Ring the changes: A new photoassisted approach to give conformationally constrained N,O,S‐polyheterocyclic scaffolds of unprecedented topologies was achieved by intramolecular [4+4] and [4+2] cycloadditions of photogenerated o‐azaxylylenes (23 examples; see scheme). The precursors can be readily assembled by simple and high‐yielding reactions, thus making this a powerful synthetic method amenable to high‐throughput diversity‐oriented synthesis.

Amplified fluorescence quenching methodology based on massive autocatalytic photo-unmasking of a dual function sensitizer-quencher is developed and adopted for photoassisted ultra-sensitive detection of molecular recognition events. The resulting binding assay, based on a molecular recognition-triggered photo-amplified cascade with concomitant decrease of fluorescence is validated with the biotin-avidin pair, achieving attomolar detection.

Externally-sensitized electron-transfer fragmentation in dithiane PPG-protected carbonyls is adopted for detection and amplification of molecular recognition events. The new methodology allows for detection of as low as 50 attomoles of avidin utilizing an imager based on a low sensitivity mass-produced consumer CCD camera. Numeric modelling is carried out to demonstrate the intrinsic limitations of 2D amplification on surfaces and the advantages of unconstrained amplification in a compartmentalized volume of spatially addressable 3D solutions.

o-Azaxylylenes have been known for half a century, [1] but remained in relative synthetic obscurity until a decade ago, when Corey and Steinhagen reported their first preparation under simple mild conditions by base-induced elimination of hydrogen chloride from derivatives of o-chloromethylaniline. [2] It was noted that "Surprisingly, simplest method possible for o-azaxylylene production … has never been reported."Synthetically useful reactions of o-xylylenes (i.e. allcarbon o-quinodimethanes) are plentiful, [3] and their generation from aromatic o-alkyl ketones by an intramolecular photoinduced hydrogen abstraction is well documented. [4] Given this fact, it was surprising to us that the equally simple photogeneration of o-azaxylylenes from o-aminoketones by an excited-state intramolecular proton transfer (ESIPT) [5] has never been utilized in synthetic chemistry.It is conceivable that such implementation was attempted but failed due to the fast back proton transfer successfully competing with bimolecular cycloadditions. Intramolecular proton transfers from one heteroatom to another (and back) are fast; a typical example is the much studied photophysics of salicylaldehyde or o-hydroxyacetophenone, although the elusive tautomeric form is not impossible to characterize. [6] We, however, found that the intramolecular cycloaddition reactions of o-azaxylylenes photogenerated by excited-state proton transfer can successfully compete with the back proton transfer. Herein, we report the first example of [4 + 2] and [4 + 4] cycloaddition reactions of photogenerated azaxylylenes with unsaturated pendant groups; this reaction offers expeditious access to a diverse array of N,S,O-polyheterocycles with mostly unprecedented topologies (Scheme 1).As the azaxylylenes are expected to act as acceptors in the inverse-electron-demand cycloaddition reactions, we synthesized photoactive azaxylylene precursors 1 and 4 by coupling o-aminoketones with furan-or thiophenepropionic acid chlorides. [7] These o-acylamido precursors have broad UV Scheme 1. Photogeneration of azaxylylenes and their intramolecular photocyclizations. Scheme 2.[4+2] and [4 + 4] cycloadditions of photogenerated azaxylylenes to the furan and thiophene pendant groups. 5' a denotes the minor hydroxy epimer in the thiophene [4 + 2] cycloadduct. Yields of the isolated products are shown.

A new strategy for encoding polypeptide libraries with photolabile tags is developed. The photoassisted assay, based on conditional release of encoding tags only from bound pairs, can differentiate between peptides which have minor differences in a form of post-translational modifications with epigenetic marks. The encoding strategy is fully compatible with automated peptide synthesis. The encoding pendants are compact and do not perturb potential binding interactions.