Cyclic benzyls

Finally, it should be of interest that the rearrangements of several cyclic benzylic sulfinates have also been described in the literature by Durst46 and Hogeveen47, and seem to proceed by a special two-step mechanism retro-Diels-Alder extrusion of S02, followed by its chelotropic addition to the unstable quinodimethane intermediate (e.g. equation 10). 

Quideau, S. Ralph, J. A biomimetic route to lignin model compounds via silver(I) oxide oxidation. 1. Synthesis of dilignols and non-cyclic benzyl aryl ethers. Holzforschung 1994, 48, 12-22. 

Table 5. Rate and equilibrium constants for the formation and reaction of cyclic benzylic carbocations [18 + ] and [20+ ] and analogous ring-substituted 1-phenylethyl carbocations (Scheme 15)°... 

Often, ethers are constructed by alkoxide displacement of a stereodefined leaving group. Hirakazu Arimoto of Nagoya University has found (Chem. Commun. 2004, 1220) that it is possible to effect diastereocontrolled construction of cyclic benzylic ethers such as 2 by oxidation of 1 to the o-quinone methide. The more stable equatorial C-glycoside product 2 is formed in near quantitative yield. 

Phenylcoumarone (VIII) has a characteristic ultraviolet and ioniza-tion-Ae spectrum, which enabled us to detect dimeric structures of this type in reaction mixtures obtained when Bjorkman spruce lignin was subjected to acidolysis for 20 hours. From the spectrophotometric estimation of the amount of the phenylcoumarone systems formed, we concluded that from a total of 100 phenylpropane units of Bjorkman lignin, about 20 are involved in phenylcoumaran systems (I) in other words, about every 10th phenylpropane unit is linked to one of its neighbors by the cyclic benzyl aryl ether linkage characteristic of I. 

Table 4 Effects of oxygen substituents on stabilities of cyclic benzylic carbocations...

Brunow G, Sipila J, Makela T (1989) On the mechanism of formation of non-cyclic benzyl ethers during lignin biosynthesis Holzforschung 43 55-59 Ede RM, Brunow G, Simola LK, Lemmetyinen J (1990) Two-dimensional H- H chemical shift correlation and J-resolved NMR studies on isolated and synthetic lignins Holzforschung 44 95-101... 

Ede, R. M., and Kilpelainen, I. (1995) Homo- and hetero-nuclear 2D NMR techniques unambiguous structural probes for non-cyclic benzyl aryl ethers in soluble lignin samples. Res Chem Intermediat 21(3-5), 313-328. 

Lundquist, K. (1992) H NMR spectral studies of lignins. Results regarding the occurrence of b-5 structures, (3-(3-structures, non-cyclic benzyl aryl ethers, carbonyl groups and phenolic groups. Nordic Pulp Paper Res. J. 7(1), 4-8, 16. 

FIGURE 7.3 Formation of non-cyclic benzyl aryl ethers by addition of a phenolic unit to a quinone methide intermediate formed during lignin biosynthesis. 

J Sipila, G Brunow. On the mechanism of formation of non-cyclic benzyl ethers during lignin biosynthesis. Part 2. The effect of pH on the reaction between a (3-0-4-type quinone methide and vanillyl alcohol in water-dioxane solutions. The stability of non-cyclic benzyl aryl ethers during lignin biosynthesis. Holzforschung 45 275-278, 1991. 

Partial desulfurization. Disulfides (cyclic, benzylic, aralkyl, and dialkyl) are converted into sulfides by reaction with the reagent at 25-80°.3 For example, when dibenzyl disulfide is treated with a 10% excess of the phosphine in refluxing benzene... 

Li found that the yields for oxidative alkylation products 37 were best when cyclic benzyl ethers were used as substrates. Li believed that the role of the InCl3 was to activate the DDQ to increase its oxidative potential, while the Cu(OTf)2 activated the malonate for the addition to the in situ generated oxonium ion. 

Using the combination of NHPI and oxygen gas as a DDQ equivalent, Li and coworkers were successful in oxidative coupling cyclic benzyl ethers 44 with mal-onates 13 using catalytic amounts of Cu(OTf)2 and InCl3 (Scheme 26) [47]. 

Scheme 26 Oxidative alkylation of cyclic benzyl ethers with malonates using oxygen gas as the terminal oxidant...

Crystalline ether hydroperoxides can be isolated after autoxidation of cyclic benzyl ethers such as isochroman333 and phthalan.334... 

Heating phthalyl alcohol (o-phenylenedimethanol) with a small amount of /7-toluenesulfonic acid gives the cyclic benzyl ether (phthalan)675 in about... 

Organic Reactions contains a review of the fission of benzyl ethers, including cyclic benzyl ethers, by hydrogenation.43 Ring fission of furans has been reviewed by Jones and Taylor.44 Burwell45 has written an excellent and detailed report on the cleavage of ethers, in which he discusses also the theoretical problems involved he concludes with some indications for the fission of sulfides, a subject that is treated also by Tarbell and Hamish.46... 

The reaction of the dialdehyde, obtained by a double ozonolysis of 118, with dibenzyl aspartate afforded the cyclic benzyl phosphinate 119. A pallado-catalyzed hydrogenolysis produced 4-azaphosphinane 107 in 85% yield (Scheme 24). 

Cyclic benzyls will initiate photo polymerizations of acrylates The process of photodecomposition and formation of initiating radicals was reported by Pappas to be as follows... 

Cyclic benzylic amines may undergo ring opening upon heating with acetic anhydride (eq 9). ... 

Pseudoindoxyls after reduction usually rearrange in acidic solution back to indoles, but it is easy to see for the particular case of dihydroisoquinamine that cyclization to a tetrahydrofuran would be the preferred pathway. The problem then is changed to what acidic conditions are required to split open this particular cyclic benzyl ether. 

---