Chiral carbamates

Deacylation or hydrolysis of chiral carbamates, carbonates and alkanoates Micelles and comicelles of N-hexadecyl-N-methylephedrinium bromide or N -myristoyl-histidine with CTABr. Rate effects and enantioselectivities examined Fomasier and Tonellato, 1984... 

To achieve diastereoselectivity in the course of allylic substitution, the controlling chiral information may not only reside in the substrate skeleton but may also be part of the allylic leaving group. Thus, a chiral carbamate has been developed as a... 

Scheme 6.43. Control of allylic substitution stereochemistry with the aid of a chiral carbamate leaving group.

Optimization of the reaction conditions was undertaken in order to find the best Sn2 /Sn2 ratio and the best substrate conversion. Initial formation of a lithium carbamate salt of 1 on treatment with MeLi, followed by treatment with a stoichiometric amount of MeGu in Et20 at 0 °G, produced clean Sn2 selectivity and isolation of the desired alkene in 75% yield. A variety of chiral carbamates 1 were investigated, the substrate with R = 1-naphthyl and X = OMe being chosen as the candidate for further studies. It is noteworthy that substrates in which X = H gave... 

Replacing the benzyl carbamate in 65 with a chiral carbamate derived from (R)-pantolactone generates diaster-eomeric molybdenum 1,2-dihydropyridine complexes 76 and 77, which are easily separable by crystallization <20000L3909>. Once separated, these chiral complexes 76 and 77 are easily converted to the more reactive methyl carbamate molybdenum 1,2-dihydropyridine complexes 78 and 79, respectively, and allow the synthesis of enantio-pure 2,6-disubstituted tetrahydropyridines (Scheme 21). 

The efficiency of this method was demonstrated in a total synthesis of the antibiotic (-r)-tetrahy-drocerulenin 28 (Scheme 8) and (-h)-cerulenin [11]. Irradiation of complex 22 in the presence of the chiral iV-vinyl-oxazolidinone 24, which is easily prepared from the amino carbene complex 23 [12], leads to the cyclobutanone 25 with high diastereoselectivity. Regioselective Baeyer-Villiger oxidation followed by base-induced elimination of the chiral carbamate yields the butenolide 26 in high enantiomeric purity. This is finally converted, using Nozoe s protocol [13], to the target molecule 28 by diastereo-selective epoxidation (- 27) and subsequent aminolysis. 

Exocyclic a,P-unsaturated lactones (e.g. 137) have been converted to the corresponding spiroaziridines 138 by treatment with ethyl AT- [(4-nitrobenzene)sulfonyl]oxy carbamate (Ns0NHC02Et) in the presence of caleium oxide <03TL4953>. In a similar vein, a novel chiral carbamate 139 based on Helmchen s alcohol has been applied to the same conditions with moderately good diastereoselectivities, as shown in the aziridination of carboethoxy-cyclopentenenone 140 <03TL3031>. 

Chiral carbamates were prepared from racemic or optically active secondary alcohols A H and used to determine the induced diastereoselectivity of the osmium-catalyzed hydroxyamination reaction93. 

Less than 5% enantiomeric excess was obtained in the osmium-catalyzed oxyamination of 2-phenyl-l-propene when chiral carbamates derived from 1-menthol and 1-borneol were used96, but the structures of the resulting products were not reported. 

The use of the corresponding thiourea in this reaction leads to equal amounts of both diastereomers, that is, no stereoselectivity is observed48. Chiral carbamates, derived from (-)-menthol and (+ )-camphor, give, in a similar reaction, a-aminoalkanephosphonic acids with optical purity up to 42 %71. 

Assorted anions. These are generated by deprotonation of allylic halides," chloromethylphosphonic esters, conjugated hydrazones, chiral carbamates,unsaturated a-aminonitriles, phosphonamides," and sulfonamides. The dianions derived from tu-haloalkanecarboxylic acids cyclize, and this reaction forms the basis of a synthesis of V-Boc cyclic imino acids. The conjugate bases of 2-(arylmethoxy)-methyl-2-oxazolines are unstable as [2,3]sigmatropic rearrangement takes place even at — 75°C. 

A 1-benzyl analogue of the preceeding structure was obtained by way of an asymmetric Pictet-Spengler ring closure of the chiral carbamate shown (a derivative of (-)-8-phenylmenthol) with an enol methyl ether of... 

Formation of diastereomeric ureas from chiral carbamate reagents... 

Altman et al. have used the Bamberger imidazole cleavage reaction of substituted imidazoles in producing enantio-enriched vicinal di-acylamines. Specifically ring cleavage of 25 with (-)-menthyl chloroformate introduces chiral carbamate substituents on the double bond in menthyl carbamates 28 and 29. Hydrogenation of the hydrolyzed product 30 leads to preferential formation of vicinal diamide 32 over 31 in a diastereomeric ratio of 5 1. 

---