Synthesis of 20S-protopanaxadiol β-D-galactopyranosides
20S-Protopanaxadiol (3β,12β,20S-trihydroxydammar-24-ene) 3-, 12-, and 20-O-β-D-galactopyranosides were synthesized for the first time. Condensation of 12β-acetoxy-3β,20S-dihydroxydammar-24-ene (1) and 2,3,4,6-tetra-O-acetyl-α-D-galactopyranosylbromide (α-acetobromogalactose) (2) under Koenigs–Knorr conditions with subsequent removal of the protecting groups resulted in regio- and stereoselective formation of 20S-protopanaxadiol 3-O-β-D-galactopyranoside, an analog of the natural ginsenoside Rh2. Glycosylation of 12β,20S-dihydroxydammar-24-en-3-one (5) by 2 with subsequent treatment of the reaction products with NaBH4 in isopropanol and deacetylation with NaOMe gave 20S-protopanaxadiol 12- and 20-O-β-Dgalactopyranosides.
Chemical Synthesis of Saponins.
Saponins are a large family of amphiphilic glycosides of steroids and triterpenes found in plants and some marine organisms. By expressing a large diversity of structures on both sugar chains and aglycones, saponins exhibit a wide range of biological and pharmacological properties and serve as major active principles in folk medicines, especially in traditional Chinese medicines. Isolation of saponins from natural sources is usually a formidable task due to the microheterogeneity of saponins in Nature. Chemical synthesis can provide access to large amounts of natural saponins as well as congeners for understanding their structure-activity relationships and mechanisms of action. This article presents a comprehensive account on chemical synthesis of saponins. First highlighted are general considerations on saponin synthesis, including preparation of aglycones and carbohydrate building blocks, assembly strategies, and protecting-group strategies. Next described is the state of the art in the synthesis of each type of saponins, with an emphasis on those representative saponins having sophisticated structures and potent biological activities.
Broadcasting Freedom: Radio, War, and the Politics of Race, 1938-1948
The World War II era represented the golden age of radio as a broadcast medium in the United States; it also witnessed a rise in African-American activism against racial segregation and discrimination, especially as they were practised by the federal government itself. In this study, the author links these cultural and political forces by showing how African-American activists, public officials, intellectuals, and artists sought to access and use radio to influence a national debate about racial inequality. Drawing on a body of national public affairs programming about African-Americans and race relations, the author uses these radio shows to demonstrate the emergence of a new national discourse about race and ethnicity, racial hatred and injustice, and the contribution of racial and immigrant populations to the development of the United States. These programmes, Savage contends, challenged the nation to reconcile its professed egalitarian ideals with its unjust treatment of black Americans and other minorities. This examination of radio's treatment of race as a national political issue also provides important evidence that the campaigns for racial justice in the 1940s served as an essential, and still overlooked, precursor to the civil rights campaigns of the 1950s and 1960s, Savage argues. The next battleground would be in the South, and on television.