Prabir C. Chakraborti

Condensed cyclic and bridged-ring systems. Part III. Regioselectivity and stereoselectivity in the acid-catalysed cyclisations of substituted benzylcyclohexanols. Stereocontrolled synthesis of some gem-carboxy–methyl-substituted hexahydrofluorene and hexahydro-5,9-methanobenzocyclo-octene derivatives

Treatment of ethyl 2-benzyl-2-hydroxy-1,3-dimethylcyclohexanecarboxylate (8) with polyphosphoric acid produced either (±)-c-2-benzyl-1,3-dimethylcyclohexane-r-1,c-3-carbolactone (6) or a mixture from which (1RS,4aRS,9aRS)2,3,4,4a,9,9a-hexahydro-1,4a-dimethyl-1H-fluorene-1-carboxylic acid (5a) was isolated in a moderate yield, depending upon the reaction temperature. Whereas aluminium chloride-catalysed cyclisation of the lactone (6) resulted in the stereospecific formation of (5SR,8RS,9SR,11RS)-5,6,7,8,9,10-hexahydro-8,11-dimethyl-5,9-methanobenzocyclo-octene-8-carboxylic acid (9), the epimeric t-2-benzyl lactone (7) produced a mixture of the hexahydrofluorene acid (5a) and the epimeric (8SR)-hexahydromethanobenzocyclo-octene acid (12), in high yields. By a similar process (±)-methyl t-2-benzyl-t-3-hydroxy-1,3-dimethylcyclohexane-r-1-carboxylate (4) gave the acid (9) and a mixture of hexahydrofluorene ester (5b) and (8SR)-hexahydromethanobenzocyclo-octene (13). Structures and configurations of the bridged-ring compounds were determined from chemical and n.m.r. spectral studies. Ready alkaline hydrolysis of the tertiary ester group in (5SR,8RS,9SR,11SR)-methyl 5,6,7,8,9,10-hexahydro-8,11-dimethyl-10-oxo-5,9-methanobenzocyclo-octene-8-carboxylate (11), through intramolecular oxo-group participation, revealed the stereochemistry of the ester group. The observed differences in the nature of products in polyphosphoric acid-catalysed cyclisation of (8) and the behavioural differences shown by the diastereoisomeric lactones (6) and (7) and the hydroxy-ester (4) in aluminium chloride-induced cyclisations are rationalised on the basis of steric and stereoelectronic factors in the intermediate cations as well as the thermodynamic stability of the products under reversible reaction conditions.

Condensed cyclic and bridged-ring systems. Part 13. Synthesis of the insect attractant hydrocarbon, 9a-carbamorphinan, and X-ray structural analyses of 9a-carbamorphinan-10-one and 9a-carba-14α-morphinan-10-one

The insect attractant bridged hydrocarbon, (±)-9a-carbamorphinan (1) and its inactive epimer, (±)-9a-carba-14α-morphinan (2), have been synthesized through (±)-9a-carbamorphinan-10-one (3), and (±)-9a-carba-14α-morphinan-10-one (4), prepared by polyphosphoric acid-catalysed cyclization of the double-bond isomeric benzyloctalins (7), derived from Huang-Minlon reduction of 1-benzyl-4,4a,5,6,7,8-hexahydronaphthalen-2(3H)-one (6), followed by the benzylic oxidation with chromic acid. The reduction of the easily accessible (±)-9a-carbamorphinan-16-one (10), the orthophosphoric acid-induced cyclization product of (6), provides an efficient alternative route to (1). The structures of the epimeric ketones (3) and (4) have been established by X-ray analysis.

Condensed cyclic and bridged-ring systems. Part IV. Stereochemically controlled synthesis of some endo-2-aryl-6-oxobicyclo[3.2.1]octanes and related compounds through intramolecular alkylations of γδ-unsaturated α′-diazomethyl ketones

The 4-arylcyclohex-3-enecarboxylic acids (4a—d), prepared through a modified Diels–Alder reaction, have been converted via the corresponding diazomethyl ketones (5a—d) into cyclopropyl ketones (6a—d) and the 2-aryl-6-oxobicyclo[3.2.1]oct-2-ene derivatives (7a—d), by oxo-carbenoid addition and boron trifluoride–ether-catalysed cyclisation, respectively, in excellent yields. In addition to the unsaturated ketones (7a and c), the hydroxy-ketones (10a and c) were also obtained in the aqueous tetrafluoroboric acid-catalysed cyclisation of the diazoketones (5a and c). Catalytic reduction of the cyclopropyl ketones and the unsaturated ketones produced the endo-2-aryl-6-oxobicyclo[3.2.1]octane derivatives (8a—d) exclusively.

Synthetic studies directed towards complex diterpenoids. Part 15. Synthesis and stereochemistry of the catalytic reduction of Δ4b(5)-gibbenes and related compounds

The synthesis of, and substituent effects in the stereoselectivity of catalytic hydrogenation of, a series of methyl- or methoxy-substituted (±)-4b,9-cyclogibba-1(10a),2,4-trien-8-ones (1a–g) and (±)-gibba-1(10a), 2,4,4b(5)-tetraen-8-ones (2a–g) are described, which lead to stereocontrolled syntheses of the respective C-4b epimeric gibbanes (3a–g) and (4a–g). The key intermediate tetrahydrofluorene-2-carboxylic acids (10a–g), prepared by cycloaddition reactions, are converted into the corresponding α-diazomethyl ketones (12a–g) which are subjected to intramolecular oxo-carbenoid addition or perchloric acid–trifluoroacetic acid-catalysed cyclisation leading to the cyclopropyl ketones (1a–g) and the Δ4b(5)-gibbenes (2a–g), respectively.