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Notes: The mechanism of the catalysis of the reversible (propargyl ester)/(allenyl ester) rearrangement (10 ⇄ 11) by silver(I) ions was investigated, using optically active and diastereoisomeric esters as well as 14C- and 18O-labelling. In order to work with crystalline materials, mainly p-nitrobenzoates (10, 11: R4 = p—O2N—C6H4) were used. In some cases the rearrangement 10 ⇄ 11 was studied using acetates (R4 = CH3). The alkyl substituents R1, R2, R3, were widely varied (cf. Tables 1, 2). The solvents in which the rearrangements were performed were in most cases dry chlorobenzene and 96% aqueous dioxane. Silver tetrafluoroborate, the benzene complex of the latter, and silver trifluoroacetate (in chlorobenzene) as well as silver nitrate (in aqueous dioxane) served as catalysts. The amounts of the silver catalysts used varied between 0,5 and 10 mol-%; reaction temperatures applied were in the range 35-95°, The results obtained are as follows: 1The rate-determining step of the (propargyl ester)/(allenyl ester) rearrangement (10 ⇄ 11) occurs in a silver(I) complex with the substrates (10, 11), which is formed in a pre-equilibrium. This follows from kinetic experiments (cf. Fig. 6, 7, 8, 10) and the fact that the rate of rearrangement (of 10a) is strongly decreased when cyclohexene is added (cf. Fig. 9). In solvents which are known to form complexes with silver(I) ions the rate of rearrangement (of 10a)is much slower than in solvents with similar dielectric constants but with small capacity for complex formation with silver(I) ions (cf. Table 4). Taking into account what is known about silver(I)-alkene and -alkyne complexes (cf. [18]), it is obvious that the (propargyl ester)/(allenyl ester) rearrangement occurs in a π-complex of the silver(I) ion with the triple bond in the propargyl ester or one of the two C,C double bonds in the allenyl ester, respectively.2The shift of the carboxyl moiety in the reversible rearrangement 10 ⇄ 11 occurs intramolecularly. p-Nitrobenzoic acid-[carboxyl-14C] is not incorporated during the rearrangement, neither in the reactant 10 nor in the product 11 and vice versa. A crossing experiment gave no mixed products (cf. Scheme 2, p. 882).3An internal ion pair can be excluded for the rearrangement 10 ⇄ 11 because the 18O-carbonyl label in the reactant is found exclusively in the alkoxy part of the product (cf. Scheme 3, p. 886, and Table 9). Thus, the rearrangement 10 ⇄ 11 occurs with inversion of the carboxyl moiety.4The rearrangement of optically active propargyl esters (10g, 10i) leads to completely racemic allenyl esters (11g, 11i). However, rearrangement of erythro- and threo-10j-[carbonyl-18O] (Scheme 3) shows that the stereospecifically formed allenyl esters erythro- and threo-11j-[18O]-epimerize rapidly in the presence of silver(I) ions. This epimerization is twice and forty times, respectively, as fast as the rearrangement of the corresponding propargyl esters (cf. Fig. 1-5). During epimerization or racemization the 18O-label is not randomized (cf. also Scheme 4, p. 898).5The equilibrium of the rearrangement 10 ⇄ 11 depends on the bulkiness of the substituents R1, R2, R3 and of the carboxyl moiety (cf. Table 2).Taking into account these facts (points 1-5), the reversible (propargyl ester)/(allenyl ester) rearrangement promoted by silver(I) ions can be described as a [3s, 3s]-sigmatropic reaction occurring in a silver(I)-π-complex with the C,C triple bond in 10 and a C,C double bond in 11. It is suggested that complex formation in 10 and 11 occurs with the π-bond which is not involved in the quasicyclic (containing six orbitals and six electrons) transition state of the rearrangement (Fig. 11). Thus, the rearrangement is of a type which has recently been called a charge-induced sigmatropic reaction (cf. [26]). Therefore, in our case, the catalysis by silver(I) ions is of a different type from that of transformations of strained cyclic molecules promoted by silver(I) ions (cf. [14] [16] [27]-[31]).Side reactions. Whereas the rearrangement of propargyl esters 10 in presence of silver tetra- fluoroborate in chlorobenzene or silver nitrate in aqueous dioxane leads to the corresponding allenyl esters 11, the rearrangement of 10 with silver trifluoroacetate, especially in the presence of trifluoroacetic acid, results in the formation of the dienol esters 12 and 13, which clearly are derived from 11 (see Scheme 1, p. 881). As shown by the rearrangement of 11 in the presence of p-nitrobenzoic acid-[carboxyl-14C], 12 and 13 arise in part from a not isolated di-p-nitrobenzoate (cf. Scheme 6, p. 905), since radioactivity is found in 12 and 13.

Notes: From the leaves of the Apocynaceae Pleiocarpa talbotii WERNHAM three new indole alkaloids were isolated and identified as 3,4,5,6-tetradehydrotalbotine (2), 5,6-dehydrotalbotine (3) and deformyl talbotinic acid methylester (4). From the stem bark of the same plant normacusine B (tombozine) was isolated.

