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87% (d.e. 3* 94%)
Ketene Claisen rearrangement of an allylthioether
The allylthioether (1.5 mmol) and about 15 mmol of Zn/Cu alloy were placed in vigorously stirred ether (15 ml) under argon and the mixture was heated to reflux. A solution of freshly distilled trichlnroacetyl chloride (3 mmol) in ether (5 ml) was added dropwise to the solution over 4h by means of a syringe pump. After cooling, the reaction solution was decanted from the residue and the thioester purified by chromatography on silica gel (87%), syn structure, d.e. 3=94%.
From  with permission.
4.5.2 [2,3] sigmatropic processes
We have mentioned previously as a stable ylide dimethyl sulfonium fluorenide (see Section 2.9). It yielded the first example (1938) of a [2,3] sigmatropic rearrangement of a sulfur ylide [505, 506] (Sommelet-Hauser rearrangement).
Many other [2,3] sigmatropic rearrangements of organosulfur compounds are known. They involve unsaturation p to a sulfur atom and a free pair of electrons a to the sulfur centre as encountered above in sulfur ylides, in carbanions, carbenes. but also in sulfoxides and sulfmamides. This synthetic utility was recognized in the late 1960s and early 1970s. Most of the papers dealing with those topics which appeared then were quoted and discussed in Block’s monograph . Some reviews by authors who made important contributions in the field are also available [253, 507, 508]; Hoffmann’s review  deals with stereochemical aspects (see also  for stereocontrol in the allylic sulfoxide sulfenate rearrangement).
The synthesis  of the terpenic Yomogi alcohol (7) illustrates these sigmatropic processes. A first rearrangement of the carbanion (2) obtained by metallation of the bis(prenyl) sulfide (1), was used to link the two prenyl moieties in the required mode. The resulting lithium thiolate (3) was methylated, yielding (4) in a 66% overall yield from (1). Oxidation (NaIO„) led to the allylic sulfoxide (5) (75% yield), which rearranged to a sulfenate (6), cleaved by the thiophilic amine to afford the Yomogi alcohol (7) in a 69% yield. The trans double bond is a consequence of a highly stereoselective sulfoxide-sulfenate transposition [507, 509].
A sequential use of two pericyclic reactions involving sulfur compounds is also implied in a general allylic amination of alkenes with Kresze’s reagent, 7V,7V'-bis(methoxycarbonyl)sulfurdiimide (8). A [2,3] sigmatropy, following an ene reaction of the S-allyl sulfinamidine (9) intermediary formed, affords the diamino sulfane (10), easily converted to the carbamate (11) and then to the amine (12) or (13) . The reactions are featured with 2-methyl-2-butene as the alkene to be functionalized. They are described in Organic Syntheses
R » C02Me
4.5.3 Erie and retro-ene reactions
Reviews of the ene reactions with thiocarbonyl compounds have been published [120, 203]. The sulfur retro-ene reactions are well covered in a general review on the synthetic applications of the retro-ene reaction
In the reaction of thiocarbonyl compounds as enophiles, one of the most striking aspects is the usual formation of a C-S rather than a C-C bond, i.e. the orientation is opposite to that observed with carbonyl compounds .
This "thiophilic" mode of reaction fits well with the other ionic and radical thiophilic additions on a thiocarbonyl group (see , and the Barton-McCombie radical thiophilic addition  previously discussed in Sections 4.2.2 and 3.2). With thioaldehydes, both C-C and C-S bonds were formed, as in the following case in which |3-pinene was used to trap thiobenzaldehyde :
For synthetic purposes very reactive enophiles have been generally used. Methyl cyanodithioformate (1), A'.N'-ditosyl sulfurdiimide (2) and N-sulfinyl benzenesulfonamide (3) are but a few examples.
A Ts-N=S=N-Ts Ph-S0o-N=S=0
(1) (2) (3)
With (3), for instance, the ene reaction occurs at 0°C and has been applied to the synthesis of allylic thiols , through LiAlH4 reduction of the ene adducts.
f° °°c , f^f°
Z' ''so Ph Et2° NHSOoPh