Cob(I)alamin als Katalysator. 6. Mitteilung [1]. Bildung und Fragmentierung von Alkylcobalaminen,ein Gleichgewichtsprozess zwischen nukleophiler Addition und reduktiver Fragmentierung

Cob(I)alamin as Catalyst. 6. Communication [1]. Formation and Fragmentation of Alkylcobalamins: the Nucleophilic Addition – Reductive Fragmentation Equilibrium Isolated olefines can be saturated using catalytic amounts of cob(I)alamin in aqueous acetic acid; as electron source an excess of zinc dust is added to the solution containing the homogeneous catalyst. During this overall hydrogenation of isolated double bonds intermediate alkylcobalamins are formed (compare e.g. Schemes 2, 4, 5, 7 and 12). Clear evidence is presented that the nucleophilic attack on the isolated double bond is carried out by cob(I)alamin and not by cob(II)alamin also present in the system (see Scheme 3b and 3c). As this catalytic saturation of olefins depends on the pH of the solution, characterized by a slow reaction at pH = 7.0 compared to the same reduction in aqueous acetic acid (see Scheme 2, 2 → 4 , and Scheme 3a), it is reasonable to accept the participation of an electrophilic attack by a proton during the generation of alkylcobalamins. – We use the term nucleophilic addition to describe the formation of alkylcobalamins from a proton, an olefin and cob(I)alamin (compare Schemes 4–7 and 12). A special sequence of experiments showed the nucleophilic addition to be regioselective. Preferentially the higher substituted alkylcobalamin revealed to be produced. Therefore, the nucleophilic addition of cob(I)alamin follows the Markownikoff rule (compare chap. 4: formation and fragmentation of β-hydroxyalkylcobalamins). Under the reaction conditions applied the intermediate alkylcobalamins can be present in base-on and base-off forms. They are known to exist as octahedral complexes and might also be stable to some extent as tetragonal-pyramidal species. In addition the base-off forms can partially be protonated at the dimethylbenzimidazole moiety in aqueous acetic acid (compare Scheme 12). From this equilibrium of intermediate alkylcobalamins three modes of decay disclosed to be possible: (i) The reductive fragmentation leading to an olefin, a proton, and cob(I)alamin is the formal retro-reaction of the nucleophilic addition (see Schemes 2, 4 and 6–12). This equilibrium of an associated alkylcobalamin and the corresponding dissociation products revealed to be a fast process compared to the reductive cleavage of the Co, C-bond cited below (s. (iii)). (ii) As the second reaction pattern an oxidative fragmentation producing an olefin, a hydroxy anion (or water, respectively) and cob (III)alamin has been observed (see Schemes 7, 8, 10 and 12). (iii) The slow reductive cleavage of the Co, C-bond, initiated by addition of electrons (see [1a] [24]), was the third reaction path observed (see Schemes 2, 4–8 and 10–12). – The stereochemistry of the three transformations originating from the intermediate alkylcobalamins is unknown up to now. The antiperiplanar pattern of the fragmentation reactions presented in the Schemes has been chosen arbitrarily (see e.g. Scheme 12).     

Aspects of the Reduction of Double Bonds Using Cob (I)alamin as Catalys

The olefinic system in 3β-methoxy-4-cholesten-6 a-ol ( 2 ) is reduced using cob (I)alamin ( 1 ( I ); see Scheme 1) as catalyst, aqueous acetic acid as solvent and metallic zinc as electron source (cf. Schemes 2 and 3). Experimental evidence for an attack of 1 ( I ) on both faces of the double bond is presented. By the same catalyst (1 R)-10, 10-dimethyl-2-pinene- 10-carbonitrile ( 9 ) is first transformed to the menthene derivative 11 (see Schemes 4 and 5). The ring opening is then followed by a fast saturation of the disubstituted olefinic system in 11 , and ultimately the remaining double bond is reduced in a slow reaction. The cis-configurated saturated menthane derivative 16 is the main final product ( 16 / 17 ≈ 10:1).     

Cob(I)alamin Differentiating Alkenes During Saturation

The olefins 2, 7, 11 , and 19 , have been reduced using catalytic amounts of cob(I)alamin( I(I) ). During a slow saturation, the catalyst is able to differentiate the two diastereotopic faces of the endocyclic double bonds in 11 (t_1/2 4 h,. cf. Scheme 3) are reduced much faster. A rationalization of the data can be obtained formulating tertiary alkylcobalamins as intermediates. Of the oxime 6 (cf. Scheme 2) and the p- bromobenzoate 23 (cf. Scheme 5) the structures have been determined by X-ray analysis.     

