Eberhard Reimann

Synthesis Pathways to Erythrina Alkaloids and Erythrina Type Compounds

The history of erythrina research begins at the end of the 19th century. During the last two decades of that time extracts from species of Erythrina have been found to exhibit curare-like neuromuscular blocking activities which are caused by alkaloids occurring therein (1, 2, 3, 4).

Protoberberines from Reissert-Compounds VIII [1]. Oxazoloisoquinolines, New and Efficient Educts for the Synthesis of 8-Oxoprotoberberines

Summary. Certain benzylated oxazoloisoquinolinones readily available from Reissert compounds provided an efficient access to 8-oxoprotoberberines in three steps. A series of these new precursors as well as several oxoprotoberberines were prepared and the scope and limitation of this procedure were investigated.

Synthese von Dibenzo[a,g]chinolizinen — Ein neuer Zugang zu Protoberberinen

SummaryA novel synthesis to prepare dibenzo-8-quinolizinones8 is reported using the intramolecular cyclisation of the benzylated Reissert compounds6 for the key step. Methylation of8 by methyllithium gives partial deoxygenated coralynes10 which in turn can be reduced by NaBH4 yielding the 8-methyltetrahydro-protoberberines11. The structures are assigned by NMR-spectroscopy.

Protoberberine ausReissert-Verbindungen, 4. Mitt. Eine neue Synthese des (±)-Alamaridins und des (±)-epi-Alamaridins

SummaryA novel synthesis of 8-methylisoquino[2,1-b][2,7]naphthyridinium salts7 is reported using the intramolecular cyclization of the 1-substituted isoquinolines5 as the key step, which in turn are obtained by alkylation of theReissert compounds1. The salts7 can be reduced by NaBH4 yielding the 8α- and 8β-methylstereomers (±)-alamaridine and (±)-epi-alamaridine9c and11c, respectively.

Protoberberine ausReissert-Verbindungen, 5. Mitt.: Synthese des (±)-Bharatamins

SummaryAlkylation of theReissert compound1 by methyl 2-bromomethylbenzoate (2) followed by spontaneous intramolecular cyclization affords dibenzo [a,g]quinolizinone4 which in turn can be reduced to benzyl ether5 by LiAlH4/NaBH4. Debenzylation of5 by ethanolic HCl furnishes the title compound6 in improved total yield.

Synthese des (±)-Rosmarinsäuremethylesters

SummaryRegioselective cleavage of the glycidic ester3 by BF3/ether to pyruvic acid ester4 followed by NaBH4 reduction affords the lactic acid derivative5a which in turn can be acylated by caffeoyl chloride8c to yield the O-protected rosmarinic acid ester9a. Alternatively,9a can be prepared by acylation of5a with diethylphosphono acetyl chloride (6c) thus generating theWadsworth-Emmons reagent7a which is subsequently reacted with the aldehyde1a. The analogous reaction using the silyl protected educts7d and1b failed to give9d. Finally,9a is debenzylated by BCl3 furnishing the title compound10a in fair total yield.

Selective Catalytic Hydrogenations and Hydrogenolyses VIII [1]: Stereoselective Synthesis of the Stereomeric Pilopyl Alcohols

Summary. The stereoselective synthesis of pilopyl- and isopilopyl alcohol is reported. The reaction of dimethyldioxanone and diethoxyphosphoryl-butyric acid ethyl ester afforded the corresponding dioxanylidenbutyric acid ester as the key intermediate. Upon treatment with mineral acid it cyclized giving 3-ethyl-4-hydroxymethylfuran-2-one which in turn could be converted either to 3-ethyl-4-methylfuranone or pilopyl alcohol with excellent stereoselectivity and quantitative chemical yield. On the other hand, hydrogenation and subsequent cyclization of the same key compound furnished isopilopyl alcohol with good stereomeric purity and yield.

Protoberberine aus Reissert-Verbindungen, 7. Mitt. [1]. Diastereoselektive Reduktion von 1-benzylierten Isochinolin-Reissert-Verbindungen und Synthese von Oxoberbincarbonsäuren

Summary. Reduction of 1-substituted Reissert compounds 1 by NaBH3CN in a mixture of trifluoroacetic and acetic acid diastereoselectively affords oxazoloquinolinones 2 as the main products; in addition, aldehydes 3 and 4 and the dibenzyl derivative 5 are formed. The structure of 2 was characterized by X-ray crystallography. Upon treatment with potassium hydroxide in ethanol, 2 is cleaved and reacylated yielding the 13a-carboxyberbinones 7. Decarboxylation of 7 occurs readily to afford a 1:2 mixture of berbinones 10 and 11.

Protoberberine aus Reissert-Verbindungen, 6. Mitt. [1]: Diastereoselektive Synthese und relative Konfiguration von 2-Benzoyl-1-cyano-1-(1-phenylalkyl)-1,2-dihydroisochinolinen

Summary. The alkylation of Reissert compounds 8 by sec-benzyl bromides 4, 7, and 10 diastereo-selectively affords the title compounds 11 and 12. X-Ray structure analysis confirms an opposite configuration of the chiral centers in 11 and 12. The benzyl bromides 4, 7, and 10 are prepared by standard procedures.

Synthese der enantiomeren (+)- und (−)-Rosmarinsäuremethylester

Summary. Starting from L-tyrosine (L-1), the phenyl lactic acid ester (−)-4 was prepared by reported procedures. (−)-4 could then be converted to the benzyl ether (−)-5. From (−)-5, the corresponding optical antipode (+)-5 is available via (+)-6 by Mitsunobu reaction. The en-antiomeric excess of 5 was determined by NMR spectroscopy from camphanic acid esters (+)- and (−)-8, respectively. The phenyl lactic acid esters (+)- and (−)-5 were acylated by caffeoyl chloride (9) to yield the O-protected enantiomeric rosmarinic acid esters (+)- and (−)-10. Deprotection of 10 by BCl3 afforded the title compounds (+)- and (−)-11 in fair yields.