Thia-Claisen rearrangements

The thia-Claisen rearrangement of allyl 2,3,4,5-tetrafluorophenyl sulfide (41) gives a mixture of dihydro-l-benzothiophenc 42 and thiochromane 43 in 30 and 40% yield, respectively.30 2,3,5,6-Tetrafluorophenyl and pentafluorophenyl allyl sulfides give product mixtures in low yield, together with tarry intractable material.36... 

Strong heating of 2-substituted 5-propargylsulfanyl-3-aryl-3//-pyrimidin-4-ones, and their 5-allylsulfanyl derivatives, has been shown to lead to 2-substituted 3-aryl-6-methyl-3//-thieno[3,2-d]pyrimidin-4-ones and their 6,7-dihydro derivatives. The proposed mechanism in both cases is a [3,3]-sigmatropic thia-Claisen rearrangement followed by tautomerization and a 5-exodig or 5-exo-trig cyclization.38... 

Why do Lewis acids promote Claisen reactions Suggest another method for accelerating aza- and thia-Claisen rearrangements. 

X = O Claisen rearrangement X = S thia-Claisen rearrangement X s N aza-Claisen rearrangement... 

For other facile retro-Claisen rearrangements see refs 48-51, for an example in the thia-Claisen rearrangement series, cf. ref 52. Flowever, it has been pointed out that the irreversibility of the Claisen rearrangement in the oxygen series is only attributed to the large stability difference between reactant and product which drives the equilibrium to the right52. 

Table 23. Thia-Claisen Rearrangement of Tertiary S-Allyl Thioamides580...

Ketene Dithioacetal Formation and Thia-Claisen Rearrangement General Procedure624 ... 

Table 27. Thia-Claisen Rearrangement of S-Crotylic a-Hydroxy Ketene Dithioacetals625...

In cyclic 1,2 thia-Claisen rearrangements bond formation occurs from the sterically less hindered side, as demonstrated by the synthesis of aza-prostacyclin analogs587,628. 

In a related thia-Claisen rearrangement of the tf-allyl derivative 36, derived from A-nitroth-ioacetyl-L-proline ethyl ester, 66% diastereomeric excess is observed637. 

As the other important exception to the applicability of the standard procedure, N-prenylation is only the minor reaction if an alkylmercapto group is located at C2. Via S-prenylation, a thia-Claisen rearrangement to the 3-position becomes the major reaction, affording 3-tcrt-prenylindoles [4, 32], as exploited in the total synthesis of amauromine [33-35] (see Chap. 4.2). 

Plieninger also found that 3-mercaptoindoles (122) can be 2-fert-prenylated (124) via thia-Claisen rearrangement after S-prenylation to 123 at 0°C [107]. 

Scheme 49 Thia-Claisen rearrangements starting from 2-alkylmercaptoindoles [32, 195]...

Scheme 50 Synthesis of amauromine (266) via bis-thia-Claisen rearrangement by Takase and co-workers [35]...

Examples of the aza-Claisen rearrangement of allyl vinyl amines and the thia-Claisen rearrangement of allyl vinyl sulfides have been reported. An efficient Lewis acid-catalysed Claisen rearrangement of zwitterionic A-acyl allylic amines... 

Thia-Claisen rearrangements of N-benzylpyrrolidine-2-thione and chiral allylic bromides derived from D-mannitol have been reported. Introduction of a bromine atom 0 onto the double bond of the allyhc bromide reverses the sense of diastereoselectivity in the [3,3]-sigmatropic rearrangement and DFT calculations rationalize the selectivity in terms of a Cieplak effect (Scheme 34). 

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