Sivagnanasundram Surendrakumar

XY–ZH systems as potential 1,3-dipoles. Part 12. Mechanism of formation of azomethine ylides via the decarboxylative route from α-amino acids

The effect of temperature, solvent, and type of aldehyde on the stereospecific or stereoselective formation of azomethine ylides by the decarboxylative condensation of aldehydes with three types of α-amino acids, (i) cyclic secondary α-amino acids, (ii) cyclic secondary α-amino acids with a benzylic carboxy group, and (iii) acyclic primary α-amino acids, has been studied. Stereospecific anti-dipole formation in (i) and temperature dependent stereoselective anti-dipole formation in (ii) and (iii) is inferred from the stereochemistry of cycloadducts of the azomethine ylides with N-methylmaleimide. These results are used to support a mechanistic scheme involving loss of carbon dioxide from intermediate oxazolidin-5-ones in a stereospecific cycloreversion reaction.

XY–ZH systems as potential 1,3-dipoles. Part 11. Stereochemistry of 1,3-dipoles generated by the decarboxylative route to azomethine ylides

The decarboxylative reaction of aryl aldehydes with cyclic secondary α-amino acids or primary α-amino acids in the presence of N-methyl- or N-phenyl-maleimide leads, via an intermediate azomethine ylide, to mixtures of bicyclic pyrrolidine cycloadducts in good yield. Cyclic secondary α-amino acids, where the carboxylic group is non-benzylic, give cycloadducts arising from a stereospecifically generated anti-dipole. Acyclic α-amino acids, and cyclic secondary α-amino acids with the carboxylic group located at a benzylic site, give rise to cycloadducts derived from both anti- and syn-configurations of the intermediate azomethine glides. The reactions show little discrimination between endo- and exo-transition states for the cycloadditions.

Distichol an antibacterial polyphenol from Shorea disticha

Abstract The structure of distichol, a new hexadehydrotrimer of resveratrol isolated from the bark of Shorea disticha , has been established on the basis of spectroscopic evidence, chemical degradations and biosynthetic considerations.

Two pyranodihydrobenzoxanthones from Artocarpus nobilis

Abstract Two new pyranodihydrobenzoxanthones have been isolated from the bark of Artocarpus nobilis and their structures established as 5,6,11-trihydro-1,3,4,8-tetrahydroxy-5-isopropenyl-11,11-dimethylbenzo[1,2:a]pyrano [2′,3′:j]xanthene-7-one(artobiloxanthone) and 5,5a,6,11-tetrahydro-1,3,8-trihydroxy-5,5,11,11-tetramethylbenzo- furo [3,3a,4:ab] pyrano [2′,3′.jlxanthen-7-one (cycloartobiloxanthone). This is the first report of the occurrence of dihydrobenzoxanthones in plants. A plausible biosynthetic route is proposed for the benzoxanthones from a simple flavone.

Polyphenols from dipterocarp species. Vaticaffinol and ε-viniferin

A dipterocarp endemic to Sri Lanka, Vatica affinis, yielded three phenols. Two of them showed anti-bacterial properties and have been shown to be the dimer and tetramer of 3,5,4′-trihydroxystilbene.

Calocalabaxanthone, the putative isoprenyl precursor of calabaxanthone from Calophyllum calaba

Abstract The bark of Calophyllum calaba var. calaba contains a new xanthone, 2,8-di-(3-methylbut-2-enyl)-1,3-dihydroxy-7-methoxyxanthone (calocalabaxanthone), the precursor of 5-hydroxy-8-methoxy-2,2-dimethyl-7-(3-methylbut-2-enyl)-2H,6H-pyano-(3,2-b)-xanthen-6-one (calabaxanthone). In addition the bark contains calabaxthone and the other constituents isolated earlier from Calophyllum calaba var. worthingtonii .

1,3-Azaprotio cyclotransfer. Nitrone-forming oxime–alkyne cyclisations

δ-Alkynyl oximes undergo a concerted 2n + 2π+ 2σ 1,3-azaprotio cyclotransfer (APT) reactiongenerating cyclic N-vinyl nitrones which have been trapped in cycloaddition reactions; MNDO and AM1 calculations and transition-state modelling provide support for the observed greater facility of oxime–alkyne reactions compared to oxime–alkene reactions.

Structure of a bipyridine alkaloid from Broussonetia zeylanica

Abstract From the benzene extract of the timber of Broussonetia zeylanica , 8-hydroxyquinoline-4-aldehyde, a new alkaloid and two unidentified minor alkaloids have been isolated. The spectroscopic evidence suggested the new alkaloid to be 3,4′-dihydroxy-2,3′-bipyridine.

Broussonetine, a bisquinolyl-γ-butyrolactone from Broussonetia zeylanica

Abstract The wood of B. zeylanica (Moraceae) contains a new alkaloid broussonetine, identified as 3,4-bis(8- hydroxyquinolin-4-yl)-γ-butyrolactone.

The decarboxylative route to azomethine ylides. Stereochemistry of 1,3-dipole formation

Stereochemical studies of cycloadducts produced from primary and secondary α-amino acids, aldehydes, and N-methylmaleimide indicate they usually arise from stereospecific or highly stereoselective formation of the anti-isomer of an intermediate azomethine ylide.