A. Srikrishna

A novel, versatile synthetic approach to linearly fused tricyclopentanoids via photo-thermal olefin metathesis

Abstract Fifteen examples of a new, speedy and general approach to linearly fused tricyclopentanoids bearing the tricyclo[6.3.0.0 2,6 ]undecane (triquinane) frame of high contemporary interest is delineated. The key concept in our synthetic sequence to triquinanes is the novel photo-thermal olefin metathesis of cheap, abundantly available Diels-Alder adducts of 1,3 cyclopentadienes and p-benzoquinones. Thus photolysis of endo -tricyclo[6.2.1.0 2,7 ] undeca-4,9-dien-3,6-diones ( 9a – 9j , 13a,b ) furnished pentacyclo [5.4.0.0 2,6 .0 3,10 .0 5,9 ]undecan-8,11-diones ( 10a – 10j , 14a,b ), which on thermal fragmentation of the cyclobutane ring gave cis , syn , cis -tricyclo [6.3.0.0 2,6 ]undeca-4,9-dien-3,11-diones ( 11a – 11j , 15a,b in just three steps and in exceptionally good yields. A few interesting transformations of the readily available parent bis-enone 11a which indicates its wider uses in syntheses, are described. Finally, a smooth thermal isomerisation of cis , syn , cis -bis-enones to cis , anti , cis -bis-enones is reported, which further enhances the scope and versatility of our synthetic theme.

Reduction of quinolines to 1,2,3,4-tetrahydro derivatives employing a combination of NaCNBH3 and BF3.OEt2

A regiospecific reduction of quinolines (and 1,10-phenanthroline) into the corresponding 1,2,3,4-tetrahydro derivatives using a combination of sodium cyanoborohydride and boron trifluoride etherate in refluxing methanol is described. Under the same conditions indole and acridine reduced to the corresponding dihydroderivatives, whereas acyl group transfer from oxygen to nitrogen atom is also noticed in the case of 8-acyloxyquinolines.

Chemoselective reductive deoxygenation of α,β-unsaturated ketones and allyl alcohols

A simple and convenient procedure for a highly chemoselective reductive deoxygenation of α,β-unsaturated ketones and allyl alcohols to olefins by sodium cyanoborohydride and boron trifluoride etherate in dry THF is described.

An enantiospecific formal total synthesis of (−)-aplysin and (−)-debromoaplysin

An enantiospecific approach to marine sesquiterpenes (−)-debromoaplysin and (−)-aplysin, starting from (R)-limonene employing a Claisen rearrangement as the key step, is described.

Application of microwave heating technique for rapid synthesis of γ,δ-unsaturated esters

The use of microwave heating technique for the acceleration of ortho ester Claisen rearrangement (a three step transformation) is described. Irradiation of a DMF solution of the allyl alcohol 5, triethyl orthoacetate and propionic acid (catalytic) in an Erlenmeyer flask for 10 minutes in a microwave oven generated the ester 8 in 83% yield. Analogously, ortho ester Claisen rearrangement of a variety of allyl and propargyl alcohols (9, 12-22) were achieved. The formation of the diester 10 from 2-butyne-1,4-diol (9) via the ortho ester Claisen rearrangement of two allyl alcohol moieties (involving sh steps) in 15 minutes, demonstrates the versatility of the microwave heating technique.

Furannulation via radical cyclisation

A three step 4-methyl furan annulation sequence is described via the radical cyclisation of bromoacetal 3 to 2-alkoxy-4-methylene tetrahydrofuran 4

A simple, ring-closing metathesis reaction based approach to (±)-1,14-herbertenediol and (±)-11-epi-herbertenolide

A total synthesis of 1,14-herbertenediol via 11-epi-herbertenolide, and a formal total synthesis of tochuinyl acetate and dihydrotochuinyl acetate, employing a ring-closing metathesis reaction based methodology, are described.

First enantiospecific synthesis of (-)-9-pupukeanone

The first enantiospecific total synthesis of (−)-9-pupukeanone, starting from (R)-carvone employing a combination of Michael–Michael reaction and an intramolecular rhodium carbenoid C-H insertion reaction as key steps, is described.

Enantiospecific first total synthesis of (−)-4-thiocyanatoneopupukeanane

Abstract The first total synthesis of (−)-4-thiocyanatoneopupukeanane starting from ( R )-carvone has been achieved, establishing the relative as well as absolute structure of the natural product.

STEREOSELECTIVE TOTAL SYNTHESIS OF ()-TOCHUINYL ACETATE AND ()-DIHYDROTOCHUINYL ACETATES

Details of the first total synthesis of the marine natural product dihydrotochuinyl acetate is described. Cyclopentenone annulation of p-methylacetophenone via a Claisen rearrangement-Wacker oxidation based sequence generated the cyclopentenone 3, a known precursor for the sesquiterpenes cuparene, laurene, α-cuparenone and β-cuparenones. Conversion of the ketone moiety into a carboxylate followed by stereoselective alkylation and reduction transformed the cyclopentenone 3 into the primary alcohol 19. Birch reduction of the alcohol 19 followed by acetylation furnished (±)-dihydrotochuinyl acetate, whereas direct acetylation of 19 furnished (±)-tochuinyl acetate.