon substituents are bulky enough [1017a] (Figure 5.42). 

Dithians. - 1,3-Dithienium fluoroborate (258) effects regiospecific alkylation of 0-silylated enolates of carbonyl compounds to produce 1,3-dicarbonyl derivatives, e.g. (259). A wider variety of substituents in 1,3-dithians is obtainable by allowing aromatic aldehydes to react with the sul-phone (260). ... 

Isoprene added at 0° to a stirred suspension of 1,3-dithienium fluoroborate in dry methylene chloride, the cooling bath removed, and stirring continued 35 min. 

The Diels-Alder-type cycloaddition between 1,3-dithienium fluoroborate (106), obtained from 1,3-dithian and trityl fluoroborate, and 1,3-dienes formed the basis of a new and efficient synthesis of A -cyclopentenones. Simple 1,3-dienes such as butadiene, isoprene, or 2,3-dimethylbutadiene reacted rapidly with (lOQ to give (107) (107b) was obtained stereospecifi-cally from (106) and isoprene in 95% yield. Treatment of the sulphonium salts (107) with butyl-lithium at -78 °C gave an ylide which rearranged on warming to the cyclopropyl-dithian (108). The latt , v/hea heated further to 200 C, underwent the well-known vinylcyclopropane to cyclopentene transformation. Hydrolysis of the dithian thus obtained furnished cyclopen-tenones. The overall yield of 3-methylcyclopent-3-enone from (106) and isoprene was greater than 83%. 

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