1.3- Dioxepane system

In comparison to the fully saturated 1,3-dioxepane system, the number of possible conformations for 4,7-dihydro-1,3-dioxepins is reduced because of the sp hybridization at carbons 5 and 6 <76BSF307). There are two chair, two twist and two boat conformations. NMR spectroscopic data indicate a rapid equilibrium between the chair and the twist conformations. It has also been shown that the conformational ratio strongly depends on the substitution pattern in positions 2, 4, and 7 <65MI 911-01,75JOC450,86ZOB144,95HCA2I48). The most Stable Conformation of benzodioxepins was determined by low-temperature NMR measurements <81CJC2283). Evidence for a twist-chair con-... 

There are relatively few entries in the non-fused dioxepin area, and most of these focus on reactions of these systems. For example the triflic acid-initiated polymerisation of 1,3-dioxepane in the presence of acetic acid and hexanedicarboxylic acid has been studied and mechanistic aspects discussed <00JPS(A)1232>. Biodegradable microspheres for the controlled delivery of drugs have been made from copolymers and homopolymer blends of L-lactide and l,5-dioxepan-2-one <00PP1628>. Ring contraction of 5-methylene-l,3-dioxepanes (eg. Ill) on reaction with trimethylsilyl trifluoromethanesulfonate in the presence of base afforded the exo tetrahydropyrans, in good yields <00TL2171>. 

It is not difficult to show by conductance measurements that when the polymerisation of dioxolan becomes of first order, roughly at the first half-life, all the available acid has reacted with monomer or oligomer to produce active centres [10]. Thus on the Mainz theory one would expect to find a number of OH groups of the same order of magnitude as the number of perchloric acid molecules introduced this is evidently not so, as long as the water concentration in the system is significantly less than that of the acid [10]. For 1,3-dioxepan the protonation is very much faster than for DXL and is complete long before the first half-life of the polymerisation. 

M. E. Butcher, J. C. Ireson, J. B. Lee, and M. J. Tyler, Seven and eight membered ring sugars and related systems The synthesis of some septanose rings from dioxepans, Tetrahedron, 33 (1977) 1501-1507. 

The monocyclic saturated peroxides 196 have been prepared by the treatment of the respective hydroperoxides with lead tetraacetate (Scheme 94) <1981S633>. Another general approach to the 1,2-dioxepan ring system involves the ozonolysis of appropriate precursors with a double bond e.g., the ozonolysis of cyclopentene gives 1,2-dioxepan (trioxane) 197 . 

Shibasaki etal reported, in 1999, the use of HCI/H2O as initiating system for the cationic ROP of l,3-dioxepan-2-one (7CC) following an AMM (Table 6, entry 9). Polycarbonates with Mn ranging from 3000 to 12 400gmor and low dispersity (1.15-1.23) could be quantitatively obtained by ROP of 7CC in dichloromethane at room temperature. Mn values could be controlled by the initial amount of water they were found to increase linearly with monomer conversion, regardless of the initial [M]/[I] ratio. According to NMR spectroscopy, no decarboxylation was noted under these conditions, a side reaction commonly observed during cationic ROP of cyclic carbonates. ... 

Although a solution seemed to be close by with these thiophenes, a technical breakthrough was achieved not until the Bayer researchers Friedrich Jonas and Gerhard Heywang decided to extend the thiophene structures to bicyclic ring systems. In other words, ring closure of two alkoxy substituents had to be accomplished to form dioxolane-, dioxane-, or dioxepane 3,4-anellated thiophenes (see Figure 4.3). 

Xu and Pan reported on a similar system. Bromine-terminated pSt was reacted with silver perchlorate to prepare a macroinitiator for the living CROP of THF [279]. Using macroinitiators of Mj =4250 and 7890,block copolymers were synthesized with Mj =9420 to 17,450 and M = 11,860 to 32,980, respectively. The were < 1.45 in aU cases. They also prepared a pSt based macroinitiator via ATRP using a hydroxy functional bromine-based initiator which was then used as a chain transfer agent for the CROP of 1,3-dioxepane (DOP, Scheme 35) [280]. The pSt macroinitiators obtained using the CuBr/bpy catalyst system had M = 6300 (M Mj =1.18) and 11,360 (M M =1.19). The ROP of DOP, using a slow addition of monomer technique, resulted in block copolymers with M = 17,540 (M /Mn=1.43) and Mj =30,880 (M M =1.46), as determined by NMR. Con-... 

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