Exo stereochemistry

Exo stereochemistry, Diels-Alder reaction and, 495 Exon (DNA), 1108-1109 Exothermic, 154... 

Benzonitrile oxide, generated by dehydrochlorination of benzohydroximoyl chloride, undergoes regio- and face-selective cycloadditions to 6,8-dioxabicyclo [3.2.1]oct-3-ene 108a yielding a 4 1 mixture of 4,5-dihydroisoxazoles 109 and 110. Both products have exo-stereochemistry, resulting from the approach of the nitrile oxide from the face opposite to the the methyleneoxy bridge. Structures of the adducts were determined by 1 H NMR spectroscopy and, in the case of compound 109, by X-ray diffraction analysis (275). 

The exo stereochemistry of compounds 127-130, although not unambiguously clear from their NMR spectra (where these have been recorded),... 

Bromonitromethane is a key starting material for the preparation of the broad-spectrum antibiotic trovafloxacin (10), which contains the interesting 3-azabicy-clo[3.1.0]hexane ring system. Pfizer chemists found that the base-catalyzed reaction of bromonitromethane with N-benzylmaleimide efficiently assembled the key bicyclic adduct 11 in high yield and with the required exo stereochemistry [14]. The exo addition product 11 could then be converted (Scheme 11.8) to trovafloxacin (10) [15]. 

The reaction of aryl azides with a number of 7-substituted norbornenes and norbornadienes has been investigated (Scheme 14).128 Although bulky substituents in the 7-position slow down addition,43-45 good yields of adducts have been obtained in most cases by carrying out the reaction for several days. NMR analysis indicates exclusive exo stereochemistry for the products, and when the 7-hydroxyl is in the anti position, the direction of azide addition is reversed (Scheme 14)."... 

The Staudinger reaction of imines derived from 7-oxanorbomenone with 2-alkoxyacetyl chlorides has been reported to afford spiro-(3-lactams with an unexpected exo stereochemistry [67],... 

From the stereochemical structure of the adduct it can be inferred that the initial ortho photocycloaddition occurs with exo stereochemistry, whereas the subsequent Diels-Alder reaction of the ortho adduct with a second molecule of maleic anhydride proceeds with endo stereochemistry. 

Atkins et al. [130] reported in 1977 that irradiation of mixtures of benzene and methyl acrylate or methyl methacrylate, both acceptors, yields mixtures of endo and exo adducts. A subsequent report from the same groups [120] describes the results of the irradiations of benzene in the presence of ethyl vinyl ether, //-butyl vinyl ether, 2,3-dihydropyran, and 1,4-dioxene. In all these cases, the major products were exo-ortho photocycloadducts. The orientations of these vinyl ethers with respect to benzene, in their loose ground-state associations, were inferred from NMR spectra. For ethyl vinyl ether, n-butyl vinyl ether, 2,3-dihydropyran, and 1,4-dioxene, the vinyl proton resonances were either unaffected by a solvent change from carbon tetrachloride to hexadeuterobenzene or appeared 4-10 Flz downfield, whereas the methyl and/or methylene signals all moved up-field by 10-25 Hz. This implies an endo arrangement of the molecules in the ground state. Thus, the ortho photocycloadducts of vinyl ethers with benzene show exo stereochemistry, even when the ground-state orientation is endo. 

In case of Paterno-Buchi reaction of cycloalkenes with prostereogenic aromatic carbonyl compounds (which show rapid ISC to the triplet excited carbonyl) less clear results were obtained. The reaction of cyclohexene with benzaldehyde was reported in the literature [51] and the spectral data of the main product 56 (35%) described as consistent with the assignment of exo-stereochemistry (Sch. 35) [110]. 

