Cyclodextrins ring opening reactions

Other compounds smdied as chiral catalysts include a- and p-cyclodextrins that were used in the hydrolysis of oxazolones although the enantioselectivity in the ring-opening reaction was rather low. When a phenyl group is present at C-2 in these systems the enantioselectivity of the reaction is somewhat higher. 

Ring-opening reactions of aziridines (and epoxides) typically require the use of a Lewis acid to catalyze the reaction. P-Cyclodextrins (Ji-CD) are very useful in creating microenvironments in which aziridines can be opened using mild conditions. The reaction of aziridines with (3-CD and sulfur nucleophiles such as thiocyanate <05SL489> or thiophenols <05TL6437> provides a mild route to such ring-opened compounds. 

Consult [4] and compare other ring-opening reactions of oxiranes, e.g. the p-cyclodextrin-catalyzed aminolysis [5] and problem 3. For an enantioselective ring-opening of oxiranes utilizing the JACOBSEN catalyst (problem 1) see [6]. 

Ring-opening with heteroatomic nucleophiles is certainly among the most thoroughly studied behavior of epoxides, and this reaction continues to be a versatile workhorse of synthetic utility. This is exemplified in the recent literature by the examples of the p-cyclodextrin-catalyzed aminolysis of simple epoxides by aniline derivatives (i.e., 53 - 54) <00SL339> and the synthesis of oxa-azacrown ethers through the treatment of Ws-epoxides 55 with diamines 56. Yields in the latter synthesis are sensitive to the size of the macrocycle and substitution on the bis-epoxide <00TL1019>. 

Cyclodextrins are formed in the degradation of starch and dextran by the action of cyclodextrin/glucanosyltran-sferases [EC 2.4.1.19]. These enzymes also catalyze so-called acceptor reactions in which the cyclodextrin ring is opened and an acceptor molecule (e.g., glucose) is added to the reducing end of the maltodextrin chain. 

The inclusion complexation of spiropyrans in cyclodextrins has also been explored as a means to control photochromic reactions.1591 Distinct differences in complexation of sulfonic acid-modified spiropyrans to various cyclodextrins were observed and the closed spiropyran form bound to (3-cyclodextrin was stable towards photochemical ring-opening. 

The cyclodextrin ring does not have a well-defined depth, so we adopted a simple procedure to turn it from an open tube into a cup. We attached some flexible groups on the primary carbons of the cyclodextrin that could partially invade the cavity and produce a flexible floor.Later we studied acylation reactions with cyclodextrins that had been modified to include a floor on the bottom of the cavity - and also with improved substrates and a somewhat better solvent - that led to a relative rate of 750,000 compared with the uncatalysed hydrolysis rate at the same pH. When we looked at related compounds in which we had frozen out one of the degrees of freedom of the substrate the improvement came up to a 3,200,000-fold acceleration compared with the hydrolysis rate in water, and with some selectivity among the geometries of the complex. ... 

Scheme 8 Functionalization of p-cyclodextrin v a DMAP catalyzed ring-opening polymerization of D,L-lactide. Top 30-60 min reaction at 120 °C and 24 h reaction in dichloromethane.

The CGTases also catalyze acceptor or transfer reactions between the cyclic maltodextrins and a carbohydrate acceptor [31,32]. In these acceptor reactions, the cyclodextrin ring is opened and the acceptor is specifically added to the reducing end of the maltodextrin chain that is formed from the cyclodextrin. The reaction is as follows ... 

---