6533b85efe1ef96bd12c0551

RESEARCH PRODUCT

Mannich Reactions with Amino Alcohols

subject

description

The condensation of resorcarenes 1 with various amino alcohols and an excess of formaldehyde was studied. The tetrabenzoxazines 2a−e were found as the only products in the reaction with 6-aminohexan-1-ol, 4-aminobutan-1-ol, and 2-aminoethanol, while 3-aminopropan-1-ol forms the tetraoxazine 3 as the main product. In the case of aminoethanols substituted at the 2-position with alkyl groups, the tetraoxazolidines 4 are the preferred reaction products, while 1-methyl aminoethanol (1-amino-propan-2-ol) yields predominantly the tetrabenzoxazine 2f. The structures of all these compounds have been confirmed by NMR spectroscopy and additionally by single-crystal X-ray analysis in the case of 2a and 4b. In [D6]DMSO, up to 60% of the corresponding, more polar tetrabenzoxazine structure is detected for compounds 3 and 4, while the equilibrium is shifted to the less polar tetraoxazine or tetraoxazolidine in CDCl3. Low-temperature NMR spectra show a C4-symmetrical conformation for the tetraoxazolidines 4 in CDCl3 owing to intramolecular OH···OH···N hydrogen bonds. For chiral residues, two epimeric conformations can be distinguished, one of which is selectively formed for larger substituents at the 2-position.