Notes: (±)-Oncinotin (19) is synthesized through the crucial intermediates 11 and 15. By comparison of the synthetic product with natural (-)-oncinotin, it is found that the latter contains an isomeric substance, represented by structure 20.

Notes: 3-Phenylisocumarine was isolated for the first time from natural source (leaves of Homalium laurifolium JACQ.).

Notes: The purely aliphatic 2,3-dipropyl-2H-azirine (1) reacts on irradiation with a mercury high-pressure lamp through a Vycor filter with methyl trifluoroacetate or acetone to form 3-oxazolines 3a, b (65%) resp. 4 (14%) (Scheme 1). 9-Azabicyclo[6.1.0]non-1(9)-ene (5) on irradiation in the presence of the dipolarophiles methyl trifluoroacetate, methyl difluoroacetate, 1,1,1-trifluoro-propanone and acetone behaves in a similar way, whereby the corresponding bicyclic 3-oxazolines 7-10 result in yields of 60-20% (Scheme 2).By analogy with the photochemical behaviour of 3-aryl-2H-azirines it is assumed that nitrile-ylides 2 resp. 6 represent intermediates. In fact irradiation of 2,3-dipropyl-2H-azirine (1, λmax 239 nm, ∊ 240) at -196° with light of wavelength 245 nm in a hydrocarbonglass gives rise to a pronounced maximum at 280 nm, for which an ∊ of ≥ 15000 can be estimated. The quantum yield for the formation of nitrile-methylide 2 is 0,8. Irradiation of the dipole 2 at -196° or warming to -150° causes the maximum at 280 nm to disappear.

Notes: 1-Hydroxy-2-methyl-2-(penta-2,4-dienyl)-1,2-dihydronaphthalene (2), on treatment with 0,75N H2SO4 in ether at 0°, underwent a [1s, 2s]-sigmatropic rearrangement to give 2-methyl-1-(penta-2,4-dienyl)-naphthalene (5), cf. scheme 2. 2-Hydroxy-1-methyl-1-(penta-2,4-dienyl)-1,2-dihydronaphthalene (4) under the same conditions gave 38% of the [1s, 2s]-product 1-methyl-2-(penta-2,4-dienyl)-naphthalene (6), together with 26% 1-methylnaphthalene, 21% 1-methyl-4-(penta-2,4-dienyl)-naphthalene (7) and 1% 1-methyl-5-(penta-2,4-dienyl)-naphthalene (8), cf. scheme 2. Most likely the latter two naphthalene derivatives at least are products of an intermolecular process.

Notes: It is known that propargyl-phenylethers rearrange at about 200° to 2 H-chromenes [1-4]. It is shown that this rearrangement occurs in benzene or chloroform at lower temperatures (20-80°) in the presence of silver-tetrafluoroborate (or-trifluoracetate). The ethers examined are presented in Scheme 1. Thus in chloroform at 61° in the presence of AgBF4, phenyl-propargylether (3) yields 2 H-chromene (13). With 0.78 molar equivalents AgBF4 in benzene at 80° the same ether 3 yields a 3:1 mixture of 2-methyl-cumaron (14) and 2 H-chromene (13). From 1′-methylpropargyl-phenylether (4) and 2′-butinyl-3,5-dimethylphenylether (5) under similar conditions the corresponding chromenes 16 and 17 resp. are obtained. Rearrangement of propargyl- and 2′-butinyl-1-methyl-2-naphthylether (6 and 7 resp.) in benzene at 80° in the presence of AgBF4 gives the corresponding allenyl-naphthalenones 18 and 19 resp. Treatment of propargyl- and 2′-butinyl-mesityl-ether (8 and 9 resp.), and propargyl- and l′-methylpropargyl-2,6-dimethyl-phenylether (10 and 11 resp.) in benzene at 80° with AgRF, yields as the only product the corresponding 3-allenyl-phenols 21, 22,24 and 25 (Scheme 3). It is shown that in the presence of μ-dichlor-dirhodiuni (1)-tetracarbonyl in benzene a t 80° the ether 4 rearranges to 2-methyl-2H-chromene (16). However with this catalyst the predominant reaction is a cleavage to phenol. No reaction was observed when ethers 3 and 12, (Scheme 7 ) were treated with the tris-(trimethylsily1)-ester of vanadic acid in benzene a t 80° (see also [8]).By analogy with the known mechanism for thc silver catalysis of the reversible propargylesterl/allenylester rearrangement [S], the silver (1)ion is assumed to form a pre-equilibrium π-complex with the C, C-triplebond of the substrate. This complex then undergoes a [3s, 3s]-sigmatropic rearrangement (Scheme 2). In the case of the others 6, 7 and 12 the resulting allenyldienones were isolated. The 2,G-dimethyl substituted ethers 8, 9, 10 and 11 resp. first give the usual allenyl- dienones (Scheme 3). These then undergo a novel silver catalysed dienon-phenol-rearrangement (Sclzenzu4) to give the 3-allenylphenols 21, 22, 24 and 25. Thc others 3, 4 and 5 with free ortho positions presumably rearrange first to the non-isolated 2-allenyl-phenols 15, 42 and 43 resp.(Scheme 7). These then rearrange, either thermally or by silver (1)ion catalysis to the 2H-chromenes 13,16 and 17 resp. The rate of the rearrangement of 2-allenylphenol (15) to 13 at room temperature in benzene or chloroform is approximately doubled when silver ions are present as catalyst.

