Attaching onto or Inserting into an Intramolecular Hydrogen Bond: Exploring and Controlling a Chirality-Dependent Dilemma for Alcohols

Prereactive complexes in noncovalent organocatalysis are sensitive to the relative chirality of the binding partners and to hydrogen bond isomerism. Both effects are present when a transiently chiral alcohol docks on a chiral α-hydroxy ester, turning such 1:1 complexes into elementary, non-reactive model systems for chirality induction in the gas phase. With the help of linear infrared and Raman spectroscopy in supersonic jet expansions, conformational preferences are investigated for benzyl alcohol in combination with methyl lactate, also exploring p-chlorination of the alcohol and the achiral homolog methyl glycolate to identify potential London dispersion and chirality effects on the energy sequence. Three of the four combinations prefer barrierless complexation via the hydroxy group of the ester (association). In contrast, the lightest complex shows predominantly insertion into the intramolecular hydrogen bond, like the analogous lactate and glycolate complexes of methanol. The experimental findings are rationalized with computations and a uniform helicality induction in the alcohol by the lactate is predicted, independent on insertion into or association with the internal lactate hydrogen bond. p-Chlorination of benzyl alcohol has a stabilizing effect on association, because the insertion motif prevents a close contact between the chlorine and the hydroxy ester. After simple anharmonicity and substitution corrections, the B3LYP-D3 approach offers a fairly systematic description of the known spectroscopic data on alcohol complexes with α-hydroxy esters.

Phosphonium-mediated synthesis of a new class of indoloquinazoline derivatives bearing C-12 aryloxy ester and spiro-γ-lactone

Indoloquinazolines functionalized at C-12 which are structure analogs of the natural alkaloid cephalanthrin B were readily constructed via a Ph3P-I2 mediated one-pot reaction of isatins with aromatic alcohols. In the presence of excess phenols, the C-12 aryloxy ester products were afforded in moderate to good yields under mild conditions. Moreover, fused bicyclic hydroxyaryl derivatives such as 8-hydroxyquinoline gave rise to novel C-12 spiro-γ-lactone derivatives. Reactive iminium cation species derived from dehydration of the C-12 hydroxy ester precursor is proposed to be transient intermediates responsible for these unprecedented transformations.

First Total Synthesis of 27-deoxylyngbyabellin A

In this study, we documented the first total synthesis of marine cyanobacteria secondary metabolite 27-deoxylyngbyabellin A in 10 linear steps with 9.7% overall yield. Key steps entailed (1) one-pot cascade reaction of (S)-2-(benzyloxy)-3-methylbutanoic acid and Boc-Ile-Thz-OMe with the building block β-azido disulfide 7 to access the critical thiazole units 3 and 4, (2) chiral oxazaborolidinone-mediated asymmetry aldol reaction to construct (S)-β-hydroxy ester 5, and (3) DPPA-mediated macrolactamization of linear precursor 2 to achieve the natural product 27-deoxylyngbyabellin A.

Synthesis and Biological Activities of Cyclodepsipeptides of Aurilide Family from Marine Origin

Aurilides are a class of depsipeptides occurring mainly in marine cyanobacteria. Members of the aurilide family have shown to exhibit strong cytotoxicity against various cancer cell lines. These compounds bear a pentapeptide, a polyketide, and an α-hydroxy ester subunit in their structure. A large number of remarkable studies on aurilides have emerged since 1996. This comprehensive account summarizes the biological activities and total syntheses of natural compounds of the aurilide family as well as their synthetic analogues.