Six-membered cyclic carbamates

Both, five- and six-membered cyclic carbamates have been synthesized by the reaction of 2-(l-haloalkyl)-oxiranes with C02 and primary aliphatic amines [81]. Notably, 5-substituted 2-oxazolidinones have been prepared in good yield (51-94%) by reacting 2-aminomethyloxiranes with C02 (0.1 MPa), in MeOH, at room temperature [82]. 

A versatile ring opening of five- and six-membered cyclic carbamates yielding acyclic, V-protected 1,2- and 1,3-amino alcohols involves protection with the AT-tert-butoxycarbonyl group, followed by cleavage with cesium carbonate in methanol13. 

NH3R) derived from a primary amine, in the presence of C02, perhydrooxazin-2-ones (six-membered cyclic carbamates) are isolated (Scheme 18) <93H(35)623>. 

Theoretical evidence for the thermal decomposition mechanism for ethyl oxam-ate, ethyl oxanilate, and ethyl A,A-dimethyl oxamate have been provided.27 Ethyl oxamate and ethyl oxanilate undergo rapid decarbonylation to give the corresponding carbamates. Ethyl A,lV-dimethyloxamate elimination reaction yields in one step, through a six-membered cyclic transition state, dimethyloxamic acid and ethylene gas. 

E2 elimination reactions occur preferentially when the leaving groups are in an anti copla-nar arrangement in the transition state. However, there are a few thermal, unimolecular sy -eliminations that produce alkenes. For example, pyrolysis of several closely related amine oxides, sulfoxides, selenoxides, acetates, benzoates, carbonates, carbamates and thio-carbamates gives alkenes on heating (Scheme 4.10). The syn character of these eliminations is enforced by a five- or six-membered cyclic transition states by which they take place. 

Let us now consider to what extent the previous reaction of phosgene with a-amino acid can be applied to (5-amino acids to yield six membered cyclic 0-acyl carbamates. 

H-3,1-Benzoxazine-2,4(1 H)-dione known as isatoic anhydride (VI) is the most popular six membered cyclic O-acyl carbamate. The synthesis of isatoic anhydride by ring closure of anthranilic acid with phosgene is well described (Ref. 250). However, we have developed an... 

N-(l-Alkoxyalkyl)-aniides or -carbamates (2 X = OR), most frequently used as stable precursors for A -acyliminium ions, are usually prepared by one of the following routes (equations 7-13). For five- or six-membered cyclic cases a simple acid-catalyzed solvolysis of the hydroxy compound provides the alk-oxy derivative (equation 7). A silicon-assisted approach involves the TMSOTf-catalyzed reaction of bis(trimethylsilyl)formamide with aldehydes (equation 8). /V-(l-Trimethylsilyloxyalkyl)formamides are thus formed in good yields, which on TMSOTf-catalyzed solvolysis lead to the /V-( 1-alkoxyalkyl)form-amides. A third method is based on the NaBH4 reduction of imidates (equation 9), and has proved useful for a total synthesis of the insect poison pederine. Addition of reactive acid derivatives to imines constitutes another method (equations 10 and 11). Acylation with acid chlorides followed by treatment with ethanol in the presence of base leads to N-(l-alkoxyalkyl)amides. A one-step protocol using diethyl dicarbonate provides the corresponding carbamates. 2... 

Although, in the past the ROP of cyclic carbamates has been rarely mentioned, it has been the subject of more detailed study more recently [56]. The six-mem-bered trimethylene carbamate was the first cychc carbamate to be successfuUy ring-opened by employing trifluoromethane sulfonate as the catalyst. Only the unsubshtuted six- and seven-membered cyclic carbamates (Scheme 5.13) can be ring-opening polymerized [68-70]. The cationic ROP has subsequently been used in block copolymerizations starting from tehahydrofuran (THF) to yield either AB- or ABA-type polymers. 

When sulfamate esters 114 are used as substrates, six-membered-ring formation is favored, and results in the selective formation of 1,2,3-oxathiazinane-2,2-dioxide heterocycles 115.251 Nevertheless, five-membered cyclic sulfamidates could be obtained when no alternative cyclization was possible. 1,3-Amino alcohols and related /2-amino acids are thus readily accessible from the same simple alcohols 113 by converting them into sulfamates 114 (Equation (90)). Furthermore, in comparison to the carbamate reaction (Scheme 9), the sulfamate substrates have... 

Phosgene reacts easily with amines containing a hydroxyl function in 2 or -3 position to afford oxazolidones (cyclic five membered carbamates) or oxazinones (cyclic six mem-bered carbamates) respectively. 

In addition to amines, some derivatives of amines can be arylated. Weakly basic amides are arylated, and intermolecular reactions between aryl bromides and tri-flates 48 with benzamide can be carried out. Combined uses of xantphos (IX-IO) and CS2CO3 in THF or dioxane gave the most successful results. The cyclic urea 49 was diarylated [40]. Sulfonamides are also arylated [41]. Intramolecular reaction of the secondary amides such as 50 or carbamates offers convenient synthetic methods for five-, six-, and seven-membered lactams like 51. As ligands, MOP (VI-12), DPEphos (IX-9), xantphos (IX-IO), and BINAP are used depending on the size of the rings. As an example, ligand IX-9 is the most suitable for the preparation of the five-membered A-carbobenzyloxy derivative 52a from 52, but BINAP was not effective. 

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