Immobilization of photocatalytic TiO2 nanoparticles inside PDMS‐based microreactors was successfully attempted by sequential mussel‐inspired surface engineering of microchannels by using CA‐PVP and commercially available TiO2 nanoparticles, respectively. TiO2‐immobilized microreactors accomplished continuous photocatalytic degradation of MB for up to 30 days without releasing TiO2 nanoparticles from the surface of the microchannels, confirming the robustness of TiO2‐immobilization in photocatalysis media. Regeneration of microreactors with diminished photocatalytic activities was also possible with simple strong acid treatment, enabling efficient cleaning of used microreactors.
Abstract:The asymmetric organocatalytic conjugate addition-cyclization reaction of malonates with o-hydroxycinnamaldehydes, which affords 4-substituted chroman-2-ols, has been established using a diphenylprolinol trimethylsilyl (TMS) ether as organocatalyst. The desired products were obtained with good to excellent yields and high enantioselectivities (up to > 99% ee). Synthetically useful chroman derivatives were formed after subsequent reactions.Keywords: asymmetric synthesis; chromans; conjugate addition; malonates; organocatalysts The chroman structural unit is common to many drugs and biologically active natural products.[1] Molecules containing chroman scaffolds exhibit a broad range of bioactivities, such as antiviral, [2] antitumor, [3] antimicrobial, [4] sex pheromone, [5] and central nervous system activities.[6] They can also be used as biodegradable agrochemicals. [7] Their importance has driven the development of numerous synthetic methods for chromans with recent research particularly focused on enantioselective approaches. [8,9] Consequently, the development of an efficient enantioselective synthetic method to obtain chroman scaffolds has attracted our attention. Herein, we disclose the first catalytic asymmetric addition-cyclization reaction of malonates with o-hydroxycinnamaldehydes using an organocatalyst that enables a facile access to enantioenriched 4-substituted chroman derivatives.We recently reported the asymmetric synthesis of chromans from o-hydroxycinnamaldehydes and organoboronic acids using an organocatalyst derived from imidazolidinone (Scheme 1). [10,11] Although arylvinylboronic acids showed good yields with high enantioselectivities in this organocatalytic reaction, arylboronic acids appeared to work with low enantioselectivities. With the aim of development of these findings, we considered the use of other nucleophiles instead of organoboronic acids for the highly enantioselective catalytic production of chroman derivatives. The reaction of malonates as nucleophiles with o-hydroxycinnamaldehydes under the action of an appropriate organocatalyst was assumed to form 4-substituted chroman-2-ols in high enantioselectivity. [12] During this process, the malonate reacts through conjugate addition with an o-hydroxycinnamaldehyde to give a chiral b-substituted aldehyde, which can be hemiacetalized to a chiral 4-substituted chroman-2-ol, [13] if the malonate serves as a nucleophile rather than the OH group of the o-hydroxycinnamaldehyde for the conjugate addition (Scheme 2). [14] The conjugate addition of dimethyl malonate 2a with o-hydroxycinnamaldehyde 1a [9c,15] was selected as a model reaction, and optimizations of catalysts and solvent were carried out, the representative results of which are shown in Table 1. The MacMillan imidazolidinone catalyst I[16] was initially tested in this reaction: the reaction did not produced the corresponding chroman-2-ol 3a and the starting material was almost completely recovered after 24 h (entry 1). When diphenylprolinol TMS ether II as catalyst...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.