Calcium borohydride reduction

Compound 7 is reduced to 2-benzamidocinnamyl alcohol by calcium borohydride in hydroxylic solvents at low temperatures. This reduction had been accomplished previously using lithium aluminum hydride in tetrahydrofuran. 

The reduction of ketones containing nitro groups to nitro alcohols is best carried out by borohydrides. 5-Nitro-2-pentanone was converted to 5-nitro-2-pentanol in 86.6% yield by reduction with sodium borohydride at 20-25°. Other nitro ketones gave 48.5-98.7% yields, usually higher than were obtained by Meerwein-Ponruiorf reduction [907]. 2-Acetamido-3-(p-nitrophenyl)-l-hydroxypropan-3-one was reduced with calcium borohydride at — 30° to 70% of threo- and 10% of er7tAro-2-acetamido-3-(p-nitrophenyl)propane-l,3-diol while sodium borohydride afforded a mixture of the above isomers in 25% and 47% yields, respectively [902]. 

More reliable therefore is reduction with lithium aluminum hydride, lithium borohydride or calcium borohydride, which do not hydrogenolyze hydroxy groups [92]. 

In this approach (17), the prochiral fragment was attached to the auxiliary at only one point making the acid-catalysed release after transformation a simple procedure. Chirality was induced by the borohydride reduction of a carbonyl group on the pro-chiral fragment in the asymmetric environment created by complexation of calcium ions between the Cl and C2 oxygens of the hexose. The diastereomeric purity was good (-70%) but separation of the diastereomers was more problematic. 

Irradiation of the 5,7-diene gave the previtamin, which was isomerized and saponified to give la-hydroxy-vitamin D3. For the last synthesis of la-hydroxy-7-dehydrocholesterol recorded here, cholesta-l,4,6-triene-3-one was again used as starting steroid.122 Deconjugation of this trienone with strong base followed by immediate reduction with calcium borohydride led to the unstable 3/3-hydroxycholesta-l,5,7-triene which, without isolation, was converted into the 1,4-addition product (265) upon reaction with 4-phenyl-l,2,4-triazoline-3,5-dione. 

S-Benzyl-y-butyrolactones (44) for which convenient preparative procedures are available, and improved techniques for their a-alkylation and a-hydroxyalkylation, provide the most common synthetic route for these lignan sub-classes (39). The Stobbe condensation (40) of aryl aldehyde with dimethyl succinate (Scheme 9) leads to the half-ester (42) which can be catalytically hydrogenated at atmospheric pressure to give the dihydro half-ester (43). Selective reduction of the potassium salt of the latter can be effectively achieved by calcium borohydride (41)... 

Calcium borohydride is generated in methanol or ethanol from CaCl2 and NaBH [BR3]. It reduces esters to alcohols, leaving acid salts intact, thus allowing the formation of lactones from hemiesters [LRl] (Section 3.2.5). It has also been used in stereoselective reduction of a,p-epoxyketones [TF2] (Section 3.2.4). 

The reagent is also suitable for reduction of aldehydes and ketones.3 Thus the stereoselective reduction of the acetamino ketone (2) to the threo diol (3) was effected by means of calcium borohydride in 70% yield 4... 

Methoxy-a-(phenylthio) ketones, e.g., 3-methoxy-l-phenyl-2-phenylthiopropanone and 5-methoxy-l-phenyl-4-phenylthio-l-penten-3-onc (Table 6), can also be reduced with syn selectivity by zinc borohydride. calcium borohydride and lithium triethylborohydride82. For zinc and calcium borohydride, at least, it is reasonable to suppose that the transition state 4 (where R3 = MOM) is reinforced by /1-chclation. These reagents are actually more successful than L-Selectride in the reduction of 3-methoxy-1-phenyl-2-phenylthiopropanone, where the L-Se-lectridc result is anomalously poor. A /5-hydroxy substrate, 3-hydroxy-2-mcthylthio-l-phenyI-propanone, could be reduced to 2-methylthio-l-phenyl-l,2-propanediol with good syn selectivity using zinc borohydride (yield 65% d.r. 93 7)81. 

