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Diethylglycine (Deg) residues incorporated into peptides can stabilize fully extended (C5) or helical conformations. The conformations of three tetrapeptides Boc-Xxx-Deg-Xxx-Deg-OMe (Xxx=Gly, GD4; Leu, LD4 and Pro, PD4) have been investigated by NMR. In the Gly and Leu peptides, NOE data suggest that the local conformations at the Deg residues are fully extended. Low temperature coefficients for the Deg(2) and Deg(4) NH groups are consistent with their inaccessibility to solvent, in a C5 conformation. NMR evidence supports a folded beta-turn conformation involving Deg(2)-Gly(3), stabilized by a 4-->1 intramolecular hydrogen bond between Pro(1) CO and Deg(4) NH in the proline containing peptide (PD4). The crystal structure of GD4 reveals a hydrated multiple turn conformation with Gly(1)-Deg(2) adopting a distorted type II/II' conformation, while the Deg(2)-Pro(3) segment adopts a type III/III' structure. A lone water molecule is inserted into the potential 4-->1 hydrogen bond of the Gly(1)-Deg(2) beta-turn.

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This paper describes the interesting structural studies on three new diterpenoids (plant products), namely, hydroxy‐dialactone nepetaefolinol (9,13‐epoxy‐6β‐hydroxy‐8α‐labdane‐16,15 :19,20‐diolactone), dehydrated nepetaefolinol (9,13‐epoxylabd‐5‐ene‐16,15:19,20‐diolactone) and isomeric tetrol (15,16‐epoxy‐labda‐13(16),14‐diene‐6β,9,17,19‐tetrol: which is the reduction product of new diterpenoid leonotinin) isolated from Indian herbal plant Leonotis nepetaefolia (collected in the flowering season from Guindy area, Madras), and are found to be the major compounds in their extractions, so they are expected to possess the interesting pharmacological properties. In all three structures, the fused ring systems adopt distorted chair‐chair, distorted chair and intermediate between 1,2‐diplanar and half‐chair and chair‐chair conformations. In compounds 1 and 2, the twisted form of lactone rings are orthogonal to each other and make dihedral angle of 88.4(1) and 87.5(2)° with each other. The primary interaction between the molecules is van der Waal's in nature.