Synthesis and easy aromatisation of 5-substituted 6-(alkylthio)-2-methoxy-2,3-dihydropyridines. A new approach to the pyridine ring

Reaction of lithiated methoxyallene, 1-ethoxyethoxyallene, 1-(methylthio)propyne and 2-butyne with methoxymethyl isothiocyanate, MeOCH₂N=C=S followed by methylation affords the imidothioates H₂C=C=C(R)C(SMe)=NCH₂OMe [R=Me, OMe, OCH(Me)OEt, SMe]. Rearrangement to the fully conjugated systems H₂C=CH---C(R)=C(SMe)---N=CHOMe and subsequent electrocyclisation of these compounds leads to the 5-substituted 6-(methylthio)-2-methoxy-2,3-dihydropyridines with good to excellent yields. In the presence of acidic catalysts or by heating at elevated temperatures these dihydropyridines eliminate methanol to afford 3-substituted 2-(methylthio)pyridines. The aroma compound 2-(methylthio)-3-pyridinol was obtained by acid-catalysed treatment of 3-(1-ethoxyethoxy)-2-(methylthio)pyridine.     

Metal‐Catalyzed Cyclization of β‐ and γ‐Allenols Derived from D‐Glyceraldehyde—Synthesis of Enantiopure Dihydropyrans and Tetrahydrooxepines: An Experimental and Theoretical Study

Regiocontrolled metal‐catalyzed preparations of enantiopure dihydropyrans and tetrahydrooxepines have been synthesized starting from β‐ and γ‐allenols derived from D ‐glyceraldehyde. The Pd^II‐catalyzed cyclizative coupling reactions of β‐allenols 1 a and 1 b with allyl bromide effectively afforded enantiopure tetrafunctionalized dihydropyrans through a 6‐endo oxycyclization protocol, whereas the gold‐, platinum‐, and palladium‐mediated heterocyclization of γ‐allenol 2 furnished tetrahydrooxepines 13 – 16 through regioselective 7‐endo‐trig oxycyclization reactions. Moreover, density functional calculations were performed to predict the regioselectivity of the γ‐allenol cycloetherification to tetrahydrooxepines on the basis of both the tether nature and characteristics of the metals, and to gain an insight into the mechanism of the oxycyclization reactions.     

Assessment of mechanistic hypotheses of 1,3-dipolar cycloaddition of (arylsulfonyl)allene to nitrilimines by DFT reactivity indices

The mechanism of 1,3-dipolar cycloaddition between 1-[2-(acetylamino)phenylsulfonyl]-1,2-propadiene 1 and a series of N-(4-substituted)phenyl-C-methoxycarbonylnitrilimines 2 has been reinvestigated on the basis of DFT calculations and a quantitative formulation of the HSAB principle. Comparison of the new computational results with experimental findings led to the formulation of a mechanistic scheme involving both 1,3-prototropic and 1,3-arylsulfonyl shifts. The role of silver cation is also pointed out.