A related two-step photochemically induced formal retro-[4+2] process was suggested by Zimmermann toexplain ring fissionof 4,4-diphenyldihydropyridin-2-ones,
10 and a similar ring opening process was suggested by Margaretha to occur during the irradiation of dihydrothiin- 3-one S-oxides.
11 The subsequent reaction pathways deviated inthe above-cited studies,but here it seems likely that the two new products 9 and 10 should combine in a Staudinger
cycloaddition reaction to af ford the correspondingblactam.
12 The exclusively transstereochemistry of 8 was attested by NMR spectroscopy,notably the observation of a coupling constant ofabout 2Hz(JH(C3)
–H(C4)
)and NOESY correlations between H(C4)and H(CH@CH2)
,and isconsisten twith the results ofStaudinger reac- tions ofimines and ketenes conducted inphotochemical condi- tions reported independen tly byPodlech
13and Xu.
14Compound8was optically inactive,consistent with the suggested mechanis mfor its formation (Scheme 4).