Staudinger reaction, cycloaddition reactions

The j -lactam nucleus can also be assembled efficiently by a ketene-imine cycloaddition known as the Staudinger reaction. The reaction of chiral imine 759 with alkoxyketenes generated tfom benzyloxyacetyl chloride or acetoxyacetyl chloride affords cw-3,4-di-substituted )S-lactams 764a (75% yield) or 764b (61% yield) with diastereoselectivities greater than 95% [218]. 

The addition of an acid chloride to an imine is an important method for the preparation of (3-lactams and is often referred to as the Staudinger reaction. The reaction allows a convenient and mild approach to the -lactam antibiotics and has therefore received considerable attention. Good stereoselectivity in favour of the cis 3,4-disubstituted product is common. For example, the p-lactam 182 was formed in reasonable yield by condensation of the acid chloride 180 and the imine 181 (3.120). The reaction is not thought to be a concerted cycloaddition with the ketene, but to take place via a zwitterionic intermediate. Almost complete asymmetric induction in the synthesis of -lactams by the Staudinger reaction using a chiral auxiliary or a chiral tertiary amine, such as benzoylquinine, has been reported. 

Recently, Delpiccolo and Mata have also explored the asymmetric synthesis of monocyclic 3,4-substituted (3-lactams (Scheme 3.15). They employed the same Staudinger-type cycloaddition reaction with the chiral glycine derivative, 4(5)-phenyloxazolidylacetyl chloride (72). The oxazolidinone moiety functioned effectively as a chiral auxiliary, and a small library of optically active p-lactams 75 were obtained following cleavage with 10% TFA/DCM and subsequent conversion to the methyl esters (diastereoselec-tivity ranged from 8/1 to >25/1). 

Asymmetric synthesis of 3-amino (3-lactams via Staudinger ketene-imine cycloaddition reaction 98KGS1448. 

Staudinger observed that the cycloaddition of ketenes with 1,3-dienes afforded cyclobutanones from a formal [2+2] cycloaddition [52] prior to the discovery of the Diels-Alder reaction. The 2+2 cycloadditions were classified into the symmetry-allowed 2+2 cycloaddition reactions [6, 7], It was quite momentous when Machiguchi and Yamabe reported that [4+2] cycloadducts are initial products in the reactions of diphenylketene with cyclic dienes such as cyclopentadiene (Scheme 11) [53, 54], The cyclobutanones arise by a [3, 3]-sigmatropic (Claisen) rearrangement of the initial products. 

The Staudinger reaction [92], a [2 + 2]-cycloaddition of a ketene and a nucleophilic imine, usually proceeds by an initial imine attack on the ketene thus forming a zwitterionic enolate which subsequently cyclizes. This reaction is an expedient route to p-lactams, the core of numerous antibiotics (e.g., penicillins) and other biologically active molecules [93]. In contrast, for Lewis-base catalyzed asymmetric reactions, nonnucleophilic imines are required (to suppress a noncatalyzed background reaction), bearing, for example, an N-Ts [94] or -Boc-substituent [95]. 

Figure 9 (a) Protein modification with PEG through a copper-catalyzed cycloaddition reaction, (b) Protein modification with fluorescein through Staudinger ligation. 

Density functional theory calculations (B3LYP/6-31G level) have provided an explanation for the stereodivergent outcome of the Staudinger reaction between acyl chlorides and imines to form 2-azetidinones (/3-lactams). When ketene is formed prior to cycloaddition, preferential or exclusive formation of ct5-j6-lactam (50) is predicted. If, however, the imine reacts directly with the acid chloride, the step that determines the stereochemical outcome is an intramolecular 5n2 displacement, and preferential or exclusive formation of trans isomer (51) is predicted. These predictions agree well with the experimental evidence regarding the stereochemical outcome for various reactants and reaction conditions. 

Azetidinones on a solid support 49 have been prepared in high yield by Staudinger reaction of a supported imine with an acid chloride in the presence of a base. The liberated p-lactams were of high purity <99TL1249>. Cycloaddition of a ketene intermediate, derived fi"om an azo compound, to an imine having an oxidatively cleavable chiral auxiliary N-substituent was used to obtain p-lactams 50. The trans. cis ratio which varied between 69 31 and 93 7, depended on the nature of the substituents R and R <99S650>. 

Staudinger reaction of imine 8 derived from 7-oxanorbomenone with 2-alkoxy-acetyl chlorides in the presence of Et3N (toluene, RT), afforded (3-lactams 9 (Scheme 3). These were obtained as single diastereomers, and no traces of the corresponding isomeric exo-(3-lactams were detected in the crude reaction products [50]. It is worth mentioning that this stereochemical outcome of (3-lactam formation with acid chlorides under Staudinger reaction conditions was opposite to the one expected from a simple [2+2]-cycloaddition reaction, which should have taken place from the exo face of compound 8. 