Notes: Pure 10β-(trans-2′-butenyl)-17β-hydroxy-estra-1,4-dien-3-one (6), 10-(trans-2′-butenyl)-2-oxo-Δ1(9),3(4)-hexahydronaphthalene (13), trans-2′-butenyl 17β-hydroxy-3-estra-1,3,5-(10)-trienyl ether (12) and trans-2′-butenyl 5,6,7,8-tetrahydro-2-naphthyl ether (14) were prepared by direct C- and O-alkylation, respectively, of the corresponding phenols (cf. [3] [10]), namely estra-3, 17β-diol and 5,6,7,8-tetrahydro-2-naphthol.The Claisen rearrangement of the ether 14 (200°, 12 h) yielded 53% 1-(1′-methylallyl)- and 34% 3-(1′-methylallyl)-5,6,7,8-tetrahydro-2-naphthol (15 and 16, respectively), whereas in the thermal (120°, 14 h) and in the acid-catalysed (boron trifluoride in ether, 20°, 20 min) reaction of the corresponding dienone 13 nearly equal amounts of 15 (53-54%) and 16 (46%) were formed by thermal and charge-induced aromatic [3s, 3s]-sigmatropic rearrangements [2].The steroid dienone 6, on heating at 120°, was rearranged by [3s, 3s]-sigmatropic processes to form 52% of 2-(1′-methylallyl)- and 48% of 4′-(1′-methylallyl)-3, 17β-dihydroxy-estra-1,3,5,(10)-triene (7 and 8, respectively). The steroid phenols 7 and 8 were carefully separated; subsequent hydrogenation (Raney-Ni in alcohol) and ozonolysis yielded 2-methylbutyric acid (9): from 7, S-(+)-9, and from 8, R-(-)-9, obtained in 88,5 and 88,0% optical purity (cf. [4a]). This means (cf. scheme 2 and Table 2) that both phenols are formed to the extent of at least 94% via a chair-like activated complex, and of at most 6% via a both-like activated complex (ΔΔG120°≠ = 2.15 kcal/mol). Similarly, the boron trifluoride-induced rearrangement of 6 (born trifluoride in ether, 0°, 45 min) yielded 7 and 8, from which S-(+)-9 and R-(-)-9 were respectively obtained in 89% and 98% optical purity. For these induced rearrangements this corresponds to at least 94,5 and 99%, respectively, of the chair-like, and to only 5.5 and 1% of the boat-like activated complex (ΔΔG0°≠ = 1.5-2.5 kcal/mol). These results demonstrate that the activated complexes of both [3s,3s]-sigmatropic processes, i.e. the pure thermal reaction at 120° and the charge-induced reaction occurring at 0°, must be very similar. Thus, the boron trifluoride accelerates the Cope-like reactions 6 → 7 + 8, but does not influence the geometries of their transition states.The Claisen rearrangement of the steroid ether 12 (200°, 15 h), yielding 7 and 8, was not influenced by the chiral steroid skeleton, because no optical induction was observed (both phenols, 7 and 8, yielded on degradation racemic 2-methylbutyric acid (9)).

Notes: 7-Chloro-2-chloromethyl-benzofuran (13) and 3, 8-dichloro-2 H-1-benzopyran (12) are the main products from the thermal rearrangement (230-260°) of 2, 6-dichlorophenyl propargyl ether (7). Compounds 17, 18 and 19 are also formed, but in much smaller amounts (scheme 2 and table 1). However, in the case of the bromo-compounds 8 and 9 the rearrangement products are the benzofuran derivatives 21 and 22, containing one bromine atom less per molecule (scheme 4).The corresponding naphthyl propargyl ethers 10 and 11 can be rearranged much more easily (180°) to the halogeno-naphthofurans 24 and 26 respectively. In the case of the bromo-ether 11, 2-methyl-naphtho[2, 1-b]furan (25) is also formed (scheme 5).If the propargylic hydrogen atoms in 7 and 11 are replaced by deuterium atoms, then after rearrangement the deuterium atoms in the products d-13 and d-26 are found in the β-positions to the oxygen atom of the furan ring (schemes 3 and 5).It is suggested that initially a [3s, 3s]-sigmatropic rearrangement of the aryl propargyl ethers to the 6-allenyl-6-halogeno-cyclohexa-2, 4-dien-1-ones (e.g. a) occurs and that from these the isolated products are formed via radical pathways (scheme 6).Under neutral conditions aryl propargyl ethers containing a free ortho-position give on heating benzopyran derivatives [2]. When this thermal reaction is carried out in sulfolane in the presence of powdered potassium carbonate, 2-methyl-benzofuran derivatives are formed (table 2). This leads to the possibility of preparing, depending on the conditions, either benzopyran or benzofuran derivatives by the Claisen rearrangement of aryl propargyl ethers. The mechanism for the formation of the benzofurans is given in scheme 9.