a De part ment of Chem is try, Chung-Yuan Chris tian Uni ver sity, Taoyuan, Tai wan, R.O.C. b De part ment of Chem is try, Soochow University, Tai pei, Tai wan, R.O.C.Tan dem cas cade re ac tions of allyl azides and olefinic dipolarophiles to give cis-fused 2,3,7-t riazabi cyclo [3.3.0]octenes (5, 6 or 7) are re ported. Therein, an intermolecular dipolar cycloaddition of azide and alkene gave a triazoline which was fol lowed by isomerization of the triazoline to a diazo es ter (4) and then an intramolecular dipolar cycloaddition from the diazo func tional group and the dou ble bond in 4 to give 5. Compound 5 may fur ther more un dergo a Mi chael ad di tion to give 7-substituted-2,3,7-triazabicyclo [3.3.0] oct-2ene ( 6) or a tautomerization to give 2,3,7-triazabicyclo[3.3.0]oct-3-ene (7). The re ac tion may be ma nip u lated to stop at a par tic u lar stage by adopt ing a suit able sol vent or an ap pro pri ate tem per a t ure.
The structures of 37 peptide crystals, containing 78 water‐peptide hydrogen bonds and 77 other hydrogen bonds involving water, were surveyed to identify the geometry of peptide backbone hydration. In the sample, hydration of peptide carbonyl occurred more frequently than hydration of peptide N–H. The most probable value of the C'=0. O water angle was near 138°, considerably greater than the 120° to the axis of a lone electron pair on the carbonyl oxygen. Associated water oxygens tended to be in the plane of the peptide bond, but not restricted to it. Cyclic association with water, to form 10‐membered rings using Ni–H and Ci + 1 =0 atoms, was common in glycine‐containing cyclic hexapeptides. The distribution of angles between two hydrogen bonds at a single water molecule, as defined by the three nonhydrogen atoms involved, was centered near the tetrahedral angle.
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