Protecting urethane

In the field of stereochemical induction, efficient new asymmetric syntheses have resulted from conjugate addition of Grignard reagents to the readily prepared chiral unsaturated systems (45) and (46). Further details and developments have appeared concerning the stereoselective a-addition of the thioacrolein dianion (47) to ketones and enones, in contrast to the random nature of the Li reagent. In the same vein, the Mg salt derived from the sterically protected urethane (48) undergoes exclusive y-attack with electrophiles [cf. species (19b) above]. ... 

Urethane [51-79-6] (ethyl carbamate) occurs as a natural by-product in fermented products such as wine, Hquors, yogurt, beer, bread, oHves, cheeses, and soy sauces. Whereas urethane has a known cancer etiology in experimental animals, no such relationship has yet been proven in humans (108,109). Alcohol may act by blocking the metaboHsm of urethane, and thus exert a protective effect in humans consuming alcohoHc beverages (110). 

W. Tobiasson, A. Greatorex, and D. VanPelt, "Wetting of Styrene and Urethane Roof Insulations ia the Laboratory and on a Protected Roof Membrane," F. J. PoweU and S. L. Matthew, eds., ia Thermal Insulation Materials and Systems STP922, ASTM, Philadelphia, Pa., 1987. 

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."... 

Certain fillers are commonly added to protect the urethane backbone from oxidative degradation. Carbon black and titanium dioxide are commonly used in conjunction with antioxidants to protect polyether polyurethanes in exterior adhesive applications that may be exposed to oxygen and light (Fig. 12). 

These base-sensitive protective groups were introduced from the chloroformate or azidoformate. They are more sensitive to base than is the Fmoc group. Cleavage times with 0.2 mL of piperidine to 0.1 mmole of urethane in 5 mL of CHCL at It occur as follows Climoc, <10 min Bimoc, <14 h Fmoc, 18 h. °... 

Analogously to these reactions of imidazolium carboxylates, the introduction of urethane-type amino-protecting groups can be accomplished with the following mesoionic azolides [194]... 

In practice, some anticoagulation agents such as heparin or antiplatelet agents, e.g. nitric oxide (NO) are delivered to sensor sites in order to reduce the risk of thrombus formation. Nitric oxide (NO), which is a potent inhibitor of platelet adhesion and activation as well as a promoter of wound healing in tissue, has been incorporated in various polymer metrics including PVC (poly(vinyl-chloride)), PDMS (poly-dimethyl-siloxane) and PU (poly-urethanes). Those NO release polymers have been tested in animals as outer protection coatings and have shown promising effects for the analytical response characteristics of the sensor devices [137],... 

Problematic functional groups, however, are thioethers and disulfides [28] as well as free amines which poison catalysts of type 1 [4c]. In case of amines this problem is easily solved by choosing either an appropriate protecting group for nitrogen (e.g. amide, sulfonamide, urethane), or simply by protonation since ammonium salts were found to be compatible with 1 [4c]. As will be discussed in Sect. 4, free amines can also be metathesized in supercritical C02 as the reaction medium [7]. 

Scheme 6. Carbohydrate based urethane protecting groups...

The suitability of the Aloe group for the construction of lipidated peptides is emphasized by the synthesis of the maleimidocaproyl-modified, S-palmi-toylated and farnesylated heptapeptide 16 which corresponds to the N-Ras C-terminus (Scheme 10).1211 In contrast to classical urethane-type protecting groups, the Aloe group can be removed in the presence of additional functional groups and under neutral conditions. It is therefore a very convenient protecting group for the synthesis of very hydro-phobic lipid-modified peptides, which are not soluble in the aqueous media required for enzyme catalyzed transformations. 

Synthesis of the Palmitoylated and Farnesylated C-Terminal Lipohexapeptide of the Human N-Ras Protein by Employing an Enzymatically Removable Urethane Protecting Group, H. Waldmann, E. Nagele, Angew. Chem. 1995,107, 2425-2428, Angew. Chem. Int. Ed. 1995, 34, 2259-2262. 

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