Enamides photochemical

The photochemical rearrangement of enamides to vinylogous amides was described (65],652). Rearrangement of cyclopropyl ketimines to enamines with acid was applied to syntheses of myosamine, apoferrosamine, mesembrine, and desmethoxymesembrine (653-656). 

Enamides 163 undergo photochemical conrotatory six-electron electrocyclic reactions to yield the dihydro intermediate 164, which in turn yields the fraws-fused cyclic product 165 (equation 105) by a (l,5)-suprafacial hydrogen shift. Several natural product syntheses like that of benzylisoquinoline and indole type alkaloids can be achieved by this type of photocyclization (equations 106163, 107164, 108165 and 109166). 

A general method for the formation of four isomeric diazaphenanthrenes (320) involves the cyclization under photochemical conditions of the enamides (321) to give (322) which... 

The photochemical rearrangement of enamides of carboxylic acids 20,113-116 is analogous to the photochemical rearrangement of enol esters (Section VII), except that it proceeds more efficiently and in much higher yield. It seems that the latter can be attributed to the high photochemical stability of the photoproducts. The )3-iminocarbonyl compound 187 formed immediately after the... 

It is important to note that a possible photochemical ring expansion by [1,3]- or [l,5]-acyl shifts in the 5-membered vinyl lactams 191 has not been observed,4,116 nor have [l,5]-shifts of acyl groups involving an aromatic ring been reported for either enol esters or enamides. However, an example of a similar [l,5]-alkyl shift, 193 -> 194, in related ketene dialkyl acetals is known.24... 

The photochemical behavior of enamines and their derivatives has been the subject of extensive mechanistic and synthetic investigations. The goal of this chapter is to summarize the photochemistry of enamines as well as enamides, enaminones, en-amidones and enaminonitriles, with particular emphasis on the application of the photochemistry of these chromophores in the synthesis of natural products. 

A mixture of enaminones 67 is obtained in good yield from the photolysis of enamides 66 (equation ll)69. It is interesting to note that the photochemical equilibrium between the E and Z isomers of 67 is different from the thermodynamic equilibrium. 

This reactivity of N-acylenamines 1 has opened up new possibilities for the use of enamides. in photochemical rearrangements2 as well as in acid-catalyzed cyclizations3,4, which lead to a variety of complex nitrogen-containing heterocycles from readily available simple precursors. These reactions have also been used to form a wide variety of natural products and polyfunctional compounds. Enamides can be also used as electrophilic reagents for amidoalkylation5,6, which can occur under certain conditions as a [4 + 2] cycloaddition to form 1,3-oxazinium heterocycles7. 

Several reviews on enamide photocyclization (11-13) and photochemical synthesis of isoquinoline alkaloids (14) have been published. 

Based on the above result, a newly isolated alkaloid alamarine (33, structure, see p. 240) was synthesized by the route involving enamide cyclization under both thermal and photochemical conditions, giving predominantly the desired lactam (32) (Scheme 28) (73). 

One of the major areas of research in enamide chemistry has been the application of enamide cyclization to the synthesis of natural alkaloids and related heterocyclic compounds. Ever since Ninomiya and co-workers discovered nonoxidative photocyclization of the A-benzoylenamine-type ena-mides in 1969 (9), several types of alkaloids have been synthesized, starting from enamides of rather simple structures, by applying enamide cyclization under both photochemical and thermal conditions. 

The cyclization of l-methylene-2-nicotinoylisoquinolines 27 under three different conditions, photochemical, thermal, and acylating conditions, are compared (73). Starting from the above enamide (27), Ninomiya et al. (73) synthesized 10-azaberbine as the major product by simple heating at 195-200°C, whereas 12-azaberbine was the major product under acylating condition as shown in Table II. Further, total synthesis of alamarine (33), the only alkaloid having an azaberbine structure, was synthesized via the route involving enamide cyclization under both thermal and photochemical conditions (73) as shown in Scheme 71. 

On the assumption that asymmetric reduction of an intermediate by a chiral metal hydride complex could occur during the course of photocycli-zation of the enamide 133, as exemplified by reductive photocyclization, Ninomiya et al. (16) have undertaken and completed the photochemical asymmetric synthesis of (—)-xylopinine. 

