Regioselectivity in oxidative cross-coupling of phenols. Application to the synthesis of dimeric neolignans
摘要:
The problems of regioselectivity in oxidative coupling of phenols are outlined, and recent research on the application to the synthesis of lignans and neolignans is reviewed. Lignans and neolignans are dimers that are typically formed by oxidative coupling of propenyl phenols. The coupling of structurally different phenols (cross-coupling) is an extension of this synthesis. Recent results show that cross-coupling can be achieved if the phenols have compatible reactivities. The regioselectivity is often quite different in cross-coupling compared to dimerization. Coupling of phenols with widely different rates of oxidation is difficult but can be achieved if the concentration of the more reactive phenol can be kept low enough during the reaction, for instance by diffusion through a dialysis membrane. (C) 2000 Elsevier Science Ltd. All rights reserved.
Regioselectivity in oxidative cross-coupling of phenols. Application to the synthesis of dimeric neolignans
摘要:
The problems of regioselectivity in oxidative coupling of phenols are outlined, and recent research on the application to the synthesis of lignans and neolignans is reviewed. Lignans and neolignans are dimers that are typically formed by oxidative coupling of propenyl phenols. The coupling of structurally different phenols (cross-coupling) is an extension of this synthesis. Recent results show that cross-coupling can be achieved if the phenols have compatible reactivities. The regioselectivity is often quite different in cross-coupling compared to dimerization. Coupling of phenols with widely different rates of oxidation is difficult but can be achieved if the concentration of the more reactive phenol can be kept low enough during the reaction, for instance by diffusion through a dialysis membrane. (C) 2000 Elsevier Science Ltd. All rights reserved.
Regioselectivity in lignin biosynthesis. The influence of dimerization and cross-coupling
作者:Kaisa Syrjänen、Gösta Brunow
DOI:10.1039/a907919j
日期:——
We have studied the regioselectivity of oxidative phenol coupling in lignin formation using an oxidation system that distinguishes between dimerization reactions and cross-coupling reactions. We found that the regioselectivity of coupling was different in the two reactions. For instance, in coniferyl alcohol dimerization the formation of β–5 coupling product has a slight prevalence over the formation of β–O-4 product; in cross-coupling the β–O-4 mode is favoured in a ratio of ≈10∶1. This ratio is higher than that found in isolated softwood lignins. The degree of cross-coupling was influenced only to a small extent by changes in the rates of conventional addition of coniferyl alcohol (Zulauf versus Zutropf conditions). We found that diffusion through a dialysis membrane did effectively suppress the dimerization of coniferyl alcohol. Of the different oxidants investigated, manganese triacetate in acetic acid yielded the highest proportion of cross-coupling product.
Regioselectivity in oxidative cross-coupling of phenols. Application to the synthesis of dimeric neolignans
作者:Kaisa Syrjänen、Gösta Brunow
DOI:10.1016/s0040-4020(00)00943-1
日期:2001.1
The problems of regioselectivity in oxidative coupling of phenols are outlined, and recent research on the application to the synthesis of lignans and neolignans is reviewed. Lignans and neolignans are dimers that are typically formed by oxidative coupling of propenyl phenols. The coupling of structurally different phenols (cross-coupling) is an extension of this synthesis. Recent results show that cross-coupling can be achieved if the phenols have compatible reactivities. The regioselectivity is often quite different in cross-coupling compared to dimerization. Coupling of phenols with widely different rates of oxidation is difficult but can be achieved if the concentration of the more reactive phenol can be kept low enough during the reaction, for instance by diffusion through a dialysis membrane. (C) 2000 Elsevier Science Ltd. All rights reserved.