Hans-Dieter Scharf

Chiral induction in photochemical reactions - 15.: Detection of stereoelectronic effects by temperature dependent measurements of the diastereoselectivity in the photosensitized [2+2]-cycloaddition

Abstract The mechanism of the diastereoselection in the photosensitized[2 + 2]-cycloaddition reaction of ethylene to 5-alkoxy-2(5H)-furanones is investigated. The dependence of product ratio on the reaction temperature as well as on structural features of the enone system is measured. The structure dependent activation parameter differences (ΔΔH≠,ΔΔS≠) obtained by this method (modified Eyring plot) are used as a tool to characterize the stereoelectronic factors, which are responsible for the diastereoselection. In detail these factors involve: pyramidalization of the β-carbon, homoanomeric effect, and the steric requirements of the alkoxy substituent in the relaxed (3ππ*)-excited furanones.

Chiral induction in photochemical reactions-XII. Synthesis of chiral cyclobutane derivatives from (+)-5-menthyloxy-2-[5H]-furanone and ethylene☆

Abstract The temperature dependence of the diastereoselectivity in the photochemical [2 + 2]-cycloaddition of (+)-5-menthyloxy-2-[5H]-furanone to ethylene is investigated and the preparative isolation of the pure (1R,4S,5S)-as well as (1S,4S,5R)-4-(+)-menthyloxy-3-oxa-2-oxobicyclo-[3.2.0]-heptane is described.

Synthesis of methyl 2,6-dideoxy-4-thio-α-D-ribo-hexopyranoside, a new thio sugar found in calichemicins

Abstract The radically induced rearrangement of the 3,4-O-thionocarbonate 4 leads to the two regioisomeric thiolcarbonates 5 and 6 , respectively. Alkaline hydrolysis affords the title compound 7 and its regioisomer 8 .

The temperature dependence of the diastereoselective reduction of 2-t-butylcyclohexanone with diisobutyl-aluminium-2,6-di-t-butyl-4-methylphenoxide

Abstract The diastereoselectivity of the reduction of 2-t-butylcyclohexanone (1) with diisobutylaluminium-2,6-di-t-butyl-4-methylphenoxide (2) is investigated as a function of the temperature and the conversion. The relevant selection steps are characterized on the basis of the isoinversion principle. The kinetically controlled formation of the alcoholate intermediates as well as the thermodynamically controlled Meerwein-Ponndorf-Verley type equilibration are recognized as the two selection steps of the reduction.

Syntheses of 4-hydroxy-5-iodo-2,3-dimethoxy-6-methylbenzoic acid and 5-bromo-4-hydroxy-2,3-dimethoxy-6-methylbenzoic acid - aromatic constituents of the calichemicins

Abstract 4-Hydroxy-5-iodo-2,3-dimethoxy-6-methylbenzoic acid ( 1 ) 5-bromo-4-hydroxy-2,3-dimethoxy-6-methylbenzoic acid ( 2 ) are the polysubstituted aromatic carboxylic acids found in calichemicin antibiotics.l The title compounds are synthesized on preparative scale from readily available methyl 4-O-benzylorsellinate ( 3 ) in six steps without any chromatographic separation needed [( 1 ): 41%, ( 2 ):44%, overall yield].

Synthese von isobutenyl-cyclobutancarbonsäuren als vierring-analoga zur chrysanthemumsäure teil i

Zusammenfassung Die racemischen 4(2-Methylpropenyl)-cyclobutan-1-carbonsauren 2,3 und 4 wurden ausgehend von Maleinaldehydsaurepseudoethylester (5) durch photosensibilisierte (2 + 2)-Cyclo-addition mit 2,3-Dimethyl- buten-2 bzw. Isobuten als Partner dargestellt. Die resultierenden Cycloaddukte fuhren zu den entsprechenden Cyclobutanaldehydcarbonsauren bzw. deren Pseudosauren, die anschliessend in einer Wittigschen Olefinierungsreaktion zu den Isobutenyl-cyclobutancarbonsaurederivaten 2, 3 und 4 umgesetzt werden. Diese zu der naturlich vorkommenden Chrysanthemumsaure1 cyclobutananalogen Carbonsauren werden untersucht.

The first total synthesis of 15-epi-annonin I

Abstract The first total synthesis of the 15-epimer 1b of the naturally occurring acetogenin annonin I 1a is described. All stereogenic centres in the bis-THF subunit are controlled via successive Sharpless asymmetric epoxidations (AE). Alkyl side chains were attached directly to the core using alkyne-epoxide couplings.

Synthesis and characterization of both enantiomers of trans-1,2-di-(2-hydroxy-2-propyl)-cyclobutane

Abstract Both enantiomers of the title compound are synthesized for the first time in four steps including a [2+2]-photocycloaddition to build up the cyclobutane backbone and a cerium supported reductive permethylation. During the cerium supported process a complete and selective change in the configuration of the cyclobutane ring takes place. The substituents are transformed from the cis - to the trans -geometry.

Die photooxidation von chlorid zu chlor mit anthrachinonderivaten in wässriger lösung : Ein beitrag zur speicherung von sonnenenergie als chemische energie

Abstract Chloride transfers an electron to photochemically excited anthraquinonesulfonic and -methanosulfonic acid in its triplet state. In presence of oxygen the obtained semiquinone radical of the anthraquinonederivative is oxidized to the ground state anthraquinone, ready for another reactioncycle. The chloride is oxidized to chlorine. The quantum yield of this photooxidation, which also proceeds in sunlight, is found to be between 0.1 and 0.13. The absorbed light energy is partly stored in chemical energy of the chlorine molecule. The quantum yield is compared to the anthraquinone phosphorescence emission and the lifetime of the triplet state.

Synthesis of cyclobutanetricarboxylic acid derivatives via intramolecular [2+2] photocycloaddition

Abstract 5-Cinnamyloxy-4-methyl-2(5H)-furanone 1 undergoes intramolecular [2+2] photocyclo-addition when irradiated in acetone as solvent and sensitiser. cis/trans-Isomerization at the double bond of the cinnamyl moiety is also observed. The time dependence of the product ratio of the photolysis of 1 is reported. A photostationary equilibrium between the cyclobutanes 4 and 5 is established by irradiation of 5. 4 (obtained in 78 % yield) is converted to the 2-methyl-4-phenylcyclobutane-1,2,3-tricarboxylic acid 7 in 73 % yield.