Iodotrimethylsilane-Promoted Conjugate Addition of Organocopper Reagents, DDQ oxidation of Arylpropenes and Synthesis of 2-Arylcoumarans
Doctoral thesis, 1998
This thesis is divided into three parts: conjugate additions of organocopper reagents, DDQ oxidations of arylpropenes, and synthesis and steric assignment of 2-arylcoumarans.
The iodotrimethylsilane(TMSI)-promoted conjugate addition of alkylcopper and copper acetylides to certain a,b-unsaturated carbonyl compounds is investigated. The RCu(LiI)-TMSI system is used in 1,4-additions to the optically active enoyl-amides of O-trityl-prolinol. Although the yields of 1,4-adducts were high (90-96%) the diastereoselectivities were only moderate (41-58%). Copper acetylides together with TMSI and LiI are an excellent reagent for 1,4-addition to certain a,.beta.-unsaturated carbonyl compounds. Aldehydes, b-alkoxy enones and a,.beta.-unsaturated ketones with a fixed or preferred s-trans conformation gave yields of the 1,4-adduct in the range of 67-98%. The presence of LiI is required to maintain the high yields. Several Cu(I) salts were studied for generation of the copper acetylides and the best results in the subsequent 1,4-addition were obtained with CuI. A survey of different halosilanes was made and the most efficient additive was TMSI. THF was found to be the best solvent for conjugate additions of alkyl copper and copper acetylides.
A method for the synthesis of cinnamaldehydes by DDQ oxidation of 1-arylpropenes and 3-arylpropenes was developed. The nature of the substituents on the aromatic ring has significant importance for the formation of aldehydes. Electron-donating substituents such as methoxy groups at the aromatic ring promoted the formation of aldehydes (yields ? 80%). The formation of intermediate mono- and bis(3-aryl-2-propenyl) ethers of 2,3-dichloro-5,6-dicyanohydroquinone (DDHQ) was studied by 1H NMR spectroscopy. The 1-arylpropenes gave DDHQ ether intermediates faster than did the 3-arylpropenes. DDQ oxidation of 3-(3,4-dimethoxyphenyl)-1-propene in the presence of methanol gave methyl (E)-3-(3,4-dimethoxyphenyl)-propenoate in 55% yield. DDQ oxidation of 1- or 3-(3,4-dimethoxyphenyl)-1-propene in the presence of acetic acid gives an acylal in high yield [the diacetate of (E)-3-(3,4-dimethoxyphenyl)-2-propene-1,1-diol].
The cis and trans forms of 2-(3,4-dimethoxyphenyl)-3-hydroxymethyl-7-methoxy-2,3-dihydrobenzo[b]fura n were synthesized. These isomers provide a basis for the steric assignment of 2-arylcoumarans by 1H NMR spectral comparisons. Neolignans of the 2-arylcoumaran type reported in the literature were assigned and reassigned.