Masked equivalent

In the synthesis of the taxol skeleton, two successive applications of the reaction on a masked equivalent for 2,3-bis(bromomethyl)-l,3-butadiene are the key steps59. 

The important point is that synthetic analysis and planning should not be restricted to the specific functionalities that appear in the target molecules. These groups can be incorporated as masked equivalents by methods that would not be possible for the functional group itself. 

Malonate derivatives have been used in the synthesis of pyranopyrandiones and dipyranopyrantriones, which are masked equivalents of triketo and tetraketo acids, respectively (Scheme 97) (66JA624, 67CC231). 

These results indicate that the sulfinyl group seems to be much more efficient in the control of the stereoselectivity of 1,3-dipolar cycloadditions (endo or exo adducts are exclusively obtained in de> 80%) than in Diels-Alder processes (mixtures of all four possible adducts were formed). Additionally, complete control of the regioselectivity of the reaction was observed. Despite these clearly excellent results, the following paper concerning asymmetric cycloaddition of cyclic nitrones and optically pure vinyl sulfoxides was reported nine years later [154]. (Meanwhile, only one paper [155], related to the synthesis of /1-nicotyri-nes, described the use of reaction of nitrones with racemic vinyl sulfoxides, but these substrates were merely used as a masked equivalent of acetylene dipolaro-phile). In 1991, Koizumi et al. described the reaction of one of the best dipolarophiles, the sulfinyl maleimide 109, with 3,4,5,6-tetrahydropyridine 1-oxide 194 [154]. It proceeded in CH2C12 at -78 °C to afford a 60 20 10 6 mixture of four products in ca. 90 % yield (Scheme 92). 

This reactivity of aryl silanes is used to convert the stable phenyl dimethylsilyl group into a more reactive form for conversion into an alcohol by the silyl Baeyer-Villiger reaction described above. Overall this makes the phenyl dimethylsilyl group a bulky masked equivalent for a hydroxyl group. This is useful because the silane will survive reaction conditions that the 1. Br2... 

The concise enantioselective total synthesis of (+)-monomorine I, a 3,5-dialkyl-substituted indolizidine alkaloid, was completed by S. Blechert et al. using a sequential cross-metathesis double reductive cyclization strategy. The enedione substrate was prepared in two steps. The Stetter reaction between the masked equivalent of acrolein and butyl vinyl ketone was followed by a retro Diels-Alder reaction under flash vacuum pyrolysis (FVP) conditions. 

The lithiated enol ether (33) is a masked equivalent of the enolate of acetaldehyde,while ethyl dithioacetate has been used as an equivalent to the dianion... 

A novel approach to estrone via D-homoestrone by S. Danishefsky and A.Nagel [Chem. Commun., 373 (1972)] is based on a novel bisannelation sequence in vrhich the 6-vinyl-2-picoline moiety serves as a masked equivalent of two moles of methyl vinyl ketone (Mi)[S. Danishefsky and R. Cavanaugh, J. Amer. Chem. Soc., 90, 520 (1968)J. 

Functional Group Manipulation of Tetrahydropyranyl Derivatives. While tetrahydropyranyl derivatives are often utilized as masked equivalents of alcohols, etc., it is possible to perform functional group manipulation on such species. In these cases the tetrahydropyranyl derivatization serves to activate the alcohol rather than protect it. 

A wide range of structurally different dienophiles and dienes have been successfully utilized in asymmetric Diels-Alder reactions in the presence of catalyst 137 [75-78]. An unusual substrate class is represented by boronic ester 140 (Scheme 17.21) [78], which serves as a masked equivalent of y5-hy-... 

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