Elucidation of the racemization mechanism of the .ALPHA.-hydroxy ketone moiety (C9-position) of optically active anthracyclinone derivatives.
作者:KATSUMI TAMOTO、SHIRO TERASHIMA
DOI:10.1248/cpb.32.4340
日期:——
In order to discriminate the possible racemization mechanisms shown in Chart 1 (for (S)-(-)-1a) for the α-hydroxy ketone moiety (C9-position of the optically active anthracyclinones ((S)-(+)-1a-c), some plausible intermediates ((±)-2 and -3) and their equivalent ((R)-(-)-12) were first synthesized. Thus, the tertiary alcohol ((R)-(-)-12) was prepared from the 1'(S), 2 (R)-diol ((-)-10) according to the reaction scheme shown in Chart 2. The isomeric seven-membered α-hydroxy ketones ((±)-2 and -3) were elaborated from the 1, 4-dihydronaphthalene (13), following the synthetic scheme shown in Charts 3 and 4 based on Dieckmann condensation, dihydroxylation, regioselective enol acetate formation, and oxidation as key steps. By subjecting the plausible intermediates ((±)-2 and -3, 5, and (R)-(-)-12) to the racemization conditions, the facile loss of optical integrity observed for (S)-(+)-1a-c was found to proceed through the ring-expanded seven-membered α-hydroxy ketones ((±)-2 and -3 for (S)-(+)-1a, which might be produced by equilibrium C (Chart 1).
为了研究光学活性蒽环酮((S)-(+)-1a-c)中α-羟基酮部分(C9位)可能的外消旋化机理(如图1中(S)-(-)-1a所示),首先合成了一些可能的中间体((±)-2和-3)及其等价物((R)-(-)-12)。叔醇((R)-(-)-12)是按照图2所示的反应路线,从1'(S),2(R)-二醇((-)-10)制备得到。异构的七元环α-羟基酮((±)-2和-3)是以1,4-二氢萘(13)为原料,通过Dieckmann缩合、双羟基化、区域选择性烯醇乙酸酯形成和氧化等关键步骤,按照图3和图4所示的合成路线制备得到。将可能的中间体((±)-2和-3,5和(R)-(-)-12)在外消旋化条件下反应,发现(S)-(+)-1a-c的光学纯度容易丧失是通过扩环形成的七元环α-羟基酮((S)-(+)-1a的(±)-2和-3)进行的,这可能是通过平衡C(图1)产生的。