The intramolecu lar [2+2] cycloadditio nof2-pyridones togive bicyclicb-lactams was firstdiscovered byCorey and Streith,
17and these strained intermedi ates have been used tomake cis-divi-nylb-lactams through ring-openi ngcross metathesis.
18Photorear-rangement ofdihydrothiin- 3-ones tothietan-3- ones has been described ,again via astrained bicyclic intermedi ate (asulfura- nyl-alkyl biradical).
19However ,the photoisome rization ofdihy- dropyrid in-3-ones via the tandem [4+2]-cyc loreversion/[2+ 2]cycloaddi tion process described here is,tothe best ofour knowl- edge,unprecedented for heterocyclic systems.
20Scheme