Achievement Of Stereoselectivity In Photodimerization Of Alkyl 2-Naphthoates Via Chiral Auxiliary And Chiral Microreactor Strategies

Enantioselectivity in organic phototransformations continues to be one of the main topics of current interest. Of the various approaches chiral auxiliary strategy and chiral microreactor strategy have shown considerable promise. In the former strategy a removable chiral auxiliary is connected to the prochiral starting substrate by a covalent bond. The chiral auxiliary in this case can bring about asymmetric induction during the photochemical transformation, resulting in diastereomers. The yielding diastereomers can be easily separated even in the case of low asymmetric induction. After the removal of the chiral auxiliary, the enantiomerically pure products can be obtained. In the chiral microreactor strategy inherently chiral hosts or chirally modified confined media are used as microreactors that provide cavities and/or surfaces to accommodate the prochiral substrate molecules and direct the photochemical reaction to proceed in an enantioselective manner. Among the various chiral microreactors thus far explored cyclodextrins (CDs) are most extensively investigated.Previously our group investigated the photodimerization of alkyl 2-naphthoates. Irradiation of these substrates results in four cubane-like phtodimers: head-to-head (HH) or head-to-tail (HT) isomers with syn and anti isomerism, denoted as anti^HH, anti^HT, syn^HT and syn^HH. The former three photodimers each consist of a pair of enantiomers. Our interest in the chirality of these photodimers originate from the fact that the cubane-like skeleton of these photodimers is strained and rigid. Thus, the derivatives of these photodimers might act as chiral ligands, and show applications in asymmetric catalysis and in molecular assemblies. In this thesis we made use of chiral auxiliary strategy and chiral microreactor strategy to achieve asymmetric induction in the photodimerizaiton of 9 alkyl 2-naphthoates with chiral auxiliaries. We are pleased to note that excellent chiral induction in the formation of anti^HH and syn^HT photodimers was obtained.1. Diastereodifferentiating photodimerization of alkyl 2-naphthoates with chiral auxiliaries: We synthesized 9 alkyl 2-naphthoates with different chiral auxiliaries. Irradiation of these 2-naphthoates in organic solution produced four photodimers: anti^HH, anti^HT, syn^HT and syn^HH. Diastereomeric excesses close to 60% were achieved in the formation of anti^HH photodimer.2. Switch and enhancement of diastereoselectivity in the photodimerization of alkyl 2-naphthoates via complexation withγ-cyclodextrin: Irradiation of alkyl 2-naphthoates with chiral auxiliaries included inγ-cyclodextrin in solid state resulted in anti^HH, anti^HT and syn^HT photodimers. Diastereomeric excesses of 94% and 88% in the formation of anti^HH and syn^HT photodimers were obtained respectively. The chirality of anti^HH photodimer is switched in comparison with that of solution photodimerization. As far as we are aware, this is the first example of the switching of product chirality and the best result of chiral induction for photochemical reactions with cyclodextrins as chiral host.