Synthesis, crystal structure, and molecular conformation of N‐Boc‐L‐Phe‐dehydro‐Leu‐L‐Val‐OCH3

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Abstract
The peptide N‐Boc‐L‐Phe‐dehydro‐Leu‐L‐Val‐OCH3 was synthesized by the usual workup procedure and finally by coupling the N‐Boc‐L‐Phe‐dehydro‐Leu‐OH to valine methyl ester. It was crystallized from its solution in methanol–water mixture at 4°C. The crystals belong to the triclinic space group P1 with a = 5.972(5) Å, b = 9.455(6) Å, c = 13.101(6) Å, α = 103.00(4)°, β = 97.14(5)°, γ = 102.86(50)°, V = 690.8(8) Å, Z = 1, dm = 1.179(5) Mg m−3 and dc = 1.177(5) Mg m−3. The structure was determined by direct methods using SHELXS86. It was refined by block‐diagonal least‐squares procedure to an R value of 0.060 for 1674 observed reflections. The C–C distance of 1.323(9) Å in dehydro‐Leu is an appropriate double bond length. The bond angle Cα–Cβ–Cγ in the dehydro‐Leu residue is 129.4(8)°. The peptide backbone torsion angles are θ1 = −168.6(6)°, ω0 = 170.0(6)°, ϕ1 = −44.5(9)°, ψ1 = 134.5(6)°, ω1 = 177.3(6)°, ϕ2 = 54.5(9)°, ψ2 = 31.1(10)°, ω2 = 171.7(6)°, ϕ3 = 51.9(8)°, ψ = 139.0(6)°, θT = −175.7(6)°. These values show that the backbone adopts a β‐turn II conformation. As a result of β‐turn, an intramolecular hydrogen bond is formed between the oxygen of the ith residue and NH of the (i + 3)th residue at a distance of 3.134(6) Å. The Boc group has a trans–trans conformation. The side‐chain torsion angles of the Phe residue are χ1 = 171.6(6)°, χ = −102.1(9)°, and χ = 78.6(10)°. The side‐chain conformational angles of dehydro‐Leu residue are χ2 = 2.7(13)°, χ = −107.3(11)°, and χ = 131.3(10)°. The torsion angles χ and χ that define the conformation of the valyl side chain are −166.16(6)° and 69.1(9)°, respectively. The crystal structure is stabilized by hydrogen bonds along the a and b axes, while the van der Waals forces are the stabilizing interactions in the c direction.