Dipole structures azomethine ylides

When azomethine ylides are generated by condensation of aldehydes with chiral a-amino acids, the stereogenic center of the latter is lost in the planar 1,3-dipole structure. To achieve diastereoselection in the addition to Ceo, an additional chiral element is therefore needed. An optically pure azomethine ylide was generated by reaction of (+)-2,3-0-isopropylidene-D-glyceraldehyde with... 

Azotnethine ylides are planar molecules composed of one nitrogen and two terminal sp carbons. At most, four geometrical isomers are possible for these transient molecules. Their cycloadditions to olefin or acetylene dipo-larophiles give rise to the formation of two sets of carbon-carbon bonds in a single step. Because of the structural complexity of azomethine ylide itself compared to other dipoles and the stereochemical selectivity in the cycloadditions, a number of stereoisomers are possible for the cycloadducts. These two points make azomethine ylides one of the most attractive 1,3-dipoles, both in the fields of ylide chemistry and synthetic organic chemistry. 

Although structural modification is quite limited, the azomethine ylides generated by the N-oxide route are complementary in their applications in organic synthesis to the identical ylides generated by other methods [e.g., the decarboxylation route (Section II,E) and the desilylation route (Section II,B)]. When the origin of the enhanced reactivity is solved, a new field will be open in the chemistry of azomethine ylide 1,3-dipoles. 

As azomethine ylide 1,3-dipoles are usually transient intermediates, direct structural investigation is impossible. Accordingly, structures are derived from a consideration of thei r stable and isolable derivatives through a stereospecihc process. Ylide structures are determined on the basis of structural analysis of their derivatives. Cycloaddition reactions are the most reliable ways to make derivatives because they usually take place stereospecifically with respect to both azomethine ylides and dipolarophiles. 

Maleimides and maleic anhydride have been most frequently employed as cyclic cis-olefin dipolarophiles in the stereochemical investigation of 1,3-dipolar cycloadditions, especially on the endo and exo selection of the reaction. They are one of the most reactive dipolarophiles toward many kinds of azomethine ylide 1,3-dipoles. Because of their structural simplicity, the only stereochemical variation possible in cycloadditions is an endo and exo selection. If a strong attractive interaction exists between the extended conjugation of these dipolarophiles and azomethine ylides, an endo-selective cycloaddition results. 

Azomethine ylides, like azomethinimines, lack a double bond in the sextet structure but have internal octet stabilization and belong to the allyl type of dipoles (121a and 121b). In terms of the PMO theory, azo-... 

In the case of cycloadditions with allyl-type" dipoles1, either one e.g., with nitrones, azomethine imines or two e.g., with azomethine ylides, additional stereocenters can be created depending on the structure of the dipole ... 

The following 1,3-dipoles will be considered (a) aryl azides (b) diazoalkanes (c) aryl nitrile oxides (d) nitrile imines (e) azomethine imines (/) azomethine oxides (g) azomethine ylides. (a) to (d) represent 1,3-dipoles with a double bond in their sextet structure, while the last three, from (e) to (g), are without a double bond . All of them have nitrogen as the central atom of the 1,3-dipole. They will be formulated as allyl-like systems, having their negative charge distributed (according to an unspecified balance) at the two sides of the positive nitrogen, e.g. 

CONTENTS Facial Diastereoselection in Diels-Alder Cycloadditions and Related Reactions Understanding Planar Interactions and Establishing Synthetic Potential, A. G. Faille and Yee-Fung Lu. Substituent and Structural Effects in the Ozonolysis of Cyclic Vinylogous Esters. W. H. Bunnelle. N-Metalated Azomethine Ylides, S. Kanemasa and Otohiko Tsuge. Azomethine Ylide Cy-cloadditlons via 1,2- Prototropy and Metallo-Dipole Formation from Imines, R. Grigg and V. Sridharan. Index. 

Interesting structures can be formed by combinations of ring and side-chain substituents in special relative orientations. As indicated above, structures (28) contain the elements of azomethine or carbonyl ylides, which are 1,3-dipoles. Charge-separated species formed by attachment of an anionic group to an azonia-nitrogen also are 1,3-dipoles pyridine 1-oxide (32) is perhaps the simplest example of these the ylide (33) is another. More complex combinations lead to 1,4-dipoles , for instance the pyrimidine derivative (34), and the cross-conjugated ylide (35). Compounds of this type have been reviewed by Ramsden (80AHCl26)l). 

---