Diastereoselective synthesis of some novel benzopyranopyridine derivatives

BACKGROUND The formation of novel N-substituted-1,2,3,4-tetrahydro[1,3]-dioxolo-[6,7]-5H-[1]benzopyrano [3,4-c]pyridines were observed unexpectedly during the acid-mediated ketal removal of ethylenedioxy ketal protected 4-piperidones. The literature revealed that benzopyranopyridine derivatives are of scientific interest and some exhibit interesting biological activities. Diastereomeric resolution was utilized to isolate optically pure chiral molecules. RESULTS The acid catalyzed deprotection of N-substituted-4,4-ethylenedioxy-3- [(1,3-benzodioxol-5-yloxy)methyl]piperidines, prepared by condensation of the corresponding phenols and mesylate derivatives, unexpectedly resulted in cyclodehydration leading to new benzopyrano derivatives, N-substituted-1,2,3,4-tetrahydro[1,3]-dioxolo-[6,7]-5H-[1]benzopyrano [3,4-c]pyridines. The process involves the deprotection of the carbonyl protecting group, and then the cyclization reaction occurs followed by dehydration to give the final product.These N-substituted-1,2,3,4-tetrahydro[1,3]-dioxolo-[6,7]-5H-[1]benzopyrano [3,4-c] pyridines were dealkylated giving the corresponding N-unsubstituted derivatives. The cis-1,3,4,4a,5,10b-hexahydro-[6,7]-2H-[1]benzopyrano [3,4-c]pyridine derivative was also obtained from the N-benzylated-1,2,3,4-tetrahydro[1,3]-dioxolo-[6,7]-5H-[1]benzopyrano [3,4-c]pyridine via catalytic hydrogenation. The resolution of the enantiomers was carried out using D-(-)-mandelic acid as chiral reagent. The absolute configuration of the S,S-mandelate salt derivative was determined by X-ray crystallographic analysis. CONCLUSION The approach led to the construction of N-substituted-1,2,3,4-tetrahydro[1,3]-dioxolo-[6,7]-5H-[1]benzopyrano [3,4-c] pyridines ring systems involving the one-pot deprotection, cyclization and dehydration of N-substituted-4,4-ethylenedioxy-3- [(1,3-benzodioxol-5-yloxy)methyl]piperidines. The hydrogenation of the N-benzylated benzopyrano [3,4-c]pyridine derivative followed by resolution led to the formation of a new compound.