CBS Oxazaborolidines - Versatile Catalysts for Asymmetric Synthesis

Introduction

CBS Oxazaborolidines 1a-c are well-established catalysts, which mediate the enantio- and diastereoselective reduction of highly functionalized ketones. [1] [2] Recently, the substrate scope has been extended to include oxime ethers [3] and ketimines. [4] Accordingly, these reagents have been extensively applied in natural product synthesis, for instance in the recently published total syntheses of (-)-laulimalide, [5] brevetoxin-B, [6] (+)-tanikolide, [7] and bistramide A. [8] Additionally, oxazaborolidines 1 were used in the kinetic resolution [9] and the dynamic kinetic ­resolution of atropoisomeric lactones. [9b] [10]

More recently, the protonated oxazaborolidinium salts 2a and 2b were used as chiral Lewis acids [11] in catalytic enantioselective Diels-Alder reactions, [12] the cyanosilylation of aldehydes, [13] Michael additions, [14] and a vinylogous ­Mukaiyama aldol reaction. [15]

Compound 1a is typically generated in situ by reaction of (S)- or (R)-2,2-diphenylhydroxymethylpyrrolidine with BH₃·THF, [2] while oxazaborolidines 1b,c may be prepared using the corresponding boronic acids. [1c] [16] In a similar fashion, reaction of the same chiral pyrrolidine substrate with o-tolylboroxine furnishes o-tolyl-CBS-oxazaborolidine, [16] which, when protonated with triflic acid or trif­limide, affords the oxazaborolidinium salts 2a and 2b, respectively. [12b] [d] The facile preparation and modification of the oxazaborolidines, along with their high reactivity and selectivity, render members of this class of molecules as important candidates for future applications in asymmetric catalysis.

References and Notes

• For reviews, see:
• 1a Cho BT. Tetrahedron  2006,  62:  7621 
• 1b Cho BT. Aldrichimica Acta  2002,  35:  3 
• 1c Corey EJ. Helal CJ. Angew. Chem. Int. Ed.  1998,  37:  1986;   Angew. Chem.  1998,  110:  2092 
• 1d Itsuno S. Org. React.  1998,  52:  395 
• CBS represents the initials of the researchers who discovered the reagents 1a-c, namely Corey, Bakshi, and Shibata:
• 2a Corey EJ. Bakshi RK. Shibata S. J. Am. Chem. Soc.  1987,  109:  5551 
• 2b Corey EJ. Bakshi RK. Shibata S. Chen C.-P. Singh VK. J. Am. Chem. Soc.  1987,  109:  7925 
• 3a Demir AS. Sesenoglu Ö. Ülkü D. Arici C. Helv. Chim. Acta  2003,  86:  91 
• 3b Demir AS. Sesenoglu Ö. Aksoy-Cam H. Kaya H. Aydogan K. Tetrahedron: Asymmetry  2003,  14:  1335 
• 3c Demir AS. Sesenoglu Ö. Gerçek-Arkin Z. Tetrahedron: Asymmetry  2001,  12:  2309 
• 4a Gosselin F. O’Shea PD. Roy S. Reamer RA. Chen C.-y. Volante RP. Org. Lett.  2005,  7:  355 
• 4b Kirton EHM. Tughan G. Morris RE. Field RA. Tetrahedron Lett.  2004,  45:  853 
• 5 Uenishi J. Ohmi M. Angew. Chem. Int. Ed.  2005,  44:  2756 ; Angew. Chem.  2005,  117:  2816 
• 6 Matsuo G. Kawamura K. Hori N. Matsukura H. Nakata T. J. Am. Chem. Soc.  2004,  126:  14374 
• 7 Kita Y. Matsuda S. Fujii E. Horai M. Hata K. Fujioka H. Angew. Chem. Int. Ed.  2005,  44:  5857 ; Angew. Chem.  2005,  117:  6007 
• 8 Crimmins MT. DeBaillie AC. J. Am. Chem. Soc.  2006,  128:  4936 
• 9a Molander GA. George KM. Monovich LG. J. Org. Chem.  2003,  68:  9533 
• 9b Bringmann G. Pabst T. Henschel P. Kraus J. Peters K. Peters E.-M. Rycroft DS. Connolly JD. J. Am. Chem. Soc.  2000,  122:  9127 
• 10a Abe H. Takeda S. Fujita T. Nishioka K. Takeuchi Y. Harayama T. Tetrahedron Lett.  2004,  45:  2327 
• 10b Bringmann G. Menche D. Kraus J. Mühlbacher J. Peters K. Peters E.-M. Brun R. Bezabih M. Abegaz BM. J. Org. Chem.  2002,  67:  5595 
• 10c Bringmann G. Ochse M. Götz R. J. Org. Chem.  2000,  65:  2069 
• 11 For a review, see: Bolm C. Rantanen T. Schiffers I. Zani L. Angew. Chem. Int. Ed.  2005,  44:  1758 ; Angew. Chem.  2005,  117:  1788 
• 12a Hong S. Corey EJ. J. Am. Chem. Soc.  2006,  128:  1346 
• 12b Ryu DH. Corey EJ. J. Am. Chem. Soc.  2003,  125:  6388 
• 12c Zhou G. Hu Q.-Y. Corey EJ. Org. Lett.  2003,  5:  3979 
• 12d Corey EJ. Shibata T. Lee TW. J. Am. Chem. Soc.  2002,  124:  3808 
• 12e Ryu DH. Lee TW. Corey EJ. J. Am. Chem. Soc.  2002,  124:  9992 
• 13 Ryu DH. Corey EJ. J. Am. Chem. Soc.  2004,  126:  8106 
• 14 Liu D. Hong S. Corey EJ. J. Am. Chem. Soc.  2006,  128:  8160 
• 15 Boeckman RK. Pero JE. Boehmler DJ. J. Am. Chem. Soc.  2006,  128:  11032 
• 16 Compounds (S)-1b, (R)-1b, and (S)-o-tolyl-CBS-oxaza-borolidine are commercially available