On the origins of triterpenoid skeletal diversity.

Intramolecular reactions of N-acyliminium intermediates

Recent advances in the preparation and synthetic applications of oxiranes

I would like to give a warning, from a theoretical point of view, about all the carbonium ion mechanisms. They are all probably concerted reactions with a beginning and an end, and the reason you write down your carbonium mechanisms in the way you do is chiefly because in writing down on paper you are bound to write something first and then something else afterwards, and the human mind is not quick and clever as the enzyme.

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A Case Study in Biomimetic Total Synthesis: Polyolefin Carbocyclizations to Terpenes and Steroids

Organic matter in tropospheric aerosols is derived from two major sources and is admixed depending on the geographic area. These sources are biogenic detritus and anthropogenic emissions. The biogenic materials in the solvent-extractable organic matter are comprised predominantly of higher plant waxes, with lesser amounts of resin and microbial detritus and the anthropogenic components are primarily vehicular emissions (e.g. oils, soot, etc.) and input from combustion (e.g. charcoal, thermally-altered biogenic matter, etc.). Both biogenic detritus and anthropogenic emissions contain organic compounds (C12−C40+), which can be identified with unique and distinguishable distribution patterns. Molecular composition analysis has been applied to such extracts after suitable chemical separation into subfractions (i.e. hydrocarbons, ketones, aldehydes, carboxylic acids, alcohols, and wax esters). Both homologous compound series and specific natural products (e.g. phytosterols, terpenes, etc.) are identified as molecular markers. Aerosols from rural and remote areas in the western United States, South America, Nigeria and Australia have been analyzed and all contained predominantly plant waxes. The loadings of hydrocarbons ranged approximately from 10–1400 ng/m3 of air, of fatty acids from 10–450 ng/m3 and of fatty alcohols from 10–1650 ng/m3. These higher molecular weight lipids primarily from flora comprise a major component of the organic carbon in rural and remote aerosols. They are thus important indicators for regional biogenic sources in the global cycling of organic carbon.

Organic matter of the troposphere - V: Application of molecular marker analysis to biogenic emissions into the troposphere for source reconciliations.

Ketoximes are converted into enimides [as (III)] in excellent yield by refluxing acetic anhydride in pyridine. Use of alumina chromatography during work-up affords the corresponding enamides [as (II)]. The latter are also prepared by reducing oxime acetates in the presence of acetic anhydride with reagents such as chromium(II). The generality of these reactions is established. Enamides show limited chemical reactivity in comparison with enamines. A particular exception is their efficient α-acetoxylation by reagents such as lead tetra-acetate. Attention is directed to the preparation and use of titanium(III) acetate.

A simple synthesis of enamides from ketoximes

(3S)-2,3-Epoxy-2,3-dihydrosqualene has been shown to be the precursor of lanosterol in pig liver, of lanosterol and ergosterol in yeast, and of cycloartenol, β-amyrin, and lupeol in germinating pea seedlings.

Investigations on the biosynthesis of steroids and terpenoids. Part XII. Biosynthesis of 3β-hydroxy-triterpenoids and -steroids from (3S)-2,3-epoxy-2,3-dihydrosqualene

Diverse carboxylic esters have been reduced with dissolving Group 1A metals. Using lithium in ethylamine, sterically hindered esters (RCO2R′) were deoxygenated giving the alkane (R′H) whereas non-hindered esters regenerated the parent alcohol (R′OH). This permitted the selective deoxygenation of diesters. Conversely, potassium–sodium eutectic solubilised with 18-crown-6 in t-butylamine and tetrahydrofuran (THF) efficiently deoxygenated both hindered and non-hindered esters. In the absence of nucleophiles at ambient temperature the principal reaction of carboxylic ester radical anions was deoxygenation.

Dissolving metal reduction of esters to alkanes. A method for the deoxygenation of alcohols

Triterpenoids and sterols of Kalanchoe spathulata

A convenient ‘one-pot’ synthesis of the corticosteroid side chain has been developed. 20-Oxopregnane oximes have been converted into 20-acetylamino-17(20)-ene derivatives, which with lead tetra-acetate in dry benzene afford 17α-acetoxy-20-acetylimines. The latter are rearranged by acid to 20-enamides [as (IV)]. Further reaction with lead tetra-acetate then introduces an acetoxy-group at C-21. The 20-acetylimines are hydrolysed to the corresponding ketones by aqueous acid. Overall yields are high. Some preliminary experiments with 16α- and 16β-methylpregnanes are outlined.

Robin B. Boar

Papers

A simple synthesis of enamides from ketoximes

Investigations on the biosynthesis of steroids and terpenoids. Part XII. Biosynthesis of 3β-hydroxy-triterpenoids and -steroids from (3S)-2,3-epoxy-2,3-dihydrosqualene

Dissolving metal reduction of esters to alkanes. A method for the deoxygenation of alcohols

Triterpenoids and sterols of Kalanchoe spathulata

A new and convenient synthesis of the 17α,21-diacetoxy-20-oxopregnane side-chain