Ylids, carbonyl

A3-Thiadiazolines (112) are unstable, losing nitrogen to yield thio-carbonyl ylids (113), which have been trapped with DMAD and other dienophiles.582 Hence, by allowing 112 to warm to room temperature in the presence of DMAD, 20% of 114 was obtained. Although... 

Similarly, nitrile oxides react with methyl acrylate 2.42 to give the adduct 2.43 with the substituent on C-5 and terminal alkenes also react in this way to place the alkyl group on C-5. Many dipoles react well with electron-rich dipolarophiles, but not with electron-poor dipolarophiles. Other dipoles are the other way round. To make matters even more complex, the presence of substituents on the dipole can change these patterns and impart their own regioselectivity. Thus the carbonyl ylid reaction 2.45 has a well defined regiochemistry determined only by the substituents, since the core dipole is symmetrical. This reaction also illustrates the point that dipolarophiles do not have to be alkenes or alkynes—they can also have heteroatoms. 

Two moles of aromatic aldehyde react with ethyl diazoacetate to form diastereomeric 1,3-dioxolanes.124 The reaction is catalysed by dirhodium(II) species, and proceeds via a carbonyl ylid. Stereo-control can be achieved using a bulky diazo substrate, and electronic effects of aromatic substituents are important. Different reactions show evidence of either a metal-stabilized ylid, a free ylid, or competition between the two. 

The synthetic applications of 1,3-dipolar cycloaddition of carbonyl ylids generated from diazo ketones have been reviewed with particular focus on pseudolaric acids.12... 

The transient intervention of a carbonyl ylids has culminated in numerous elegant approaches to total syntheses of natural products, such as the synthesis of the tigliane ring system (61-63) by Dauben [53] (Scheme 28). 

It is worthy of note that this reaction is still the subject of solid and productive interest, as shown by the following recent examples. Chiu has exploited a rhodium carbene-promoted intramolecular formation of a carbonyl ylid - cycloaddition cascade as the key reaction in the synthesis of the nucleus of the cytotoxic diterpenoids pseudolaric acids A and B [54]. Although the diastereoselectivity was preferential for the undesired isomer 64, use of Hashimoto s chiral rhodium catalyst Rh2(SBPTV)4 reversed the selectivity in favor of 65 (64 65, 1 1.4) [55] (Scheme 29). 

The photochemical reactivity of the epoxides (189) and (190) and (191) have been studied. A detailed examination of the photo-ring opening of the epoxides (192) to yield carbonyl ylid has been reported by George ei Laser flash photolysis of... 

Carbonyl ylids have been proposed as intermediates trappable by alkenes in the energy transfer and electron transfer sensitised opening of styrene oxides. 

The final class of dipoles discussed here are the carbonyl ylids. Padwa et al. generated these reactive intermediates from a rhodium (Il)-catalyzed reaction with l-diazopentanediones.388 a synthetic example of this reaction is taken from Padwa s synthesis of ribasine,389 where diazo compound 497 was treated with Rh2(tfa)4 [tfa = trifluoroacetic acid ligand] to give carbonyl ylid 498. In the presence of dimethylacetylene dicarboxylate (DMAD), cycloadduct 499 was isolated in 82% yield. Interestingly, formation of the carbonyl ylid and cycloaddition was very dependent on the rhodium catalyst used, and when Rh2(OAc)4 was used, 482 was not produced. 

Figure D.7 illustrates the application of the rules to delocalized systems A8a-1, A8b-1, A8c-1, A9a-1, A9b-1, and A9c-1. In particular, 1,3-dipoles are prominent members of the tricentric cases. Azomethine ylids, azomethine imines, nitrones, carbonyl ylids (e.g. A8c), carbonyl imines, carbonyl oxides fit prototype A8 bent nitrile ylids, may also be treated as A8. The linear 1,3-dipoles are treated as a triply-bonded systems with two mutually orthogonal sets of paired faces.

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