THF/DMPU

The monolithium salt of 4-hydroxy-4-(phenylethynyl)-2.5-cyclohexadienone (12), prepared in situ by the addition of lithium acetylide to /7-benzoquinone, was treated with methylmagnesium chloride in l HF-TMEDA or in THF —DMPU. The syn-, 4-addition adduct 13, derived from intramolecular delivery of the carbon nucleophile by the hydroxy oxygen, as well as the <7s-1,4-diol 14, obtained via intermolecular 1,2-addition, were obtained in varying amounts depending on the conditions. The selectivity on 1,4- to 1,2-addition increased by the addition of cation chelating agents such as DMPU, TMEDA, and 15-crown-5. Although the 1,4 to 1,2... 

THF/DMPU mixture. The attack of the Reformatsky reagent from the less hindered exo face dictates the overall stereochemical outcome. 

Nitropropanoic acid esters can be converted into acrylic acid derivatives by treatment with a slight excess of DBU in THF or DMSO at room temperature. If 3-nitro-propanoates are treated with two equivalents of LDA at -78 °C in THF/DMPU or THF/HMPA, however, the resulting dianion can be cleanly C-alkylated, and the product isolated without elimination of the nitro group [160, 161] (Scheme 5.57). Without the addition of a cosolvent (DMPU, HMPA, or quinuclidine N-oxide [161]) alkylation of the 3-nitropropanoate dianion does not proceed sufficiently quickly [160,161],... 

Fig. 13.17. Highly "Z"-selec-tive generation of ester enolates in a THF/DMPU solvent mixture (DMPU, /V,/V -dimethyl-propyleneurea). The transition state A of this deprotonation with a metal-free diisopropy-lamide anion (in solution) corresponds to the calculated transition state B of the deprotonation of propionic aldehyde with a hydroxide anion (in the gas phase).

Fig. 10.14. Highly Z -selective generation of ester enolates in a THF/DMPU solvent mixture. DMPU, N,N -dimethylpropyleneurea.

As shown in Table 16, (i )-propargyl mesylate (ee > 95%) reacts with cyclohexanecarboxaldehyde in the presence of Ini and 5 mol% of a palladium catalyst to give, via an allenylindium intermediate, the adduct with high enantiomeric excess.55,56 The addition is most efficient in 3 1 THF-HMPA and 1 1 THF-DMPU solvents. The Anti syn ratios are excellent with a-branched aldehydes but only modest with unbranched and conjugated aldehydes. 

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