Cob(I)alamin as Catalyst. 8th Communication. Cob(I)alamin and Heptamethyl Cob(I)yrinate During the Reduction of α,β-Unsaturated Carbonyl Derivatives

Hydrogen bonds as presented in Figure 2 cannot account for the enantioselective attack of cob(I)alamin ( 4 ( I )) or heptamethyl cob(I)yrinate ( 5 ( I )) on one of the two enantiotopic faces of the substrates. The attack of the strongly nucleophilic 3d orbital is preferentially directed to the re-side of the starting materials with (Z)-configuration and leads, after the highly stereoselective reductive cleavage of the Co, C bond, to saturated products with (S)-configuration in varying enantiomeric excesses (see Schemes 1, 3 and Table 1).     

Skeletal Migrations Observed during the Cob(I)alamin-Catalyzed Reduction of 4β-(tert-Butyl)-1β-(1-methylvinyl)cyclohexanecarbaldehyde

During the cob(I)alamin( 1(I) )-catalyzed reduction of 3 , intermediate formation of 2 and final generation of 4–10 was observed (see Scheme 1, cf. Tables 1 and 2). Identical products in similar ratios were generated starting from either 2 or 3 . Accepting the intermediate formation of six interconnected cobalt complexes, i.e. A–F (cf. Scheme 2), the generation of all the products observed can be explained.     

Cob(I)alamin als Katalysator. Retention der Konfiguration bei der reduktiv-protonenübertragenden Spaltung der Co,C-Bindung eines Alkylcobalamins. 7. Mitteilung [1]

Cob(I)alamin as Catalyst. 7. Communication [1]. Retention of Configuration during the Reductive Cleavage of the Co, C-Bond of an Alkylcobalamin Using catalytic amounts of cob(I)alamin (see Scheme 1) in aqueous acetic acid (?)-α-pinen ( 1 ) and (?)-β-pinen ( 2 ; s. Scheme 3) have been reduced. A large excess of metallic zinc served as electron source. The saturated products 5–8 (see Scheme 3) and the mechanistic aspects of their generation are discussed. The relative amounts of cis- ( 5 ) and trans-pinane ( 6 ) lead to the conclusion that the reductive cleavage of the Co, C-bond accompanied by H⁺ transfer in an alkylcobalamin occurs with retention of configuration. This result is in agreement with the corresponding cleavage of the Co,C-bond of an alkyl[hydroxy-diazaoctahydroporphinato]cobalt complex [9].     

Cob(I)alamin als Katalysator, 5. Mitteilung [1]. Enantioselektive Reduktion α,β-ungesättigter Carbonylderivate

Cob(I)alamin as Catalyst. 5. Communication [1]. Enantioselective Reduction of α,β-Unsaturated Carbonyl Derivatives The cob(I)alamin-catalyzed reduction of an α,β-unsaturated ethyl ester in aqueous acetic acid produced the (S)-configurated saturated derivative 2 with an enantiomeric excess of 21%. The starting material 1 is not reduced at pH = 7.0 in the presence of catalytic amounts of cob(I)alamin (see Scheme 2). It is shown that the attack of cob(I)alamin and not of cob(II)alamin, also present in Zn/CH₃COOH/H₂O, accounts for the enantioselective reduction observed. All the (Z)-configurated starting materials 1 , 3 , 5 , 7 , 9 and 11 have been transformed to the corresponding (S)-configurated saturated derivatives 2 , 4 , 6 , 8 , 10 and 12 , respectively. The highest enantiomeric excess revealed to be present in the saturated product 12 (32,7%, S) derived from the (Z)-configurated methyl ketone 11 (see Scheme 3 and Table 1). The reduction of the (E)-configurated starting materials led mainly to racemic products. A saturated product having the (R)-configuration with a rather weak enantiomeric excess (5.9%) has been obtained starting from the (E)-configurated methyl ketone 23 (see Scheme 5 and Table 2). The allylic alcohols 16 and 24 have been reduced to the saturated racemic derivative 17 .     

