Cascade Synthesis of 2-Azafluorene, Azocine, and Azabicyclononane Derivatives by Reaction of Activated Acetylenes with Some β-Amino Ketones

Abstract

Double Mannich bases obtained from α-indanone and α-tetralone contain a bis-β-amino ketone fragment and are promising intermediate compounds for the synthesis of nitrogen heterocycles via reactions with dimethyl acetylenedicarboxylate and methyl propiolate. The products of these reactions are determined by the activated acetylene structure. The reaction begins with attack of the activated acetylenic compound on the tertiary nitrogen atom of the Mannich base to give 1,3-zwitterionic intermediate. The subsequent domino reaction is driven by neutralization of the charge on the nitrogen atom through formation and/or rupture of several C–C bonds. Intramolecular attack of the negatively charged center of the 1,3-zwitterion derived from the indanone base and dimethyl acetylenedicarboxylate on the carbonyl carbon atom of the indanone fragment is followed by domino process, leading to the formation of dimethyl 2-methyl-1,2-dihydro-9H-indeno[2,1-c]-pyridine-3,4-dicarboxylate. The reaction of the same Mannich base with methyl propiolate involves 1,3-sigma-tropic rearrangement with neutralization of the positive charge on the nitrogen atom in the intermediate structure and negative charge transfer to the carbonyl oxygen atom. Next follows aldolization/crotonization with the second β-amino carbonyl fragment to produce methyl 2′-methyl-1-oxo-1,1′,2′,3,5′,11′-hexahydrospiro-[indene-2,6′-indeno[2,1-c]azocine]-4′-carboxylate. One more reaction direction leading to the formation of linear enamines via intramolecular proton migration from the indan fragment to the C-anionic center in the 1,3-zwitterion with simultaneous elimination of 2-methylideneindan-1-one has been revealed by HPLC/MS analysis of both reaction mixtures. Such enamines, methyl 3-{methyl[(1-oxo-1,2,3,4-tetrahydronaphthalen-2-yl)methyl]amino}prop-2-enoate and dimethyl (2Z)-2-{methyl[(1-oxo-1,2,3,4-tetrahydronaphthalen-2-yl)-methyl]amino}but-2-enedioate, were isolated in the reactions of activated acetylenic compounds with the Mannich base derived from tetralone. Previously described synthesis of 1-azabicyclo[3.3.1]nonanes from Mannich bases has been reproduced, and detailed analysis of the reaction mixtures has revealed products resulting from elimination of one alkylaryl group from the zwitterionic intermediate. The formation of linear enamines, dimethyl (2Z)-2-[methyl(3-oxo-3-phenylpropyl)amino]but-2-enedioate and dimethyl (2Z)-2-{methyl-[3-oxo-3-(thiophen-2-yl)propyl]amino}but-2-enediate, suggests facile retro-aldol reaction of 3-aroyl-4-aryl-1-methylpiperidin-4-ols as the necessary step. PASS online predicted diverse biological activities of all newly synthesized compounds with a probability of 65–80%.