Cyclic urethanes

Derivatization of cyclic urethanes with (B0C)20 makes the urethane carbonyl susceptible to hydrolysis under mild conditions and leaves the amine protected as a BOC derivative."... 

Methoxy-l-naphthylamide 2-CHjO-C oH6-NRCO-This group was removed from a cyclic urethane with CAN. 

The acylation of enamines derived from cyclic ketones, which can lead to the acyl ketone or ring expansion (692-694), was studied by NMR and mass spectroscopic analysis of the products (695,696). In a comparative study of the rates of diphenylketene addition to olefins, a pronounced activation was observed in enamines (697). Enamine N- and C-acylation products were obtained from reactions of Schiff s bases (698), vinylogous urethanes (699), cyanamides (699), amides (670,700), and 2-benzylidene-3-methylbenzothiazoline (672) with acid chlorides, anhydrides, and dithio-esters (699). 

Rearrangements of vinylogous urethanes to vinylogous carbonic acids and decarboxylation are other interesting enamine rearrangements which may be synthetically useful in the formation of cyclic enamines (623,624). 

The 2-oxotetrahydro-l,3-oxazines can be classified as cyclic urethanes. They have been relatively extensively investigated, because of their expected biological activity. 

Condensation of methyl urethane, H2NC06CH3j with CH20 gives rise to the methylene diurethane (III) and this compd, under different specific conditions of refluxing, resulta in the various specific linear (IV V) and cyclic products (I II) as shown. The observations of relevance here are that the various products, both linear and cyclic, are interconvertible under acid catalysis including / into IF and conversely. The... 

The process proceeds through the reaction of pairs of functional groups which combine to yield the urethane interunit linkage. From the standpoint of both the mechanism and the structure type produced, inclusion of this example with the condensation class clearly is desirable. Later in this chapter other examples will be cited of polymers formed by processes which must be regarded as addition polymerizations, but which possess within the polymer chain recurrent functional groups susceptible to hydrolysis. This situation arises most frequently where a cyclic compound consisting of one or more structural units may be converted to a polymer which is nominally identical with one obtained by intermolecular condensation of a bifunctional monomer e.g., lactide may be converted to a linear polymer... 

The direct conversion of alcohols and amines into carbamate esters by oxidative carbonylation is also an attractive process from an industrial point of view, since carbamates are useful intermediates for the production of polyurethanes. Many efforts have, therefore, been devoted to the development of efficient catalysts able to operate under relatively mild conditions. The reaction, when applied to amino alcohols, allows a convenient synthesis of cyclic urethanes. Several transition metal complexes, based on Pd [218— 239], Cu [240-242], Au [243,244], Os [245], Rh [237,238,246,247], Co [248], Mn [249], Ru [224,250-252], Pt [238] are able to promote the process. The formation of ureas, oxamates, or oxamides as byproducts can in some cases lower the selectivity towards carbamates. 

Cope and LeBel (60JA4656) prepared the cis laetone 81 from eyclo-pentane-c -l,2-dicarboxylic anhydride and converted it to the hydrazide 82. Curtius degradation of 82 gave the cyclic urethane 83. 

Rearrangement of vinylogous urethanes to give cyclic enamines [193, 194]. 

The volatility of difunctional isocyanates (such as tolylene diisocyanates, hexamethylene diisocyanate, etc.) creates many environmental problems in the urethane industry. These difficulties can be overcome by preparation of NCO-terminated oligomers with low vapor pressure. One approach is the preparation of NCO-ter-minated oligomers by partial cyclotrimerization of difunctional isocyanates. Usually this is achieved by a multi-step process which includes also deactivation of the catalyst at a certain conversion. During our work on cyclotrimerization of isocyanates we found that cyclic sulfonium zwitterions are very active cyclotrimerization catalysts (2). Recently we found that cyclic sulfonium zwitterions under certain reaction conditions act as anionic initiators. This behavior of cyclic sulfonium zwitterions permits preparation of isocyanate oligomers containing isocyanurate rings by a one-step procedure, eliminating the deactivation step. 

Polyoxypropylene triol-based urethane networks are simulated. Simulations show that cyclic molecules are present in substantial amounts in the sol fraction when the vhnmotr-. tin rss1.0. Simulations also show that the fractions of loops are much higher than those obtained from modified cascade theory. 01 lc r... 

Lipatova and coworkers reported that cyclic urethane-based polystyrene rotax-ane) 26 was not soluble in benzene and DMSO whereas polystyrene itself is soluble [64-66]. This is probably because of the introduction of strong intermolecular hydrogen bonding between die cyclic urethane units in the crystalline domains. 

Urea, 3-oxo-A4-1,2,4-thiadiazolin-5-yl-synthesis, 6, 592 Urea, thio-cyclic derivatives toxicity, 1, 138-139 toxicity, 1, 139 Urethane foam catalysts, 1, 405-406 Urethanes... 

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