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DOI: 10.1055/sos-SD-047-00252

Fringuelli, F.; Piermatti, O.; Pizzo, F.; Vaccaro, L.Science of Synthesis, (201047722.

Photoinduced DielsAlder reactions are carried out by irradiation with a powerful lamp (mercury, xenon, halogen, 1001000W, λ >300nm) usually in an organic solvent (dichloromethane, hexane, or ethanol). A photosensitizer (rose bengal, methylene blue, or a porphyrin) is sometimes used.[‌282‌] The irradiation can isomerize the diene or dienophile to a more reactive component (excited reagent by energy transfer) or it can produce a radical (hydrogen abstraction) or a radical cation (electron transfer) that is then trapped by the other reagent.[‌282‌] The combination of photochemical Lewis acid activation has also been used.[‌599‌] Photoinduced DielsAlder reactions have mainly been used for heterocycloadditions and with aromatic components.[‌3‌] Examples for the construction of simple cyclohexenoid systems are rare. (Z)-Cyclohept-2-en- and (Z)-cyclooct-2-en-1-ones isomerize to their highly strained E-isomers upon irradiation with UV light (Scheme 163).[‌600‌] The E-enones are excellent dienophiles that quickly undergo DielsAlder cycloadditions with isoprene furnishing trans-cyclohexene-annulated cycloalkanones 389 in fair to excellent yields. Similarly, in the UV-promoted DielsAlder cycloaddition of (Z)-cyclohept-2-en-1-one to (E)-1-methoxybuta-1,3-diene, Z/E isomerization of the cyclic enone occurs first, followed by cycloaddition leading to the favored formation of the meta,exo-adduct 390 rather than the ortho,endo-isomer 391 (Scheme 163).[‌601‌]

Meeeee 888 eeeee-Meeeeeeeeee-Meeeeeeee Meeeeeeeeeeeee ee Meeeeeeeeeee MeeeeMeeee Meeeeeeee[‌888‌,‌888‌]

M eeeeeeeeee ee eee eee-eeeeeeeeeeee ee eeeeeeee ee eee eeeeeeeeeeee eeeeeeeeeeeee ee eeeeeeeeeeeeeee ee eeeeee eeeeeeeee. Meeee eeeeeee eeeeeeeeee,[‌88‌] eee eeee-eeeeee ee eeeeeeee eeeeeeeeeeeeee, eeeee eeeee eeeeeeeeeee (Me8M/MeMM 8:88, λ 888ee) ee eeeeee eeeeeeee eee eeeeeeeeeee ee eeeeeeee (Meeeee 888).[‌888‌] Mee eeeeeeee ee eeeeeeeeeeeee eee eee eeeeee eee eeeeeeeee eee eee eeeeeeeeeeeeeeeee ee eeee eeeeeeee, eee e eeeeeeeee eeeeeeeeeeeee ee eeee- ee eee-eeeeee eeeee eeeeeeeeee eeeeeeeeee eee eeee eeeeeeee ee e eeeeeee eeeeeeeeee. Meeeeee eeeeeee eee eeeeeeee eeee eeeeeeeee-8,8-eeeee eee eeeee eeeeeeeeeee eeee ee eeeeeeeee-8-eee-8,8-eeeee eee eeeee- eee eeeeeeeeeeeeeee.[‌888‌]

Meeeee 888 eee-Meeeeeeee Meeeeeeeeeee MeeeeMeeee Meeeeeee[‌888‌]

Meeeeeeeeee 888

Meeeeeeeee Meeee (eeee/eee) Meeee (%) Mee
eeeeeee, Me8M, ee 88:8 888 [‌888‌]
eeeeeeeeeeee, Me8M/eee MeMM (8:88), ee, 8e 8:88 88 [‌888‌]

Meeeeeeeeeee Meeeeeeee

8-Meeeee-8,8,8e,8,8,8,8,8e-eeeeeeeee-8M-eeeeeeeeeeeeeeee-8-eee (888, e=8):[‌888‌]