Reductions of cis- and /ran -oxadiazine-5-carboxylates (154 R = C02Me) (Section 6.17.7.2.3.1) with calcium borohydride in THF-ethanol proceed in high yields (90%) without epimerization. Swern oxidation of the resulting alcohols (154 R = CH20H) yields the stereoisomeric aldehydes (154 R = CHO) <89TL5507>. 

Asymmetric aldolysation of glycolaldehyde has been achieved using the asymmetric acetal derivative (1) with triethylamine or calcium hydroxide the mixture of tetritol stereoisomers (2) obtained after borohydride reduction showed small stereoselectivities for... 

Reduction of Carbonyl Groups. Rapid reduction of salts of carboxylic acids to alcohols with 2 molar equivalents of borane in tetrahydrofuran (THF) has been reported. Full details have appeared of the improved procedure for reduction of esters to primary alcohols, using borane dimethyl sulphide (6,157) in THF at reflux, which allows the dimethyl sulphide that is liberated to distil off during the reaction. Further suggested procedures for the reduction of esters to primary alcohols use lithium borohydride in diethyl ether or THF, calcium borohydride in THF, and sodium borohydride in a t-butyl alcohol-methanol mixed solvent. The reactions using lithium borohydride-diethyl ether can be catalysed by lithium 9-boratobicyclo[3.3.1]nonane (1), lithium triethylboro-hydride, or the Lewis acid 9-methoxy-9-borabicyclo[3.3.1]nonane (2). ... 

Monodisperse particles present the advantage of uniform active site distribution and can be considered as models for heterogeneous catalytic reactions. Monodisperse metals, metal oxides or metal borides can now be easily obtained using microemulsions, vesicles, polymers or normal micelles (refs. 1-4). Microemulsions were used to obtain monodisperse particles of platinum (refs. 5-7), palladium (refs. 5,6), rhodium (refs. 5,6), iridium (ref. 5) and gold (ref. 8) by reducing the precursor metal ions with hydrogen, hydrazine, sodium borohydride or solvated electrons. Monodisperse nickel boride (refs. 1,9-12), cobalt boride (refs. 1,10,13-17), nickel-cobalt boride (refs. 1,10,15-17), and mixtures of iron boride and iron oxides (refs. 1,18) were prepared by sodium borohydride reduction of the precursor metal ions. Iron oxides (ref. 19), magnetite (ref. 20), calcium carbonate (ref. 21) and silver chloride (ref. 22) were obtained by precipitation reactions. 

Good yields of phenylarsine [822-65-17, C H As, have been obtained by the reaction of phenylarsenic tetrachloride [29181-03-17, C H AsCl, or phenyldichloroarsine [696-28-6], C3H3ASCI25 with lithium aluminum hydride or lithium borohydride (41). Electrolytic reduction has also been used to convert arsonic acids to primary arsines (42). Another method for preparing primary arsines involves the reaction of arsine with calcium and subsequent addition of an alkyl haUde. Thus methylarsine [593-52-2], CH As, is obtained in 80% yield (43) ... 

Palladium catalysts resemble closely the platinum catalysts. Palladium oxide (PdO) is prepared from palladium chloride and sodium nitrate by fusion at 575-600° [29,30]. Elemental palladium is obtained by reduction of palladium chloride with sodium borohydride [27, 31], Supported palladium catalysts are prepared with the contents of 5% or 10% of palladium on charcoal, calcium carbonate and barium sulfate [32], Sometimes a special support can increase the selectivity of palladium. Palladium on strontium carbonate (2%) was successfully used for reduction of just y, (5-double bond in a system of oc, / , y, (5-unsaturated ketone [ii]. 

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