Amino acyclic

Vinylogous urethanes, j8-amino-acyclic esters are sensitive to hydrogenolysis. Rhod-ium-on-alumina, 5% Pd-on-carbon are used with success, but at below 100°C. ... 

The acid-catalyzed reaction of acetophenone with acyclic secondary amines results in the formation of the expected enamine and a rearrangement product. The latter product arises from the transfer of one of the amino N-alkyl groups to the cnamine s carbon to produce a ketimine (53a). 

On the other hand there have been isolated salts of either the acyclic amino ketone form or the cyclic enamine form, namely 6-methylamino-l-a-naphthyl-l-hexanone (95, = 5) and 12-methylamino-l-a-naphthyl-l-dodecanone (95, n=ll), or l-methyl-2-a-naphthyl-l-aza-2-cycloheptene... 

The aldol reactions of enamines may be formally considered to proceed via acyclic amino aldehyde or amino ketone forms, in spite of the fact that the cyclic enamine forms can also take part in aldol reactions. 

The mechanism of the Cook-Heilbron reaction between a-aminonitriles and dithioformic ester 6 proceeds via an acyclic intermediate 7, as proven by its isolation in several cases. Nucleophilic attack of the amine function on the sulfur-bearing carbon leads to the elimination of hydrogen sulphide. Cyclization of the acyclic thiacetoamide results in a five membered ring which aromatises favourably to give 5-amino-2-benzylthiazole 8. 

Occasionally it happens that the oxo compound, produced by oxidation, forms a hydrate which is further oxidized to a dihydroxy compound. Attention must be given to the possibility (so far unreported) that when the hydrated species is in equilibrium with a trace of the ring-opened structure a sufficiently fast oxidation rate of the amino-aldehyde (i.e. the acyclic structure) could lead to the incorrect conclusion that the original material was not cyclic. 

Recently, Charette et al. have also demonstrated this behavior in the stereoselective cyciopropanations of a number of enantiopure acyclic allylic ethers [47]. The high degree of acyclic stereocontrol in the Simmons-Smith cyclopropanation has been extended to synthesis several times, most notably in the synthesis of small biomolecules. Schollkopf et al. utilized this method in their syntheses of cyclopropane-containing amino acids [48 a, b]. The synthesis of a cyclopropane-containing nucleoside was also preformed using acyclic stereocontrol [48c]. 

Nitro and 3-amino sugars have been prepared via stepv/ise constnicdon from acyclic precursors by the nitro-aldol strategy as shovm in Scheme 3 8... 

Conjugated cyclohexenones [6] have also been easily prepared by combining the cycloaddition of dimethylaminobutadiene 4 and several cyclic and acyclic dienophiles followed by the elimination of the amino group from the cycloadducts under acidic conditions. Scheme 2.3 summarizes some of these results. 

The finding that substantial 2.5-helicity may be retained in water upon the introduction of a limited number of acyclic or y9 -amino acid residues at chosen positions in the sequence, further expand the side-chain array available for functionalization of the 2.5-helical scaffold. In water, y9-heptapeptides 107 and 108 which contain two -amino acid residues [184a] and two y9 -amino acids [184b], respectively, still display a CD spectrum and NOE coimectivities characteristic of the 2.5-helix. However, the addition of a third acycHc amino acid is detrimental to the formation of the 2.5-helix in water. 

While conformation II (Fig. 2.34) of Uke-y -amino acids is found in the 2.614-helical structure, conformation I, which similarly does not suffer from sy -pen-tane interaction, should be an appropriate alternative for the construction of sheet-like structures. However, sheet-like arrangement have not been reported so far for y-peptides composed of acyclic y " -amino acid residues. Nevertheless, other conformational biases (such as a,/9-unsaturation, cyclization between C(a) and C(y)) have been introduced into the y-amino acid backbone to restrict rotation around ethylene bonds and to promote extended conformation with formation of sheets in model peptides. Examples of such short chain y-peptides forming antiparallel (e.g. 152 [208]) and parallel (e.g. 153-155 [205, 208]) sheet-hke structures are shown in Fig. 2.38. 

Brzezinska KR, Deming TJ (2001) Synthesis of ABA triblock copolymers via acyclic diene metathesis polymerization and living polymerization of alpha-amino acid-N-carboxyanhy-drides. Macromolecules 34 4348 354... 

The PNPH coupling constant, which is 3—4 Hz for the acyclic phos-phinimine (81), is quadrupled (13—14 Hz) when the amino-groups are included in a small ring, as in (82). ... 

MeLi to cyclohexenone with p-amino sulfide ligand 88-9 MeMgl to acyclic enone with arene-thiolatocopper(l) complexes 90-1 mesylamide ligand (camphor-derived) 174, 176... 

Pratt, R. F. Govardhan, C. P. P-Lactamase-catalyzed hydrolysis of acyclic depsipeptides and acyl transfer to specific amino acid acceptors. Proc. Natl. Acad Sci. USA 1984, 81, 1302-1306. 

Organic-Base Catalyzed. Asymmetric direct aldol reactions have received considerable attention recently (Eq. 8.98).251 Direct asymmetric catalytic aldol reactions have been successfully performed using aldehydes and unmodified ketones together with chiral cyclic secondary amines as catalysts.252 L-proline and 5,5-dimethylthiazolidinium-4-carboxylate (DMTC) were found to be the most powerful amino acid catalysts for the reaction of both acyclic and cyclic ketones as aldol donors with aromatic and aliphatic aldehydes to afford the corresponding... 

The process is not limited to acyclic amino acids, as cyclic compounds such as proline and pipecolic acid derivatives can also be employed, though in these cases more complex heterobicyclic substances are formed. 

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