High Pressure Mediated Intramolecular Diels-Alder Reactions of Furans with Unactivated Methylenecyclopropane Terminators

Heterocycles from Alkylidenecyclopropanes

“…The furfuryl derivatives 52, with an allyl ether (X ) O) or allylamine (X ) NMe) type chain linked to an ACP moiety, readily undergo intramolecular Diels-Alder reactions at 10-12 kbar to yield spirocyclopropane-annelated tricyclic structures 53 in excellent yields and with high diastereoselectivity (Table 5). 23 At ambient pressure, 52a did not undergo intramolecular [4+2] cycloaddition even at temperatures up to 150 °C. The use of Lewis acids to promote the Diels-Alder reactions was only marginally successful.…”

Biobased C6-Furans in Organic Synthesis and Industry: Cycloaddition Chemistry as a Key Approach to Aromatic Building Blocks

“…During reflux in benzene over 5 days, the 2-aminomethyl-5-methylfuran derivative with the but-2-ene fragment undergoes intramolecular cyclization with a moderate yield of 35% of the corresponding epoxyisoindoline (Scheme A) . In the case of a more sterically loaded terminal cyclopropane fragment, a similar reaction proceeds with greater difficulty and requires increased pressure (Scheme B) . Introduction of the (phenylsulfonyl)­acrylate moiety to the side chain of the monobenzylated BHMF derivative results in intramolecular cycloaddition with good yield (Scheme C) .…”

“…103 In the case of a more sterically loaded terminal cyclopropane fragment, a similar reaction proceeds with greater difficulty and requires increased pressure (Scheme 5B). 104 Introduction of the (phenylsulfonyl)acrylate moiety to the side chain of the monobenzylated BHMF derivative results in intramolecular cycloaddition with good yield (Scheme 5C). 105 It was shown that during the interaction of 2-(hydroxymethyl)-5-methylfuran with maleic anhydride, ester formation occurs before [4 + 2] cycloaddition (Scheme 5D).…”

Correlation between pressure and steric interactions in organic reactions

“…They involved furans tethered by bicyclopropylidene and methylene cyclopropane moieties (Scheme 8). [35][36][37] When R in 2 and 3 is changed from H to OMe, the pressure sensitivity of the cycloaddition increases ( Table 4). …”