588699-98-3Relevant academic research and scientific papers
Biradicals/zwitterions from thermolysis of enyne-isocyanates. Application to the synthesis of 2(1H)-pyridones, benzofuro[3,2-c]pyridin-1(2H)-ones, 2,5-dihydro-1H-pyrido[4,3,-b]indol-1-ones, and related compounds
Li, Hongbin,Yang, Hua,Petersen, Jeffrey L.,Wang, Kung K.
, p. 4500 - 4508 (2007/10/03)
Thermolysis of benzannulated enyne-isocyanates 13 and enyne-isocyanates 36 and 37 promoted the cycloaromatization reactions to generate in situ O,4-didehydro-2-hydroxyquinolines and O,4-didehydro-2-hydroxypyridines, respectively, as reactive intermediates. These cycloaromatized intermediates could be captured either as biradicals and/or as zwitterions depending on the nature of the substituent at the alkynyl terminus. The intermediate derived from cycloaromatization of 13a bearing a phenyl substituent could be regarded as biradical 14, which then abstracts hydrogen atoms from γ-terpinene leading to 2(1H)-quinolinone 15. Alternatively, the same intermediate could also be regarded as zwitterion 14′, which then undergoes an initial hydride abstraction from γ-terpinene followed by protonation to produce 15. The presence of a 2-phenylethyl substituent in 13b and 37a or a 2-methylphenyl substituent in 37b also allowed the resulting intermediates to be captured intramolecularly either as biradicals or as zwitterions, producing 2(1H)-quinolinone 19, 2(1H)-pyridone 39, and benzopyranopyridine 43, respectively. On the other hand, with a 2-methoxyphenyl, a 2-(dimethylamino) phenyl, or a 3-methoxypropyl substituent, the chemical behavior of the cycloaromatized adduct could be best accounted for in terms of a zwitterionic intermediate leading to benzofuro [3,2-c]quinolin-6(5H)-one (20), 5,11-dihydro-11-methyl-6H-indolo-[3,2-c]quinolin-6-one (25), benzofuro[3,2-c]pyridin-1(2H)-one 44, 2,5-dihydro-2,5-dimethyl-1H-pyrido-[4,3-b] indol-1-one 46, and related compounds. Interestingly, thermolysis of 37f bearing a 2-(methoxymethyl)phenyl substituent at the alkynyl terminus produced the unexpected benzopyranopyridine 56 as the major product in a process involving the cleavage of the bond between the methoxyl oxygen and the adjacent methylene carbon. The efficiency and selectivity of the cycloaromatization reaction could also be enhanced by the introduction of 1.1 to 10 equiv of dimethylphenylsilyl chloride to the reaction mixture to capture the resulting zwitterion.
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 (2007/10/03)
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.
