Carbamates nitrogen nucleophile cyclization

Additions to l,3-dienes6 (12, 367-368 14, 249-250 15, 245). This reaction can be used to effect intramolecular cyclization of cyclic 1,3-dienes substituted by a suitable nitrogen nucleophile. Thus reaction of the amido diene 1 with lithium acetate catalyzed by Pd(OAc)2 (with benzoquinone as reoxidant) provides the ds-fused heterocycle cis-2, in which the acetoxy group is cis to the ring fusion, formed by an overall trans-1,4-oxyamidation of the diene system. Addition of a trace of LiCl improves the yield and results in an overall cis- 1,4-oxyamidation (equation I). Acetamides and carbamates can also be used in place of amides. 1,4-Chloroamidation can also be effected by use of 2 equiv. of LiCl. 

Cleavage of the product from initial addition of a nitrogen nucleophile can also lead to difunctionalized products. This cleavage has been conducted in some cases with an oxidant. For example, the carbamate in Equation 16.124 cyclizes in the presence of PhI(OAc)j to form the acetoxy oxazolidinone. This strategy can also be followed in an intermolecular fashion, as shown in Equation 16.125. ... 

C.ii.b. Intramolecular Attack by Nitrogen Nucleophiles. The nitrogen nncleophiles, which are compatible with the reaction conditions (HOAc and 1,4-benzoquinone), consist of electron-deficient ones such as amides, carbamates, ureas, and sulfonamides. Because of the low nucleophilicity of these, no intermolecular 1,4-additions involving C—bond formation is known. However, if the nitrogen is tethered to the diene, a cyclization takes place in good yields. 

Cyclization can also be performed via nucleophilic attack of nitrogen nucleophiles as shown for the release of cyclic urea derivatives. Dressman et al. examined a methodology to cleave hydantoins 222 via treatment of carbamate linkers with an intramolecular amide functionahty and base [188]. In the case of resin-bound substrate 221 (Scheme 33), the resulting heterocycle is a five-membered ring system but other heterocycles containing a urea-type unit can also be synthesized as published by diverse other groups (c.f. benzimidazolone-synthesis of li et al. [199]). 

In some carbamates their nitrogen was found to participate in cyclizations similar to those reported with oxygen nucleophiles [8]. 

The ease of cyclization of the 3-butenyl ureas (1, R2 = H, n = 2) was dependent mainly on the steric and electronic effects of the nitrogen substituents R1 and R2 when R1 is methyl, the best results were obtained when R2 is isopropyl, cf 6 when R1 is hydrogen no cyclization occurred and methanol (solvent) acted as the nucleophile. Generally, long reaction times were necessary, and the cyclizations were often incomplete. Furthermore, cyclization fails for /V-(5-heptenyl)-amides and (3-butenyl) carbamates. 

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