Marc Boeykens

Two 6,7-dioxygenated coumarins from Pterocaulon virgatum

Abstract From the petrol and chloroform extracts of the aerial parts of Pterocaulon virgatum , two new coumarins, virgatenol and virgatol, were isolated, as well as the four known coumarins, 7-(3-methyl-2-but-enyloxy)-6-methoxycoumarin, 7-(2,3-dihydroxy-3-methylbutoxy)-6-methoxycoumarin, ayapin (6,7-methy-lenedioxycoumarin) and scopoletin (7-hydroxy-6-methoxycoumarin). Their structures were elucidated on the basis of spectroscopic methods.

Structural revision of four coumarins from Pterocaulon species

Abstract The structures of two 5,6,7-trioxygenated coumarins named sabandinol and sabandinone isolated from Pterocaulon virgatum have been revised. The new structures assigned were regioisomers and were identified on the basis of additional spectroscopic data and X-ray crystallographic data. Also the structural revision of two trioxygenated coumarins from Pterocaulon balansae and P. lanatum is presented.

Synthesis of 3,3-dimethylazetidine-2-carboxylic acid and some derivatives

Abstract γ-Chloro-α-(N-alkylimino)esters were reduced by sodium cyanoborohydride in methanol in the presence of acetic acid with complete selectivity to give rise to either γ-chloro-α-(N-alkylamino)esters (reaction at 0°C) or 1-alkyl-3,3-dimethylazetidine-2-carboxylic esters (reaction at reflux). The isolable γ-chloro-α-(N-alkylamino)esters are suitable sources for 1-(N-alkylamino)-2,2-dimethylcyclopropane-1-carboxylic esters via base-induced 1,3-dehydrochlorination, while the former substrates as transient species undergo 1,4-dehydrochlorination to the corresponding azetidines. The latter process was used for the synthesis of 3,3-dimethylazetidine-2-carboxylic acid, a new non-proteinogenic sterically hindered α-amino acid, via hydrogenolysis of methyl 1-benzyl-3,3-dimethylazetidine-1-carboxylate and subsequent acidic hydrolysis. Reduction of alkyl 4-chloro-3,3-dimethyl-α-(N-alkylimino)butanoates with lithiumaluminiumhydride in diethyl ether afforded 1-alkyl-3,3-dimethyl-2-(hydroxymethyl)-azetidines.

Selective transformation of α,α-dibromomethyl ketones into α-monosulfenylated ketones

Abstract When α,α-dibromomethyl ketones are treated with sodium thiolates only the α-monosulfenylated ketones are formed. Evidence is put forward that the reaction mechanism proceeds by an initial nucleophilic substitution of one bromo atom and reduction by single electron transfer (SET) - hydrogen atom abstraction of the second bromo atom.

1,4-Dehydrochlorination of α-chloroimines as a suitable procedure for the synthesis of unactivated 2-aza-1,3-dienes and 3-aza-1,3,5-trienes

N-Benzyl and N-allyl α-chloroimines were dehydrochlorinated in a 1,4-fashion to afford the corresponding 2-aza-l,3-dienes and 3-aza-l,3,5-trienes, respectively.

Regiospecific synthesis of α-diones, α,α-dialkoxyketones and α-alkoxy-α-sulfenylated ketones

Abstract A convenient synthesis of α-diones and their monoprotected acetals, i.e. α-ketoacetals, was developed by mercury induced solvolysis of regiospecifically formed α-chloro-α-(alkylthio)ketones. Analogously, α-alkoxy-α-sulfenylated ketones were formed when reacting α-chloro-α-sulfenylated ketones with an alkaline alcoholic medium. α-Alkoxy-α-sulfenylated ketones themselves could be transformed into α-diones or α-ketoacetals, which in turn were hydrolyzed under anhydrous conditions into the corresponding α-diones.

Revision of the structure of obtusifol

Abstract The recently isolated 6,7,8-trioxygenated coumarin, named purpurasol, has been shown to be identical to the previously reported obtusifol. The structure of purpurasol has been confirmed and that of obtusifol revised for a second time.

Synthesis of 2,2-dialkylcyclopropylamines from β-chloroimines and application towards the synthesis of1-amino-2,2-dialkylcyclopropane-carboxylic acids

Abstract Base-induced 1,5-dehydrochlorination of β-chloroimines, having a relatively acidic hydrogen atom at carbon-1 of the N-substituent (e.g. benzyl, methoxycarbonylmethyl, α-methylbenzyl), afforded N-cyclopropylimines which were easily hydrolyzed into cyclopropylamines. This synthetic methodology was applied to the synthesis of the potentially plant growth regulating 1-amino-2,2-dialkylcyclopropanecarboxylic acids via oxidation with catalytic ruthenium(IV) oxide/ sodium periodate of suitably N-protected 1-aryl-2,2- dialkylcyclopropylamines.

Synthesis of Pantonine

Abstract A convenient synthesis of the naturally occurring α-amino acid pantonine, i.e. 2-amino-4-hydroxy-3,3-dimethylbutyric acid, was developed starting from 3-chloro-2,2-dimethylpropanal. Pantonine itself underwent cyclization into the corresponding γ-lactone by reaction with gaseous hydrogen chloride in methanol.

OXIDATIVE TRANSFORMATION OF FUNCTIONALIZED METHYL KETONES INTO ALPHA -KETOCARBOXYLIC ESTERS

Abstract Methyl ketones are oxidatively transformed into α-keto esters by metal ion assisted alcoholysis of the corresponding β-keto-α, α-dichloroalkyl sulfides.