Cob(I)alamin-Catalyzed Skeletal Migrations Observed during the Reduction of 4β-(tert-Butyl)-1α-(1-methylvinyl)cyclohexanecarbaldehyde

The cob(I) alamin (1(I)) -catalyzed² transformation of the aldehyde 2 has been studied (cf. Table 1). Kinetic examinations showed a rapid isomerization of 2 to 3 (cf. Fig. 1 and 2). From the transformations in glacial AcOH, the two cyclopropanols 5 and 7 were isolated as main products (cf. Tables 1–3 and Fig. 1 and 2). Using large amounts of 1(I) , the aldehyde 4 was very slowly transformed. Accepting the intermediate formation of 6 interconnected Co-complexes, i. e. A , B , C , D , E and F (cf. Scheme), the generation of all the products observed can be explained.     

Asymmetric Catalysis by Vitamin B12. The Mechanism of the Cob(I)alamin-Catalyzed Isomerization of 1,2-Epoxycyclopentane to (R)-Cyclopent-2-enol

The isomerization of 1,2-epcxycyclopentane ( 1 ) to enantiomerically enriched (R)-cyclopent-2-enol ( 2 ) in protic solvents is catalyzed by cob(I)alamin. The enantiomeric excess (e.e.) of (R)- 2 is usually ca. 60%; it is only slightly dependent on the temperature, but increases with decreasing dielectric constant ε of the solvent. Standard kinetic methods show the reaction to be first order in vitamin B₁₂ and zero order in 1 . The rate constant increases exponentially with increasing ε of the solvent. An Arrhenius plot at ε = 40 gives activation parameters ΔH^≠ = 78 ± 4 kJ·mol^?1 and ΔS^≠ = ?49 ± 1 J·mol^?1·K^?1. The isomerization 1 → 2 proceeds in two steps (Schemes 2 and 7): (i) The epoxide ring is first opened by the proton-assisted fast and irreversible nucleophilic attack of the chiral Co^I catalyst to form diastereoisomeric (1R,2R)- and (1S,2S)-(2-hydroxycyclo-pentyl)cob(III)alamins 6 in a ratio of ca. 4:1 which are the dominant species in the steady state; (ii) The intermediates 6 then decompose in the rate-limiting step to form 2 and recycled catalyst. Experiments with specifically ²H-labeled 1 showed the hydro-cobalt elimination 6 → 2 to be non-stereoselective. It proceeds via reversible Co? C bond homolysis to a free 2-hydroxycyclopentyl radical from which stereoelectronically controlled H-abstraction by Co¹¹ takes place.     

Cob (I)alamin als Katalysator 4. Mitteilung. Reduktion von α,β-ungesättigten Nitrilen

Cob(I)alamin as Catalyst. 4. Communication. Reduction of α,β-Unsaturated Nitriles Using catalytic amounts of cob (I)alamin and an excess of metallic zinc as source of electrons 1-naphthonitril ( 5 ) has been reduced to (1-naphthyl)methylamin ( 6 ) and in small amounts to (1-naphthyl)methanol ( 7 ) and (1,2,3,4-tetrahydro-1-naphthyl)methanol ( 8 ) (5 ½ h, CH₃COOH/H₂O; s. Scheme 3). Starting from cyclododecylideneacetonitrile ( 15 ) similar conditions (68 h, CH₃COOH/H₂O) produced the amines 16–19 as well as the nitrogen free saturated aldehyde 20 , the corresponding allylic alcohol 21 and the saturated derivative 22 (s. Scheme 6). It is deduced that the first attack of cob (I)alamin on an α,β-unsaturated nitrile might occur on both the nitrile dipole as well as on the carbon atom in β-position. Cob (I)alamin in aqueous acetic acid saturates the isolated double bonds in allylic alcohols and amines. In a slow reaction the two different aromatic rings of (1-naphthyl)methanol ( 7 ) have been reduced giving the corresponding tetrahydronaphthalene derivatives 8 and 12 , and in one case the production of the octahydroderivative 14 has been observed in a low yield (s. Scheme 5).     

