Paul Brassard

Reactions of ketene acetals-14 The use of simple mixed vinylketene acetals in the annulation of quinones

Abstract α,β- β, γ-unsaturated esters can be converted by strong base and chlorotrimethylsilane to the corresponding mixed vinylketene acetals which are shown to be particularly useful and generally applicable reagents for the regiospecific annulation of halogenoquinones. The reaction proceeds readily with a variety of substrates including benzoquinones.

Regiospecific syntheses of quinones using vinylketene acetals derived from unsaturated esters

Abstract A general method of annellating quinones in high yield has been devised using mixed vinylketene acetals obtained directly from the enolate ions of unsaturated esters.

A convenient synthesis of naturally occurring quinizarins

Abstract A general and regiospecific method for the preparation of quinizarins involves the cycloaddition of electron-rich dienes. Advantageous syntheses of several natural products, 2-methylquinizarin, islandicin, digitopurpone, erythroglaucin, 5-0-methylislandicin and 8-0-methyl-digitopurpone illustrate this procedure. A structure attributed to ventinone B is incorrect and 1,4,8-trihydroxy-6-methylanthraquinone is different from a natural substance so described.

Reactions of ketene acetals 15. Regiospecific syntheses of erythrolaccin and “7-hydroxyerythrolaccin”.☆

Abstract A new diene, 1,2-demethoxy-1-trimethylsiloxy-l,3-pentadine, has been prepared from crotonaldehyde via the cyanohydrin nand the β γ-Unsaturated eater. The usefulness of the reagent is demonstrated by advantageous syntheses of the naturally occuring title-compounds and of various partially methylated analogues. The electron-donating substituents on the diene are disposed in order to illustrate the consequence of their oppossing electronic effects.

Regiospecific additions of some simple dienes to haloquinones

Abstract Some simple dienes have been shown to react regiospecifically with haloquinones; the adducts can be oxidized without loss of the oxygenated function or aromatized to a variety of natural products and useful intermediates.

A new synthesis of α-pyrones

A simple two-step synthesis of α-pyrones from αβ-unsaturated carbonyl compounds has been devised.

An integrated approach to the synthesis of contiguously substituted xanthopurpurins, pachybasins and purpurins

Abstract Alkylation, hydroxyalkylation and acylation of 3-methyl-, 3-methoxy- and 3,4-dimethoxycrotonates can be induced to occur exclusively in the α-position. Conversion of the products to dienes then provides, through cycloaddition, a wide variety of substitution patterns. This approach is illustrated by simplified syntheses of a number of naturally occurring quinones and confirms the structures proposed for vismiaquinone C, 7-geranylemodin, cinnalutein, 4,5-dihydroxydigitolutein, 2-hydroxyislandicin 1-methyl ether and calyculatone 1-methyl ether.

Reactions of Ketene Acetals. IV. A New Synthesis of α-Pyrones. III

A simple one-step synthesis of α-pyrones and 3-chloro-α-pyrones from β-functionalized α, β-enones and ketene acetals has been devised. The method has also been adapted to the preparation of some 4-methoxy-α-pyrones. Finally the mechanism of the reaction has been investigated.

Regiospecific α-substitution of crotonic esters synthesis of naturally occurring derivatives of 6-ethyljuglone

Abstract Methyl β-methoxycrotonate is alkylated regiospecifically, through the anion, in the α-position. Enolsilylation of the resulting β,γ-unsaturated ester affords the corresponding 1,1,2,3-tetrasubstituted butadiene. Cycloaddition of the latter to the appropriate halogenated benzoquinone, followed by various transformations, provides the first recorded syntheses of several natural products derived from 6-ethyl-7-methoxyjuglone.

Reactions of keten acetals. Part VI. Total syntheses of the anthraquinones (±)-nalgiovensin, (±)-isorhodoptilometrin, and (±)-rhodoptilometrin

The three title compounds have been synthesized and compared with the natural products. Emodin trimethyl ether(11) was brominated and converted into the propenyl derivative (15). The latter could then be transformed into the 3-(2-hydroxypropyl) compound (19) either by oxymercuration–demercuration or by epoxidation and reduction. (±)-Nalgiovensin [1,8-dihydroxy-3-(2-hydroxypropyl)-6-methoxyanthraquinone](24) was then obtained by partial demethylation. The completely demethylate(±)-isorhodoptilometrin (25) was prepared analogously from the propenylanthraquinone (15) after demethylation and acetylation. For the synthesis of (±)-rhodoptilometrin [1,3,8-trihydroxy-6-(1-hydroxypropyl)anthraquinone](34), the propenyl derivative (15) was hydrogenated, de-methylated, and brominated; substitution of the bromine by acetate and saponification gave the desired product. Conditions for the preparation of the starting material (11) have been further investigated.