Photochemistry of Allenyl Salicylaldehydes

Abstract

The intramolecular photocycloaddition of aryl aldehydes containing allene side chains is a versatile reaction as it provides a rapid and efficient access to original complex structures such as 1,3,4-tetrahydro-1,4-epoxy-5-alkylidene-2-benzoxepines 2 and substituted 2-oxa-tricyclo[5.2.2.0] 1,5 undeca-4,8,10-triene-9-carbaldehydes 3. Novel polycyclic structures are characterized, and optimized strategies for the synthesis of the substrates 1 are described.Creating complexity by the formation of new cores, stereocenters, and strained rings in one step from simple precursors is one of the many challenges of organic chemistry. Photochemistry is particularly attractive from this perspective because it may provide access to polycyclic structures without the need of aggressive reagents such as oxidants, reductants, acids, and bases. Tandem photocycloaddition/ fragmentation reactions and their applications in synthesis have been widely reviewed, but there is still a considerable potential for discovery. 1 In the past few years, we have been interested in the formation of unusual ring systems. The 1,4-cycloaddition of alkenes with arenes (para cycloaddition) has been well documented, but the similar process with allenes is much less understood. Our interest has focused on the photochemical reactivity of aryl aldehydes containing an allene side chain. In particular, when the substrate contains an aldehyde, the competition with a Paternò-Büchi-type reaction has to be considered. We report here the photochemical reactivity of ortho-allenyl benzaldehydes.For this purpose, we prepared the o-allenyl-benzaldehyde 6a-f substrates (Scheme 1) following a strategy described

Arene–Alkene Cycloaddition

“…181,182 Both oxygen-and nitrogen-containing tethers gave predominantly bicyclo[2.2.2]-octadiene derivatives (Scheme 96).…”

Electrochemical switching fluorescence emission in rhodamine derivatives

“…The synthesis of 2-(Prop-2-yn-1-yloxy)benzaldehyde (4) (CAS: 29978-83-4) was based on a literature procedure [27] (Scheme 2).…”

Photocycloaddition of Arenes and Allenes

“…28 The reaction was also carried out on a preparative scale (0.15-0.79 mmol), and the identity of the products were established by full spectroscopic analyses; the identity of the core structures was confirmed by X-ray analysis of 2l and 3b. 16 Substitution on position 3 of the aromatic ring with electron releasing substituents ( Table 3, entries [2][3][4] accelerates the reaction considerably and enhances the formation of the para-photocycloaddition product, whereas the substituents on the position 4 show an opposite effect. For example irradiation of the 4-methyl-or 4-methoxy-substituted analogue leads to remarkably longer reaction times and no formation of the bicyclo[2.2.2]octadiene product (Table 3, entries 5 and 6).…”

“…15 In a preliminary communication, we reported on a very robust intramolecular para-cycloaddition of aromatic aldehydes with allenes, which is remarkably tolerant of a variety of substituents on either the allene or the arene partners (Scheme 1). 16 In the present work, we explore the scope and limitation of this reaction, in particular by studying the influence of the chromophore and the nature of the allene tether. This reaction was used to prepare a series of rigid and highly complex cores, bearing several branching points and reactive functionalities, different features that are attractive for diversity-oriented synthesis (DOS).…”