Xiaojun Wang, T. Fang, Xiaofeng Tong
May 27, 2011
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Angewandte Chemie
Abstract
Substituted pyran rings are important structural units found in biologically active compounds and natural products. Given their high importance, many excellent methods have been developed to synthesize substituted pyrans. However, new reactions leading to functionalized pyrans with stereogenic center(s) are still in high demanding and remain challenging, especially in terms of asymmetric catalysis and atom economy. In the past decades, various Lewis bases have been extensively explored as nucleophilic catalysts for application in novel reaction protocols. In this context, Lewis base catalyzed reactions of allenoates have attracted much attention as new synthetic methods for generating diversity and complexity. In sharp contrast to the well-developed phosphine catalysis of allenoates, the corresponding amine analogue is relatively rare and only a few examples, as far as we know, have been reported. For all these cases of amine catalysis, the first mechanistic step is proposed as the addition of the amine catalyst to the allenoate to form zwitterionic intermediates A and B (Scheme 1), which are subsequently captured by various electrophiles such as aldehydes, imines, or activated alkenes. During our study of the phosphine-catalyzed [4+2] annulations, we found that readily available oxodiene 2a worked well as a 1,4-amphiphilic reagent to react with phosphoniumcontaining zwitterions. Thus, we envisioned that compound 2a would also be capable of interacting with ammoniumcontaining zwitterions A and/or B (Scheme 1). Eventually, when a solution of allenote 1 and 2a in toluene was treated with 20 mol% of 1,4-diazabicyclo[2.2.2]octane (DABCO) at room temperature, compound 3,4-dihydro-2H-pyran (E)-3a was isolated in 97 % yield [Eq. (1)]. Therefore, the amine-