Nitrones were the first as well as the most widely used dipoles in asymmetric cycloadditions. The first report on the use of enantiomerically pure vinylsulf-oxides as dipolarophiles was due to Koizumi et al. [153], who described in 1982 the reaction of (-R)-vinyl p-tolyl sulfoxide 1 with acyclic nitrones 191. The reactions required 20 h in refluxing benzene to be completed, yielding a mixture of only two compounds, 192 and 193 (Scheme 91). They exhibited identical endo or exo stereochemistry (which was not unequivocally assigned), deduced from the fact that their reduction yielded enantiomeric thioethers. The major component, 192, exhibits (S) configuration at C-3, determined by chemical correlation. The authors claim this paper [153] to be the first example of 1,3-dipolar cycloaddition using chiral dipolarophiles. 

Apeloig et al. have recently provided X-ray crystallographic evidence for the structure of the product exo-2a, from tetrachlorocyclopropene and 1-acetoxybuta-l,3-diene, and established that only the evo-product was obtained. The products from 1 b and Ic have the same stereochemistry, because they can be related chemically to exo-2a. The exo stereochemistry in these reactions is apparently caused by unfavorable interaction between the diene and the 3,3-dihalo groups of the cyclopropene in the endo transition state. ... 

The exo stereochemistry of four other cyclopropene adducts 3-6 has also been demonstrated by X-ray crystallography. ... 

The product has exo stereochemistry with the carbonitrile group syn to the ether bridge. 

Given the published studies that indicate that the insertion of nonfunction-alized norbomenes into metal-carbon bonds occurs with exo, exo- stereochemistry (e.g., Fig. 9.1) we sought to determine whether the presence of an endo functionality would change the insertion stereochemistry. Because of the relative instability of the catalytically active palladium species, detailed studies encompassing the coordination and insertion of norbornene derivatives were carried using the model... 

With some 1,3-dipoles, two possible stereoisomers can be formed by syn additions differing in the relative orientation of the reacting molecules. In several cases where disubstituted diazomethanes have added to olefins, mixtures are obtained.This is comparable to the situation of endo versus exo stereochemistry in the Diels-Alder reaction. 

The copolymers obtained with various cationic systems modified with chelate diphosphine or dinitrogen ligands always showed cis-exo-stereochemistry. Cooligomers with the spiroketal stmctnre were obtained in methanol as the solvent using systems formed... 

First, we studied the reaction between upper rim 1,2-functionalized calix[4]arene 22a, prepared in 72% overall yield by reduction of the corresponding 1,2-dinitro compound [29] and subsequent reaction with two equivalents of a-chloroacetyl chloride, and cavitand 23. When this reaction was performed in a 1 1 ratio of 22a and 23 the diastereomeric products 24a and b in which the calix[4]arene moiety is coupled to the cavitand in a 1,2- (proximal) fashion with the endo and exo stereochemistry were isolated in 20 and 32% yield, respectively. In addition to these 1 1 reaction products small amounts of the three possible isomeric 2 1 products 25a-c were formed. Products in which the calix moiety is coupled in a 1,3- (distal) fashion to the cavitand could not be detected. When the reaction between 22a and 23 was carried out in 2 1 ratio only the 2 1 products 25 were isolated in an almost statistical ratio of endo-endo (25a), endo-exo... 

Addition of Propargyltrimethylsilane A novel stereoselective carbon chain extension reaction at the C-6 position of 1,6-anhydromannose triacetate 53, prepared from D-matmose in three steps [38], was reported by Nishikawa et al. [39]. Compound 54 was prepared by photobromination of 53 according to the Perrier [40] procedure. The reaction of 54 with propargyltrimethylsilane 55 in the presence of AgOTf gave only one allenylated compound 56 (exo-stereochemistry, l) at C-6 position. Thus, ozonolysis of aUene 56, followed by reduction with NaBH, led to the alcohol 57, which was efficiently transformed into L-D-Hepp hexaacetate 58 after acetylation and subsequent acetolysis (Scheme 2.15). 

Retention of configuration was demonstrated in the Baeyer-Villiger reaction shown in Equation 3.11 [49]. Perbenzoic acid initiates migration of the norbornyl group in a concerted fashion, allowing the exo stereochemistry of the starting material to be maintained. 

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