3-Enaminoesters cyclization

As shown by the two examples represented in Scheme 99, chiral enaminoesters are good candidates for the synthesis of saturated or partially saturated oxazolo[3,2- ]pyridine derivatives (Scheme 99) acylation of 374 with acryloyl chloride or conjugate addition of 376 to 377 afford in both cases substrates that readily undergo cyclization, respectively to 375 <2002TL2521> and 378 <1996JOC1890>. 

A different type of indole ring synthesis was accomplished via an intramolecular cyclization of enaminoketones that proceeds through intermediate arylpalladium complexes (90H911). /3-Hydrocarbolines (33) were obtained as the final products. Similar results were obtained in the photo-cyclization of enaminoesters (91TL6129) and enaminolactones (95H1939). Results obtained in the cyclization of A-arylenamines to indoles with... 

Studies of reactions of 3-chloro-2-propenyliminium salts (141) with enaminonitriles and enaminoesters (59CCC2385 72ZC417 76JPR705) established that the cationic center in 141 attacks the /3-position of enam-ines, and this is followed by a cyclization to 1,4-dihydropyridines (142) with... 

The /i-enaminoesters such as 202 undergo similar loss of ethanol upon FVP and the resulting iminoketene 203 cyclizes to afford 204102. Simple imidazole esters also undergo loss of an alcohol to give imidazole ketenes103. Thus FVP of 205 at 750-820 °C gives... 

Allyl enaminoesters and ketones were cyclized using IVNaX Oj. <95JOC7357> Dehydro-halogenation leads to pyrrole formation. When the allyl group is incorporated into a cyclohex-2-enyl... 

Although the reactivity of enaminones is not always the same as that of typical enamines due to the additional conjugative interaction with the carbonyl group, the anodic oxidation of enaminones seems useful in organic synthesis since they yield dimerized or cyclized products upon anodic oxidation. In anodic oxidation of the enaminones or enaminoesters in methanol containing sodium perchlorate, for instance, derivatives of pyrrole are formed via initial dimerization (equation 4)5. 

Reaction of a primary cyclic enaminoester with aryl isocyanates yield urea derivatives, which can be cyclized to fused uracils10 (equation 6). 

Oxoindolizines are obtained by reaction of ethyl pyrrolidinylideneacetate with several acyclic unsaturated carbonyl compounds by cyclization of the formed Michael adducts104 (equation 73). When this reaction was modified by changing the ring size of the enaminoester, the substitution pattern of the enone structure and by varying the conditions, different products were isolated105. N-acylation could be accomplished by reaction of acyl chlorides in the presence of pyridine. Bicyclic lactams are yielded by Michael addition of acrylic esters and NaH (equation 74). 

Cyclization is even successful with enaminoesters of Z configuration160 (equation 109). 

Reductive cyclization of aliphatic and aromatic bis-enaminoesters via electrochemical methods takes place on cathode to give heterocyclic derivatives227 (Scheme 146). 

Moore and MitchelH prepared soluble poly(enaminoester) (X) from a.a -bis(carbomethoxy) diacetyl benzene and aromatic diamines. The polymers were subsequently thermally cyclized (Conrad-Limpach reaction) to poly(quinolines) (XI), which is analogous to the conversion of poly(amic acids) to polyimides in terms of converting a soluble prepolymer to a stable polymer of cyclized, rigid ring structures with the formation of volatile by-products. The cyclized product however, was infusible and insoluble. 

A second new route to DPP Pigments was developed and published 1983 In this route, succinic ester is condensed in a pseudo-Stobbe condensation with an aromatic nitrile in the presence of strong base to afford the desired DPP in good yield (Stdieme 11—4). Mechanistically, the formation of a DPP unit from succinic esters is believed to proceed along the pathway shown in Scheme 11—4. TTie initially formed enaminoesters 5C/5D cyclize to the pyrrolinone esters 6B, which further react under basic conditions with another benzonitrile. Subsequent ring closure affords the DPP 2. 

This sequence is thought to proceed through (1) Ag(I)-catalyzed propargyl-CiAisEN rearrangement to produce allenic ketoesters 95, (2) condensation of 95 with the primary amine to give allenic P-enaminoesters 96, and (3) Au(I)-catalyzed 5-endo-dig cyclization of 96 to the 2-methylpyrrole-4-carboxylate 94. 

A final illustration of how Dieckmann cyclizations have been utilized in yohimbine alkaloid synthesis is found in the work of Winterfeldt and coworkers (59, 60). In these studies, this methodology was employed to construct the pentacyclic alkaloid skeleton found in thioketal 335 (Scheme 3.55). Reaction of j5-enaminoester 327, prepared by condensation of tryptamine and... 

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