Stereoselective C-N Ring Construction
Several new methods for C-N ring construction based on enantioselective addition to imines have been reported. Michael J. Krische of University of Texas at Austin has used (J. Am. Chem. Soc. 2007, 129, 7242. ) his Rh-mediated reductive dimerization of acetylene to effect enantioselective homologation of N-sulfonyl imines such as 1 to give dienes such as 2. N-Alkylation followed by alkene metathesis converted 2 into 3. William D. Wulff of Michigan State University, in another sort of imine addition, has established (J. Am. Chem. Soc. 2007, 129, 7216. ) that by using a super stoichiometric amount of Lewis acid to trap the product from the addition of 4 to 5, a catalytic amount of the VAPOL (6) effectively directed the addition. Ramón Gómez Arrayás and Juan C. Carretero of the Universidad Autónoma de Madrid have described (J. Am. Chem. Soc. 2007, 129, 1480. ) a conceptually-complementary approach. Glenn C. Micalizio of Yale University has shown (J. Am. Chem. Soc. 2007, 129, 7514. ) that Ti-mediated coupling of 8 with 9 proceeded with ≥ 50:1 diastereoselectivity, leading to the piperidine 11. Coupling of 8 with the (mismatched) enantiomer of 9 led to a mixture of three diastereomers in a ratio of 6 : 3 : 1.
Just three years ago, intramolecular hydroamination, as in the conversion of 12 to 13, was just being developed as a synthetic method. Now, Laurel L. Schafer has devised (Angew. Chem. Int. Ed. 2007, 46, 354. ) a Zr catalyst that directed the cyclization of the prochiral diene 12 not just with high enantioselectivity, but also with substantial diastereocontrol. F. Dean Toste of the University of California, Berkeley, has created (J. Am. Chem. Soc. 2007, 129, 2452, ; Science 2007, 317, 496. ) enantioselective Au catalysts for the electronically very different hydroamination of allenes such as 15.
Amir H. Hoveyda of Boston College has created (Angew. Chem. Int. Ed. 2007, 46, 4534. ) enantioselective Ru alkene metathesis catalysts. Using such a catalyst, he was able to convert prochiral bicyclic protected amines such as 17 into piperidines such as 18 with high ee.
Other routes to highly-substituted pyrrolidines have been reported. Shu Kobayashi of the University of Tokyo has devised (J. Am. Chem. Soc. 2007, 129, 5364. ) a catalyst for the Michael addition of amino ester derivatives such as 20 to acrylates such as 19. The enolate so generated added to the imine to give 21 with high relative and absolute stereocontrol. Babak Borhan of Michigan State University observed (J. Am. Chem. Soc. 2007, 129, 3794. ) that acetylide added to the aziridine aldehyde 22 to give 23 with high diastereoselectivity. Under carefully defined conditions, 23 underwent Payne rearrangement to liberate the sulfonamide anion, which then added to the pendant alkyne, to give 24. Professor Borhan has also (J. Am. Chem. Soc. 2007, 129, 1996. ) reported an alternative approach to highly substituted pyrrolidines.