Enamine carbaldehyde

Enamine carbaldehyde 76 (Scheme 20) by sequential photochemical cycloaddition and iminium ion-propargylsilane cyclimtion furnishes allenes 77a-c in good yield and with high diastereoselectivity (92X2081). (Gas chromatographic fR values and thin-layer chromatographic Rp values have been reported.) Radical cyclization of ca-iodoalkyl isoquinolone 78a under... 

Enamine carbaldehydes (317) have been used in the synthesis of 1,4-dihydropyridines (318) via photochemical cycloaddition with alkenes183,184. 

High-pressure absorption spectroscopy is widely applicable for studying the reactivity and the selectivity of chemical transformations. Some results are shown concerning the investigation of the //ctc/ o-Diels-Alder reactions between the enamine carbaldehyde 1 and enamino ketones 2 with ethyl vinyl ether 3, which lead to the diastereomeric dihydropyrans 4-1-5 and 6 + 7, respectively. Scheme 6.7-1 (Buback et al., 1988 Buback et al., 1989). 

Hydroxy-l,2,3,4-tetrahydropyridines (161) are obtained on irradiation of enamine carbaldehydes (162) in the presence of a wide range of alkenes (163) the cycloaddition proceeds with high regioselectivity and in almost... 

Quantitative yields of the adducts (109) are obtained by low temperature, -20 to 40°C, irradiation of the enamine carbaldehyde (110) with the alkenes (111). The enamide (112) also undergoes photoaddition of acrylonitrile. The adduct from this, on catalytic hydrogenation, provides a synthetic route to the carboxamides (113). ... 

A negligible pressure effect on the diastereoselectivity was also observed for the cycloaddition of the enamine carbaldehyde (81a) carrying an electron-withdrawing group at position 3 and 61a to yield the dihydropyrans 82 and 83 (Scheme 8.21). This reaction was again studied by direct quantitative infrared spectroscopy up to 300 MPa between 45 and 95 °C in different solvents. The activation volume was found to be —(25.1 1.7) cm mol in dichloromethane and —(25.0 + 1.8) cm mol in isodurene. Thus, in this reaction solvent polarity had no influence on the pressure dependence of the rate coefficient in addition, the ratio of the two diastereo-meric products is not changed under high pressure thus the AAV value is very small (AAV < 1 cm mol ). 

The oxidation of 2,3-dimethylindole with peroxodisulfate and peroxomonosulfate yields 3-methy-lindole-2-carbaldehyde (136) (88JCS(P2)1065). The reaction proceeds in three steps, namely electrophilic attack of the peroxide at C-3 to give an indolenine intermediate (134), then peroxide attack on the enamine tautomer (135), and hydrolysis (Scheme 26) (93JOC7469). 

The imines derived from 3-hydroxynaphtho[l,8-c]thiopyran-2-carbaldehyde and the corresponding 2-acetyl compound exist in solution in the keto-enamine 270, R1 = H, shown (Scheme 24). Irradiation at 436 nm results in reversible Z -E isomerization about the exocyclic double bond <1995ZOR1559>. 

This catalytic cascade was first realized using propanal, nitrostyrene and cinnamaldehyde in the presence of catalytic amounts of (9TMS-protected diphenylprolinol ((.S )-71,20 mol%), which is capable of catalyzing each step of this triple cascade. In the first step, the catalyst (S)-71 activates component A by enamine formation, which then selectively adds to the nitroalkene B in a Michael-type reaction (Hayashi et al. 2005). The following hydrolysis liberates the catalyst, which is now able to form the iminium ion of the a, 3-unsaturated aldehyde C to accomplish in the second step the conjugate addition of the nitroalkane (Prieto et al. 2005). In the subsequent third step, a further enamine reactivity of the proposed intermediate leads to an intramolecular aldol condensation. Hydrolysis returns the catalyst for further cycles and releases the desired tetrasubstituted cyclohexene carbaldehyde 72 (Fig. 8) (Enders and Hiittl 2006). 

