Synthesis of the optically active hexahydrobenzofuran nucleus of the avermectins

Synthesis of the South Unit of Cephalostatin. 7. Total Syntheses of (+)-Cephalostatin 7, (+)-Cephalostatin 12, and (+)-Ritterazine K1

“…Chemoselective oxidation of the C 3 alcohol was next investigated in hopes of separation of the spiroketals. Both Swern oxidation and IBX oxidation 17 failed to achieve the desired transformation although the literature reports cases where an alcohol was oxidized in the presence of a thioether moiety. Fortunately, the C 3 alcohol could be chemoselectively oxidized to ketone in 91% yield (7% SM recovered) upon treatment with pyridine−CrO 3 for 5 min.…”

The conversion of carbohydrate derivatives into functionalized cyclohexanes and cyclopentanes

“…The yield was 80% showing the preference for cis-ring fusion in closures of this type. 45 A further approach to the oxahydrindan unit of the avermectins relies on a process induced by thiolate attack at the /3-position of the alkene function of the furanone 85, which was prepared from methyl 2,3-0isopropylidene-/3-D-ribofuranoside. The "ate" species derived by reaction of trimethylaluminum with lithium thiophenate gave 86 in high yield as a single product.…”

Diastereoselective synthesis of the C29–C41 fragment of karlotoxin 2

“…Accordingly, benzyl groups were removed with lithium in the presence of naphthalene in freshly distilled THF at À20°C to obtain diol 4 in 85% yield (Scheme 3 Primary alcohol was selectively oxidized with TEMPO/BAIB in CH 2 Cl 2 to afford aldehyde, which was immediately purified by flash chromatography on silica gel and treated with homoallyl magnesium bromide in the presence of a catalytic amount of CuI at À78°C in anhydrous THF to obtain 1,3-diols 16 and 16a (9:1) in 85% yield. 13 The selectivity was intrigued due to chelation controlled 1,3-induction by an internal addition of nucleophile (Scheme 4). 14 At this stage, the Rychnovsky method 15 was utilized to assign geometry of the newly created stereogenic center.…”