Reductive alkylation of pyridinium salts. Part 2. Utilisation of di-, tetra- and hexa-hydropyridine esters

Abstract

4-Benzyl-4-ethoxycarbonyl-1-substituted piperidines 1 (R¹= PhCH₂, PhCO; R²= PhCH₂) cyclise with polyphosphoric acid (PPA) to give spiro[indane-2,4′-piperidin]-1-ones 3 (R = PhCH₂, PhCO), while 2-benzyl-2-ethoxycarbonyl-1-methylpiperidine 4 gives the N-methylspiro[indane-2,2′-piperidin]-1-one 5. 3,4,4a,5-Tetrahydrobenz[g]isoquinolin-10(2H)-one 8 arises from PPA treatment of 1-benzoyl-4-benzyl-5-ethoxycarbonyl-1,2,3,4-tetrahydropyridine 7 (R = PhCO) while o-chloranil converts 1-benzoyl-4-benzyl-4-ethoxycarbonyl-1,4-dihydropyridine 6 (R = PhCO) into 4-benzyl-3-ethoxycarbonylpyridine 9. Phenyl(tribromomethyl)mercury reacts with 1-benzoyl-4-benzyl-4-ethoxycarbonyl-1,4-dihydropyridine 2 (R¹= PhCO, R²= PhCH₂) yielding 2-benzoyl-5-benzyl-7,7-dibromo-5-ethoxycarbonyl-2-azabicyclo[4.1.0]hept-3-ene 11, and with 1-benzoyl-4-benzyl-3-ethoxycarbonyl-1,4-dihydropyridine 6 (R = PhCO) to give 2-benzoyl-5-benzyl-7,7-dibromo-4-ethoxycarbonyl-2-azabicyclo[4.1.0]hept-3-ene 12. The structure of the latter is confirmed by X-ray crystallographic analysis. Catalytic hydrogenation of 2-benzoyl-5-benzyl-7,7-dichloro-5-ethoxycarbonyl-2-azabicyclo[4.1.0]hept-3-ene 16 yields 2-benzoyl-5-benzyl-7,7-dichloro-5-ethoxycarbonyl-2-azabicyclo[4.1.0]heptane 21 which cyclises with PPA to give the tetracyclic product 22 in good yield. When 2-benzoyl-5-benzyl-7,7-dichloro-4-ethoxycarbonyl-2-azabicyclo[4.1.0]hept-3-ene 17 is hydrogenated it yields mainly 2-benzoyl-5-benzyl-7,7-dichloro-4-ethoxycarbonyl-2-azabicyclo[4.1.0]heptane 25 but the dibromo analogue 12 under the same conditions gives two components thought to be 2-benzoyl-5-benzyl-7-endo-bromo-4-ethoxycarbonyl-2-azabicyclo[4.1.0]hept-3-ene 24 and 2-benzoyl-5-benzyl-7-exo-bromo-4-ethoxycarbonyl-2-azabicyclo[4.1.0]hept-3-ene 23.