The process mechanism as shown in Figure 2.23 consists of an initial activation of the aldehyde (66) by the catalyst [(5)-67] with the formation of the corresponding chiral enamine, which then, selectively, adds to nitroalkene (65) in a Michael-type reaction. The following hydrolysis liberates the catalyst, which forms the iminium ion of the a,(3-unsaturated aldehyde (62) to accomplish the conjugate addition with the nitroalkane A. In the third step, another enamine activation of the intermediate B leads to an intramolecular aldol condensation via C. Finally, the hydrolysis of it returns the catalyst and releases the desired chiral tetra-substituted cyclohexene carbaldehyde (68). 

Hydrazones (166) react with chloromethyleneiminium salt (I) giving, after hydrolysis, aldehydes (167 Scheme 12). In this transformation the hydrazones (166) are clearly behaving as aza-enamines. - Hydrazones (168) and (169) react with chloromethyleneiminium salt (1) at both the methyl group and at nitrogen to yield the corresponding iminium salts (170) and (171), which cyclize with loss of dimethylamine to provide iminium salts (172) and (173), respectively. Hydrolysis of these salts (172) and (173) affords pyrazole-4-carbaldehyde derivatives (174) and (175 72-96% Scheme 13). Pr uct (174) can also be prepared from semicaibazone (176). 

Route 3 (123) A quaternary carbon atom can be set up by irradiation of the A-methylaniline enamine 395 of adamantane-2-carbaldehyde, giving the indoline 396 in 34% yield. Oxidation of this product gave the spiro-oxindole 403 in low yield, together with the yV-formylindoline 397 (42% yield). 

The synthesis of the tricyclic core of the cytotoxic marine alkaloid Madangamine required an efficient method to generate the central quaternary carbon function. Weinreb employed an aza-Claisen rearrangement in the presence of a palladium catalyst [21c]. After treatment of ketone 116 subsequently with TOSMIC and DIBALH, the formed carbaldehyde 117 was reacted with diallylamine in the presence of Pd(OCOCFj)2/PPh,. Initially, the enamine 118 was formed, which underwent diastereoselective aza-Claisen rearrangement The unsaturated imine 119 was cleaved with aqueous HCl and the corresponding aldehyde 120 was isolated in 68% yield. Several further steps allowed one to complete the synthesis of the core fragment 121 of the natural product (Scheme 10.27). 

Another example deals with the direct synthesis of pyr-rol-2-carbaldehydes 598 and 601 to avoid installation of these moieties into preformed pyrroles. Therefore, it is necessary to change from a normal p-ketocarbonyl compound to a stabilized ketonitrile 596 to ensure only one possible reaction pathway toward the enamine (Scheme 13.150) [270]. 

The use of this catalyst allowed the same authors to elaborate an asymmetric domino Michael-Michael-aldol reaction, involving two aldehydes and a nitroalkene on the basis of an enamine-iminium-enamine activation. The corresponding cyclohexene-carbaldehydes were isolated with virtually complete diastereo- and enantioselectivities, as shown in Scheme 1.63. 

A study of the preparation of ocimene by isomerization of a-pinene has been reported. a-Azidoacetophenones have been converted to 2-aryl-l,3-oxazole-4-carbaldehydes by rearrangement of the carbon framework on exposure to DMF/POCI3. The rearrangement occurs via alkenyl azides and 2//-azirines. Enamine rearrangement of pyridinium salts to indoles has been studied.Kinetic stabilization and reactivity of a series of singlet 2,2-dialkoxy-l,3-diphenyloctahydropentalene-l,3-diyls enable cis-selective formation of the ring-closed compounds (Scheme 105). ... 

Aldehydes and ketones react with ammonia and primary amines to form imines. Reaction with aldehydes yields aldimines, and ketimines arise in reactions with ketones (Figure 8.28). Imines derived from aliphatic carbonyl compounds are generally unstable and are transformed to more stable products, such as amines, diamines and others. Secondary amines react with aldehydes and ketones with the formation of enamines (Figure 8.29). Imines are similarly flavour-active compounds derived from furan-2-carbaldehyde. For example, the aromas of Ar-furfuryl(isobutylidene)amine, N-(furfuryl)isopentyhdeneamine (8-155) and N-(furfurylidene)isobutylamine (8-156) reportedly resemble chocolate. 

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