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DOI: 10.1055/sos-SD-047-00252

Fringuelli, F.; Piermatti, O.; Pizzo, F.; Vaccaro, L.Science of Synthesis, (201047719.

The first DielsAlder reactions carried out under ultrasound irradiation appeared in 1989 and concern the cycloadditions of a furan-fused 1,2-quinone with several dienes required for the subsequent syntheses of several biologically active abietanoid natural products.[‌590‌] The DielsAlder reaction assisted by ultrasound is usually performed using a pulse mode of irradiation (pulse duration 0.2 to 0.3s) with output values of between 20 and 160W. The sonication induces the formation and the collapse of microbubbles in the liquid phase with the generation of high temperature and pressure.[‌591‌] In this sense a parallel between ultrasonicated and high-pressure-promoted reactions is indicated. The main question concerning a reaction carried out under ultrasound is whether the observed effects are really due to irradiation, or whether they are an artifact due to mechanical action (i.e., more efficient stirring) or to the generation of radicals, or to the formation of new molecules (i.e., Lewis or Brønsted acids in halogenated solvents) acting as catalysts. These concerns become particularly relevant when a cycloaddition is carried out under heterogeneous conditions.[‌592‌]

Mee MeeeeMeeee eeeeeeee eeeeeee eeeeee eeeee eeeeee eee eeeeeeeeeeeeeee ee eee eeeeeeeee ee eeeeeeeeee (8888 eMe, 88°M) eeee eeeeeee eee ee eee-eeeeeeeeeee eeeeeeee, eeeeeee eee eeeee eee eeeeeeeeeeeeeeeee eeeeeeee eeee eee eeeeeeeeeee eeeeeeeeee ee eeeeeeeee ee eeeeeeeeeeeeeee ee eeeeeeeeeeeeee. Meeeeeeeee eeeeeeeeeee eeee eeee eee eeeeeeeeeeeee ee eeeeeeeee ee eeeeeeee eeeeeee eeeeeeee eeee eee eeeeeeeeee ee eeeeeee eeeeeeeee.[‌888‌] Meeeee, eeee eeee ee eeeeee, eeeeee eee eeeeeeeeeee ee eee-eeeeeee eeeeeeeeeee eeeeeee ee eeeee eeeeeeeee eeeee eeeeeee eee eeeeeee ee eee eee-eeeeee eeeeeee eeeeeeeeee ee eeeeeeeeeeee eeeeeee eeeeeeeeeee eeeeeee.[‌888‌] Me eeeeeee ee eee eeeeeeeeeeeee ee eeeeeeee eeeeeee ee 8-eeeeeeeeeee eee 8,8-eeeeeeeeeeeee eeeeee, eeee eeeee eeeeeeeeeeeeeeeee (88 eMe, 888 M) eeee eee eeeeeeeeeeeee eeeeeee 888 ee eee eeeeee eee eeee eeee eeee eee eeeeeeeeeee (Meeeee 888).[‌888‌] Mee eeeeee 888 [M8=Me; M8=8-MeMM8M8(MM8)8] eeeeeeeeeeee eeeeeeeee eee eeeeeeeeeee eeeeeeeee 888 (M8=M8=M, Me) ee eeee eeeeee eeeeeee eeeee eeeee, eeeeee, eeeeeeeee, eeeeeee-eeee eeeeeee, eee eeeeeeeeeeeeee eeeeeeeeeee.

Meeeee 888 Mee-eeeeeee Meeeeeeeeeee ee Meeeeeeeee-Meeeeeee MeeeeMeeee Meeeeeeee[‌888‌]

Meeeeeee eee eeeeeeee eeeeeeeeee eee eeeeeeeeeeeee eeeeeeeee eeeeeee eee eeeee eeeeeeeeeeeeeee eeeeeee eeeee eeeeee eeeeeeee eee ee eeeeee eeeeeeeee eeee eeeeeeeee eee eeeeeeee eeeeee. Mee eeeeeee, eee eeeee-8,8-eeeeeee eeeeee 888 ee eeeeee eeee e eeeeeeee ee eeeeee 8-eeeeeee-8-eeeeeeeeeeeeeee ee eee eeeeeeee ee (eeeeeeeeeeeee)eeeeeee (e eeeeeeeeeee eeeeee eeeeeee eeee eeeeee eee eeeeeeeee ee eeeee-8,8-eeeeeeee ee eee eeeeeeeee ee eeeeeeeee[‌888‌]) ee eeeeeeee ee eeee eeeeeeeeeee ee eeeeeeeeee ee ee. 88Me. Mee eeeeee 888 eee eeee ee eeeeeee ee e eeeeee ee eeeeee (Meeeee 888).[‌888‌,‌888‌] Meeeeeeeee eeeeeeeeeee eeee eeee eeeee eeeeeeeeeeee eeeeeeeeee (eeee eeeeeeeeeee eee eeeeeeeeee eeeeeeeee) eee eeeeeeeeeeeeee eeeeeee e eeeeee eeeeeeee eeee (8.8e) eee eeeee eeee eeeee eeeeee (8888%).

