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.

We report on a Nd:YLF laser that operates at 1053 nm without optical intracavity elements for the suppression of the stronger 1047-nm transition. The Nd:YLF crystal is end pumped by a fiber optically coupled 10-W diode-laser bar. The different thermal-lensing focal lengths of the two main lasing wavelengths in a plane-parallel resonator were used to achieve the selection by tilting the end mirror slightly from its optimum position for maximum output power. With 9.8-W cw diode-laser-pumping power the 1053-nm Nd:YLF laser produces a maximum output power of 1.9 W in cw operation and nearly 1 W of average power at a Q-switch repetition rate of 15 kHz. The highest slope efficiency of 47% achieved in cw operation.

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In memoriam Hans Heusser (8.VI. 83) SummaryOn singlet excitation (A = 254 nm, acetonitrile) the diepoxydiene (E)-7 undergoes photocleavage to the carbonyl ylide VII and the carbenes X and XI. The carbonyl ylide VII rearranges to the thermally labile dioxabicyclo [3.2.1 Ioctene 20 or fragments via VIII to the aldehyde 9 and propyne. The carbene X, showing behaviour typical of vinyl carbenes, undergoes addition to the adjacent double bond furnishing the cyclopropene 11. The carbene XI, however, undergoes an insertion reaction into the neighbouring oxirane C, C-bond leading to the oxetene (E)-21 which can be isolated at -78", but at room temperature is rapidly transformed to the aldehyde 10.On triplet excitation (acetone, A > 280 nm), however, (E)-7 shows the typical behaviour of epoxydienes, undergoing C, 0-cleavage of the oxirane and isomerization to 22,23 and (E/Z)-24.

On flash vacuum thermolysis at temperatures between 390 and 585", the epoxyenones 1-9 and the epoxydienes 10-12 undergo various types of reactions involving C-C and/or C-0 bond cleavage in the oxirane ring. Thus, the compounds 1,4-9,11, and 12 were transformed to the divinyl ethers 13,20,21,24,25,29, and 38 by a reversible [I ,5] homosigmatropic H-shift. On thermolysis of the epoxides 1-12, several products formed via carbonyl-ylide intermediates were also isolated. The extent of the formation of ylide products is clearly related to the conjugating ability of the functional groups neighboring the oxirane. Thus, the epoxides 3, 5, and 7-10, bearing a C(3)=C (4) bond, a 5-0x0 function, a 3,4-epoxy or a 3,4-methano group, preferentially underwent reactions via a carbonylylide intermediate. As a further reaction pathway, the epoxides 1-12 undergo cleavage of the C-0 bonds of the oxirane, which, however, is presumably an acid-catalyzed rather than a thermal reaction.