C-Glycosylidene derivatives (exo-glycals): their synthesis by reaction of protected sugar lactones with tributylphosphonium ylids, conformational analysis and stereoselective reduction

Diastereoselective Total Synthesis of (±)-Basiliolide B and (±)-epi-8-Basiliolide B

“…Ketone 19 was found to be unreactive with methyl-2-(trimethyl­phos­phoranylidene)­acetate in refluxing benzene or toluene (entry 4), and it decomposed slowly in refluxing mesitylene (entry 5). Switching to methyl 2-(tri- n -butyl­phos­phoranylidene)­acetate in refluxing toluene also resulted in no reaction (entry 6). A trace amount of the olefination product ( 25 or 26 ) was observed by LC-MS after refluxing with chlorobenzene (entry 7), and raising the reaction temperature to 150 ° C with refluxing mesitylene afforded the olefination product 26 in only 5% yield (entry 8).…”

“…However, the ketone ( 27 ) was also found to be too hindered for olefination and decomposition of the substrate was observed with a variety of phosphonate/base systems or triphenylphosphine-based stabilized ylides. Methyl and tert -butyl 2-(tri- n -butyl­phos­phoranylidene)­acetate were finally found to be effective for olefination of 27 in toluene at 100 °C and afforded the expected ( E )-olefins ( 28a , 28b ) in good yields. The methyl lactol moieties were then oxidized with Jones reagent directly to lactones 29a and 29b …”

“…Methyl and tert -butyl 2-(tri- n -butyl­phos­phoranylidene)­acetate were finally found to be effective for olefination of 27 in toluene at 100 °C and afforded the expected ( E )-olefins ( 28a , 28b ) in good yields. The methyl lactol moieties were then oxidized with Jones reagent directly to lactones 29a and 29b …”

“…Achmatowicz rearrangement of 41 gave the corresponding lactol, which was then methylated and provided 42 as a mixture of lactol diastereoisomers. After olefination with methyl 2-(tri- n- butyl­phos­phoranylidene) acetate, Jones oxidation of the methyl acetal to the lactone afforded the 2 H -pyran-2,5-dione moiety of 43 . Alkene migration with DABCO followed by intramolecular DA cycloaddition afforded 44 , which was characterized unambiguously with 2D NMR studies.…”

“…Methyl 2-( n -butylphos­phoranylidene)­propanoate (6.75 g, 24.6 mmol) was added to a stirred solution of 27 (2.00 g, 4.92 mmol) in toluene (50 mL). After stirring at 100 °C for 1.5 h, TLC showed consumption of the starting material was complete, and then the reaction was cooled to room temperature and quenched by addition of a saturated aqueous NH 4 Cl solution.…”