Templated Enzymatic Synthesis of δ-Cyclodextrin

While alpha-, beta-, and gamma-cyclodextrin (CD) are ubiquitous hosts employed by supramolecular chemists, delta-CD (formed from nine alpha-1,4-linked glucopyranose units) has received very little attention. alpha-, beta-, and gamma-CD are the major products of the enzymatic breakdown of starch by cyclodextrin glucanotransferase (CGTase), but delta-CD forms only transiently in this reaction, as a minor component of a complex mixture of linear and cyclic glucans. In this work, we show how delta-CD can be synthesized in unprecedented yields by employing a bolaamphiphile template in an enzyme-mediated dynamic combinatorial library of cyclodextrins. NMR spectroscopy studies revealed that delta-CD can thread up to three bolaamphiphiles forming [2]-, [3]-, or [4]-pseudorotaxanes, depending on the size of the hydrophilic headgroup and the length of the alkyl chain axle. Threading of the first bolaamphiphile occurs in fast exchange on the NMR chemical shift time scale, while subsequent threading occurs in slow exchange. To extract quantitative information for 1:2 and 1:3 binding events occurring in mixed exchange regimes, we derived equations for nonlinear curve fitting that take into consideration both the chemical shift changes for species in fast exchange and the integrals for species in slow exchange to determine K-a1, K-a2, and K-a3. Template T1 could be used to direct the enzymatic synthesis of delta-CD due to the cooperative formation of a 1:2 complex-the [3]-pseudorotaxane delta-CD center dot T1(2). Importantly, T1 is recyclable. It can be readily recovered from the enzymatic reaction by precipitation and reused in subsequent syntheses enabling preparative-scale synthesis of delta-CD.