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Laccase-catalyzed oxidation of substituted catechols followed by reaction with 4-hydroxy-pyrone/-benzopyrone afforded substituted benzofuran regioisomers whose structures with only two aromatic protons in total prevent a rapid structural assignment. Based on the evaluation of (1)H-(13)C long-range coupling constants a rule of thumb could be deduced for an easy and unambiguous differentiation between the possible regioisomers formed. DFT frontier orbital calculations of the reactants offer an interesting tool to explain the regioselectivity of the key reaction.
The silyl ketene acetal 5 (M = SiMe,) is formed first and is transformed into the propionate 4 b during workup (1 N HCI). Absence of the acid treatment allows the silyl ketene acetal to be isolated.
The addition of the chiral nucleophiles Na-2 and Li-5 to pyridinium compounds 1 and the subsequent ring closure (PictetSpengler cyclization) to indoloquinolizines 3a and ?b, respectively, proceed stereoselectively (d.e. for 3a: 54 %; for 7b: 2 9 5 %). Proof for the absolute configuration of these products is provided by their CD spectral data and the transformation of 7b into the indole alkaloids (Elegant and convergent syntheses of various indole alka-I. Addition of uchirul nucleophiles to chiral pyridinium loids were carried out by Wenkert and coworkers by start-compounds (preferentially with a stereodifferentiating ing from (electrophilic) pyridinium compounds connected to an (electron-rich) indole system ['.2]. This structural "built-up" enables the formation of several C-C bonds in a one pot reaction. According to this fundamental scheme izines D are obtained via 1,4-dihydropyridines B by the addition of appropriate nucleophiles to pyridinium salts A.group at C-3).
NH Nu NHA 0
C
DThe mechanistic details of this initial step [single-electron transfer (SET) vs. ionic pathway] have not been determined yet. So far this method has almost exclusively been used in the synthesis of the racemates of the corresponding natural products. Our investigations focused on its use for a diastereoselective synthesis of indole alkaloids. There are two strategies for the synthesis of enantiomerically pure indole alkaloids of the yohimbane, heteroyohimbane, corynanthe, or vallesiachotamane type according to the strategies I and 11.The prerequisites for these strategies are regioselective addition (i. e. addition at C-4 leading to the formation of the 1 ,Cdihydropyridine B) and stere~electronically[~] controlled bond formation between the intermediary iminium species C (obtained after protonation of B) and the indole moiety to yield indolo [2,3-u]quinolizine D (kinetically controlled Pictet-Spengler reaction)L5I. Preferential attack at C-4 of the pyridinium ring was observed not only in the case of deprotonated malonic diester but also with a series of other nucleophiles (e. g. lithiated ester enolates and trimethylsilyl enol ethers)L6]. Alkyllithium compounds add diastereoselectively to chiral 3-substituted pyridines. The intermediary 1 ,Cdihydropyridines can be trapped with acyl halides to give N-substituted chiral l ,4-dihydropyridine~[',~].
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