Monensin Diels-Alder reaction

Recent work from Ireland s group directed to the synthesis of monensin subunits takes as well advantage of the suitability of hetero Diels-Alder reactions for generating spiroketals [495]. In situ generation of the highly labile dienophile 7-19 from 7-18, subsequent cycloaddition to acrolein which acted as 1-oxa-1,3-butadiene and reduction yielded the spiroketal 7-21 accompanied by small amounts of diastereomeric byproducts. A mild acid catalysed rearrangement is the next key transformation to the spiroketal subunit 7-20 of monensin 7-22 (Fig. 7-5). 

A total synthesis of the monensin lactone is summarized in Scheme 50. The crucial point of this synthesis is the hetero Diels-Alder reaction of triethylsilyloxy diene (179) with aldehyde (180), carried out with 10 mol % of Yb(fod).i to give the syn Diels-Alder cycloadduct (181) as the only isomer. Compound (181) is treated with HF in pyridine and MeOH to obtain ketone (182), which is converted into the monensin lactone (183). 

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