Group 13 sulfides indoles

The Parikh-Doering oxidation provides a very high regioselectivity for the oxidation of alcohols. Oxidation-sensitive functionalities, like indoles,99a,c sulfides,109 and selenides 110 as well as oxidation-sensitive protecting groups, like dithioacetals,111 PMB104 and dimethoxybenzyl ethers109b, do not react. 

Indok synthesis. Gassman and van Bergen have used a modification of the above-mentioned procedure for synthesis of 2-substituted indoles from anilines. The aniline IS treated as above with a chlorinating reagent at — 65° and then an equivalent of methyl-thio-2-propanone at the same temperature. An equivalent of a base (usually triethyl-amine) is added. Workup affords the indole derivative (2) in 60-70% yield. The thio-methyl group is removed with Raney nickel (>70% yield). The keto sulfide can be varied thus nse of methyl phenacyl sulfide [CH3SCH2C(=0)C5H5]4 in the synthesis leads to 2-phenylindoles. 

Desulfurization of 3-indolyl sulfides. Nonreductive removal of thio groups uses CF3COOH to protonate the 3-indolyl sulfides, thus allowing thiophilic attack by a thiol reagent (e.g., o-mercaptobenzoic acid). The corresponding 3-unsubstituted indoles are produced. 

The above reaction sequence (Scheme 11), sometimes called the Katritzky method, has been exploited by a number of research groups for the preparation of 2-substituted IV-unsubstituted indoles [83, 102, 130, 218, 240-245], Gribble and Bergman used this chemistry to prepare 2-bromoindole and 2-iodoindole [246-248]. Bergman and Janosik further explored this sequence to prepare indol-2-yl sulfides and selenides [57,249,250]. Hudkins has used this chemistry to prepare 2-hydroxyatkylindoles en route to indolocarbazole derivatives [251, 252], The first step in Bergman s short total synthesis of yuehchukene used the Katritzky method and aldehyde 59, which gave 2-(hydroxyalkyl)indole 60 (Scheme 12) [253, 254]. 

Kita s group also described an intermolecular version that resembles the procedure of Engler [8]. This is shown in Scheme 5 and again features phenyliodine(ni) bis(trifluoroacetate) (FIFA) reaction with Al-tosylaniline, followed by nucleophilic attack by an activated olefin, and cyclization to give either Af-tosylindoline (equation 1) or Af-tosylindole (equation 2). The latter is the product when phenyl vinyl sulfides are employed. The thiophenol byproduct was oxidized to diphenyl disulfide with excess FIFA. Four indoles that were prepared in this study are also shown. 

The synthesis of 1-substituted carbazole sulfides formed by an unusual Pummerer rearrangement has been reported. The acyloxy group is displaced by an internal nucleophile, in this case, the indole 84. Tautormerisation followed by the addition of a nucleophile then regenerated the indole and resulted in sulphides 87a-d. A range of nucleophiles can be employed, giving good yields and moderate diastereoselectivity. 

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