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37.4.1.6.2 Method 2: High-Pressure Diels–Alder Cyclizations of Furans and Activated Alkenes

DOI: 10.1055/sos-SD-037-00286

Ding, K.; Wang, Z.Science of Synthesis, (200837521.

Under ambient conditions, the cycloadditions between furans and less activated alkenes often take place slowly and with very poor to mediocre yields. High reaction temperatures cannot be used to accelerate these sluggish reactions because cycloaddition products derived from furan are generally susceptible to thermal cycloreversion to the starting materials. The use of high pressure, on the other hand, is a valuable technique to promote these reactions that have large negative activation volumes. Typically, a compressible sample tube containing a dichloromethane solution of furan and a dienophile is kept under hydrostatic pressure (1.5 GPa) at room temperature in a hydraulic press for a specified period of time. Good yields of cycloadducts 149 can be obtained using otherwise inert dienophiles (Scheme 63). For the reaction of acrylic dienophiles with furan, the use of high pressure generally affords the cycloadduct in higher yield within a shorter reaction time as compared to normal thermal conditions. Moderate yields are also realized in the reaction of propenal or methyl vinyl ketone with furan to give products 149 (R1=R2=H; R3=Ac, CO2Me) (Scheme 63). However, the crotonic or methacrylic dienophiles only give poor yields of cycloadducts 149 (R1 or R2=Me, respectively), presumably owing to the steric hindrance and electronic deactivation induced by the methyl groups. For dienophiles with two ester groups, excellent yields of cycloadducts 149 (R1=H; R2=R3=Ac, CO2Me) are obtained. For each case of the high-pressure reaction, approximately 1:1 ratios of exo/endo isomers are produced. A limitation to this procedure is that the 1.5 GPa apparatus is not commonly accessible in many laboratories. Although the general procedure for this high-pressure process is known, the detailed workup of the cycloadducts is not reported.[‌94‌]

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