Dimethyl acetylenedicarboxylate photocycloaddition

The thermal addition of dimethyl acetylenedicarboxylate to indoles, unlike the photocycloaddition (see Section 3.2.1.4.1.1.), proceeds via a polar stepwise process to yield, initially, 3,4-benzo-2-azabicyclo[3.2.0]hepta-3,6-dienes which in some cases are isolable,13 141 but which, in general, ring open in situ to give the indolylacrylates 3 and/or undergo electrocyclic ring expansion to 1-benzazepines.21... 

Photodecomposition of 1,2-benzisothiazole (17) affords the ring-opened disulfide 7.40 If the 3-position is blocked by substitution, then more interesting products may result. Photocycloaddition reactions of 3-phenyl-1,2-benzisothiazole (23) with alkenes afford excellent yields of 2,3-dihydro-1,4-benzothiazepins such as 24.41 With electron-rich alkynes, thiazabicyclohep-tadienes (25) are produced, but no reaction occurs with 2-butyne, 1-phenylpropyne, or dimethyl acetylenedicarboxylate.42 An analogous S—N bond fission occurs with 2-aryl-1,2-benzisothiazolinones.43... 

Davis PD, Neckers DC (1980) Photocycloaddition of dimethyl acetylenedicarboxylate to activated indoles. J Org Chem 45 456 62... 

Benzonorbornadienes and Related Benzopolycyclics.—Cycloaddition routes to benzopolycyclics have been extended by investigations, which show the synthetic importance of [4 + 4] photocycloaddition, and by further examples of [4 + 2] thermal cycloaddition. Addition of dimethyl acetylenedicarboxylate to 2-substituted anthracenes provides a satisfactory route to substituted dibenzobarrelenes. Increasing methyl substitution in a series of polymethyl-naphthalenes facilitates Diels-Alder addition of maleic anhydride, "" and as expected the kinetically favoured product is the endo-addact but the thermodynamically favoured product is the exo-adduct. Details of the preparation of (347) by a benzyne addition and of further additions of dimethyl acetylenedicarboxylate to substituted naphthalenes are given.In order to investigate their conformational equilibria, (348) and (349) were synthesized by [4 + 2] cycloaddition of benzocyclobutadiene to 2,7-dimethylanthracene and by [4 + 4] cycloaddition of o-xylylene to 2,7-dimethylanthracene, respectively. The estimated barrier to inversion in (349) is 8 kcal mol . ... 

---