Cis-Vinyl epoxides

The electrophilic addition of I2 to 2,3-allenols 340 in Et20 was highly regioselec-tive with respect to the terminal C=C bond, leading to the diiodination products 341 with a preponderance of the Z-isomer. The diiodide 341 may be further converted to trans/cis vinylic epoxide 342upon the treatment with a base (Scheme 10.137) [159]. 

A number of enantiomerically pure complexes have been made, and this chemistry has been used in several natural product syntheses. Enantiopure complexes are readily available from the corresponding vinylic epoxides, and in cases where diastereoselective complexation is possible, diastereoselectivities tend to be moderate (typically 3 1 -4 1). The rationale for the origin of this diastereoselectivity has been proposed to derive from a preferential complexation of a Fe(CO)4 fragment to the alkene anti to the epoxide. Since the initial vinyl epoxide is conformationally flexible, four diastereomeric itt-complexes would be produced as a consequence of anti or syn complexation to the s-trans or s-cis conformers. Isomerization of these initial 7r-complexes to alkoxy- 7r-allyl species would then enable interception of an iron-bound carbonyl ligand by the alkoxide to afford diastereomeric lactone complexes. Fortunately, equilibria between the two possible trans itt-allyl complexes and their more stable cis Tr-aUyl analogs simplifies the outcome significantly. Thus, for trans vinyl epoxides, the major diastereomer typically is the one designated as endo cis (the C-1 substituent points toward the iron atom) the minor diastereomer corresponds to the exo cis isomer (the C-1 substituent points away from the iron atom) (Scheme 51). For cis vinyl epoxides, this outcome is reversed - the exo cis isomer is the major product. 

In contrast to reactions with vinyl epoxides and palladium catalysts, the reactions with rhodium retain the stereochemistry of the alkene fragment during the reaction [20]. This is illustrated by the reactions of trans-37a/h and cis-37a/b, which give only one product possessing the same olefin geometry as the starting epoxides (Eqs. 4 and 5). The retention of olefin stereochemisty has also been documented in allylic functionalizations with iridium catalysts, indicating that similar modes of action may be present [21, 22]. 

Cleavage of vinyl epoxides to cis-diols. In the presence of a Pd(0) catalyst, prepared by reaction of BuLi (2 equiv.) with Pd(OAc)2 (1 equiv.) and P(0-/-Pr)3 (excess) in THF, carbon dioxide reacts with vinyl epoxides to give carbonates of vinyl cw-l -diols.1... 

Various unsaturated compounds, such as CO2, isocyanates and aldehydes, undergo Pd-catalysed cycloaddition with vinyl epoxides. Reaction of CO2 with 127 affords cyclic carbonates 128 with retention of the configuration at C(3), offering a method of cis hydroxylation of epoxides [66], and has been used for the synthesis of the side-chain unsaturated (—)-exo-brevicomin (129) [67], The tetrahydrofuran 131 was prepared by [3+2] cycloaddition of activated alkenes such as benzylidene malononitrile (130) with vinyl epoxide via Michael addition and allylation [68],... 

Carboxylates behave as an O-nucleophile and are allylated. Reaction of AcONa with cyclopentadiene monoxide (139) proceeds with retention of stereochemistry as shown by 140, to give the 3,5-c -disubstituted cyclopentene 141 [74], Although alcohols are rather unreactive nucleophiles for the Pd-catalysed allylation, the alkoxide anions generated by the treatment of silyl ethers with TBSF are easily allylated. Desilylated alcohol from 142 reacts intramolecularly with the cis and trans vinyl epoxides to give the cis and trans pyrans 143 and 144 regio- and stereoselectively, and... 

An unusual syn addition to epoxides occurs when 1,3-diene monoepoxides are treated with organozinc reagents. Thus, the cyclic vinyl epoxide 72 was converted to the cis-ethyl-cyclohexenol 75 with diethyl zinc in methylene chloride and trifluoroacetic acid. The syn addition is believed to derive from an initial coordination of the oxiranyl oxygen to the organozinc compound, which then delivers the alkyl group to the same face. This transfer is facilitated by a relaxation of the sp3 hybridization brought about by the Lewis acidic zinc center and the allylic character of the incipient carbocation <020L905>. 

The chemistry of this class of complexes is dominated by the 7r-allyliron tricarbonyl lactone complexes (155), which have been studied extensively and reviewed. Compounds (155) are normally prepared by reaction of vinyl epoxides with a source of Fe(CO)4 , such as (1) and photolysis, (2) and heat, or (2) in Tetrahydrofinan (THF) or with ultrasonication (see Sonication) the latter two methods are considered to be superior. It has been reported that butene-1,4-diols, particularly the cis isomers, (with or without Lewis acids), 3-butene-1,2-diol sulfites, 3,6-dihydrooxazmes, or related compounds also give (155) with (2). The complexes are isolated as anti isomers at C-1. Substituents at C-3 are oriented syn when not constrained in a ring, whereas... 