Eine chiral ökonomische Totalsynthese von (R)- und (S)-Muskon via Epoxysulfoncyclofragmentierung

A chiral economic synthesis of (R)- and (S)-muscone using the cyclofragmentation of epoxysulfones Starting with isobutyric acid (2) and using a microbiological oxidation with pseudomonas putida (S)-β-hydroxy-iso-butyric acid (3) has been prepared. From this /pseudosymmetrical: (see text) intermediate the two enantiomeric bromo derivatives 8 (R) and 20 (S) have been synthesized (cf. scheme 4) by altering the sequence of the reactions (cf. scheme 3). A Grignard reaction starting from the two bromo compounds 8 and 20 and from cyclododecanone 1 produced after hydrogenolysis the two enantiomeric dialcohols 9 and 21 (1 + 8 → 9, 1 + 20 → 21 , cf. scheme 5). The subsequent transformations led to the two enantiomeric olefin derivatives 12 and 24 . Oxidation of 12 with peracid produced a mixture of the two epoxy-sulfones 13 and 14 (cf. scheme 6). The olefin-derivative 24 was oxidized to the corresponding mixture of 25 and 26 . A one pot cyclofragmentation (cf. [4] and scheme 6) produced a mixture of (E)- and (Z)-3-methylcyclopentadec-4-en-1-one (13 + 14 → 15 + 16, 25 + 26 → 27 + 28) . The final hydrogenation led to natural (R)- and unnatural (S-muscone (3-methylcyclopentadecanone). The achiral starting material has been transformed to the desired optically active target products without loss of material with undesired absolute configuration. The authors used the notion of chiral economic synthesis to characterize synthetic sequences with the above mentioned features.     

the Mechanism of the Oxidation of Cob(I)alamins

Abstract

Spectroelectrochemical investigations of the reoxidation sequence of the reduced cob(I)alamin to the oxidized cob(III)alamin show that two different cob(II)alamin intermediates are formed during the processes which appear to correlate to base-on and base-off cob(II)-alamin species.     

Oxidative Cyclopropanol Opening by Cob(III)alamin

Starting from the cyclopropanol 2 , the isomeric cyclopropanol 4 and the β, γ-unsasturated aldehydes 7 and 8 have been produced by a cobalamin-dependant transformation. In traces, the two acetoxycyclopropanes 3 and 6 , the saturated aldehydes 5 and 11 and the β,γ-unsaturated aldehyde 9 could be detected (cf. Structural Formulae and Table). Starting from 4 the same products in a rather similar distribution were obtained. The isomerization 2?4 as well as the transformations leading to 7,8 , and 9 are shown to be mediated by cob(III)alamin (1(III)) . The results are explained on the basis of rearranging Co-complexes. The migrations might be driven by the electrophilic nature of the central Co(d⁶)-atom.     

1,3-Dipolare Cycloadditionen von 2-(Benzonitrilio)-2-propanid mit 4,4-Dimethyl-2-phenyl-2-thiazolin-5-thion und Schwefelkohlenstoff

1,3-Dipolar Cycloadditions of 2-(Benzonitrilio)-2-propanide with 4,4-Dimethyl-2-phenyl-2-thiazolin-5-thione and Carbon Disulfide Irradiation of 2,2-dimethyl-3-phenyl-2H-azirine ( 11 ) in the presence of 4,4-dimethyl-2-phenyl-2-thiazolin-5-thione ( 7 ) yields a mixture of the three (1:1)-ad-ducts 8 , 12 and 13 (Schemes 3 and 6). The formation of 8 and 12 can be explained by 1,3-dipolar cycloaddition of 2-(benzonitrilio)-2-propanide ( 1b ) to the C, S-double bond of 7. Photochemical isomerization of 12 leads to the third isomer 13 (Schemes 3 and 7). Photolysis of the azirine 11 in the presence of carbon disulfide gives a mixture of two (2:l)-adducts, namely 12 and 13 (Scheme 4). A reaction mechanism via the intermediate formation of the 3-thiazolin-5-thione b is postulated. The structure of the heterocyclic spiro compound 13 has been established by single-crystal X-ray structure determination (cf. Fig. 1 and 2).     

