Alkaloids phase-transfer reaction

A number of other types of compounds have been used as chiral catalysts in phase-transfer reactions. Many of these compounds embody the key structural component, a P-hydroxyam-monium salt-type structure, which has been shown to be crucial to the success of the above described cinchona-derived quats. Although they have not been as successful as the cinchona catalysts, the ephedra-alkaloid derived catalysts (see 20, 22, 23 and 25 in Charts 3 and 4) have been used effectively in several reactions. In general, quats with chirality derived only from a single chiral center, which cannot participate in a multipoint interaction with other reaction species, have not been effective catalysts [80]. 

Optimization of the alkaloid phase-transfer catalysts included both the development of improved reaction conditions and the design of more efficient organocatalysts. Addressing this latter issue, O Donnell observed the first remarkable improvement of the enantioselectivity on use of modified alkaloid organocatalysts with an O-substituent, in particular an O-allyl or O-benzyl substituent, for example 23 and 24, respectively. This positive effect of O-alkylated structures was discovered during a detailed mechanistic study [22]. In this study it was found that O-alkylation of the previously used alkaloid catalysts, e.g. 21, and N-alkylated derivatives thereof, e.g. 22, by reaction with an alkyl halide (which is used in 1.2-5... 

The direct enantioselective organocatalytic a-fluorination can also be performed with cinchona alkaloid derivatives as catalyst under phase-transfer reaction conditions [25]. The fluorination reaction by NFSI of / -ketoesters 21, readily enolizable substrates, generated a stereogenic quaternary C-F bond in high yields and with enantioselectivities up to 69% ee for the optically active products 26 (Eq. 6). 

Reaction of 4-hydroxyquinoline-2-one 598 with oxalyl chloride gave oxazoloquinoline 599 (970PP211). The oxazoloquinoline 600 was obtained as a byproduct during the synthesis of pyranoquinoline alkaloids 601 by reaction of 598 with 2-methyl-2-chlorobutyne under phase transfer catalysis (87JHC869) (Scheme 101). 

Arai and co-workers have used chiral ammonium salts 89 and 90 (Scheme 1.25) derived from cinchona alkaloids as phase-transfer catalysts for asymmetric Dar-zens reactions (Table 1.12). They obtained moderate enantioselectivities for the addition of cyclic 92 (Entries 4—6) [43] and acyclic 91 (Entries 1-3) chloroketones [44] to a range of alkyl and aromatic aldehydes [45] and also obtained moderate selectivities on treatment of chlorosulfone 93 with aromatic aldehydes (Entries 7-9) [46, 47]. Treatment of chlorosulfone 93 with ketones resulted in low enantioselectivities. 

Table 1.12 Cinchona alkaloid-derived phase-transfer catalysts for asymmetric Darzens reactions.

The asymmetric aziridination of a, P-unsaturated carboxylic acid derivatives is a direct route to optically active aza-cyclic a-amino acids, and this class of chiral aziridines can also be used as chiral building blocks for the preparation of other amino acids, P-lactams, and alkaloids. Prabhakar and coworkers carried out an asymmetric aziridination reaction of tert-butyl acrylate with O-pivaloyl-N-arylhydroxylamine 25 in the presence of cinchonine-derived chiral ammonium salt 2e under phase-transfer conditions, which furnished the corresponding chiral N-arylaziridine 26 with moderate enantioselectivity (Scheme 2.24) [46],... 

Currently, the chiral phase-transfer catalyst category remains dominated by cinchona alkaloid-derived quaternary ammonium salts that provide impressive enantioselec-tivity for a range of asymmetric reactions (see Chapter 1 to 4). In addition, Maruoka s binaphthyl-derived spiro ammonium salt provides the best results for a variety of asymmetric reactions (see Chapters 5 and 6). Recently, some other quaternary ammonium salts, including Shibasaki s two-center catalyst, have demonstrated promising results in asymmetric syntheses (see Chapter 6), while chiral crown ethers and other organocatalysts, including TADDOL or NOBIN, have also found important places within the chiral phase-transfer catalyst list (see Chapter 8). 

