Asymmetric cyclopropanation of 1-alkenylboronic esters and its application to the synthesis of optically active cyclopropanols

Kinetic Enzymatic Resolution of Cyclopropane Derivatives

“…Instead of starting from racemic acetate rac-25a, we decided to use enantiomerically enriched starting material (1R)-24a that was readily available from enantiomerically pure alkenylboronic ester 29 via a diastereoselective cyclopropanationoxidation sequence (Scheme 11). [69] A moderate (though improved) enantiomeric excess of the product was observed, but the material was available in high yield (92%; 75% ee). [70,71] Acylation and enzymatic resolution gave the enantiomerically pure (within the experimental error) cyclopropanol (1R)-24a; slightly impure product was obtained in 73%.…”

“…[59±63] (known compounds: 22b, [65] 22h, [77] and 22i [63] ), cyclopropylboronic esters 23 (known compounds: 23b [65] and 23h [77] ), cyclopropanols 24 (known compounds 24a [70] and 24c [69] ), and vinyl acetate 26.…”

Chemo- and diastereoselective cyclopropanation of allylic amines and carbamates

“…The resulting mixture was stirred at rt for 24 h then concentrated in vacuo. Purification by flash column chromatography (eluent 30e40 C petrol/Et 2 O, 19:1) gave 51 as a white solid (600 mg, 50%, >99:1 dr); mp 130e132 C. (7)), 9.7 (C(6)), 16.3 (C(1)), 16.9 (C(4)), 24.8 (C(5)), 28.0 (C(3)), 28 Following General procedure 1, 60 (217 mg, 1.0 mmol), ZnEt 2 (2.0 mL, 2.0 mmol) and CH 2 I 2 (0.32 mL, 4.0 mmol) in CH 2 Cl 2 (2.0 mL) were reacted for 1 h to give, after purification by flash column chromatography (eluent 30e40 C petrol/EtOAc, 1:0 to 9:1), 64 as a white solid (229 mg, 99%, >99:1 dr); mp 101e103 C; Following General procedure 1, 61 (231 mg, 1.00 mmol), ZnEt 2 (2.0 mL, 2.0 mmol) and CH 2 I 2 (0.32 mL, 4.0 mmol) in CH 2 Cl 2 (2. Following General procedure 1, 62 (201 mg, 1.0 mmol), ZnEt 2 (2.0 mL, 2.0 mmol) and CH 2 I 2 (0.32 mL, 4.0 mmol) in CH 2 Cl 2 (2.0 mL) were reacted for 1 h to give, after purification by flash column chromatography (eluent 30e40 C petrol/EtOAc, 7:3 to 1:1 gradient elution), 66 as a white solid (215 mg, quant, >99:1 dr); mp 122e124 C; n max (KBr) 3295 (NeH), 1625 (C] (2 mL) at rt and the resultant suspension was heated at reflux for 16 h. The reaction mixture was then allowed to cool to rt, filtered and concentrated in vacuo.…”

“…10b Diastereoselective cyclopropanation employing a zinc carbenoid, relying upon delivery of the incoming methylene group by the binding of an allylic hydroxyl group to the zinc atom, has long been exploited. Other groups including a,bunsaturated acetals, 14 amides 15 and boronates 16 have also been shown to enable diastereoselective reaction. Although allylic amines have the same potential for directing cyclopropanation, the competing formation of a zinc-complexed ammonium ylide often thwarts cyclopropanation.…”

Selektive C‐C‐Bindungsspaltung zur stereoselektiven Synthese acyclischer Systeme

“…Enantiomerenreine Cyclopropanole [40] 33 wurden unter Erhaltung der Enantiomerenverhältnisse in offenkettige Alkohole 34 umgesetzt (Schema 14). [41] Die stereospezifische Oxymercurierung von Cyclopropanolen, [41] in Kombination mit einer durch Sauerstoff dirigierten Cyclopropanierung acyclischer Alkohole, [42,43] schuf einen direkten und stereoselektiven Zugang zu verschieden substituierten Polypropionatfragmenten. Diese bahnbrechenden Arbeiten auf dem Gebiet der konformativ flexiblen Cyclopropylcarbinole gestatten eine vollständige Inversion der Konfiguration des Dreirings 35 unter Anwendung von Quecksilber(II)-trifluoracetat (Schema 15).…”