Propan asymmetric carbonylation

Walsh orbitals6 are the MOs corresponding to the cyclopropane C-C bonds. We begin by supposing that carbon orbitals are sp2 hybrid. Indeed, many studies show that cyclopropane is more like ethylene than propane or cyclobutane. For example, it adds bromine slowly, whereas cyclobutane is inert. Hydrogenation of cyclopropane occurs at 120°and cyclobutane at 200°. Asymmetric CH2 stretching modes appear in the IR spectrum of cyclopropane at 3050 and in that of ethylene at 3080 cm-1. Methyl cyclopropyl ketone shows a carbonyl stretch at 1695 cm-1. Finally, the HCH angle is 116°6 in ethylene, and 118° in cyclopropane. 

Asymmetric cyclopropanation. The ability to effect ligand exchange between rhodium(II) acetate and various amides has lead to a search for novel, chiral rhodium(II) catalysts for enantioselective cyclopropanation with diazo carbonyl compounds. The most promising to date are prepared from methyl (S)- or (R)-pyroglutamate (1), [dirhodium(ll) tetrakis(methyl 2-pyrrolidone-5-carboxylate)]. Thus these complexes, Rh2[(S)- or (R)-l]4, effect intramolecular cyclopropanation of allylic diazoacetates (2) to give the cyclo-propanated y-lactones 3 in 65 S 94% ee (equation 1). In general, the enantioselectivity is higher in cyclopropanation of (Z)-alkenes. 

Nikkomycins B 61 and Bx 62 are nucleoside peptide antibiotics isolated from the culture of Streptomyces ten-They contain a (3-amino carbonyl moiety. Hayashi and co-workers developed a three-component, Mannich-type reaction mediated by L-proline and applied it to an asymmetric synthesis of the A -terminal amino acid moiety of nikkomycin B 61 and Bx 62 (formal total synthesis). As shown in Scheme 27.10, a three-component Mannich reaction using 2-furylaldehyde 55, reri-butyl-dimethylsi-loxyaniline 56, and propanal 39 gave the Mannich adduct 58 in 96% ee. In the particular case of 2-fiirylaldehyde 55, pyridine (1.5 equiv) was reported to be effective for obtaining excellent enantioselectivity. 

---