Tetracyclic diterpenoids with a new carbon skeleton forming products of the cyclization of labdane diterpenoids
From the product of the cyclization of a mixture of isomeric ent-copalols (i) [i] with a mixture of formic and concentrated acids, we have isolated an oxygen-containing fraction (yield 47%) consisting of a mixture of five alcohols and their formates. Three of the alcohols proved to be known: isoagath-12-en-15-ols epimeric at Cz4 [2] and 14~-hibanol [3]. For the other two alcohols, with the composition C2oH3~O, structures (2) and (3) were shown, these being based on a new carbon skeleton [4].
Cationic cyclization of bicyclic diterpenes
Manool and agathadiol cyclize in formic acid to the 13-epimeric pimara-8,15-dienes and 14α-hydroxyhibane derivatives. The mechanistic and biosynthetic implications of these results are discussed in relation to other work involving cations in tetracyclic diterpenes. The nuclear magnetic resonance spectra of relevant tricyclic and tetracyclic diterpenes are documented.
Synthesis of 2,4-dideoxy-.beta.-D-erythro-hexopyranosyl nucleosides
The synthesis of 1-(2,4-dideoxy-β-D-erythro-hexopyranosyl)thymine (12) 1 was accomplished using two different synthetic routes. It was obtained starting either from 1,2:5,6-di-O-isopropylidene-α-D-glucofuranose or from tri-O-acetyl-D-glucal. The other modified nucleosides, with either a cytosine, guanine, or adenine moiety, were synthesized using the second reaction scheme. Deoxygenation reactions were accomplished with the (2,4-dichlorophenoxy)thiocarbonyl derivatives generated in situ