engleski

Spyrobipyridopyrans and indolinospiropyridopyrans : synthesis, X-ray crystal structure, separation of enantiomers and barriers to thermal cleavage of the C(sp3)- O bond

The novel chiral spirobipyridopyrans 1 and 2 were synthesized by the acid catalysed aldol type condensation of 5-deoxypyridoxal with the appropriate ketone and subsequent reaction of the resulting pyrilium salt with base. The indolinospiropyridopyrans 3-5, which contain modified B6 unit, were prepared by aldol reaction of 5-deoxypyridoxal with 1, 3, 3- trimethyl-2-methylen-indolines. Analytical separation of enantiomers was accomplished by low-pressure liquid chromatography (LPLC) on triacetylcellulose. The barriers to thermal racemization were determined by on-line measurements of the enriched enantiomers after LPLC. Gibbs energies of activation deltaG# for reversible cleavage of the Cspiro-O bond in 1, 3 and 4 were found to be in the range 108-103 kJ/mol. The lower limits of the barriers in 2 and 5 were estimated to be greater than 102 and 109 kJ/mol by an attempted thermal racemization. The increase of the barriers from 3 to 4 and 5 was explained by the influence of electron withdrawing groups to decreased stability of the ring-opened transition states to C(sp^3)-O bond cleavage. Geometrical data from X-ray structure analysis showed that the angle /C3-C2-C3´/ around the spiro carbon atom increases with elongation of the chain in the C3-C3´ bridge. This angle widening is explained by ring strain effect which is greater in the fivemembered ring involved in the skeleton of 16 than in the six- and sevenmembered ring of 1 and 2

spyrobipyridopyrans ; indolinospiropyridopyrans ; synthesis ; x-ray crystallography ; separation of enantiomers ; barriers to racemization

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