The 3-oxa-di- i -methane rearrangement is far less common. As a symmetrical system, the product should be a vinyl epoxide (equation 23). So far, no 3-oxa-di-T -methane products have been observed from the divinyl ethers depicted in the equation. It was shown that the solution photochemistry of (21a) derives from its singlet i , i -state and parallels that of the unsubstituted divinyl ether (21d) and of furan in the gas phase. All products could be rationalized in terms of the initial formation of a singlet vinyl-vi-nyloxy radical pair (equation 24). Triplet sensitization brings about cis-trans isomerization and consequent deactivation. Had the 3-oxa-di- ir-methane product, i.e. the vinyloxirane (22), been formed, it would have decomposed via Griffin fragmentation as confirmed by irradiation of an authentic sample. 

Rama Rao, A V, Gurjar, M K, Devi, T R, Kumar, K R, A formal synthesis of a novel immunosuppressant ISP-1 stereocontrolled Pd(0) catalyzed cis hydroxyamination of carbohydrate derived vinyl epoxide. Tetrahedron Lett., 34, 1653-1656, 1993. 

In general, Sn2 displacement on cyclic vinyl epoxides by organocopper reagents has been found to proceed with inversion anti pathway). However, acyclic vinyl epoxides can react via either s-cis or... 

The acid-catalysed hydrolysis reactions of vinyl epoxides (140 = 1-4) have been shown to follow an A-l mechanism, via the intermediate allylic cations (141). ° The distributions of products from both (140 = 1) and (140 = 3) were similar, comprising all four possible cis- and trans-1,2- and -1,4-diols, whereas cyclohexadiene oxide (140 = 2) gave mainly the trans-... 

Aromatic amines are converted into carbamate esters on treatment with an alcohol and CO (1 atm) in the presence of 0 , CuClo, and catalytic amounts of PdCl2 (Scheme 64). Vinyl epoxides react with CO2 in the presence of a palladium(O) catalyst, generated in situ from Pd(0H. )., P(0Pr )T, and BuLi, to give the corres-ponding vinyl carbamates (Scheme 65). The epoxide stereochemistry is retained and this methodology corresponds overall to a cis-dihydroxylatlon process. 

The synthesis of (-)-A -acetyl-O-methylacosamine, " an amino sugar, is an interesting application of the process of cis-hydroxyamination catalyzed by palladium complexes of olefins developed by Trost and Sudhakar. Enantiomerically pure aldehyde 263 was first transformed to vinyl epoxide 264. The latter reacted smoothly with p-toluene sulfonyliso-cyanate in the presence of 1-3 mol% of palladium and 6-18 mol% of tiiisopropyl phosphite in THF at room temperature. As thermal reaction favored 0-aIkylation, these conditions led exclusively to A-p-toluenesulfonyl-2-oxazolidone 265 as shown in Scheme 60. 

Sn2 VersusSN2 Reactionof2-TMS-l,3-Dithiane With Vinyl Epoxides. In 2002 Smith et al. demonstrated that 2-lithio-2-TMS-1,3-dithiane reacts with vinyl epoxides in an Sn2 fashion exclusively, while other, larger silyl dithianes afford mixtures of Sn2 and Sn2 products. Particularly useful, the large 2-lithio-2-triisopropyl-1,3-dithiane provides solely the Sn2 product. Furthermore, it was found that trans epoxides furnish syn products and cis epoxides produced anti products, albeit in modest yields (eq 28). However, under the reaction conditions (THF, HMPA) the TMS group underwent a 1,4-Brook rearrangement to afford a 1 1 mixture of the anticipated homoaUyhc alcohol and the rearranged silyl ether. 

Achiral tellurides were used in the catalytic epoxidation of aldehydes by Huang and coworkers in 1990. With diisobutyl telluride as a catalyst (20mol%), various aromatic, heteroaromatic, and non-primary aliphatic aldehydes can be epoxidized directly with aUyhc bromide to the corresponding vinyl epoxides in good yields with predominantly cis-selectivity [59], Later, they successfully extended this method to the cyclopropanation of a,(3-unsaturated ketones with aUyl bromides [60], The asymmetric version of the cyclopropanation was realized by Tang and coworkers in 2003 [61], using salt 43 directly as a catalyst (20mol%) in the reaction of chalcones with silylated aUyl bromide. The desired vinyl cyclopropanes were... 

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