Dihydropyrones, formation

Another enantiospecific synthesis of longifolene shown in Scheme 13.32 used an intramolecular Diels-Alder reaction as a key step. An alcohol intermediate was resolved in sequence B by formation and separation of a menthyl carbonate. After oxidation, the dihydropyrone ring was introduced by 7-addition of the ester enolate of methyl 3-methylbutenoate, followed by cyclization. 

The kinetic control approach to constructing the CD-spiroacetal segment envisaged a hetero-Michael cyclisation of the dihydropyrone 28 (derived from 26 by sequential oxidation to the P-diketone, PMB removal and cyclisation to provide the D-ring), where axial attack might be favoured (Scheme 7). In practice, treatment of 28 with DBU led to installation of the C-ring via hetero-Michael reaction, with a small preference (60 40) for formation of the desired spiroacetal 29 over 30. Despite the modest selectivity observed in this mode of spiroacetal formation, the endeavour highlighted an important new means for... 

A clear two step formation of a pyrone by an enantioselective Mukaiyama-aldol and acid catalysed aldol dihydropyrone annulation using aliphatic and aromatic aldehydes and l-methoxy-3-trimethylsilyloxy-l,3-butadiene in the presence of a tryptophan-derived oxazaborolidine was described by Corey et al. [115]. The resulting pyrone which could be assigned as a formal Diels-Alder adduct was obtained with a 67-82% ee and 57 -100% yield. 

Dihydropyrones are valuable intermediates for the synthesis of a variety of substituted tetrahydropyran rings. Recently, stereoselective aldol reactions of p-chlorovinyl ketones using the dienol boronate derivative derived from chiral IpC2BOTf was utilized for enantioselective formation of dihydropyrones. No detectable racemization was reported on the cyclization step (eq 4). ... 

Formation of Stereogenic Centers inside the DihydropYrone Ring ... 

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