Consequently, Dehmlow and coworkers modified the cinchona alkaloid structure to elucidate the role of each ofthe structural motifs of cinchona alkaloid-derived chiral phase-transfer catalysts in asymmetric reactions. Thus, the quinoline nucleus of cinchona alkaloid was replaced with various simple or sterically bulky substituents, and the resulting catalysts were screened in asymmetric reactions (Scheme 7.2). The initial results using catalysts 8-11 in the asymmetric borohydride reduction of pivalophenone, the hydroxylation of 2-ethyl-l-tetralone and the alkylation of SchifF s base each exhibited lower enantiomeric excesses than the corresponding cinchona alkaloid-derived chiral phase-transfer catalysts [14]. 

Asymmetric epoxidation catalyzed by chiral phase-transfer catalysts is another reaction which has been extensively studied following an initial report by Wynberg [2,44]. Shioiri et al. further improved the enantioselective epoxidation of naphthoquinones under cinchona alkaloid-derived chiral phase-transfer catalysis [45],... 

Dehmlow and coworkers [17] compared the efficiency of monodeazadnchona alkaloid derivatives 14a-c in the enantioselective epoxidation of naphthoquinone 50 with that of cinchona alkaloid-derived chiral phase-transfer catalysts 15a-c (Table 7.7) (for comparison of the alkylation reaction, see Table 7.1). Interestingly, the non-natural cinchona alkaloid analogues 14a-c afforded better results than natural cinchona alkaloids 15a-c. The deazacinchonine derivatives 14a,b produced epoxidation product 51 in higher enantioselectivity than the related cinchona alkaloids 15a,b. Of note, catalyst 14c, which possessed a bulky 9-anthracenylmethyl substituent on the quaternary nitrogen, afforded the highest enantioselectivity (84% ee). 

The chiral phase-transfer catalyst 3 afforded product 61 in 49% ee. The same group studied this reaction further by employing monodeazacinchona derivatives 14a-c [17]. The newly prepared non-natural analogues of cinchona alkaloids effectively promoted the hydroxylation reaction, although the enantioselectivity was lower than with natural cinchona alkaloid-derived chiral phase-transfer catalysts (Scheme 7.15). 

The intramolecular alkylation of the enolate derived from phenylalanine derivatives 22a,b to form P-lactams 23a,b has also been achieved using Taddol as a chiral phase-transfer catalyst (Scheme 8.11) [23]. In this process, the stereocenter within enantiomerically pure starting material 22 is first destroyed and then regenerated, so that the Taddol acts as a chiral memory relay. Taddol was found to be superior to other phase-transfer catalysts (cinchona alkaloids, binol, etc.) in this reaction, and under optimal conditions (50 mol % Taddol in acetonitrile with BTPP as base), P-lactam 23b could be obtained with 82% et. The use of other amino acids was also studied, and the... 

Esters 16b,c are used in reactions catalyzed by cinchona alkaloid-based phase-transfer catalysts, since the size of the ester is important for efficient asymmetric induction in these reactions [35], However, the syntheses of esters 16b,c adds considerable cost to any attempt to exploit this chemistry on a commercial basis. Fortunately, it was possible to develop reaction conditions which allowed the readily available and inexpensive substrate 16a to be alkylated with high enantios-electivity using catalyst 33 and sodium hydroxide, as shown in Scheme 8.18 [36]. The key feature of this modified process is the introduction of a re-esterification step following alkylation of the enolate of compound 16a. It appears that under... 

The first example of the use of an alkaloid-based chiral phase-transfer catalyst as an efficient organocatalyst for enantioselective alkylation reactions was reported in 1984 [3, 4]. Researchers from Merck used a cinchoninium bromide, 8, as a catalyst... 

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