Allyl vinyl sulfides, thio-Claisen rearrangement

The [3,3] sigmatropic rearrangement of allyl vinyl sulfides (thio-Claisen) is particularly easy activation enthalpies can be 10 kcal mob1 lower than for the oxygen analogues [486], and the reaction which was pioneered by Brandsma [487] occurs usually at room temperature and with excellent yields. 

Chirahty transmission can also be induced in thio-Claisen reactions by a chiral center adjacent to the allyl vinyl sulfide core. The group of Metz-ner examined the diastereoselectivity of the rearrangement of acyclic S-allyl ketene dithioacetals bearing a chiral center adjacent to carbon 6. hi these substrates, the sigmatropic shift proceeded smoothly and with modest syn anti diastereoselectivity [74]. In contrast, higher selectivities are foimd for thio-Claisen precursors bearing a chiral center adjacent to carbon 1 mainly due to steric effects and allylic strain [75]. In 1991, the group of Beslin examined the effect of a hydroxyl substituted chiral center attached to C-1 of Z and E S-allyl ketene dithioacetals [76,77] (Scheme 7). Under the reaction conditions, these... 

Scheme 11.35 Activation enthalpy, activation entropy and reaction enthalpy for the thio-Claisen rearrangement of allyl vinyl sulfide predicted on different levels oftheo7 (1996).

Scheme 11.37 The thio-Claisen rearrangement was studied computationally by Arnaud and Vallee in 1997 [51]. Compared to the only slightly exothermic Claisen rearrangement of allyl vinyl sulfide, the presence of the 2-amino function increases the exothermicity of the rearrangement.

Allyl vinyl sulfide in dry tetrahydrofuran treated at -78° with 5ec-butyllithium in pentane, then with benzyl bromide at -78° for 0.5 hr. and at -30° for the same period, the resulting crude intermediate dissolved in 3 1 dimethoxyethane-water in the presence of ca. 3 equivalents Ca-carbonate, and refluxed 12 hrs. -> product. Overall Y 62%. F. e. s. K. Oshima et al.. Am. Soc. 95, 2693 (1973) 7-ketoaldehydes via thio-Claisen rearrangement cf. ibid. 95, 4446 y,5-ethylene-carboxylic acid esters via stereospecific dithio ester thio-Claisen-rearrangement cf. ibid. 95, 5803 syntheses via a-alkylation of dihydro-zd -thiopyrans with sec-butyllithium in the presence of TMEDA s. P. L. Stotter and R. E. Hornish, ibid. 95, 4444. 

Thio-Claisen rearrangements (TCRs) of vinyl allyl sulfides 136 are useful in the synthesis of unsaturated aldehydes [96]. 

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