ISSN:
0009-2940
Keywords:
Singlet carbenes
;
[4 + 1] Cycloadditions
;
Isopyrazoles
;
Diels-Alder reactions
;
[4 + 2] Cycloreversion
;
Chemistry
;
Inorganic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Nucleophilic Singlet Carbenes in the [4 + 1] Cycloaddition with 1,2,4,5-Tetrazines: a New Synthesis of IsopyrazolesA series of 3,6-disubstituted 1,2,4,5-tetrazines 6, including C6H5, SCH3, SO2CH3, N(CH3)2, CF3, CO2CH3 groups, has been submitted to [4 + 1] cycloaddition with the nucleophilic singlet carbenes 4, 15, and 20, which are generated from the precursors 1, 11 and 19, resp. In all cases isopyrazoles (4H1-pyrazoles) 9, 10, 18, and 21 are isolated in good yields. They are formed in a two-step reaction sequence with the [4 + 1] cycloadducts of type 8 as intermediates which eliminate nitrogen by subsequent [4 + 2] cycloreversion. The acceptor-substituted isopyrazole 9e is characterized as an electron-deficient diene by some Diels-Alder reactions with inverse electron demand leading to the expected azo-bridged cycloadducts 23, 25, 27, 29, and 31 without acid catalysis or application of higher pressure. With cyclooctyne (32), the [4 + 2] cycloaddition is followed by a [4 + 2] cycloreversion with formation of the cyclopentacyclooctene 34.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/cber.19931260325
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