Peptides 1K, YKGGGAAAAAAAAKAAAAAAAAAGGGK-NH 2 ; 2K, YKGGGAAAAAKAAAAA-KAAAAAAGGGK-NH 2 ; and 3K, YKGGGAAAAKAAAAKAAAAKAAAGGGK-NH 2 have been prepared by solid-phase synthesis, purified, and characterized by amino acid analysis, MALDI mass spectrometry, and ultracentrifugation. Their circular dichroism (CD) spectra of unaggregated solutions are reported for measurements in 0.01 M NaCl at 2, 25, and 60 °C and at 2 °C in aqueous guanidinium hydrochloride (0-3 M) and aqueous trifluoroethanol (TFE, 0-15 mol %). The CD spectra exhibit a helical signature in 0.01 M NaCl or in water-TFE at 2 °C, and the intensities of the mean residue ellipticities at the minimums of 222 nm in 0.01 M NaCl are (1K) -9100, (2K) -18 100, and (3K) -19 900 deg cm -1 dmol -1 . These ellipticities are accurately modeled using a Lifson-Roig algorithm by the helical propensities previously reported by Renold et al.

Intramolecular ring opening reactions of 2-phenyl-3-oxetanols have been studied. The starting materials were prepared by the photocycloaddition of benzaldehyde and various silyl enol ethers. The intramolecular nucleophile was either incorporated into the silyl enol ether prior to the Paternò−Büchi reaction (oxetanes 3, 16) or was later installed by functional group interconversion (oxetanes 5, 12). With anionic heteroatom nucleophiles (O, N, S) which were attached to the carbon atom C-3 of the trimethylsilyl-protected oxetanol via an alkyl chain, a substitution at the less substituted position C-4 was observed and the corresponding heterocycles (6, 8, 13, 19) were obtained in moderate to good yields (52−91%). Upon acid catalysis a ring opening of the Boc-protected 3-oxetanol 23 to cyclic carbonates occurred. The reaction did not proceed stereospecifically and resulted in a mixture of diastereomeric products 24 and 25.

The facial diastereoselectivity in the Paternò−Büchi reaction of chiral silyl enol ethers and benzaldehyde was studied. The substituents (RS, RL) at the stereogenic carbon atom (−C*HRSRL) attached to the β-position of the silyl enol ether were varied in order to evaluate the influence of steric bulk and electronic effects. The combined yields for the two diastereomeric 3-(silyloxy)oxetanes a and b range between 44% and 76%. In accordance with the 1,3-allylic strain model the facial diastereoselectivity (diastereomeric ratio (dr) = a/b) was best with large (RL = t-Bu, SiMe2Ph) and polar (RL = OMe) substituents at the γ-position of the silyl enol ether (dr up to >95/5). Two regioselective ring opening reactions were applied to the product oxetanes 29a, 50, and 51. They furnished diastereomerically pure diols (52, 53) and triols (31) in excellent yields.

No abstract

A single element of chirality determines the configuration of the three newly formed stereogenic centers in the title reaction [Eq. (a)]. In particular, large (R = SiMe2Ph,tBu) and polar (R = OMe) substituents induce good diastereoselectivities. Simple ring‐opening reactions facilitate a stereoselective approach to acyclic products.