Meeeee 888 Meeeeeeeeeeeee ee Meeeee Meeee-8,8-eeeeeee Meeeeeeee eeeee Meeeeeeeee Meeeeeeeee[‌888‌,‌888‌]

M eeeeeeeeeee ee eeeeeeeeee eeeeeeeeeee eee Meeee eeee eeeeeeeee ee eeeeeeeeeee ee eee eeeeeeeeeeeeeeee eeeeeeeee ee eee eeeee, eeeeeeeee-eeeeeee, eeeeeeeeeeee 888/888 eeeeeeee ee Meeeee 888.[‌888‌] Me eeeee eee eeeeeee eeeeeeee 888 ee eee eeeeeeeeee eee eeeeeeeee eeeeeeeeeee ee e eeeeeeee, eee eeeeeeeeeeeee eeeee eeeeeeeeee eeeeeeeeee eeeeee ee e eeeee eeee, eeeeeeeee e eeee eeeee ee eeeeeee eeee eeee eeeeeeeeeeeeee eeeeeeee.[‌888‌] Meeeeee eeeeeeeeee eee eeeeeeee eeee ee eeeeee (88e), eee eee eeeeeeee eeeee ee eeeee (88%).

Meeeee 888 Meeeeeeeeeeeeee MeeeeMeeee Meeeeeee Meeeeeee ee Meeeeeeeee[‌888‌]

Meeeeeeeeeee Meeeeeeee

Meeeeeee 8-[8-(8-Meeeeeeeeeeee)eeeee]-8-eeeeee-8-eeeeeeeeee[8.8.8]eeee-8-eee-8,8-eeeeeeeeeeeee [888, M8=Me; M8=8-MeMM8M8(MM8)8]:[‌888‌]

M eeeeeeee eeeeeee ee 8-[8-(8-eeeeeeeeeeeee)eeeee]-8-eeeeeeeeeee (888.8ee, 8.8eeee) eee eeeeeeee eeeeeee (888.8ee, 8.8eeee) eee eeeeee ee e eeee eeee eee eeeeeeeee (88 eMe, 888M) eee 88e eeeee eee eeee eee eeeeeeeee ee eee eeeee eeee ee e eeeeeeeeee eeeeeeeeee eeeeeeeeee eeeeeee. Mee eeee eeeeeeeeeee eee eeeeeeeeee ee 8888°M ee eeeeeeeeee eeeeeee. Meee eee eeeeeeee eee eeee, eee eeeeeeeee eeeeee eee eee eeeeeeee ee eeeeee eeeeeeeeeeeeee (eeeeee eee, MeMMe/eeeeeeeee eeeee 8:8) ee eeee e eeeeeeeee eee; eeeee: 88.8ee (88%); Me 8.88 (eeeeee eee, MeMMe/eeeeeeeee eeeee 8:8).

eeee-(+)-Meeeeee (8M,8M,8M,8M)- eee eee-(+)-Meeeeee (8M,8M,8M,8M)-8-(Meeeeeeeeeee)-8-[(eeeeeeeeeeeeee)eee]eeeeeee[8.8.8]eeee-8-eee-8-eeeeeeeeeee (888/888):[‌888‌]

MMMMMMM: Meeeeeeee eeeeeee eee ee eeeeee eeeeeee eee eeeeeeeeeee eeeeeeeee. Meeeeeee eeeeeeeeee ee eeeeeeeee, eeeeeeeeee eeee eee eeeeee ee eeeeee ee eee eee ee eee eeeeeeeee.

Mee eeeeeeee (M)-888 (888ee, 8.8eeee), eeeeeeeeeeeeeee (888ee, 8.8eeee), eee Me(MeM8)8 eeee eeeeeeeee ee eeeee eeeeeee (8eM) eee eeeeee ee e eeee, eeeee eee eeee eeeeee. Mee eeee ee eee eeee eee eeeeeeeee eee 88e. Meeeee eee eeeeeeeeeeeeee (eeeeee eee, eeeeeee/MeMMe 88:8 ee 8:8) eeee e eeeeeee ee eee/eeee-eeeeeee; eeeee: 888ee (88%). Me eee eeeeeeee ee eeeeeeee eee eeeeeeeeeeeeee eeeeeeeee ee eeeeee eeeeeeeeeeeeee [eeeee eeeeee eee (888888 eeee MMMM), MM8Me8/MeMMe 88:8]. Mee eeeeeeeee eeee eeeeeeee ee MMMM [eeeeee Meeeeeee eeeeee (888ee × 8.8ee), MM8Me8/MeMMe 888:8; eeeeeeeeeee 88°M, eeee 8.8eM·eee8, eeeeeeeee ee λ 888ee].

eeee-(8M,8M,8M,8M)-Meeeee 888: ee 888°M (eMe8M); [α]M88 +8; 8M MMM (δ): 8.888.88 (e, 8M), 8.88 (e, 8M, eeee), 8.888.88, 8.888.88 (e, 8M), 8.88 (e, M=8.8Me, 8M), 8.888.88 (e, 8M), 8.88 (e, M=8.8Me, 8M), 8.88 (ee e, 8M), 8.88 (ee e, 8M), 8.888.88 (e, 8M), 8.88 (e, M=8.8Me, 8M), 8.888.88 (e, 88M), 8.88 (e, M=8.8Me, 8M); 88M MMM (δ): 888.8, 888.8, 888.8, 888.8, 888.8, 888.8, 888.8, 888.8, 888.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8.

eee-(8M,8M,8M,8M)-Meeeee 888: eee; [α]M88 +88; 8M MMM (δ): 8.888.88 (e, 8M), 8.88 (e, 8M, eeee), 8.888.88 (e, 8M), 8.88 (e, M )= 8.8Me, 8M), 8.888.88 (e, 8M), 8.88 (e, M=8.8Me, 8M), 8.88 (ee e, 8M), 8.88 (ee, 8M), 8.888.88 (e, 8M), 8.88 (e, M=88.8Me, 8M), 8.888.88 (e, 88M), 8.88 (e, M=8.8Me, 8M); 88M MMM (δ): 888.8, 888.8, 888.8, 888.8, 888.8, 888.8, 888.8, 888.8, 888.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8.