Imine additions protic solvents

Dialkyl esters of cystine (39) and lanthionine (40) undergo a surprising thermolysis reaction at between 25 C and 80 °C to afford cis and trans methyl 2-methylthiazolidine-2,4-dicarboxylates (43) in protic solvents. A two stage process is proposed for this transformation. An initial i-elimination reaction gives the thiol (41) and the enamine (42). Thiol addition to the imine tautomer of (42) is then followed by loss of ammonia and an intramolecular cyclisation to give (43) <96CC843>. 

Discussion Catalyst 62 was synthesized in five steps in 86% overall yield from commercially available starting materials, with a single chromatographic purification [68]. The reaction as described by Jacobsen appears to be relatively insensitive to dilution, reagent stoichiometry, and rate of nucleophile addition. Different low-polarity solvents can be used with little effect on product ee-value. In highly polar aprotic solvents, however, the -values were significantly lower, whereas in protic solvents the imine rapidly decomposed. Aliphatic N-Boc imines were not examined due to a lack of useful methods for their synthesis. The inherent elec-trophilicity of the imine was shown to be important, with less-electrophilic N-allyl... 

The Knoevenagel reaction is strongly solvent-dependent. The first step, the formation of the enolate from the 1,3-dicarbonyl and its addition to the carbonyl (or imine) is facilitated in solvents of high polarity and the second step, 1,2-elimination, is inhibited by protic solvents. Thus, dipolar aprotic solvents such as dimethylformamide are especially useful in Knoevenagel condensations. - ... 

The ultraviolet (UV) absorption spectra of the heteroaromatic N-imines are dependent upon many factors, such as the nature of the parent heterocycles, substituents on the imino nitrogen, and solvent used. Usually pyridine N-acylimines show two intense absorption maxima at 230-235 and 330-360 nm in aprotic solvents such as dioxane, benzene, or chloroform, the latter band of which disappears by addition of acid.13,20,21,75,77 In protic solvents such as alcohol or water a hypsochromic shift and a hypochromic effect of the long-wavelength absorption are observed for example, pyridine N-benzoylimine shows a maximum at 352 nm (e 9700) in dioxane but at 313 nm (e 4650) in ethanol. 13,21 The long-wavelength absorption of pyridine N-ethoxycarbonylimine has been ascribed to a n-n transition on the basis of a Pariser-Parr-Pople (PPP) calculation and measurements of the absorption spectra of preoriented pyridine N-imines in polarized UV light.125... 

Insertion of isocyanides into zirconacyclopentanes and -enes is complete within a few minutes at room temperature to afford iminoacyl complexes such as 47 (Scheme 5.15). With most nitrogen substituents (especially MesSi and Ph), these rearrange to the n -imine complexes 50 which can be trapped by protonolysis to afford 51, or by the insertion of alkynes, alkenes, and ketones (see Section The rearrangement is induced by the addition of protic solvents, and so quenching the iminoacyl complexes 47 with methanol gives the amines 51 (in around 50% yield). 

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