Light turns the Lewis acid on: The Lewis acidity of a boron atom integrated into a cyclic dithienylethene photoswitch is modulated by light: 1 a has low Lewis acidity since the p orbital of the boron center is partially occupied by delocalized π electrons, whereas the rearrangement of the π electrons in 1 b reduces the electron density at the boron center and turns the Lewis acid “on”.

[reaction: see text] Photolysis of the amino acid derived symmetrical and unsymmetrical diacyl peroxides at 254 nm at low temperature (-78 to -196 degrees C) generates various bis(amino acids) in a concise manner and with orthogonal protection. The methodology was applied to the synthesis of (4R)-5-propyl-L-leucine (PrLeu), a component of HUN-7293.

Licht macht sauer: Die Lewis‐Azidität eines Boratoms in einem cyclischen Dithienylethen‐Photoschalter wird durch Licht moduliert: 1 a hat eine niedrige Lewis‐Azidität, weil das p‐Orbital des Borzentrums teilweise durch delokalisierte π‐Elektronen besetzt ist; in 1 b ist die Elektronendichte am Borzentrum reduziert und die Lewis‐Säure eingeschaltet.

Peresters generate ethers in good yields when photolyzed in the absence of solvent using short wavelength UV light. At -78 degrees C or below, the process proceeds predominantly with retention of configuration at the site adjacent to the carbonyl where the decarboxylation occurs, but increase in temperature results in loss of stereochemical control. Chiral acyclic acetals can be prepared using precursors derived from tartaric or malic acids.