Rapid and Enantioselective Synthetic Approaches to Germanicol and Other Pentacyclic Triterpenes
作者:Karavadhi Surendra、E. J. Corey
DOI:10.1021/ja802730a
日期:2008.7.1
routes to the key intermediate 2 for the synthesis of the pentacyclic triterpene germanicol 1 have been developed. In the first, the ( S)-epoxide of farnesyl bromide is transformed in just three steps to the tetracyclic intermediate 7, which is converted to chiral 2 by treatment with polyphosphoric acid. The second synthetic route to 2 involves the coupling of the ( S)-epoxide 8 with vinyl iodide 9 to give
sulfenylation/deiodination/aromatization of cyclic alkenyl iodides with sulfonyl hydrazides. In the absence of external catalysts and additives a range of 4‐iodo‐1,2‐dihydronaphthalenes reacted with sulfonyl hydrazides to give structurally diverse 2‐naphthyl thioethers in good yields. Mechanistic studies showed that at an early stage sulfonyl hydrazides decomposed completely to thiosulfonates and disulfides
Studies on the oxidation of hydrazones with iodine and with phenylselenenyl bromide in the presence of strong organic bases; an improved procedure for the synthesis of vinyl iodides and phenyl-vinyl selenides
The oxidation of hydrazones by iodine in the presence of strong organic bases (guanidines) has been studied. Improved yields of vinyl iodides can be obtained by inverse addition using dry solvents and with a final heating period where appropriate. The reaction has been extended to various dihydrazones with interesting results.
The oxidation of ketone hydrazones by iodine in the presence of a base to furnish vinyliodides has been considerably improved. The three factors responsible are (1) absence of water, (2) the use of strong guanidine bases and (3) inverse addition.