Corunnine, Nandazurine, ( )-Caaverine, ( )-Isoboldine, ( )-Thalicmi-dine, and ( )-Domesticine. Relative to the aporphine synthesis by enamide photocyclization, Kupchan and O Brien (141) have developed the oxidative photochemical synthesis of aporphine alkaloids. 

A detailed report of the photochemical conversion of the enamides (188) into the spiro compounds (189) has been... 

Enamides of various structural types can be photochemically cyclised to give alkaloid ring systems this year, for example, it has been reported that the dimethoxyprotoberberines (190) have been prepared by irradiation of (191) in which the enamide is an N-vinyl benzamide which cycllses non-regioselectively onto the non-equivalent orfho-positions of the benzamide phenyl ring. In the... 

N-Oxides of heteroaromatics, reactions with ketenes 79H(12)819. Photochemical electron transfer in reactions of N-heterocycles 82YZ716. Photochemical transformations of heterocyclic enamides 78S489. Photochemistry of N-heterocycles 82AHC(30)239. 

The structure of alamarine [( )-58], a typical member of this benzopyri-doquinolizine family, was confirmed by its synthesis featuring thermal or photochemical cyclization of the enamide 134 (1,116). Chowdhury (227) synthesized isoalamarine (59) from the isomeric enamide 135 via a parallel route. For the syntheses of ( )-alangimaridine [( )-61] and alangimarine... 

Reactions of 1,2-Diketones and other 1,2-Dicarbonyl Compounds. - Further details of the control of the photocyclizations of enamides such as (142) affording (143) have been described. The results obtained are influenced by the nature of the chiral auxiliaries used. The best results are obtained using ionic and covalent auxiliaries in the crystalline phase or in zeolites. " Photochemically induced a-cleavage occurs on irradiation of the ketoamides (144) in water. This results in the release of the carboxylic acid moiety. The by-product of the reaction is the hemiacetal (145). " The irradiation of propellane in the presence of... 

Several reviews whose topics include photochemical reactions of aromatic compounds have appeared during the year. The preparation of indole alkaloids by enamide photocyclisation reactions has been surveyed. A very useful four volume work dealing with the theory and experimental practicalities of light-induced electron transfer reactions has been published the third volume of this set is... 

Miscellaneous Rearrangements.—An improved non-photochemical route to 13-epi-androstanes and -oestranes involved conversion of the appropriate 17-oximino-compounds with boiling acetic anhydride-pyridine into the enamide (176) and the enimide (177) which were readily hydrolysed to the 17-oxo-... 

Following photochemical pathways, the enamides 89 afford 8-oxyberbine compounds (91 and 92) in a 45-96% yield with elimination of the substituent A in the ortho position (651) (Scheme 34). This reaction has been extended to form 8-oxyprotoberberine or 13-methyl-8-oxyprotoberberine. 

The enamide photocyclization reaction has been widely exploited for the synthesis of protoberberine and other alkaloids (see Vol. 4 of these Reports and ref. 7). It has now been found that enamides of the type (160 X = OMe) undergo photochemical elimination of methanol to produce in good yield protoberberine precursorsofthetype(161). Forexample,compounds(161 R = R = H, = OMe),2 2 (151. r4 H r2 1 3 OCHjO), and (161 ... 

Photochemical electrocyclization of unsaturated enamides is a well-documented reaction of high synthetic value in the field of alcaloids [56]. The reaction can be viewed as a conrotatory photocyclization process leading to a dipolar intermediate which can be transformed into the final products either through a thermal, suprafacial, 1,5-sigmatropic shift or by an intermolecular protonation process. 

The phenanthridine ring of 45 is constmcted via a hydrogen bond-controlled enamide photocyclization. Alkyation of the enamide nitrogen is necessary, otherwise, the substrate decomposes <97SL547>. Spirocyclohexylisoquinolines are obtained photochemically from N-arylenamides of cyclohexyl aldehyde <97SC453>. A dihydroisoquinoline is formed in the photo-NOCAS reaction of isobutylene with 1,4-dicyanobenzene in acetonitrile <97JOC8432>. 

These transformations represent the first direct photochemical preparation of a quaternary protoberberine salt from an enamide. 

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