Protecting groups prenyl esters

Hydroxyl protection. By using the title reagent in the Mitsunobu reaction alcohols are protected. The prenyl group of the derived esters is removable by DDQ and subsequent addition of Yb(OTf)3 promotes lactonization to liberate the alcohols. ... 

Nucleophilic tert-prenylation. When using prenylboranes instead of tert-prenylboranes, tert-prenylation of 3-chloroindolenines becomes possible. The Danishefsky tert-prenylation has also been extensively used for the synthesis of 3-tert-prenylindoles which can be obtained from 2-tert-prenylated precursors (see Sect. 4). The 1995 synthesis of the acyl-CoA-cholesterol acyltransferase inhibitor gypsetin (94) [91, 92] was the first occasion to publish that elegant reaction (Scheme 19) [88, 93]. On treatment of phthalimide-protected tryptophan methyl ester (85) with tcrt-BuOCl, the 3-chloroindolenine is formed in sim, which is nucleophilicaUy attacked by prenyl-9-BBN with regioinversion of the prenyl group. Hydrazinolysis afforded 2-tert-prenyltryptophan methyl ester (91). 

Deprotection of prenyl esters to carboxylic acids is successfully achieved by using a catalytic amount of (la) [156]. This method is tolerant to various functionalities such as protected a-amino groups, other esters, and carbon-carbon multiple bonds. 

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