Cycloaddition /reactions silyl ketenes

Romo et al. have used Lewis acids to catalyze the formation of a-silyl-/ -lactones in their synthesis of potential inhibitors of yeast 3-hydroxy-3-methyl glutaryl-coenzyme A (HMG-CoA) synthase <1998BMC1255>. In addition to various Lewis acid catalysts, a chiral promoter based on the chiral diol (l/ ,2R)-2-[(diphenyl)hydroxymethyl]cyclo-hexan-l-ol was introduced to the reaction in an attempt to improve the stereoselectivity. A variety of chiral 2-oxetanones were formed, with enantioselectivities ranging from 22% to 85%. Dichlorotitanium-TADDOL catalysts 113 and 114 have also been used in an attempt to encourage the stereoselective [2+2] cycloaddition of silyl ketenes and aldehydes (TADDOL = (—)-/ra r-4,5-bis(diphenyl-hydroxymethyl)-2,2-dimethyl-l,3-dioxolane), although this method only afforded 2-oxetanones in moderate yields and optical purity (Equation 41) <1998TL2877>. 

The 2+2 cycloadditions of benzyne to cis- and trani-propenyl ether gave cis- and fran -benzocyclobntanes as the main products, respectively [ 117,118], Stereospecific [2+2] cycloaddition reactions were observed between the benzyne species generated by the halogen-Uthium exchange reaction of ort/io-haloaryl triflates and the ketene silyl acetals (Scheme 23) [119],... 

Theoretical calculations at DFT level agree that the reactions of nitrones with silyl ketene acetal proceeds via 1,3-dipolar cycloaddition followed by the transfer of the silyl group, yielding an open-chain product (641). 

No reaction occurs with vinyl ethers, silyl enol ethers, or ketene silyl acetals, usually used in thermal [2 + 2]cycloaddition reactions, but the present case is the first example of the preparation of a chiral cyclobutanone by a cycloaddition route. 

As mentioned in Section 7.2, when the electron transfer reaction between electron-rich alkenes and excited carbonyl compounds is energetically favorable, the RI pair becomes an important intermediate in photochemical [2 + 2] cycloaddition reactions (Scheme 7.5). The regioselectivity of these reactions may differ from that observed for the PB reaction involving 1,4-triplet biradical intermediates. Typical examples of PB reactions with very electron-rich alkenes, ketene silyl acetals (Eox = 0.9 V vs SCE), have been reported (Scheme 7.11) [27]. Thus, 2-alkoxyoxetanes were selectively formed as a result of the PB reaction with benzaldehyde or benzophenone derivatives, whereas a selective formation of 3-alkoxyoxetanes was observed in less electron-rich alkenes (see Scheme 7.9). When p-methoxybenzalde-hyde was used in the photochemical reaction, the regioselectivity was less than that observed in the case of benzaldehyde. This dramatic decrease in regioselectivity provided evidence that the selective formation of 2-alkoxyoxetanes occurred via RI pair intermediates. It should be noted that the stereoselectivity is also completely different from that associated with triplet 1,4-biradicals (vide infra). 

Paternd-Biichi reactions [152] this competition has been investigated for electron-rich alkene substrates for several combinations of carbonyl compounds and electron-donors, e.g. a-diketones and ketene acetals [153], aromatic aldehydes and silyl ketene acetals, and enol ethers. In polar solvents, the assumption of a 1,4-zwitterion as decisive intermediate is reasonable. This situation then resembles the sequence observed for ET-induced thermal [2 -I- 2]-cycloaddition reactions [154]. Both regio- and diastereoselectivity are influenced by this mechanistic scenario. The regioselectivity is now a consequence of maximum charge stabilization and no longer a consequence of the primary interaction between excited carbonyl compound and alkene. Whereas 3-alkoxyoxetanes are preferentially formed from triplet excited aldehydes and enolethers, 2-alkoxyoxetanes result from the reaction of triplet excited ketones or aldehydes and highly electron-rich ketene silylacetals (Scheme 40) [155]. 

Lactams can be prepared by [2 + 2] cycloadditions of mines and ketenes50- 52, iniines and ester enolatcs53, imines and silyl ketene acetals, alkenes and isocyanates54 56 and carbodi-imides and ketenes. The stereospecificity of these reactions has been extensively investigated due to the tremendous practical importance of 0-lactam antibiotics57 -59,1I3. 

The same group found that a four-membered ring fused to a benzyne also had a powerful regioselective directing effect in the [2+2] cycloadditions of benzyne, presumably through the severe ring strain [97]. In the reaction of ketene silyl acetal... 

Neier et al. made use of the Diels-Alder reaction to generate an allyl silyl ketene acetal in situ (Scheme 4.62) [62]. Diels-Alder cycloaddition of a silyl ketene acetal derived from a dienol propionate yielded an intermediate allyl silyl ketene acetal which underwent in situ Ireland-Claisen rearrangement. The pentenoic acid products were formed in 60% yield as a 47 29 24 mixture of diastereomers, from which the major diastereomer shown below was isolated. 

Photoinduced reactions of cyclic a-diketones with different alkenes takes place via [2 + 2], [4 + 2] or [4 + 4] photocycloaddition pathways. Photoaddition of electron deficient silyl ketene acetals to 2-, 3- and 4-acetylpyridine generates oxetanes as major products. The reaction is favoured in non polar solvents. The photoreaction between silyl enol ethers and henzil affords [2 + 2] cycloaddition products, while in the case of 9,10-phenanthrenequinone [4 + 2] cycloacidition predominates. Photocycloaddition of p-henzoquinones to hicyclopropylidene affords spirooxetanes (21) as the primacy products further irradiation leads to rearranged spiro[4.5]deca-6,9-diene-2,8-diones. With 9,10-anthraqui-none, in addition to the spirooxetane, a spiro[indan-l,l -phthalan]-3 -one is also obtained. ... 

A possible mechanism for the catalytic [2+2] cycloaddition reaction catalyzed by Tf2NH is depicted in Scheme 4.9. The Mukaiyama-type Michael addition of silyl enol ether to enoate catalyzed by silyl triflic imide aHbrds the corresponding silyl ketene acetal, and then it proceeds successively to the intramolecular silyl oxonium carbon to... 

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