Preparation of Optically Active Apoverbenone and Verbenone from Nopinone by Use of the Sulfenylation-Dehydrosulfenylation Method. Stability and Reactivity Attributable to Absolute Configuration at the Sulfur Atom in Sulfoxides.
Apoverbenone (4) and verbenone (5) in optically active forms are potentially useful compounds as the chiral source in enantioselective synthesis. Starting with (+)-nopinone (1), readily available from (-)-beta-pinene, (+)-apoverbenone (4a) and (+)-verbenone (5a) were prepared in synthetically satisfactory overall yields, using commonly the sulfenylation-dehydrosulfenylation method directed toward construction of an enone function. This methodology could be applicable to preparation of their enantiomers (-)-4b and (-)-5b on starting with (-)-nopinone. While configuration of the phenylsulfinyl group in sulfoxides 10a,b, and 18a,b were assigned by their (1)H NMR spectra and NOE correlations, absolute configurations at the sulfur atom were performed by comparison in (1)H NMR spectra with their homologues whose absolute configurations are well-defined, that is, 10a with 13a, 10b with 13b, 18a with 19b, and 18b with 19a. As a result, it was proved that, in 3-(phenylsulfinyl)nopinones, thermodynamic stability of isomers is dependent on absolute configuration at the sulfur center; that is, the trans-isomer 10a possesses an R(s)-phenylsulfinyl group and the cis-isomer 10b possesses an S(s)-phenylsulfinyl group, and it was proved that, in (4R)-4-methyl-3-(phenylsulfinyl)nopinones, both cis-isomers 18a,b are stable, irrelevant with absolute configurations at the sulfur atom. In elimination of phenylsulfenic acid from sulfoxides 10 and 18, the use of purified sulfoxides was essential, because the competing Pummerer reaction proceeded to give 3-(phenylthio)verbenone (11) as a byproduct, when acidic contaminants were present. The sulfoxides 10a,b and 18a provided smoothly 4a and 5a, respectively, in a syn elimination manner, whereas 18b gave a mixture of decomposed products as a major part, probably because of instability of 18b to heat as well as because of its conformational requirement.
Kosugi
Hiroshi
H
Institute for Chemical Reaction Science, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan.
Ku
Jaseung
J
Kato
Michiharu
M