C. Subrahmanyam

The structures of lignans from Gmelina arborea Linn

Five new lignans isolated from Gmelina arborea Linn have been characterised. The parent compound arboreol, is 2a,6e-dipiperonyl-1e,2e-dihydroxy-3,7-dioxabicyclo-[3,3,0]-octane. It is accompanied by its 2-O-methyl ether, the 2-O-ethyl ether and its 2-epimer, isoarboreol,. The fifth substance, gmelanone, is the first reported example of a lignan derived from 3,6-dioxabicyclo-[3,2,1]-octane.

Crystalline constituents of euphorbiaceae—XII: Isolation and structural elucidation of three new lignans from the leaves of Phyllanthus niruri Linn.

Abstract Three more new lignans from the hexane extract of Phyllanthus niruri Linn. are now reported besides phyllanthin and hypophyllanthin. The structure of niranthin is now established as 3,3′,4′,9,9′-pentamethoxy-4,5-methylenedioxy-8,8′-butyrolignan ( 2 ). Nirtetralin and phyltetralin are shown to be 1-phenyl tetralins. On the basis of NMR and mass spectra, nirtetralin is assigned the structure ( 7 ) and phyltetralin ( 11 ).

New diterpenes from a new species of Lobophytum soft coral of the South Andaman coast

Abstract Two new cembrenoid diterpenes (10) and (13) and one novel neodolabellane diterpene (16) have been isolated from a new species of Lobopytum soft coral of the South Andaman Coast, along with the known diterpenes (2) and (3) and lipids (1), (4), (5), (6), (7), (8) and (9). The structures were determined from spectral data and chemical conversions.

Diterpenes from the marine mangrove Bruguieragymnorhiza

Abstract Steviol and five new diterpenes have been isolated from the outer layer of the rootbark of Bruguiera gymnorhiza Lam of the Andaman and Nicobar Islands. They are ent-kaur-16-en-13-hydroxy-19-al; 15 (S)-isopimar-7-en-15,16-diol, ent-kaur-16-en-13,19-diol,methyl-ent-kaur-9 (11)-en-13,17-epoxy-16-hydroxy-19-oate; 1β,15 (R)-ent-pimar-8(14)-en-1,15,16-triol. Their structures were established by means of spectral studies, chemicalreactions and, in case of the last compound, by X-ray analysis.

Umbraculolides B–D, further Briarane Diterpenes from the Gorgonian Gorgonella umbraculum

Abstract Three new briarane diterpenes, umbraculolides B–D, have been isolated from the Indian Ocean gorgonian Gorgonella umbraculum. Their structures have been established by spectral data.

A new polyoxygenated steroid from the gorgonian Gorgonella umbraculum

A new polyoxygenated steroid, 1, has been isolated from the Indian ocean gorgonian Gorgonella umbraculum and its structure established from its spectral data.

Reactions of dialkylbromoboranes with alkali metal hydrides in the presence of alkenes. Synthesis of ‘mixed’ trialkylboranes (RA2RBB) in solution

Dialkylbromoboranes react steadily with hydroxide-free sodium hydride in diglyme, in the presence of alkenes, to give nearly quantitative yields of partly mixed trialkylboranes. The reaction proceeds too slowly to allow prior clean preparation of dialkylboranes, so reaction in the presence of alkenes possessing hydride-sensitive functional groups is not possible, although terminal alkynes can be hydroborated in situ by a modified experimental procedure. Potassium hydride reacts much more rapidly with dialkylbromoboranes, the reactions being complete within minutes at 20 °C, but the yields are lower. Addition of dibenzo-18-crown-6 leads to an increase in yield but the results are still inferior to those obtained using sodium hydride. Reaction of dialkylbromoboranes with lithium hydride is very sluggish.

The chemistry of organoborates. Part 7. Protonation studies of alkynyltrialkylborates

Protonation of trialkylalkynylborate salts followed by oxidation is a very general route to ketones that proceeds under mild conditions. Hydrolytic work-up gives olefins in good yields. When a phenyl group migrates then the Z-olefin results but in all other cases mixtures of isomers are obtained. However the protonation of dialkyl(phenyl-ethynyl)-(1,1,2-trimethylpropyl)borates gives Z-olefins, but from alkynyldialkyl-(1,1,2-trimethylpropyl)borates the E-olefin predominates. Different acids give different stereochemical results with simple alkynyltrialkylborate salts. The intermediate boranes, not availble by other methods, may be used to produce olefins by reaction with iodine and base.

The chemistry of organoborates. Part 5. Alkylation of alkynyltrialkylborate salts

Alkynyltrialkylborates undergo β-alkylation with alkyl migration from boron to the α-carbon atom under mild conditions to give dialkylvinylboranes which were previously unobtainable. Oxidation of these intermediates provides a versatile route to regiospecifically α-substituted ketones. Hydrolysis yields alkenes in high yields as E–Z mixtures. The reaction is more stereoselective with borates derived from phenylethyne and with alkynyldialkyl(thexyl)borates. Formation of the major products appears to be kinetically controlled and, in all cases, the the major product has alkylating agent and the migrating group on the same side of the double bond. Whilst the rate of reaction is dependent on the alkylating agent, the rearrangement appears to have much less polar character than expected from reactions of carbocations, as the isomer ratio (E:Z) is essentially independent of the alkylating agent and solvent. MINDO/3 calculations are used to support a two-stage mechanism (Scheme 4) in which rate-determining electrophilic attack produces a bent vinyl cation-like intermediate; this then undergoes rapid, exothermic rearrangement with preferential retention of stereochemistry at the migration terminus.

Reaction of higher dialkyl(methylthio)boranes with bromine: a new synthesis of dialkylbromoboranes

Higher dialkyl(methylthio)boranes, readily prepared in situ from the corresponding trialkylboranes, react rapidly at low temperature with bromine to give dialkylbromoboranes and dimethyl disulphide. Removal of the latter under reduced pressure gives a pure product in excellent yield.