Herrn Prof. T. Reichstein zum 80. Geburtstag gewidmet (9.1X.77) Photoinduced Vinylogous B-Cleavage of Epoxy-enones of the Ionone Series SummaryThe photochemistry of the a,/?-unsaturated y , 6-epoxy-enones 1-3 is determined by: (i) C(y)-0-scission of the epoxide (vinylogous /?-cleavage of Type A ) ; (ii) C (y)-C(G)-cleavage of the oxirane (vinylogous P-cleavage of Type B); (iii) (E/Z)-isomerization of the enone chromophore. In contrast, 4 with tertiary C (6) shows no Type B cleavage.Type A cleavage is induced both by n,n*-and n,n*-excitation and arises probably from the TI-state, but Type B cleavage is observed only on x, n* -excitation and represents presumably a S2-reaction.On Type A cleavage 1-4 undergo 1,2-akyl-shifts to 1,5-dicarbonyl compounds (15-18, 25-28, 34 and 35) or rearrange to dihydrofuranes (7 and 30). The isomerization 1 -+ 7 proceeds by a stereoselective [ 1,3]-sigmatropic shift.On Type B cleavage 1-3 isomerize to a bicyclic enol-ether (8, 29) or to a monocyclic enol-ether (9; product of a homosigmatropic [ 1,5]-shift) or undergo fragmentation to isomers such as allenes 10,22 and 31 or cyclopropenes 11 and 21.The non-isolated, unstable (Z)-epoxy-enones# 14, 19, 24 and 38 isomerize by fragmentation to the furanes 12,23,33 and 39 respectively, on contact with traces of acid or by heating. However, for 19 and 4, Type B cleavage may lead to the furanes 23 and 39.On UV. irradiation of the epoxy-enone 4 the initially formed (EI2)-isomers 34 and 35 yield on n, n*-excitation the enones 37 and 40 by a vinylogous P-fragmentation. In addition, on n, n*-excitation 34 isomerizes to 35, which decarbonylates exclusively to the enone 37.The reactions of 1-4 with BF, . 0 (C2H5), were also studied (see appendix). The epoxy-enones 1 and 2 isomerize by an 1,2-alkyl shift in good yield to the 1,4-') Auszugsweise vorgetragen von B. Frei an der Herbstversammlung der Schweizerischen Chemischen Gesellschaft in Bern am 7.Oktober 1977. Schema 3 15(E); 16(Z) 1 7 ( E ) ; 18(Z) 7 12 0 i a 3 4 7 n r n 0 1>347nm/He 6 ) 193 Wurde die Eduktlosung parallel zur Photolyse im Dunkeln aufbewahrt, so trat keine Reaktion ein. 24) Das Rohprodukt stellte der 'H-NMR.-Analyse zufolge ein ausserst komplexes Gemisch dar. Neben dem AB-System des Eduktes liessen sich eindeutig nur H-C(8') und -C(Y) des Dihydrofurans 30 (d bei 5,46 und 5,65 ppm) identifEieren. Fur das Furan 33 konnten weder die Methylsingulette bei 1,31 und 1,98 pprn, noch die olefinischen Wasserstoffatome (AB-System bei 6,20 ppm) nachgewiesen werden. 2s) 25-28 und 31 konnten auch gas-chromatographisch nicht in reiner Form abgetrennt werden.
Die Photoreaktionen der Enone (Ia)‐(Ic) a) bei 254 nm b) bei 347 nm zu den angegebenen Produkten sowie die des Enons (II) unter gleichen Bedingungen werden eingehend studiert (die Photoreaktion von (Ib) bei 2 347 nm ist bereits untersucht worden).
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