Synthesis of Nanotubule-Forming Cyclic Octapeptides via an Fmoc Strategy

New syndiotactic cyclic octapeptides, namely cyclo(–D-Phe-L-Asp–D-Phe–L-Asn–D-Phe–L-Asp–D-Phe–L-Asn–) (1) and cyclo(–D-N-MeAla–L-Asp–D-N-MeAla–L-Asn–D-N-MeAla–L-Asp–D-N-MeAla–L-Asn–) (2), have been prepared, and preliminary structural studies have been conducted. The synthesis of the linear peptides was performed by using Fmoc chemistry, and head-to-tail cyclization was accomplished by using an orthogonal protection strategy and a support-bound cyclization step. Acidification of aqueous solutions of cyclic octapeptide (1) initiated formation of needlelike crystals whose morphology and infrared absorption behaviour suggested that they were hydrogen-bonded nanotubular aggregates of (1).