K Selectride

K-Selectride 548 K20s042H20 685 K3Fe(CN)6 677,682 Katsuki epoxidation 361 kempene-2 667 ketalization 692, 701 ketene 70 f. 

Replacement of hydrogen by alkyl groups gives compounds like lithium triethylborohydride (Super-Hydride ) [100], lithium tris sec-butyl)borohydride [101] (L-Selectride ) and potassium tris sec-butyl)borohydride (K-Selectride ) [702], Replacement by a cyano group yields sodium cyanoborohydride [103], a compound stable even at low pH (down to 3), and tetrabutylammonium cyanoborohydride [93],... 

Another approach to secondary amines has been reported (J. Am. Chem. Soc. 125 16178, 2003) by Masakafsu Shibasaki of the University of Tokyo. Addition of methoxyamine to a chalcone 7 (alkyl enones work in slightly lower ) gives the amine 8. The amine 8 can be reduced with high stereocontrol to the amino alcohol 9. K-Selectride gives the complementary diastereomer. 

To a solution of2 (340 mg, 0.9 mmol) in THF (30 mL) cooled to —78°C K-Selectride (1.34 mL of a 1 -M solution in THF) was slowly added (1 h) under nitrogen atmosphere. The reaction mixture was stirred for an additional 1 h, quenched with an aqueous saturated solution of NaHC03 (20 mL) and allowed to attain room temperature. The product was extracted with ethyl acetate (4 x 20 mL), the combined extracts were dried (MgS04) and concentrated to dryness. The residue was dissolved in dichloromethane (30 mL), then triethylamine (500 p,L, 3.6 mmol), acetic anhydride (250 p.L, 2.6 mmol), and a crystal of AJA-dimethylaminopyridine (DMAP) were added, and the mixture was left under nitrogen at room temperature. After 24 h the solution was concentrated to dryness and the residue was chromatographed on a silica gel column with hexane-ethyl acetate (3 1) to give 25,35,4/ ,6/ -3,4-diacetoxy-2-(l-menthyloxy)-6-(2-furyl)-tetrahydropyran 4 (290 mg, 79%) mp 127°-129°C, [a]D - 5.3° (c 0.8, CHC13). 

The sterically hindered reducing agent K-Selectride favors 1,2-addition from the face opposite the Req substituent. 

Reduction with NaBH4 or DIBAL-H furnished a product mixture in which the equatorial allylic alcohol predominated. However, reduction using K-Selectride produced the axial alcohol as the major product (88% yield, 9.8 1 mixture of diastereomers). 

Unactivated halopyridines including 3-halopyridines undergo substitution at room temperature using thioborates prepared in situ from K-Selectride and phenylethanethiol in 1,2-DME in moderate to good yield. 3-Chloropyridine is converted to the thiol in 63% yield using these conditions <1999TL5565> (Equation 99). 

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