Lactones, iodotrimethylsilane

The structurally simplest silicon reagent that has been used to reduce sulphoxides is the carbene analog, dimethylsilylene (Me2Si )29. This molecule was used as a mechanistic probe and did not appear to be useful synthetically. Other silanes that have been used to reduce sulphoxides include iodotrimethylsilane, which is selective but unstable, and chlorotrimethylsilane in the presence of sodium iodide, which is easy to use, but is unselective since it cleaves esters, lactones and ethers it also converts alcohols into iodides. To circumvent these complications, Olah30 has developed the use of methyltrichlorosilane, again in the presence of sodium iodide, in dry acetonitrile (equation 8). A standard range of sulphoxides was reduced under mild conditions, with yields between 80 and 95% and with a simple workup process. The mechanism for the reaction is probably very similar to that given in equation (6), if the tricoordinate boron atoms in this reaction scheme are replaced... 

Few examples of intramolecular enol silyl ether or silyl ketene acetal cyclizations to oc,3-enones have been reported. Notable, as exemplified in Scheme 34, is the iodotrimethylsilane-mediated intramolecular cyclization of 5-(iodoacetoxy)-a,3-enones (211) to 5-lactones (214). These cyclizations proceed with in situ generated silyl ketene acetals (212) arising from iodotrimethylsilane reduction of the iodoacetoxy moiety.87... 

Cleavage of MEM ethers.1 2-Methoxy ethoxy methyl (MEM) ethers are cleaved by NaI/ClSi(CH3)3 in CH3CN at —20 or 25° in moderate to high yield. Iodotrimethylsilane (commercial) is less effective. Fairly selective cleavage of MEM ethers is possible in the presence of lactones, methyl or benzyl ethers, and methyl esters. 

Hydroxy-lactone 6.75 was prepared in several steps from glutamic acid (see chapter 2, section 2.2) and then opened by treatment with iodotrimethylsilane to give 6.76.Displacement of iodide with sodium azide, followed by reduction to the amine and hydrolysis, gave R-GABOB in 56% overall yield (>99%ee). 

A range of carboxylic esters undergo mild and neutral transesterification when treated with iodotrimethylsilane followed by an alcohol. The reaction is limited to primary and secondary aliphatic alcohols, and yields between 40 and 98% are obtained. Another non protic trans-esterification method involves treatment of the methyl esters of various carboxylic acids with tetra-n-alkylammonium halide at 140 °C, leading to the production of the corresponding n-alkyl esters. The generality of this method clearly depends on the availability of the required tetra-n-alkylammonium halide. Lactones react with iodotrimethylsilane in the presence of an alcohol to provide a convenient route to iodoalkyl esters in good yield. ... 

Ester Cleavage. Although esters are readily cleaved with iodotrimethylsilane, reaction of esters with TMS-Br under similar conditions gives somewhat lower yields of silyl esters or acids upon hydrolysis (eq 4). Lactones, however, react with TMS-Br at 100 °C to afford < >-bromocarboxylic acids after hydrolysis of the silyl ester (eq 5). ... 

In an alternative approach, the Diels-Alder adduct (144), obtained from the enedione (143) with l-methoxy-3 [(trimethylsilyl)oxy]-1,3-butadiene, was further transformed into the a-iodo ester (145). Exposure of the latter to iodotrimethylsilane furnished the 8-lactone (146), an attractive ABCD ring precursor to the quassinoids (106). 

A similar intramolecular approach for the synthesis of 8-lactones was reported by Watt et al. [118] (Scheme 70). After screening of several conditions, iodotrimethylsilane was found to effect the desired cyclization. Thus, treatment of a-iodoacetate 316 with iodotrimethylsilane led to the formation of lactone 317, probably through the formation of O-trimethylsilyl ketene acetal 318. 

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