Protecting groups, deprotection

Reduction of 2,2-dimethyl-cyclohexane-1,3-dione (50) with Baker s yeast gave alcohol (ee 98.3%) whose tetrahydropyranyl derivative on methoxycarbonylation produced (51) quantitatively. Michael addition of (51) with methyl vinyl ketone followed by heating the adduct under reflux with pyrrolidine in benzene yielded (52) in 85% yield as stereoisomeric mixture whose separation presented problems. In order to eliminate the complexity due to a chiral center in tetrahydropyranyl protective group, deprotection of (52) was achieved by treatment with p-toluenesulphonic acid in methanol. The product obtained was a mixture of the lactone (53) and hydroxy ester (54). Probably the stereoisomer of... 

The C18-C28 segment 502 was synthesized starting from epoxide 497 as shown in Scheme 71. The thioketal 500 was synthesized by coupling of 498 and 499, which were derived from epoxide 497. The thioketal 500 was converted into hemiketal 501 via adjustment of the silyl protective group, deprotection of thioketal, oxidative cleavage of the double bond, and acetylene formation. Hydrostannylation/iodination of acetylene 501 gave vinyliodide, and protective group manipulation afforded 502. 

Abstract The synthetic uses of molecular iodine are described. This chapter discusses the versatile uses of iodine in heterocyclic synthesis. Iodine is a universal oxidizing agent, a mild and nontoxic Lewis acid catalyst, and it catalyzes various organic reactions with high efficiency and selectivity. Also, iodine acts as an electrophile-nucleophile dual activator in the reactions. Further applications are the introduction of protecting groups, deprotection, iodocyclization, C-C bond formation, and formation of heterocycles. It is quite clear from the growing number... 

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