Trichloroimidates

The use of trichloroimidates for the preparation of ethers is an effective method for O-alkylation of alcohols [27]. This method has found widespread use in the protection of alcohols as benzyl ethers since the corresponding trichlorobenzylimi-date is inexpensive and commercially available. The mechanism involves activation of the imidate with a catalytic amount of a strong acid (typically TfOH) which leads to ionization of the electrophile and the formation of carbocation which is rapidly trapped by an alcohol. For the preparation of sec-sec ethers, this protocol has been limited to glycosidation reactions, due to the SN1 nature of the reaction which often leads to diastereomeric mixtures of products [26],... 

An iodine-mediated 6(0) " -exo-Trig cyclization has been used in the synthesis of (25, 4/ )-4-hydroproline from (S)-O-benzylglycidol219. Stereocontrolled 5(0) " -endo-Trig iodoetherification has provided an entry to traw.s-2.5-disubstituted tetrahydrofuran derivatives220. On treatment with iodine, trichloroimidates of primary a-allenic alcohols are converted into oxazolines with high stereoselectivity221. 

In another synthesis of the rat brain Thy-1 GPI anchor, Schmidt and coworkers [88] have adopted the linear strategy to construct the pentasaccharide building block. As shown in O Scheme 11, after all monosaccharide units were obtained, they were assembled sequentially by the trichloroimidate glycosylation method to form pentasaccharide 130. It was manipulated... 

The transition metai cataiyzed Overman rearrangement allows the reaction to take place at or around room temperature, so thermally sensitive substrates can be used. In the laboratory of M. Mehmandoust, this approach was applied for the synthesis of enantiomerically pure ( )- 3,y-unsaturated a-amino acids, which are potent enzyme inhibitors. ° The trichloroimidate substrates were derived from optically pure monoprotected diallylalcohols and were exposed to 10 mol% of Pd " -salt. The rearrangements took place rapidly at room temperature with complete transfer of chirality. 

The asymmetric total synthesis of the phenanthroquinolizidine alkaloid (-)-cryptopleurine was reported by S. Kim et al. One of the key steps in the sequence was the thermai Overman rearrangement which took place in refluxing toluene in nearly quantitative yield and without any loss of the optical purity of the allyl trichloroimidate substrate. 

Glycosylation. Protected glycosyl fluorides and trichloroimidates undergo glycosylation, whereas the correspionding glycosyl bromides and phosphates are inert. 

Kr, , and trichloroimidates undergo >> niides and phosphates are inert. i ketones with alkynyllithiums and < OEu have been compared. The ut of the benzenesulfonyl group of... 

Glycosyl trlchloroacetimldates have been used to prepare glycosyl phosphates by treatment with mono- or dl-esters of phosphoric acid the reaction normally proceeded with predominant inversion (Scheme 3) but in the case of the trichloroimidate (15) with monocetyl phosphate retention was observed. 

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