Usha Ranjan Ghatak

Influence of methoxy-and methyl-aromatic substituents on stereochemistry of the products in the acid-catalyzed cyclization of 2-(2-arylethyl)-1,3,3-trimethylcyclohexanols: stereocontrolled total synthesis of (±)-nimbidiol and (±)-nimbiol

Abstract The distributions of the trans -and the cis -podocarpatrienes ( 5c - g ) and ( 6c - g ) in the cyclialkylation reaction of the easily accessible cyclohexanols ( 4c - g ) under a mild condition have been investigated. The cyclohexanol precursors having unactivated aromatic ring proceeds with high stereoselectivity leading to the respective trans -products, while the substrates with an electron donating methoxy or a methyl substituent with respect to the site of electrophilic attack result in the corresponding cis -and the trans -product mixtures. Consistent mechanisms for these stereochemical results have been advanced. Based on these results simple syntheses of the modified diterpenes (±)-nimbidiol (7) and (±)-nimbiolmethylether (18) have been realized.

Synthetic studies towards complex diterpenoids—VI : New synthetic routes to tetracyclic bridged-bicyclo[3.2.1]octane intermediates by intramolecular alkylation reactions through α-diazomethyl ketones of hydroaromatic γ,δ-unsaturated acids

Abstract Two new synthetic routes to a number of tetracyclic intermediates, for the total synthesis of some diterpenoids incorporating the bicyclo[3.2.1]octane moiety, are described. The syntheses of the bicyclo[3,2,1]octane derivatives begin with preparation of the hydroaromatic γ,δ-unsaturated acids 6, 15 and 17, and proceed via the α-diazomethyl ketones to the corresponding pentacyclic ketones 20, 24 and 26 by an intramolecular carbenoid insertion reaction, followed by a regiospecific acid-catalysed cleavage of the aromatic conjugated cyclopropane bond to the respective unsaturated ketones 27, 29 and 30. The second route to these unsaturated ketones involves a single step boron trifluoride etherate catalysed intramolecular alkylation in the corresponding α-diazomethyl ketones. The tetracyclic ketones 31, 34 and 35 were obtained in quantitative yields by a regiostereospecific hydrogenolytic cleavage of the aromatic conjugated cyclopropane bond in the respective pentacyclic precursors with PdC in ethanol. Under the same conditions, reduction of the styrenoid bond in the ketones 29 and 30 proceeds stereospecifically leading to 34 and 35 respectively, whereas the unsaturated ketone 27 gave a mixture of epimeric ketones 31 and 32 in a ratio of 69:31.

Synthetic studies directed to gibberellins and related compounds

Abstract A stereoselective total synthesis of (±)1β,4a-α-dimethyl-cis-1,2,3,4,4a,9a-hexahydrofluoren-9-one-1 1α-carboxylic acid (II), has been described. The intermediates involved in this synthesis have been thorougly investigated and evidence for their stereochemistry presented.

A simple synthesis of α,β-unsaturated aldehydes by 1,3-carbonyl transposition through one carbon homologation

Abstract α,β-Unsaturated aldehydes are conveniently prepared from ketones through their β-diethoxymethyl derivatives by sodium borohydride reduction followed by acid-catalysed rearrangement of the resulting diacetal carbinols.

Synthetic studies towards complex diterpenoids. Part 18. Total synthesis of (±)-isopisiferin and the related compounds

A simple convergent and general method has been developed for the synthesis of (±)-isopisiferin (1c), a rearranged abietane diterpene, having a hexahydrodibenzo [a,d] cycloheptene ring system, and the related model systems (1a) and (1b), through the respective enolisable tricyclic ketone mixtures (12c) and (13c), (12a) and (13a), and (12b) and (13b), obtained from the corresponding easily accessible 2-arylethyl-3,3-dimethylcyclohexanones (7c), (7a), and (7b). Demethylation of the styrenoid ethers (15b) and (15c) under acid conditions gave the tetracyclic dienones (17b) and (17c) through Ar1-5 cyclisation.

An Efficient Palladium Catalyzed Stereocontrolled Synthesis of 4a-Angular Methyl Substituted Hydrofluorenes

A high yield stereoselective synthesis of 4a-angularly methyl substituted cis-hexahydrofluorenes 5a-h and tetrahydrofluorenes 7a,b has been developed by palladium-catalyzed intramolecular Heck-type cyclization of the respective exo-olefins 4a-h and 4a,b through exclusive 5-exo-trig mode.

First total synthesis of the novel cytotoxic benzocycloheptenes (±)-deoxofaveline and (±)-faveline methyl ether

Abstract Simple and efficient syntheses of the cytotoxic benzocycloheptenes (±)-deoxofaveline(1) and (±)-faveline methyl ether(2) have been developed by a convergent route from the ketone 7 via the tricyclic ketone12.

Synthesis studies on resin acids—VII: Synthesis of two isomers of 4,10-dimethyldecalin-4-carboxylic acids

Synthetic of two stereoisomers of 4,10-dimethyldecalin-4-carboxylic acid have been described. The stereochemistry of these acids has been established.

Synthesis of Some Angularly Cyclopentanone Fused Hydrophenanthrene and Hydrofluorene Derivatives by Acid-Catalyzed Intramolecular C-Alkylation of γ,δ - Unsaturated α′-Diazomethyl Ketones

Abstract A facile synthesis of a few new angularly cyclopentanone fused hydrophenanthrene and hydrofluorene derivatives is described based on intramolecular C-alkylation of γ,δ-unsaturated α′-diazomethyl ketones.

Synthetic studies towards complex diterpenoids-171: synthesis and oxidative cleavage of (+)-19,20-cycloabieta-19-oxo-8,11,13-triene

Abstract Oxidative cleavage of the enolates of ( + )-19,20-cycloabieta-19-oxo-8,11, 13-triene ( 1a ) and the related tetracyclic ketone ( 1b ) with molecular oxygen leads to ( + )-abieta-8,11,13-triene-19,20-dioic acid ( 20a ) and the dicarboxylic acid ( 20b ) respectively. The synthesis of the tetracyclic ketone ( 1a ) has been realized by two different methods through Ni(acac) 2 catalyzed intramolecular carbon-hydrogen insertion of the α-diazomethyl ketone ( 13 ), prepared from ( + )-20-nor-4-epidehydroabietic acid ( 7 ) via lactone 6 and the stereo-specific rearrangement of the cyclobutanone ( 16 ), obtained from the easily accessible unsaturated acid ( 4 ) via the unsaturated cyclobutenone ( 15 ). Lithium-ammonia reduction of 4 gave ( + )-20-nor-dehydroabietic acid ( 11 ) exclusively, whereas catalytic hydrogenation of 4 over Pd-C (10%) resulted in the A/B-ring cis-acid ( 9 ) as the major product, which was obtained exclusively from the lactone 6 by hydrogenolysis under the same condition.