Cyclorearrangement of 3 4 5-Tri-O-acetyl- (1 2 43 5)-cyclopentanepentol-1 2-O-acetoxonium Tetrafluoroborate.код для вставкиСкачать
pivalate, on the other hand, affords (8b) (43%) and (9b) (57 "/,). The ion (86) exhibits a rapid rearrangement (8b) .+ (lob) (see Table 1) whereas the ion (9b) does not. The salt (Pb), from which (1,2,3/0)-cyclopentanetriol is easily accessible, can be obtained by crystallization from a mixture of the hexachloroantinionates of ( 8 b ) and ( 9 b ) . Thecation (11) also exhibits a rapid valence isomerization (11) +(12) (see Table 1). ( I l l is best prepared as the tetrafluorobordte by removal of hydride from 3-O-acetyl-1,2-O-ethylidene(1,2/3)-cyclohexanetriolwith (C6Hs)3C@BFd0. 4-Aceioxymethyl-2-mefhyI-I,3-dioxolnn-2-ylium hexachloroantimonate (Sa) SbC15 (4.5 ml) is dissolved in CHzCl2 ( 5 ml) and then added dropwise with stirring at -10 "C to a solution of glycerol triacetate (7.7 g) in anhydrous CHzCl2 (7 ml). After 24 h a t room temperature the acetoxonium salt (5a) crystallizes (yield SO%, m.p. 145-148 "C). Received: August 29, 1969 [Z 98a IE] German version: Angew. Chem. 81, 905 (1969) [*I Prof. Dr. H. Paulsen and Dip].-Chem. H. Behre Institut fur Organische Chemie der Universitat 2 Hamburg 13, Papendamm 6 (Germany) [l] Part 7 of Carboxoniurn Compounds in Carbohydrate Chemistry. Part 6: F. Garrido Espinosa, W. P. Trautwein, and H . Paulsen, Chem. Ber. 101, 191 (1968).  H . Meerwein, V . Hederich, and K. Wunderlich, Arch. Pharmaz. 291, 541 (1958); H. Meerwein, K . Bodenbenner, P. Borner, F. Kunerf, and K . Wunderlich,Liebigs Ann. Chem. 632,38 (1960); H . Meerwein, V . Hederich, H . Morschel, and K . Wunderlich, ibid. 635, 1 (1960). [ 3 ] C. B. Anderson, E. C . Friedrich, and S . Winstein, Tetrahedron Letters 1963, 49. Cyclorearrangement of 3,4,5-Tri-O-acetyl(1,2,4/3,5)-cyclopentanepentol-l,2-O-acetoxonium Tetrafluoroborate By H . Paulsen and H . Behre[*] In acyloxonium salts of 1,2,3-triols a rapid valence isomerism takes place between the two possible dioxolanylium cations [I]. The AG* value of this rearrangement is lower in the case of (1,2/3)-cyclopentanetriolderivatives than it is in the case of glycerol or (1,2/3)-cyclohexanetriolcompounds, since the neighboring group reaction 121 in the cyclopentane ring is apparently sterically favored. As was to be expected, we have found that a total valence isomerization takes place in the cyclopentanepentol system. Penta-O-acetyl-(l,2,4,'3,5)-cyclopentanepentol reacts heterogeneously with SbCIS. We therefore prepared 3,4,5-tri-Oacetyl-l,2-O-et hylidene-(l,2,4/3,5)-cyclopentanepentol ( I ) by reaction of the free pentolE31 with acetaldehyde diethylacetal and subsequent acetylation. [ ( l )consists of 65% of the ex0 form (m.p. 120 "C) and 35 % of the endo form (m.p. 110 "C)]. The acetal ( I ) reacts with (C6H5)3CBBFqQ in CH3CN with loss of hydrideE41 to give the BF40 salt of (2). By successive neighboring group reactions the cation (2) can be converted into a second cation that is structurally identical with (2). The prerequisites for a neighboring group reaction are fulfilled at each stage of the conversion. After ten conversion steps the starting material is re-formed. The N M R spectrum of the acetoxonium salt (2) in CD3CN at room temperature contains a n acetoxonium methyl signal a t low field strength with T = 7.13 (3H) and two signals due to normal methyl groups at 7 = 7.90 (6 H) and T = 7.95 (3 H). At 80 OC all signals are considerably broadened, thus indicating that at this temperature the rearrangement is rapid according to the N M R time scale. Careful integration of the changed methyl signals shows that all the acetyl methyl groups participate in the process; this can be considered as proof that the substance undergoes rearrangement in the manner mentioned. The exact coalescence temperature cannot be determined since (2) rapidly decomposes at temperatures above 8OoC. The value of AG+ can be estimated as being approximately 18 kcal/mole by extrapolation and comparison with the heating curves of the acetoxonium salts of 1,2,3-triols. We have also examined whether "jumping" takes place during the cyclic rearrangement, i.e. whether a C-atom can be "jumped over" via 1,3-neighboring group reaction. If this were the case then other isomers of cyclopentanepentol would have to be formed. G L C analysis of the hydrolyzed reaction mixture showed that other isomeric cyclopentanepentols 151 are formed in amounts of 3 %. Received: August 29, 1969 [Z 98b IEI German version: Angew. Chern. 81, 906 (1969) [*] Prof. Dr. H. Paulsen and DipL-Chem. H. Behre Institut fur Organische Chemie der Universitat 2 Hamburg 13, Papendamm 6 (Germany) [l] H . Paulsen and H. Behre, Angew. Chem. 81, 905 (1969); Angew. Chem. internat. Edit. 8, 886 (1969).  S . Winstein and R . E . Buckles, J. Amer. chem. Soc. 64, 2780, 2787 (1942). [ 3 ] H . Z . Sable, Th. Anderson, B. Tolbert, and Th. Posternak, Helv. chim. Acta 46, 1157 (1963).  H . Meerwein, V. Hederich, H . Morschel, and K . Wunderlich, Liebigs Ann. Chem. 635, 1 (1960). 151 Th. Posternak and G. Wolczunowicz, Naturwissenschaften 55, 82 (1968). II,C OAc k?