Synthesis and structural features of cyclobutane-containing chiral bicyclic ureas

Efficient and stereoselective synthetic routes have been developed for the preparation of chiral N-monoprotected cyclobutane bicyclic ureas in which one of the NH groups is protected as a benzyl or tert-butyl carbamate. Ureas in both enantiomeric forms were obtained from a common chiral precursor via the selective manipulation of functional groups. These compounds have been subjected to a structural study in solution and in the solid state. NMR, IR and TEM techniques evidence a strong tendency to aggregation in solution giving regular assemblies, which is a result of intermolecular urea N–H⋯OC hydrogen bonding. In the solid state, X-ray analysis shows that two urea molecules interact through only one hydrogen bond yielding infinite chains. This fact and the almost complete coplanarity of both the urea and the carbamate carbonyl groups determine the crystal packing to be formed by a parallel molecular arrangement. All these structural features are well supported by theoretical calculations that allow us to conclude that the formation of a network based on hydrogen bonding is energetically favourable.

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(1S,5R)-tert-Butyl 3-oxo-2,4-diazabicyclo[3.2.0]-heptane-2-carboxylateC10H16N2O3[α]D = +42.8 (c 0.56, CH2Cl2)Source of chirality: chemoenzymatic hydrolysis of a meso precursorAbsolute configuration: (1S,5R)

(1R,5S)-Benzyl 3-oxo-2,4-diazabicyclo[3.2.0]heptane-2-carboxylateC10H14N2O3[α]D = −60 (c 0.59, MeOH)Source of chirality: chemoenzymatic hydrolysis of a meso precursorAbsolute configuration: (1R,5S)