132 The sequence, outlined in Scheme 1.17, started with alkylation of cyclohexa-1,4-diene ( 244 ) with 3-bromo-1,1-diethoxypropane. Recently, Fujioka reported the fourth synthesis of clavolonine. As a consequence the enzyme's activity and its selectivity may undergo a change. When applying an enzyme and a hydrophilic zeolite together in a liquid phase reaction the zeolite will partly “dry” the enzyme. Here catalyst and sorbent are not in contact.Īnother approach in ester and acetal synthesis is to apply a membrane reactor equipped with a water-selective zeolite A membrane. Interactions between catalyst and adsorbent are avoided when a Soxhlet technique is applied. In the last method, the reactants should fit in the Y zeolite, which moreover should be chosen with a relatively high Si/Al ratio, i.e., relatively hydrophobic, in order to prevent water accumulation in the catalyst. Preferred combinations consist of KA together with a solid acid catalyst: a sulfonic acid resin, silica–alumina or a large-pore zeolite such as HY or rare earth (RE)-Y. Problems may arise, however, from cation exchange leading to catalyst deactivation. Catalyst: chemo-catalyst, enzyme or large-pore zeolite.įor Brönsted acid-catalyzed water-forming reactions – acetalization, esterification – an obvious combination would seem, at first sight, a conventional homogeneous proton catalyst together with zeolite KA. DCKA catalysis has been further developed as a polymer support that is recyclable. This catalyst is an alternative to Lewis acids and is efficient particularly for the conversion of α,β-unsaturated acetals to monothioacetals. Dicyanoketene ethylene acetal (DCKA) is also used to catalyze the monothioacetalization of acetals. In this chapter, no comparison is made between ethers and quaternary acetals. Additionally, tetrahydropyranyl (THP) and methoxymethyl (MOM) ethers are less reactive than benzylic and trisubstituted acetals. Competitive monothioacetalizations demonstrate preferential reaction with benzylic acetals over tertiary carbon centered O, O-acetals which are more reactive than quaternary carbon centered O, O-acetals. Catalytic lithium bromide reacts chemoselectively with acetals to produce the corresponding monothioacetals efficiently. As an alternative, two milder catalytic approaches have been developed as shown in Table 1. Generally, monothioacetals have been synthesized by Lewis acid mediated transacetalization of acetals such as RSH/BF 3♾t 2O, RSH/MgBr 2, Me 2BBr/RSH/Pr iNEt 2, Bu 4− nSn(SPh 2) n/BF 3♾t 2O, and PhSH/Et 3Al.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |