Chiral synthesis via organoboranes. 13. A highly diastereoselective and enantioselective addition of [(Z)-.gamma.-alkoxyallyl]diisopinocampheylboranes to aldehydes

Asymmetric synthesis of 6-(2′,3′,4′,5′,6′-pentafluorophenyl)-δ-lactones via “allyl”boranes: application for the synthesis of fluorinated analog of key pharmacophore of statin drugs

“…It was stirred at À100 8C for 2 h and then warmed to À78 8C and stirred overnight. The reaction mixture was oxidized with 12.0 ml of 3.0 M sodium hydroxide and 12.0 ml of 30% hydrogen peroxide and stirred for 10 6-146.8 (m, 5C), 133.0, 120.9, 92.8, 80.1, 71.9, 68.5, 67.3, 58.8; 19 Alcohol 3a (0.65 g, 2.7 mmol) was dissolved in CH 2 Cl 2 (6.0 ml) and cooled to 0 8C. Acryloyl chloride (0.3 ml, 4.1 mmol) and triethylamine (0.9 ml, 6.8 mmol) were added to it at 0 8C and stirred for 1 h at room temperature.…”

Total Synthesis of Rapamycin

“…At this stage, orthogonal protection of the newly formed alcohol as its PMB ether under acidic conditions gave 66 (the final conclusion regarding the use of a C32-Alloc protecting group had not yet been reached by this time). Pleasingly, treatment of the acetal in 66 with NBS in wet acetonitrile successfully generated the desired lactol (67), from which productive elaboration appeared straightforward. Unfortunately, in the event, we were thwarted yet again by the stubborn stability of the cyclic form of 67.…”

“…Although separable, isolating 88 by purification on silica gel was tedious and in the end impractical. Fortunately, reagent control via asymmetric Brown alkoxyallylation [67] of the same aldehyde, followed by removal of the acetate protecting group (necessary to facilitate purification), furnished 90 as a single diastereomer. [68] The yield over these two steps was somewhat disappointing.…”

γ-Fluorophenyl-GABA derivatives from fluorobenzonitriles in high diastereomeric and enantiomeric excess

“…IV. Imine 2a was mixed with IV [28], followed by the addition of 1 equiv. of methanol and the reaction was monitored with 11 B NMR spectroscopy.…”