Chemoselective rhodium(II)–carbenoid cyclisation reactions in the stereoselective construction of rigidified cyclic α-amino acid derivatives

Stereoselective Synthesis of Quaternary Proline Analogues

“…[85] Later, Undheim et al demonstrated an alternative annulation procedure involving rhodium(II)-catalyzed carbenoid insertions into diazoketone substrates 162. [86] The insertion reactions occurred with complete chemoselectivity at the adjacent annular nitrogen in preference to C-C double bond additions or C-H insertions, and no racemization was observed either in the carbenoid reaction or in the subsequent hydrolysis.…”

“…[63] Metallated azomethine ylides, derived from aliphatic or aromatic aldehydes and aliphatic or aromatic α-amino acids, underwent highly endo-diastereoselective cycloadditions with the acrylamide of proline benzyl ester (86). An illustrative example is depicted in Scheme 22.…”

Recent progress on the stereoselective synthesis of cyclic quaternary α-amino acids

“…Hydrolysis of 638a,b with 3 M HCl, followed by treatment with (Boc) 2 O and triethylamine, produced the protected dipeptides 639a,b (Scheme 138). 252…”

Rh(II)-carbenoid insertion into chiron substrates for stereoselective amino acid construction

“…10,11 We have in earlier work observed exclusive five-and six-membered ring formation involving the adjacent annular nitrogen atom. 6,7 In the present case, annulation to the adjacent nitrogen atom would have yielded a seven-membered ring structure. C-H insertion at the a-carbon in the R-group would also lead to an unfavourable seven-membered ring formation.…”

“…[3][4][5] In the most recent examples, the amino nitrogen was incorporated into the ring, the products being derivatives of proline and pipecolic acid. 6,7 The conformational freedom of the peptidic material will be significantly affected on insertion of quaternary a-amino acids. Hence the preparation and properties of this class of a-amino acids have attracted great attention.…”