Photochemische Reaktionen. 117. Mitteilung [1] Zur Photochemie von 5,6-Epoxydienen und konjugierten 5,6-Epoxytrienen

Photochemistry of 5,6-Epoxydienes and of Conjugated 5,6-Epoxytrienes On singulet excitation (δ = 254 nm) the 5,6-epoxydiene 6 and the conjugated 5,6-epoxytrienes 7 and 8 exclusively give products arising from cleavage of the C, C-bond of the oxirane (cf. 6 → 9 , 10 , 11 ; 7 → (E)- 15 , 16 , 17 ; 8 → 18 (A+B) , 19 (A+B) , 20 , 21 ). The dihydrofuran compounds 11 and (E/Z)- 15 are formed by cyclization of a ketonium-ylide a and d , respectively. Photolysis of a gives the carbene b which yields the cyclopropene 9 , whereas d forms photochemically the carbenes f and g which yield the methano compounds 16 and 17 . The isomeric cyclopropene derivatives 20 and 21 are products of the intermediates h and i , respectively, which are formed by photolysis of the ylide e . The cyclopropene 21 isomerizes by intramolecular cycloadditions to 18 (A+B) and 19 (A+B) . - On triplet excitation (λ?LD nm; 280 nm; acetone) 6 undergoes cleavage of the C(5), O-bond and isomerizes to 12 and 14 . However, 7 is converted by cleavage of the C, C-bond of the oxirane to yield 15 . On treatment with BF₃O(C₂H₅)₂ 6 gives 14 , whereas 7 yields 22 , and 8 forms 23 and 24 .     

Total Synthesis of β-Bulnesene and 1-Epi-β-bulnesene by Intramolecular Photoaddition

dl-β-Bulnesene (1) and dl-1-epi-α-bulnesene (15) have been synthesized starting from the bromide 4 (Schemes 2 and 3). In the key step 9→10 the bonds of the final product were formed by an intramolecular photoaddition. The synthesis was completed by the fragmentation 12→14 and the Wittig reaction 14→15+1 .     

Basenkatalysierte Cyclisierungen von 2-(2-Propinyl)oxybenzamiden

Base Catalysed Cyclizations of 2-(2-Propynyl)oxy-benzamide Systems 2-(2-Propynyl)oxy-benzamides were cyclized under base catalysis to 6- or 7-membered ring compounds, depending on the reaction conditions. Treatment of 2-(2-propynyl)oxy-benzamide ( 10 ) with sodium methylsulfinylmethanide (NaMSM) in DMSO gave two isomeric oxazepinons 11 (34%) and 12 (7%), while the transformation with sodium-2-propanolate in 2-propanol afforded the oxazinone 13 (34%) and with lithium cyclohexyl-isopropylamide (Li-CHIP) in N-methylpyrrolidone 11 (48%) exclusively (Scheme 4). N-Methyl-2-(2-propynyl)-oxy-benzamide ( 14 ) behaved similarly. In the reaction of 14 with sodium 2-propanolate in 2-propanol yielding the benzoxazinone 16 , the allenyloxy-benzamide 17 could be isolated as an intermediate (Scheme 5). The N-phenyl-compounds 18 and 22 treated with NaMSM/DMSO were converted to 3-anilino-2-methyl-benzo- and naphtho-pyran-4-ones, respectively (Schemes 6 and 7). The mechanisms for these reactions are discussed (Schemes 8, 9 and 10).     

Unexpected Thermal Transformation of Aryl 3‐Arylprop‐2‐ynoates: Formation of 3‐(Diarylmethylidene)‐2,3‐dihydrofuran‐2‐ones

