Feodor Scheinmann

Studies in the xanthone series. Part XII. A general synthesis of polyoxygenated xanthones from benzophenone precursors

Various 2-mono-, 2,3- and 1,7-di-, 1,3,7-, 1,5,6-, 1,6,7- and 2,3,4-tri-, 1,3,4,7-, 1,3,5,6-, 1,3,6,7-, 1,5,6,7- and 2,3,4,5-tetra-, and 1,3,5,6,7-penta-oxygenated xanthones have been synthesised by preparation of 2-hydroxy-2′-methoxybenzophenones under Friedel–Crafts conditions, and subsequent base catalysed cyclisation to eliminate methanol. Acid or base catalysed selective demethylations of polymethoxyxanthones, and of benzophenones lead to efficient synthesis of natural hydroxymethoxyxanthones, and of hydroxymethoxybenzophenones. Selective methylation procedures are also given.

Biflavanoids and a flavanone-chromone from the leaves of Garcinia dulcis(Roxb.) Kurz

From the leaves of Garcinia dulcis(Roxb) Kurz a series of biflavanoids were isolated. Morelloflavone was characterised as its heptamethyl ether; it forms an octa-acetate by ring opening of ring I-C. GB-2a forms an octamethyl ether having a naringenin unit linked to a chalcone. By methylation, volkensiflavone and amentoflavone were isolated (as their hexamethyl ethers); methylation of a minor component gave the novel I-4′,I-5,II-5,I-7,II-7-pentamethoxyflavanone[I-3,II-8]chromone.

Extractives from Guttiferae. Part XXVI. Isolation and structure of six xanthones, a biflavanoid, and triterpenes from the heartwood of Pentaphalangium solomonse Warb

Two new naturally occurring xanthones, 6,11-dihydroxy-3,3-dimethylpyrano[2,3-c]xanthen-7(3H)-one (1) and its probable biogenetic precursor, 1,3,5-trihydroxy-4-(3-methylbut-2-enyl)xanthene-9-one, have been isolated from Pentaphalangium solomonse Warb. Other xanthones found were 1,3,7-trihydroxy-2-(3-methylbut-2-enyl)-,1,3,6,7-tetrahydroxy-, and 5-hydroxy-1-methoxy-xanthen-9-one. The isolation of a 3–8 linked biflavanone, GB-1a, and of a sterol mixture is reported.

Isomerisation of allyl phenyl ethers and allylphenols with transition metal catalysts

Isomerisation of allyl phenyl ethers with PdCl2(PhCN)2 in boiling benzene gives quantitatively phenyl prop-1-enyl ethers with the cis-isomer predominating. Varying the substituents in all three positions of the allyl group shows that the reaction is sensitive to steric effects and the results are discussed in terms of an intermediate π-allyl(hydrido)palladium complex and also a sigmatropic [1,3] hydrogen shift within a π-olefinpalladium complex. The effect of other transition metals on the isomerisation is summarised. The isomerisation of allylphenols to trans-(prop-1-enyl)phenols is discussed in terms of these mechanisms and conformations of the side chains.

Extractives from Guttiferae. Part XXIX. Synthesis of celebixanthone methyl ether and related 1,5,6,7-tetraoxygenated xanthones

The total synthesis of celebixanthone methyl ether [1,5-dihydroxy-6,7-dimethoxy-8-(3-methylbut-2-enyl)xanthen-9-one](2) is described. The route involves preparation of 2-hydroxy-2′,3,4,5,6′-pentamethoxybenzophenone (8) and subsequent base catalysed cyclisation to give 1,5,6,7-tetramethoxyxanthen-9-one (4). Selective demethylation and methylation gives 1,5-dihydroxy-6,7-dimethoxyxanthen-9-one (3). Preparation of the 5-(3-methyl-2-butenyloxy)xanthen-9-one (10) followed by a Claisen rearrangement gives celebixanthone methyl ether.

Extractives from guttiferae. Part XXVII. Structure and synthesis of new biphenyls from Pentaphalangium solomonse warb

Two new natural biphenyls in Pentaphalangium solomonse Warb., isolated as their methyl ethers, have been identified as 3,4′,5-trihydroxy-(1c) and 3,5-dihydroxy-4-(3-hydroxy-3-methylbutyl)biphenyl (1d). A synthesis of 4-(3-hydroxy-3-methylbutyl)-3,4′,5-trimethoxybiphenyl is described.

Studies in the norbornene series. Part II. Synthesis and reactions of some norbornanes carrying fused heterocycles

Some norbornanes carrying fused heterocycles have been synthesised by 1,3-dipolar additions to substituted norbornenes. Attempts to prepare heterocycles by reactions of electrophilic reagents with the strained double bond in norbornenes were unsuccessful owing to the occurrence of rearrangements. In the reaction of iodine with 5-endo-aminomethylnorborn-2-ene, rearrangement may be prevented by prior nucleophilic attack of the amine group on the cation intermediate to give a 5-azatricyclo[4.2.1.0]nonane (32).

Studies in the norbornene series. Part I. Elucidation of the structure of norbornene derivatives by use of the nuclear magnetic resonance shift reagent trisdipivaloylmethanatoeuropium(III), the nuclear Overhauser effect, and mass spectrometry

An assessment of n.m.r. and mass spectral methods shows that the nuclear Overhauser effect and the shift reagent trisdipivaloylmethanatoeuropium(III) are particularly valuable in assigning the structure and stereochemistry of substituted norbornenes and of norbornanes carrying a fused heterocycle. Mass spectra support the structural assignments.

Use of the two enantiometers of 7,7-dimethylbicyclo[3.2.0]hept-2-en-6-one to form complementary optically active synthons in a convergent synthesis of leukotriene-B4

The acetate (±)-4 was resolved by an enantioselective hydrolysis catalysed by porcine pancreatic lipase. The resultant alcohol (–)-3 and the optically active ester (+)-4 were converted into the ketones (+)-2 and (–)-2 respectively. The ketone (+)-2 was elaborated to produce the diester 9 while the ketone (–)-2 was transformed into the phosphonium salt 21. A Wittig reaction between 9 and 21, followed by deprotection and chromatography furnished leukotriene-B4.

Chemoenzymatic, enantiocomplementary, total asymmetric synthesis of leukotrienes-B3 and -B4

The ketone 3 has been resolved using various enzyme-catalysed reactions and the enantiomer (+)-3 was transformed into the benzoate 8 while (–)-3 was converted into the esters 10 and 11; compounds 8 and 10 are complementary sections of leukotriene B4 while compounds 8 and 11 are late-stage synthons for leukotriene B3.