Acetal, bicyclic

The cationic polymerization of DBO was studied by three independent groups 34,35,36). 

It was demonstrated that DBO polymerizes with Lewis acids as initiators. BF3 OEt SbCl5 and PFS were used and the best results were obtained with PFS however, no systematic studies of the kinetics of polymerization or molecular weights have been published yet. The stereochemistry of DBO polymerization was investigated in detail, and it was shown that DBO polymerizes exclusively through opening of the 5-membered ring  

Polymerization of DBO may lead to a- and p-stereoisomers, due to the presence of the assymetric C-l atom  

In the a- anomer the hydrogen atom on carbon C-l occupies the equatorial position, while in the p-anomer it is located in the axial position  

The EQ-H % index indicates that polymer prepared at —40 °C and isolated at low conversions contains 91 % of oc-linked units (polymer prepared at —80 °C contains almost exclusively a-linkages independent on conversion), whereas at 94.6% conversion, polymer contains 84 % of a- and 16 % of P-linkages. The proportion of P-linkages increases further although both conversion and molecular weight do not change appreciably. 

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Sumitomo, H. and Okada, M. Ring-Opering Polymerization of Bicyclic Acetals, Oxalactone, and Oxalactam. Vol. 28, pp. 47—82. 

Ring-Opening Polymerization of Bicyclic Acetals, Oxalactone, and Oxalactam... 

In connection with studies on the ring-opening polymerization of cyclic acetals, we have undertaken investigations on the polymerization of bicyclic acetals, bicyclic oxalactone, and bicyclic oxalactam, which yield polysaccharide analogs, macrocyclic oligoesters, and a hydrophilic polyamide, respectively, some of which can be expected to be useful as novel speciality polymers. The monomers employed in the studies were prepared via synthetic routes presented in Scheme 1, starting from 3,4-dihydro-2H-pyran-2-carbaldehyde (acrolein dimer) I. 

Polymerization of 6,8-dioxabicyclo[3.2.1]octane, 2, has been most extensively studied among bicyclic acetals. This monomer is readily prepared from 3,4-dihydro-2H-pyran-2-carbaldehyde 1 by reduction with sodium borohydride followed by add-... 

The difference in behavior of the cis- and trans-isomers 43 and 45 is ascribed mainly to the greater ring strain in the trans-isomer which results in a more favorable free energy change for its polymerization. This result is similar to that reported for the polymerization of cis- and frans-8-oxabicyclo[4.3. OJnonane 48 and 4936. The molecular weights of the polymers of the bicyclic ether 49 were, however, much higher than those of the polymers of the bicyclic acetal 45. 

The bicyclic acetals 43 and 45 can be regarded as 4,5-disubstituted-l,3-dioxolanes. In connection with their pdymerizabilities, it is interesting to note here that cis-4,5-dimethyl-l,3-dioxolane has a slightly greater tendency to polymerize than its trans-counterpart22, 37. The polymerization of 45 is an equilibrium reaction and the system is completely reversible. From the temperature dependence of the equilibrium... 

Iodoetherification (vide supra) of ( )-aIlyl alcohols 99 followed by transannular radical cyclization in a 5-exo-trig mode was reported to provide ci s-fused bicyclic acetals with high diastereoselectivities. To illustrate, an example is given below <00SL1193>. 

Very interesting rearrangements of fused bicyclic acetals (243), which occur by the action of boron trifluoride etherate, were found and studied in detail by Chlenov et al. (401) (Scheme 3.161). 

Further experiments with diols such as 42 and 46 showed that bicyclic acetals 44 and 48 are formed (Scheme 15.9) [21]. With AgN03 the dihydropyrans 45 and 49 were always observed as side-products. 

Apart from those reviewed in the chapter written by Mori and in reference 479 of this chapter, only a few syntheses of the bicyclic acetals mentioned above have been published [504-506]. 

Unsymmetrically 2,5-disubstituted furan derivatives 61 reacted with 1-Me in good to very good yields, but gave mixtures of regioisomers and diastereomers endo/exo-62y endo/exo-65,the bicyclic acetals 62c-d showed a distinct tendency to be cleaved upon chromatographic separation. Dimethylfuran 61b, surprisingly, is less reactive towards 1-Me than furan (57) which is probably due to steric reasons (Scheme 16) [46]. 

Other cyclic acetals that have been studied are 1,3,5-trioxane, 1,3,5-trioxepane, 1,3,6,9-tetraoxacycloundecane, and 1,3,5,7-tetroxocane (also referred to as 1,3,5,7-tetroxane) [Kawakami and Yamashita, 1979 Munoz-Escalona, 1978 Schulz et al., 1984 Szwarc and Perrin, 1979]. Polymerization of bicyclic acetals has been of interest for synthesizing polysaccharides [Good and Schuerch, 1985 Hirasawa et al., 1988 Okada, 1991 Okada et al., 1989 Sumitomo and Okada, 1984]. 

Sumitomo, H. and M. Okada, Sugar Anhydrides and Related Bicyclic Acetals, Chap. 5 in Ring-Opening Polymerization, Vol. 1, K. J. Ivin and T. Saegusa, eds., Elsevier, London, 1984. 

Padwa et al. (44) studied the diazo-decomposition of 119 and found that the cyclic ylide 120 could be trapped by a variety of heterodipolarophiles such as ethyl cyanoacetate (Mander s reagent) to provide aminal 121 or with benzaldehyde to generate the bicyclic acetal 122. In both cases, only a single isomer was formed, with the regiochemistry easily predicted from frontier orbital considerations. Nair et al. (45) were able to employ the highly functionalized o-quinone 125 for the trapping of carbonyl ylide 124 to provide the highly complex cycloadduct 126 in 76% yield. 

D-Camphor-lO-sulfonic acid (CSA) has been utilized to form aj-fused bicyclic acetals from hexopyranosides (Scheme 20) <1997TL849>. A silver terrafluoroborate-promoted cyclization of a series of 3-hydroxyalkyl-2-cyanopiperidines has been used to prepare the octahydropyrano[2,3- ]pyridine of upenamide (Scheme 21) <2004EJ01057>. Related pyranopyr-idines have been prepared using an intramolecular Wadsworth-Emmons cyclization <2003TL8545>. 

Although not directly related to this article, a brief summary of the hydrogenolysis of bicyclic acetals, a synthetically useful reaction, is given. 

C. Audin, J. M. Lancelin, and J. M. Beau, Radical cyclization on carbohydrate pyranosides A controlled formation of functionalized ring-fused bicyclic acetals. Tetrahedron Lett. 29 3691 (1988). 

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