Diazoketones enantioselectivity

The Cu semicorrin complex (68a) has been successfully used as the catalyst for cyclization of alkenyl diazoketones, though the reactions of some substrates showed modest enantioselectivity (Scheme 74).276 Shibasaki et al. have successfully used the cyclization of diazoketone with Cu bis(oxazoline) (101) for the construction of the CD ring skeleton of phorbol.277... 

Good yields of the bridged tetrahydropyran-3-one 38 are obtained when the a-diazoketones 37 are decomposed by chiral Rh(II)-catalysts in the presence of DMAD. It is proposed that an enantioselective intermolecular 13-dipolar cycloaddition follows the generation of a carbonyl ylide which is bound to the rhodium (Scheme 21) <99JA1417>. 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

However, the chemical yield increases considerably as the proportion of THF is reduced, although the stereoselectivity is slightly decreased. Interestingly, the sense of enantioselection in the formation of ds-81 in hexane is opposite to that in THF. Katsuki et al. applied this reaction to the intramolecular cyclopropanation, when the irradiation of alkenyl diazoketones in the presence of 80 in THF afforded bicyclo [3.1.0]hexan-2-ones in a highly enantioselective cyclopropanation, and in moderate yields (Table 4.3) [60]. 

Treatment of the alkoxy diazoketones 218 with rhodium(II) acetate affords 3(2//)-furanones in suitable yields (89TL1749). The carbenoid cycliza-tion reaction to form 2,5-disubstituted 3(2//)-furanones exhibits a stereoselection favoring the cis isomers. This phenomenon was exploited in an enantioselective synthesis of (-t-)-muscarine (89TL1753). The formation of furanone 219 illustrates the clear preference for the five-membered ring when two ether oxygens are present to activate the C—H bonds, leading to either the five- or six-membered rings. 

A similar transformation has been reported using enantiomericaUy pure copper catalysts, where the diazoketone (9.126) forms an initial oxonium yhde, which then rearranges to the cyclic product (9.127), with reasonable enantioselectivity, using a copper complex of ligand (9.128). ... 

Strategy of Rh-triggered qrcloaddition cascade. In this case, the Rh-catalyzed transformation of a-diazoketone 132 into an oxatetracycUc key cycloadduct 133 through intramolecular [3+2]-cycloaddition of an in-situ generated carbonyl yUde was achieved. Further, the regioselective conversion of the cycloadduct 133 into a tropolone derivative led to an efficient enantioselective access to colchicine (Scheme 40). 

Another example of orthoester rearrangement for the construction of a quaternary center is given in the enantioselective synthesis of (-t)-valerane 148 [34]. In this synthesis, ii-(-)-carvone 149 is used as starting material. Rearrangement of allylic alcohol 150 afforded ester 151. Diazoketone cyclization is then followed by a ring enlargement affording the bicyclo[4.4.0]decane system (Scheme 6.22). 

In the first enantioselective synthesis of triquinane (-)-cucumin H 168 [38], a quaternary center is introduced via an orthoester rearrangement. AUylic alcohol 170 is prepared from (J )-limonene 169 by a known sequence of reactions. Orthoester rearrangement occurred in a highly stereoselective manner anti to the iso-propylidene side chain and afforded ester 171. Then diazoketone cycUzation followed by cationic cyclization gave rise to the desired tricyclic system (Scheme 6.25). 

The Schiff base catalyst 4 was the first chiral catalyst for the intramolecular cyclopropanation of diazoketone, although low enantioselectivity (8% ee) and only moderate yield (64%) were obtained (125). A major breakthrough occurred when semicorrin 6 was employed. Although the yield remained moderate, high... 

Spirodicycloalltylphenyl derivatives. A highly enantioselective N-H insertion reaction of a-diazoketones (178) has been developed by using cooperative catalysis by chiral spiro phosphoric acid (180) and dirhodium(ii) carboxylate (181). This reaction provided a new access route to diverse chiral a-aminoketones (179) with fast reaction rates (1 min), good yields and high enantioselectivity under mild and neutral conditions (Scheme 47). ... 

---