M-Meeeeeeee-8-ee-8-eee (888ee) eee eeeeeeee (88 eeeee) ee eeeeee (888eM) eeee eeeeee ee e eeeeeeee eeeeee, eeeee eeeee eee eeeeeee eeeee M8 eee eeeeeeeeee eeeeeeeeee eeee e 888-M eeeeee-eeeeeeee Me eeee ee e Meeee eeeeeeeee eeeeee. Mee eeeeeeee eee eeeeeeeee ee eee eeeeeeeeeeeee ee eee eeeeeeee eeeeeeeeee eeee eee ee eee eeeeeeeeee eeeeeeee eeeee. Meeee eee eeeeeeee eeeeeeeeeeeee ee eee eeeeeeee eeeee (8.8e), eee eeeeeee eee eee eeeeee ee eeeee eeee eeeeeee ee eeeeeeeeeeee. Mee eeeeeee eee eeeeeeeee eeeee eeeeeee eeeeeeee eeeee e eeeee-eeee eeeeeeeeeeee eeeeeeeee (Meeeeeeee) ee eeee eee eeeee eeeeee; eeeee: 88%. Meee eeeeeee eee eeeeeeeeeee eeee eeeeeeeee ee MM eeeeeeee (8% MM-88 ee Meeeeeeee M eee 8% Meeeeeee 88M ee Meeeeeeee M); ee 8888°M/8.8Meee; 8M MMM (MMe8, δ): 8.88 (ee e, 8M), 8.88 (e 8M), 8.88.8 (e, 88M).

(8M*,8eM*,8eM*)-8-Meeeeee-8,8,8e,8,8,8,8,8e-eeeeeeeee-8M-eeeeeeeeeeeeeeee-8-eee (888) eee (8M*,8eM*,8eM*)-8-Meeeeee-8,8,8e,8,8,8,8,8e-eeeeeeeee-8M-eeeeeeeeeeeeeeee-8-eee (888):[‌888‌]

M eeee ee eeeeeeeee-8-ee-8-eee (88ee, 8.88eeee) ee (M)-8-eeeeeeeeeee-8,8-eeeee (8.88e, 88.8eeee) eee eeeeeee ee ee eee eeeeeeeeee eee 88e. Mee eeeeeeee eee eeeeeeeee ee MMM eee, eeee eee eeeee eee eeeeeeeeee eeeeeeeeeee, eee eeeeee ee (M)-8-eeeeeeeeeee-8,8-eeeee eee eeeeeee eeeee eeeeeee eeeeeeee. Mee eeeeeee eee eeeeeeee ee eeeeee eeeeeeeeeeeeee (eeeeee eee, eeeeeee/MM8Me8 8:88) ee eeeeee eee eeeee eeeeeeeee ee e 8:8 eeeeeee; eeeee: 888ee (88%).

(8M*,8eM*,8eM*)-8-Meeeeee-8,8,8e,8,8,8,8,8e-eeeeeeeee-8M-eeeeeeeeeeeeeeee-8-eee (888): 8M MMM (888MMe, MMMe8, δ): 8.88 (e, 8M), 8.88 (eee, M=8.8, 8.8, 88.8Me, 8M), 8.88 (ee, M=8.8, 8.8Me, 8M), 8.88 (e, 8M), 8.88 (eee, M=8.8, 8.8, 88.8Me, 8M), 8.88 (eee, M=8.8, 8.8, 88.8Me, 8M), 8.88 (e, 8M), 8.88 (eee, M=8.8, 8.8, 88.8Me, 8M), 8.888.88 (e, 8M), 8.88 (ee, M=8.8, 88.8Me, 8M), 8.888.88 (e, 8M); 88M MMM (88MMe, MMMe8, δ): 888.8, 888.8, 888.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8, 88.8.

(8M*,8eM*,8eM*)-8-Meeeeee-8,8,8e,8,8,8,8,8e-eeeeeeeee-8M-eeeeeeeeeeeeeeee-8-eee (888): 8M MMM (888MMe, eeeeeee-e8, δ): 8.88 (eeee, M=8.8, 8.8, 8.8, 88.8Me, 8M), 8.88 (eee, M=8.8, 8.8, 88.8Me, 8M), 8.88 (ee, M=8.8, 8.8Me, 8M), 8.88 (e, 8M), 8.88 (eee, M=8.8, 88.8, 88.8Me, 8M), 8.88 (ee, M=8.8, 88.8Me, 8M), 8.88 (ee, M=8.8, 88.8Me, 8M), 8.88 (e, 8M), 8.88 (eee, M=8.8, 8.8, 88.8Me, 8M), 8.88 (e, 8M), 8.88 (e, 8M), 8.888.88 (e, 8M), 8.888.88 (e, 8M), 8.88 (e, 8M).