3-bromo-2-pyrone: an easily prepared chameleon diene and a synthetic equivalent of 2-pyrone in thermal diels-alder cycloadditions

Asymmetric pyrone Diels–Alder reactions enabled by dienamine catalysis

“…25 These nal examples (45-47) collectively reveal the ability of the method to construct optically active products containing two fully substituted vicinal carbon atoms in a single step. 26 With this body of results in hand, a transition state analysis to account for the stereoselection observed for the process is provided in Fig. 2; here, we have focused in particular on explaining the variations in dr observed based on the structural differences in the two respective partners.…”

Inverse‐Electron‐Demand Diels–Alder Reactions of 2‐Pyrones: Bridged Lactones and Beyond

“…Through reaction of 3-bromo-2-pyrone 125 a with 2,2-dimethyl-1,3-dioxole 132 a and 1,1-dimethoxyethylene 13 a under thermal conditions, lactone 133 was obtained with good endo-selectivity,w hereas lactone 134 was achieved with excellent regioselectivity,a ss hown in Scheme 30. [41] Ta ndem Diels-Alder reactions have attracted much synthetic attentioni nt he past decades because of their powerful potential to rapidly increase molecular complexity in as ingle operation. In 2006, Cho and co-workers reported that 3-bromo-5vinyl-2-pyrones underwent tandem Diels-Alderr eactions with allyl vinyl etherst op repare variousn ovel tetracyclic lactones.…”

“…Primary investigations on thermal IEDDA reactions of 3‐bromo‐2‐pyrone with electron‐rich dienophiles were carried out by Posner and co‐workers in 1991. Through reaction of 3‐bromo‐2‐pyrone 125 a with 2,2‐dimethyl‐1,3‐dioxole 132 a and 1,1‐dimethoxyethylene 13 a under thermal conditions, lactone 133 was obtained with good endo ‐selectivity, whereas lactone 134 was achieved with excellent regioselectivity, as shown in Scheme 30 [41] …”

Diels–Alder Cycloadditions of 5‐Hydroxy‐2‐pyrones: 2H‐Pyran‐2,5‐diones and 5‐(tert‐Butyldimethylsilyloxy)‐2‐pyrones as Synthons

“…Without a Lewis acid, DA cycloaddition between compound 2 and MVK required 1 d in toluene at 80°C for completion and gave a 73 % yield of 21 exclusively in the form of the endo product (Entry 12). [18] Although the thermal DA cycloaddition of 2 is less efficient than the Lewis-acid-promoted DA cyclizations, the silyl enol ether moiety in 21 could provide a useful handle for the development of cascade cyclization reactions.…”