Synthesis of Monosubstituted (S)‐Oxiranes in High Optical Purity

Abstract

rate with high vapor pressure at T>2500"C, thus accounting for their low content; the high melting impurities Ta and W, on the other hand, remain in the niobium. The samples R-Nb-1 were electrolyzed; the major impurities Ta and W are thus removed, but metals such as Cr, Fe, and Co are apparently involved by contamination. The samples R-Nb-2 were additionally zone refined and annealed, which leads to a marked decrease in the Cr, Fe, and Co content.The niobium thus obtained can be additionally characterized by its specific electrical resistivity of @ (4.2 K)=6 x 0 cm in the normal state; the total content of non-metals is about 100 ppb. These findings show that niobium of particularly high purity and structural perfection can be prepared by the above method. The analytical method developed here for the determination of Cr, Fe, and Co is characterized by its extremely high sensitivity and freedom from error regarding contamination and losses.

An Eco‐Friendly and Switchable Carbon‐Based Catalyst for Protection‐Deprotection of Vicinal Diols

“…Hydrolysis reactions proceeded with very good yields (>84 %) for acetonides 15 , 17 and 21 (entries 1, 2 and 5). Substrates 19 and 20 , however, were transformed into their corresponding diols in slightly lower yields (79 % for 12 and 81 % for 13 ), probably due to the high volatility of the starting acetonides [25,26] . It should be noted that neither of them required any further purification step by chromatography.…”

“…Substrates 19 and 20, however, were transformed into their corresponding diols in slightly lower yields (79 % for 12 and 81 % for 13), probably due to the high volatility of the starting acetonides. [25,26] It should be noted that neither of them required any further purification step by chromatography. In all cases, 1 H NMR spectra of the hydrolyzed compounds evidenced the absence of signals attributable to isopropylidene groups.…”

Síntese de regioisômeros quirais a partir de D-manitol: obtenção de uma mistura de álcoois acetilênicos

“…Na etapa seguinte, o composto 3 foi reduzido com boroidreto de sódio conduzindo ao derivado acetonídeo hidroxilado 4. 20 O cetal cíclico 4 foi alquilado via derivado tosilato 5 através da reação com organocuprato e n-butil lítio, 20,21 obtendo-se o composto 6. Posteriormente, realizou-se a desproteção do cetal alquilado 6, com ácido clorídrico diluído.…”

Total Synthesis of Tricolorin A

“…Secondly, methyl 11( S )-jalapinolate [methyl 11( S )-hydroxyhexadecanoyl ester ( 25 )] was prepared employing a chiral pool approach starting from d -(+)-mannitol (Scheme ). 2( S )-Hydroxyheptanyl tosylate ( 14 ), readily obtained from d -mannitol in seven steps in a yield of 6.7% according to Barry's procedure, was treated with t -BuLi to afford the 1,2( S )-epoxyheptane ( 15 ) (overall yield of 4.7% from d -mannitol) . On the other hand, transformation of undecylenic acid to ω-bromononanyl ( tert -butyl dimethyl) silyl ether ( 21 ) was easily realized in six steps (overall 61% yield): reduction with LiAlH 4 , acetylation, oxidative cleavage of the double bond by KMnO 4 , Hunsdieck decarboxylic bromination, deacetylation, and protection of 1-OH with TBDMS.…”

“…The mixture was extracted with Et 2 O (70 mL × 3), washed with brine, dried over Na 2 SO 4 , and concentrated. Flash chromatography on a silica gel column (pentane−Et 2 O, 100:3) furnished a yellow oil 15 (1.429g, 70%): [α] 20 D −9.46 ( c 0.43, CHCl 3 ) (lit …”