588699-90-5Relevant articles and documents
Cascade cyclizations via N,4-didehydro-2-(phenylamino)pyridine biradicals/zwitterions generated from enyne-carbodiimides
Li, Hongbin,Petersen, Jeffrey L.,Wang, Kung K.
, p. 5512 - 5518 (2003)
Thermolysis of the enyne - carbodiimides 7 having the central carbon - carbon double bond incorporated as part of the cyclopentene ring favors the formation of the corresponding N,4-didehydro-2-(phenylamino)pyridine intermediates, either as the σ,π-biradicals 8 or as the zwitterions 8′, for subsequent synthetic elaborations. By placing appropriate substituents at the acetylenic terminus, a variety of the intramolecular decay routes are available for the initially formed σ,π-biradicals/zwitterions, leading to the 5,6-dihydrobenzo[c][1,8]naphthyridine 21, the 1,2,3,4-tetrahydro[1,8]naphthyridine 24 and related compounds 25 and 26, the 5,6-dihydrobenz[f]isoquinoline 28, and the benzofuro[3,2-c]pyridine 30. Surprisingly, the use of the dimethylamino group of the 2-(dimethylamino)phenyl substituent to capture the carbocationic center in the zwitterion 8e′ furnished the 5H-pyrido[4,3-b]indole 32 in only 14% yield. The majority of the products were the 1H-pyrrolo[2,3-b]quinolines 34 and 35, isolated in 48 and 7% yields, respectively. However, it was possible to redirect the reaction toward 32 by conducting thermolysis of the enyne - carbodiimide 7e in the presence of 5 equiv of dimethylphenylsilyl chloride. Under this reaction condition, the 2-pyridone imine 37 was isolated in 86% yield, which on exposure to silica gel was converted to 32 in essentially quantitative yield. Thermolysis of the enyne - carbodiimide 42 having a methoxymethyl substituent at the acetylenic terminus led to the formation of 46′ as a pyridine analogue of orthoquinone methide imine. An intramolecular hetero-Diels - Alder reaction of 46′ then furnished the tetrahydro[1,8]naphthyridino[2,1-c][1,4]benzoxazine 47.