A number of aryl 3‐arylprop‐2‐ynoates 3 has been prepared (cf. Table 1 and Schemes 3 – 5). In contrast to aryl prop‐2‐ynoates and but‐2‐ynoates, 3‐arylprop‐2‐ynoates 3 (with the exception of 3b ) do not undergo, by flash vacuum pyrolysis (FVP), rearrangement to corresponding cyclohepta[b]furan‐2(2H)‐ones 2 (cf. Schemes 1 and 2). On melting, however, or in solution at temperatures >150°, the compounds 3 are converted stereospecifically to the dimers 3‐[(Z)‐diarylmethylidene]‐2,3‐dihydrofuran‐2‐ones (Z)‐ 11 and the cyclic anhydrides 12 of 1,4‐diarylnaphthalene‐2,3‐dicarboxylic acids, which also represent dimers of 3 , formed by loss of one molecule of the corresponding phenol from the aryloxy part (cf. Scheme 6). Small amounts of diaryl naphthalene‐2,3‐dicarboxylates 13 accompanied the product types (Z)‐ 11 and 12 , when the thermal transformation of 3 was performed in the molten state or at high concentration of 3 in solution (cf. Tables 2 and 4). The structure of the dihydrofuranone (Z)‐ 11c was established by an X‐ray crystal‐structure analysis (Fig. 1). The structures of the dihydrofuranones 11 and the cyclic anhydrides 12 indicate that the 3‐arylprop‐2‐ynoates 3 , on heating, must undergo an aryl O→C(3) migration leading to a reactive intermediate, which attacks a second molecule of 3 , finally under formation of (Z)‐ 11 or 12 . Formation of the diaryl dicarboxylates 13 , on the other hand, are the result of the well‐known thermal Diels‐Alder‐type dimerization of 3 without rearrangement (cf. Scheme 7). At low concentration of 3 in decalin, the decrease of 3 follows up to ca. 20% conversion first‐order kinetics (cf. Table 5), which is in agreement with a monomolecular rearrangement of 3 . Moreover, heating the highly reactive 2,4,6‐trimethylphenyl 3‐(4‐nitrophenyl)prop‐2‐ynonate ( 3f ) in the presence of a twofold molar amount of the much less reactive phenyl 3‐(4‐nitrophenyl)prop‐2‐ynonate ( 3g ) led, beside (Z)‐ 11f , to the cross products (Z)‐ 11fg , and, due to subsequent thermal isomerization, (E)‐ 11fg (cf. Scheme 10), the structures of which indicated that they were composed, as expected, of rearranged 3f and structurally unaltered 3g . Finally, thermal transposition of [¹⁷O]‐ 3i with the ¹⁷O‐label at the aryloxy group gave (Z)‐ and (E)‐[¹⁷O₂]‐ 11i with the ¹⁷O‐label of rearranged [¹⁷O]‐ 3i specifically at the oxo group of the two isomeric dihydrofuranones (cf. Scheme 8), indicating a highly ordered cyclic transition state of the aryl O→C(3) migration (cf. Scheme 9).     

Preparation and Electrophilic Substitution of 1-Trimethylsilylpentadienyllithium

Pentadienyllithium (16) was regioselectively and efficiently transformed to 1-trimethylsilyl-2,4-pentadiene (17) by reaction with chlorotrimethylsilane (Scheme 5). Deprotonation of 17 and subsequent electrophilic attack furnished the regioisomeric products 12 and/or 13 in good yields (Schemes 5 and 6). The utility of the reaction 18 → 12 for the convergent assembly of the 1-silyl-1,3-butadiene unit with an olefinic dienophile is further illustrated by the smooth intramolecular Diels-Alder reaction 19 → 20 .     

Double-Bond Shifts in [4n]Annulenes as a New Principle for Molecular Switches: First Results with Dimethyl Heptalene-1,2- and -4,5-dicarboxylates

A new concept for molecular switches, based on thermal or photochemical double-bond shifts (DBS) in [4n]annulenes such as heptalenes or cyclooctatetraenes, is introduced (cf. Scheme 2). Several heptalene-1,2- and -4,5-dicarboxylates (cf. Scheme 4) with (E)-styryl and Ph groups at C(5) and C(1), or C(4) and C(2), respectively, have been investigated. Several X-ray crystal-structure analyses (cf. Figs. 1–5) showed that the (E)-styryl group occupies in the crystals an almost perfect s-trans-conformation with respect to the C?C bond of the (E)-styryl moiety and the adjacent C?C bond of the heptalene core. Supplementary ¹H-NOE measurements showed that the s-trans-conformations are also adopted in solution (cf. Schemes 6 and 9). Therefore, the DBS process in heptalenes (cf. Schemes 5 and 8) is always accompanied by a 180° torsion of the (E)-styryl group with respect to its adjacent C?C bond of the heptalene core. The UV/VIS spectra of the heptalene-1,2- and -4,5-dicarboxylates illustrated that it can indeed be differentiated between an ‘off-state’, which possesses no ‘through-conjugation’ of the π-donor substituent and the corresponding MeOCO group and an ‘on-state’ where this ‘through-conjugation’ is realized. The ‘through-conjugation’, i.e., conjugative interaction via the involved s-cis-butadiene substructure of the heptalene skeleton, is indicated by a strong enhancement of the intensities of the heptalene absorption bands I and II (cf. Tables 3–6). The most impressive examples are the heptalene-dicarboxylates 11a , representing the off-state, and 11b which stands for the on-state (cf. Fig.8).