Synthesis of 5,6-dihydropyran-2-ones
A Cu(I)-catalyzed direct vinylogous aldol reaction (DVAR) of β,γ-unsaturated esters and various aldehydes enables a synthesis of chiral α,β-unsaturated δ-lactones. For aromatic aldehydes, a one-pot reaction consisting of DVAR, isomerization of the unsaturated carbon-carbon double bond, and subsequent intramolecular transesterification was required, whereas for α,β-unsaturated and aliphatic aldehydes, the reaction proceeded directly.
H.-J. Zhang, L. Yin, J. Am. Chem. Soc., 2018, 140, 12270-12279.
Highly stereoselective 2-oxonia-Cope rearrangement reactions between rationally designed nonracemic vinylogous aldolation synthons and aldehydes provide δ-hydroxy-α,β-unsaturated esters with excellent enantioselectivities and unprecedented Z- and E-selectivities.
A. Padarti, H. Han, Org. Lett., 2018, 20, 1448-1452.
An AgOTf-catalyzed intramolecular cyclization of phenoxyethynyl diols affords multisubstituted α,β-unsaturated-γ-lactones in good yields under mild conditions. This method was also applicable to the synthesis of α,β-unsaturated-δ-lactones. Replacement of AgOTf with a stoichiometric amount of N-bromosuccinimide furnishes α-bromo-substituted α,β-unsaturated lactones.
M. Egi, Y. Ota, Y. Nishimura, K. Shimizu, K. Azechi, S. Akai, Org. Lett., 2013, 15, 4150-4153.
Highly functionalized δ-lactones were produced in the presence of a catalytic amount of the Cu(OAc)2-DIFLUORPHOS complex through three-component assembly of dialkylzincs, allenic esters, and unactivated ketones. This CAMCR (catalytic asymmetric multicomponent reaction) constructs two C-C bonds and one tetrasubstituted chiral center simultaneously.
K. Oisaki, D. Zhao, M. Kanai, M. Shibasaki, J. Am. Chem. Soc., 2007, 129, 7439-7443.
In a construction of 6-substituted 3-fluoro-5,6-dihydropyran-2-ones under mild conditions, the key step is a Julia-Kocienski olefination.
S. K. Mandal, A. Sarkar, P. Chakraborty, A. P. Chattoapdhyay, Synlett, 2018, 29, 75-78.
A mild, enantioselective hetero-Diels-Alder (HDA) reaction of the Brassard diene with aldehydes in the presence of a chiral titanium(IV) tridentate Schiff-base complexe has been developed. The mechanism is discussed.
Q. Fan, L. Lin, J. Liu, Y. Huang, X. Feng, Eur. J. Org. Chem., 2005, 3542-3552.