Deshmukh et al. [134] have investigated the use of D-(+)-glucose derived chiral ketenes in the stereoselective synthesis of spiro-(3-lactams 226-227. The D-(+)-glucose acid chloride 224, serving as a ketene precursor, in the Staudinger cycloaddition reaction with appropriate imines 225 afforded the diastereomeric mixture of spirocyclic-(3-lactams 226-227 in 70 30 ratio, respectively. This reaction has cleanly produced only two diastereoisomers instead of theoretically possible four... 

In conclusion, the CAI activity of spiro-(3-lactams, their antiviral and antibacterial properties, their potential as efficient (3-tum nucleators and (3-tum mimetics, and their application as synthons for a,a-disubstituted (3-amino acids motivated synthetic and medicinal chemists to design novel spirocyclic (3-lactams. Several approaches to the stereoselective synthesis of spiro-(3-lactams have been described in this review. However, ketene-imine cycloaddition (Staudinger Reaction) shows much versatility for the access to diversely functionalized spiro-(3-lactams. In addition, we have developed a facile route to novel spiro-(3-lactams by using... 

Scheme 26 Synthesis of sugar-based monocyclic [S-lactams by Staudinger [2+2] cycloaddition reaction...

Reaction of D-phenylalanine ethyl ester with cinnamaldehyde has been reported to give a chiral Schiff base, that underwent an asymmetric Staudinger [2+2] cycloaddition reaction with phthalimidoacetyl chloride to give the monocyclic... 

A combined theoretical and experimental study has been reported for the formation of silylated (3-lactams, via Staudinger [2+2] cycloaddition reaction from silylketenes and imines, in the presence or in the absence of a Lewis acid... 

Y/ .v-(3-Lactains have been reported to be regioselectively synthesized by [2+2] Staudinger cycloaddition reactions of imine such as (3,4-dimethoxybenzylidene)-(4-methoxyphenyl)-amine and ketenes derived from different acyl chlorides and triethylamine [110]. 

A stereocontrolled Staudinger cycloaddition reaction has been reported to be performed on vinylketenes, possessing a y-heteroatom, and imines to produce frans-vinyl-(3-lactams [112]. The vinyl side chain adopted stereoselectively the (Z) configuration in the transition state, stabilizing the vinyl ketene and leading, exclusively, to the frans-3-vinyl-(3-lactam (Scheme 37). 

Compared with the single-bond construction approach of (3-lactam synthesis, the ketene-imine cycloaddition, which includes carbenoid insertion and the Staudinger reaction, have been widely used [56, 65]. Due to the ready availability of both imines and ketenes, the Staudinger reaction has provided a useful and economical approach for the synthesis of (3-lactams. In addition, the ketene-imine cycloaddition is efficient, which constructs the (3-lactam four-member ring in just one-step... 

Gallop et al. [80] reported the preparation of p-lactams via a [2+2] cycloaddition reaction of ketenes with resin-bound imines derived from amino acids (Scheme 9). This is another solid-phase adaptation of the Staudinger reaction, which could lead to the synthesis of structurally diverse 3,4-bis-substituted 2-azetidinones [81]. In addition, a novel approach to the synthesis of A-unsubstituted-p-lactams, important building blocks for the preparation of p-lactam antibiotics, and useful precursors of chiral p-amino acids was described [82]. 

Two possible [2+2] cycloadditions can be envisaged for the synthesis of (3-lactams (Scheme 1). Interestingly, the same fragmentations have been observed in the mass spectra of these compounds [10, 11], One possibility consists of the [2+2] cycloaddition between ketenes (2) and imines (3) to yield (3-lactams (1). This reaction has been explored experimentally and it is also known as the Staudinger reaction between ketenes and imines [12-15]. In an alternative approach, the [2+2] cycloaddition between alkenes (5) and isocyanates (4) leads to (3-lactams (1). This reaction has been less extensively used, but it has proven to be useful in the chemical synthesis of interesting compounds [16-19]. 

A perfunctory study of the Staudinger reaction ( rich alkene + ketene — cyclobuta-none)49 can lead us to believe in a failure of FO theory. The alkene, being rich in electrons, will react preferentially by its HOMO and the ketene by its LUMO. Also, as nco is lower than jtcc, the cycloaddition should give a methylene oxetane and not a cyclobutanone. 

Another common method for the synthesis of 2-azetidinones is the cycloaddition of imines with ketenes, which is known as the Staudinger reaction . Although commonly described as a [2 + 2]... 

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