Chiral tetraketone ligands are obtained by Claisen-type condensation of 4-bromoacetophenone with the acetone ketal of L-tartraic acid diethylester and lead with gallium(III) or iron(III) ions in self-assembly processes to dinuclear helicate-type cryptands which are able to bind lithium cations.
The chiral tartaric acid derived bis(b-diketonate) ligands 1-3-H 2 diastereoselectively form dinuclear triple-stranded helicates with a series of lanthanide(III) ions
A series of enantiomerically pure bis-b-diketone derivatives was prepared by a Claisen-type condensation between protected tartaric acid diesters and several ketones. Hereby the tartaric acid unit is introduced as either a 2,2-dimethyldioxolane or as a 5,6-dimethoxy-5,6-dimethyl-1,4-dioxane derivative. The yields of the Claisen reaction are relatively low, but the starting materials are readily available, so that the one-step procedure leads to the bis-b-diketone derivatives in up to gram quantities.b-Diketonates are a class of compounds that represents one of the most powerful chelating units for the coordination of metal ions. The obtained complexes are applied in different fields, due to their special magnetic, electrochemical, or photophysical properties. Chiral derivatives are used as NMR shift reagents but are also efficient catalysts for stereoselective reactions. Thus the interest in bdiketonates and their derivatives starts with synthetic chemistry, structural coordination chemistry, and finally enters the field of materials science. 1 Working in the field of supramolecular chemistry and being specially interested in the stereochemistry of selfassembly processes, we were searching for an entry to prepare chiral linear bis-b-diketonate ligands in order to obtain enantiomerically pure helicates or other supramolecular structures, like trinuclear coordination compounds and clusters. 2,3 The obtained oligonuclear complexes with transition or f-block metals could be promising candidates for chiral catalysis or novel luminescent materials.Herein we describe a simple synthetic approach towards a series of bis-b-diketonates applying a Claisen-type condensation reaction as a key step of the synthesis. Hereby we use ester-substituted heterocycles as bis-electrophilic chiral building blocks, which are derived from protected tartaric acid derivatives. Scope and Limitations1,3-Dicarbonyl compounds can be prepared by the reaction of an enolate with carboxylic acid derivatives like esters or acid chlorides. In order to prepare the chiral dioxolanes 3a-g or the dioxanes 4c,d, we performed the reaction of the methyl ketones 1a-g with the tartaric acid derivatives 2a and 2b (see Scheme 1, Figure 1), which are easily prepared according to literature procedures. 4,5 Reaction of acetone (1a) with diester 2a gave 3a as an oily product. In order to prepare different bis-b-diketones, the substituent R at the starting methyl ketones 1 was systematically varied. Thus, the reaction of the acetophenone derivatives 1b-f with 2a proceeded similarly. However, now the corresponding b-diketones could be recrystallized from ethanol and the crystalline materials were characterized by elemental analysis as well as standard spectroscopic methods (see Table 1).In order to extend the p-system in the periphery of the bisb-diketones, we introduced the 4-tolanyl methyl ketone (1g) which was prepared in 69% from phenylacetylene and 4-bromoacetophenone in a Sonogashira coupling reaction. 6 In the following reaction step, the tolanyl-substituted b...
We describe a stereoselective synthesis of the triquinane (±)-cameroonanol using the Lewis acid mediated [3+2] cycloaddition of an allylsilane and a modified Fleming-Tamao oxidation as key steps.In 1998 Weyerstahl and co-workers isolated the sesquiterpene (-)-cameroonan-7a-ol (1) from the essential oil of the rhizomes of Echinops giganteus var. lelyi, an endemic species of Cameroon and Nigeria. 1 Cameroonanol (1) is an example of structurally related angular triquinanes biogenetically deriving from farnesyl pyrophosphate (2) by a sequence of various rearrangement steps (Scheme 1). 1 Although a large number of sesquiterpenes has been identified in the essential oil of E. giganteus, cameroonanol (1) is considered to be the main contributor to its patchoulilike, woody fragrance. The relative and absolute configuration has been assigned based on NMR spectroscopy, a comparison with co-occurring substances and rearrangement to related compounds of known structure. 1-3 As a result of their unique architecture, presenting significant synthetic challenges, triquinanes have received considerable attention in organic synthesis. 3,4 A few years ago, Coates et al. reported the first total synthesis of (±)-cameroonanol [(±)-1]. 3 Herein, we describe the second synthesis of (±)-1 using our allylsilane [3+2]-cycloaddition methodology.Scheme 1 Biogenetic pathway to triquinane sesquiterpenes.In our seminal studies, we have developed a new methodology for the construction of silyl-substituted five-membered rings by Lewis acid promoted [3+2] cycloaddition of allylsilanes, 5,6 which subsequently found diverse applications. 7 Further functionalization of the products became feasible by extending the scope of the classical FlemingTamao oxidation. 8 We devised reaction conditions, which convert even highly sterically hindered silyl groups, e.g. triphenylsilyl, tert-butyldiphenylsilyl and di(isopropyl)phenylsilyl, into hydroxy groups (modified FlemingTamao oxidation). 9 In the present communication, 10 we describe the total synthesis of (±)-cameroonanol (±)-(1) using our [3+2] cycloaddition of allyl-tert-butyldiphenylsilane followed by modified Fleming-Tamao oxidation.Scheme 2 Reagents and conditions: a) 1. TiCl 4 , CH 2 Cl 2 , -20°C, 10 min; 2. allyl-tert-butydiphenylsilane (6), -78°C, 24 h, 43% (ratio of anti-7:syn-7 = 7:1).Enone 5 was prepared using a modification of Paquette's procedure reported by Coates. 3,11 The titanium tetrachloride promoted [3+2] cycloaddition of allyl-tert-butyldiphenylsilane (6) and enone 5 provided the tricyclic silylcyclopentane 7 as a mixture of two diastereoisomers (ratio of anti:syn = 7:1) along with a disilylated byproduct (Scheme 2). The assignment of the relative configuration of 7 is based on our previous studies, 6 using the chemical shift of the silylcyclopentane a-carbon atom in the 13 C NMR spectrum, 12 and by an X-ray crystal structure analysis at a later stage of our synthesis (vide infra). For our synthetic strategy, we could continue with the diastereoisomeric mixture of 7.The next step requ...
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