防已科植物アルカロイド研究 (第188報)

Abstract

Some time ago, Tomita and Kunitomo derived l- and d-N, O, O-trimethylcoclaurine (II and IIa) from D-(-)- and L-(+)-laudanosine, whose absolute configuration had already been elucidated, and determined the absolute configuration of coclaurinetype bases. Based on this result, absolute configuration of the two asymmetric centers in the majority of biscoclaurine-type bases was also clarified.Of these, the oxyacanthine-type base having methylenedioxy as the characteristic atom group in the molecule, cepharanthine (IV), was submitted to cleavage reaction with metallic sodium in liquid ammonia, by Tomita and Sasaki, and d-1-(p-hydroxybenzyl)-2-methyl-7-isoquinolinol (V) and D-(-)-N, O-dimethylcoclaurine (VI) were obtained as the cleaved bases. This proved that the absolute configuration of one (in B) of the two asymmetric centers in cepharanthine was a D type but that of the other (in A) had not been confirmed.In order to elucidate this point, cleavage reaction with metallic lithium in liquid ammonia was carried out on D-(-)- and L-(+)-N, O, O-trimethylcoclaurine (II and IIa), whose absolute configuration had been clarified in a previous work. (II) and (IIa) were respectively derived to D-(-)- and L-(+)-N, O-dimethylisococlaurine (VII and VIIa), formed by demethylation of the methoxyl group in the 6-position of the isoquinoline ring, and the Ullmann reaction of (VII) and (VIIa) afforded the phenyl ethers (VIII and VIIIa), which were again submitted to cleavage reaction with metallic sodium in liquid ammonia to be derived respectively to l- and d-1-(4-methoxybenzyl)-2-methyl-7-methoxy-1, 2, 3, 4-tetrahydroisoquinoline (IX and IXa) (cf. Table I). The O, O-dimethyl compound of (V), one of the bases obtained by cleavage reaction of cepharanthine (IV), was found to be identical with the above (IXa). This has proved that the absolute configuration of the unidentified asymmetric center at A in cepharanthine (IV) is the L type. It therefore follows that the absolute configuration of the two asymmetric centers in cepharanthine is indicated by (L, D) or (S, R), as shown in formula (IV).