Boronic amino esters

Strained bicyclic carboxylic or boronic amino esters [122] [Eq. (58)] as well as fluorinated amino esters [123,124] were then synthesized. The reaction could also be applied to allenynes to produce alkenyl alkylidene bicyclo[3.1.0]hexanes [125]. 

Ring-opening of aziridine-2-carboxylates with alcohols has been reported to give (3-alkoxy-a-amino esters [16, 102]. Treatment of as-aziridine 127 (Scheme 3.45) with alcohol in the presence of a catalytic amount of boron trifluoride etherate afforded P-alkoxy-ot-amino esters 128 in 57-100% yields [16,102a], The reaction is both regio- and stereoselective, affording 128 as the only product. 

Perez-Fuertes et al.6 8 have suggested the possibility of using the three-component chiral derivatizing approach for prediction of the absolute configuration of amines. Comparison of aH NMR spectra has shown significantly more deshielded signals of (S,S)-imino-boronate esters, derivative of a-amino esters than (S,R) diastereomers. 

In order to introduce a second hydroxyl group al C-l 1 of the ester (99) its sodium salt was treated with p-nitrobenzene diazonium chloride in methanol. The resulting product on methylation and then reduction with potassium dithionate afforded the amino ester (100). Diazotization and methanolysis produced ethyl ( )-camosate dimethyl ether (101), which on saponification produced camosic acid dimethyl ether (102). Wenkert [22] accomplished demethylation of dextrorotatory camosic dimethyl ether with boron tribromide. Thus the work of Meyer lacks only the... 

Asymmetric aza-DA reactions using chiral imines derived from a-amino esters an carbohydrates were extensively studied by Waldmann [310] and Kunz [311], re P o tively, at the end of fhe nineteen-eighties. After these significant works Yamamoto e al. reported the utility of homochiral N-(l-phenylefhyl)imines in asymmetric cy dition [219, 312]. In fhe presence of a stoichiometric amount of chiral boron tors 83 and 84, the cycloaddition of Danishefsky s diene 102 to these imines rea iz s almost complete diastereocontrol in the matched cases (Scheme 10.115), a e... 

In their synthesis of the pyrimidine segment of the potent antitumor antibiotic bleomycin, Umezawa, Ohno and coworkers have described the reaction of highly functionalized imine (96) with malonic acid monoethyl ester to afford 3-amino ester (97 equation 16)." The low yield of (97) is largely due to elimination of the amino side chain, giving the corresponding acrylate derivative. A subsequent modification of this reaction using a boron enolate overcomes this problem and will be discussed in Section 4.1.3.2.2.ii. 

P-Amino esters. By means of douM and a chiral BINOL-boronate catalyst, th a simple method for the synthesis of 0-a Hydroxy-/3-amino esters are similarly aco... 

Transesterification of boronic acid esters s. 44, 52 2-Amino-1,3-dioxanes from 1,3-diols s. 44, 941... 

A homochiral boronate derived from the chiral imidazolidinone template yields various biaryls with little or no loss of stereochemical integrity. Although the amino ester shown in Eq. 38 undergoes partial racemization during biaryl coupling, a recent method of Novartis in USA using Seebach s imidazolidinone made the assessment of the dias-tereoselectivity exceedingly simple (Eq. 70).The imidazolidinones... 

To the best of our knowledge, there are very few reports on the use of other metal-based systems. However, in 2007, Sames and coworkers [42] reported the ruthenium-catalyzed decarboxylative ary-lation of cyclic 2-amino esters. This process provides a rapid avenue to a variety of 2-arylpyrrolidines and piperidines from commercially available proline, hydroxyproline, and pipecolinate esters (Scheme 3.25). Examination of the substrate scope also showed that many arylboronic acids and boronate esters serve as coupling partners. It was also demonstrated that the required amidine- or... 

The Petasis reaction is a multicomponent condensation occurring between boronic acids, amines and aldehydes. The asymmetric version of this reaction is very attractive for the synthesis of chiral a-amino acids.In this context, Schaus and Lou reported the use of chiral biphenols as organocatalysts for the asymmetric Petasis reaction of ( )-diethyl styrylboronate with secondary amines and ethyl glyoxylate. The corresponding a-amino esters were obtained in high yields and enantioselectivities of up to 97% ee by using a vaulted biaryl phenol such as (5)-VAPOL as the organocatalyst in the presence of 3-A molecular sieves (Scheme 2.59). 

Macrocyclic Lactams.— Two spermidine alkaloids have been prepared by the reaction of spermidine itself with a thiazolidine 2-thione diamide derivative under high-dilution conditions the two regioisomers produced were separated by chromatography.An interesting boron-template cyclization of the amino-ester (173) to the lactam (174) has been used to prepare several members of the celacinnine group of natural products (Scheme 85). ... 

Three-component Petasis reactions among salicylaldehydes, amines, and organo-boronic acids have been catalysed with a chiral thiourea BINOL catalyst to prepare alkylaminophenols with yields and ee values of up to 92% and 95%, respectively. Formation of amino esters Et02CCH(Ar)NR2 by Petasis combination of Et02CH0, R2NH, and ArB(OH)2, catalysed by Cu(I), is believed to involve transmetallation from boron to copper from which Ar is delivered intramolecularly to the iminium group of the coordinated intermediate. ... 

The enantioselective inverse electron-demand 1,3-dipolar cycloaddition reactions of nitrones with alkenes described so far were catalyzed by metal complexes that favor a monodentate coordination of the nitrone, such as boron and aluminum complexes. However, the glyoxylate-derived nitrone 36 favors a bidentate coordination to the catalyst. This nitrone is a very interesting substrate, since the products that are obtained from the reaction with alkenes are masked a-amino acids. One of the characteristics of nitrones such as 36, having an ester moiety in the a position, is the swift E/Z equilibrium at room temperature (Scheme 6.28). In the crystalline form nitrone 36 exists as the pure Z isomer, however, in solution nitrone 36 have been shown to exists as a mixture of the E and Z isomers. This equilibrium could however be shifted to the Z isomer in the presence of a Lewis acid [74]. 

Aziridine esters are a- and -amino acid derivatives at the same time. A characteristic reaction of a-amino acids is their reaction with triethylboron to give boroxazolidines. We showed that aziridinecarboxylic acids exhibit the expected behavior in their reaction with triethylboron, viz., that they form stable boroxazolidines 34 (Scheme 19) [29]. These boron heterocycles can be reconverted into the free amino acids by treatment with 8-hydroxyquinoline. 

An approach to the preparation of asymmetrically 1,2-disubstituted 1,2-diamines has been reported the zinc-copper-promoted reductive coupling of two different N-(4-substituted)phenyl aromatic imines, one bearing a 4-methoxy and the other a 4-chloro substituent, in the presence of either boron trifluoride or methyltrichlorosilane, gave a mixture of the three possible 1,2-diamines, where the mixed one predominated [31 ]. Low degrees of asymmetric induction were observed using 1-phenylethylamine, phenylglycinol and its 0-methyl ether, and several a-amino acid esters as the chiral auxiharies meanwhile the homocoupling process was not avoided (M.Shimizu, personal communication). 

Alkenylboronic esters undergo regio- and stereoselective 1,3-dipolar cycloadditions with nitrones. These reactions lead to boronic ester-substituted isoxazolidines, which can be converted by oxidation with H202 to the corresponding 4-hydroxy derivatives (Eq. 8.48).69 The high selectivity could be the result of a favorable interaction between the boronic ester and the amino group. 

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