Stereochemistry of the Allene-Forming Cycloelimination of 4-Diazo-3,5-Diethyl-3,5-Dimethyl-1-Pyrazoline
Hoover, Dennis Jay
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https://hdl.handle.net/2142/67242
Description
Title
Stereochemistry of the Allene-Forming Cycloelimination of 4-Diazo-3,5-Diethyl-3,5-Dimethyl-1-Pyrazoline
Author(s)
Hoover, Dennis Jay
Issue Date
1980
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Organic
Language
eng
Abstract
The goal of this investigation was the determination of the stereochemical course of the decomposition of diazopyrazoline 1a to 3,5-dimethyl-3,4-heptadiene (2). To achieve this goal, a stereospecific synthesis of 1a which would permit its optical resolution was devised. Cyclization of tosylhydrazine and 4,5-epoxy-5-methyl-3-heptanone (3) provides a mixture of diastereomeric pyrazolols 4a and 5a, which upon oxidation to the
corresponding ketone and selective reduction affords isomer 4a. This racemate was resolved by separation (crystallization or HPLC) of the diastereomeric 1-(1-naphthyl)ethyl carbamates and their trichlorosilane cleavage. Reaction of resolved 4a with trimethylaluminum produces resolved 6a with complete stereospecificity. The synthesis is also equally effective for other racemates 6 (R = Me, n-Pr, n-hexyl, i-Bu, 2-phenethyl, i-Pr and Ph). Methyllithium and methylmagnesium bromide are less effective in converting compounds 4 to compounds 6, as competing elimination occurs. Resolved 6a is converted via the ketone 7a to hydrazone 8a, which upon nickel peroxide oxidation affords diazopyrazoline 1a. 1a is also
generated by the methoxide-induced decomposition of 11a, which was derived via oxime 9a and amine 10a from ketone 7a. The stereospecificity of the decomposition of 3(S),5(S)-1a to (S)-(+)-2, {(alpha)}(,D) = 0.80(DEGREES), is solvent-independent. The configuration of 1a was determined unequivocally by the correlation of (-)-5(R)-4a to (S)-(+)-3-methyl-1-pentyn-3-ol via 4(R),5(S)-3. The configuration of (+)-2 follows from its sign of rotation, its formation ({(alpha)}(,D) = 5.96(DEGREES)) in the reaction of (S)-3-methyl-4-heptyn-3-yl acetate with dimethylcopperlithium, and its conversion to methyl (R)-2-methoxy-2-methylbutyrate (12) upon methoxymercuration, ozonolysis, and
diazomethane esterification. The enantiomeric enrichment of 1a-derived 2 is established at between 2% and 13%, and is estimated 4% from the observed enrichment of 12. This cycloelimination thus proceeds with a conrotation opposite to that previously observed for the diphenyl analog of 1a, and diazocyclopropanes, suggesting that a simultaneous loss of both nitrogen molecules occurs. The reaction of a 2-pyrazoline with peracid is examined. The LiAlH(,4) reduction of compounds 4, bromination of the 1-pyrazoline products, and cyclization of the resulting halohydrins stereospecifically provides pyrazolenine epoxides. A potentially general ('1)H NMR method for e.e. and absolute configuration determination of 1-pyrazolines is discussed.
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