Lewis Acid-catalysed Aza-Diels-Alder versus Mannich Reactions of N-Diethyl Phosphoryl Imino Dienophiles with Oxygenated Dienes and Application of a Chiral Lewis Acid

Abstract

N-Phosphoryl imines react with oxygenated dienes in the presence of Lewis acids to provide either the formal aza-Diels-­Alder adduct, or the acyclic Mannich-type addition product, depending upon the catalyst and work up used, which is in agreement with the hypothesis that these reactions are mediated by zwitterionic Lewis acid-complex intermediates, rather than a concerted cyclo­addition transition-state followed by hydrolytic ring opening. Stoi­chiometric asymmetric induction can be achieved using a zinc(II)-binol complex, producing up 77% ee.

Key words:

heterodienophile - Diels-Alder reaction - Mannich ­reaction - asymmetric induction

References

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10

1% TFA quench followed by silica gel chromatography.

11

All spectroscopic and analytical properties were consistent with the structures assigned.

12

Developed for the purpose of high throughput screening avoiding an aqueous work up.

13

The crystals of 12b repeatedly cracked on cooling at about 125 K, which indicated a phase transition at this temperature hence the X-ray data were finally collected at 200 K. The molecule 12b in crystal (Figure [1] ) shows severe disorder of both ethyl-groups and terminal carbonyl group. The molecule has a trans-configuration around C3=C4 double bond, the conformation around P-N bond is characterised by a torsion angle for O1,P1,N1,C1 of -349(3)°. The molecules of 12b in the crystal are linked together by strong hydrogen bonds N-H···O = P(x + 1, y z) (N···O 2873(3)Å), forming chains parallel to a-axis.

15

Guillarme G., Whiting A., see following paper.

16

Asymmetric induction was determined using chiral HPLC on a Chiralpak OD column using i-PrOH-hexane as eluant (96:4) for 3a and 3b; (94:6) for 3c; and (97:3) for 3e.