Aziridine Opening

Scheme 12.20 Influence of the nitrogen substituent (sulfonyl, acyl) on the regioselectivity of aziridine opening.

Another problem with the reaction of phenols with aziridines is the selectivity between O-alkylation vs C-alkylation. A recent report has identified that the use of (ArO)3B selects for C-alkylation <06OL2627>. Most of the examples reported in this paper showed less than 5% of the O-alkylation product. What is interesting about this report is the stereochemistry of the product. While the mechanism is not known, the product is formally an SNl type product. Generally less than 5% was the product of inversion of configuration (the Sn2 product). In addition to the A-tosyl, both the A-Cbz and A-Dpp aziridines gave excellent yields of aziridine-opened product. 

The following equations show the difference a strategically located ester group can make in the direction of aziridine opening [76]. The ester favors deprotonation at the conjoint benzylic position, leading to the benzazepine as the major product. 

The essential aspects of reaction (25) are depicted in Fig. 6. Huisgen et al. (1967) have provided a beautiful example of an odd electrocyclic change in (26). The aziridine opens up to a dipolar four-electron allylic species. Since the HOMO is b (Fig. 1), the thermal change is conrotatory and the excited state process is disrotatory. To avoid equilibration of the dipolar ions, these workers trap them with an acetylenic ester in a stereospecific cycloaddition, which we shall discuss presently. 

For two further aziridine opening reactions please refer to transesterification and silicon activation section above. 

In order to describe the ring opening of the aziridine 6.55, we need to define what suprafacial and antarafacial mean when applied to a p orbital. This is shown in Fig. 6.9, and applied there to the conrotatory aziridine opening. When both lines are drawn into the same lobe it is suprafacial, and when there is one line drawn into the top lobe and one into the bottom, it is antarafacial. Since this is neither a n nor a a orbital, it is given the Greek letter uj. The same designations apply whether the orbital is filled (on the left) or unfilled (on the right), and whether it is a p orbital or any of the sp" hybrids. 

In the aziridine opening shown in Fig. 6.9, the aryl group behind the nitrogen atom is left out for clarity. The dashed lines are drawn from the large lobes of the conrotatory mode, so the dashed line on the left goes up, to overlap with the... 

Methyl 2-indolecarboxylate 560 was found to react on the silica gel surface with iV-tosylvinylaziridine in 68% yield. The solid-phase aziridine opening constituted a key step in the synthesis of the P-carbolin-l-one mimic of Pancratistatin (Equation 135) <2005JOC3490>. 

The high sensitivity of lanthanide reagents to steric factors is also observed in the cyanosilylation reaction of ketones catalyzed by ytterbium cyanide, Yb(CN)3 (Eq. 7) [10], Other reactions, for example epoxide and the aziridine opening by tri-methylsilyl cyanide, TMSCN, are also efficiently catalyzed by Yb(CN)3 [11]. This Yb reagent is not regarded as a Lewis acid but as the active species in these reactions. 

For the epoxide opening reactions, see a) S. Matsubara, H. Onishi, K. Utimoto, Tetrahedron Lett. 1990, 31, 6209-6212 for the aziridine opening reactions, see b) S. Matsubara, T. Kodama, K. Utimoto, Tetrahedron Lett. 1990,31, 6379-6380. 

The 1,3-dipolar reagent can in some cases be generated by the in situ opening of a suitable three-membered ring system. For example, aziridines open to give a zwitterion, such as 186, which can add to activated double bonds to give pyrrolidines. ... 

Aziridine opening.. Azin opened with MsOH and the pro... 

Aziridine opening. Aziridines containing electronically biased substituents are opened with MsOH and the products may be transformed into vicinal amino alcohols. ... 

Aziridine opening. Hydroxylated compounds (alcohols, water) open iV-tosylaziridines in the presence of a Lewis acid [Sn(OTf)2 or BFj.OEt2]. 

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