Asymmetric Synthesis of Bi(thio)xanthylidene Overcrowded Alkenes

Abstract

Overcrowded alkenes are a fascinating class of inherent dissymmetric molecules that attract considerable interest for instance as chiroptical molecular switches and unidirectionally rotary motors. A practical synthesis route towards enantiomerically pure overcrowded alkenes is an important goal. We report here the development of an asymmetric synthesis of bi(thio)xanthylidenes. The upper and lower part of the desired alkenes were first coupled to a chiral template. Intramolecular coupling with partial stereocontrol, separation of diastereoisomers and subsequent removal of the templates gave stable enantiomers of bithioxanthylidene. The absolute configuration was determined by X‐ray analysis. In the case of bixanthylidene, stereocontrol during the intramolecular coupling reaction was complete, and it was found that the presence of a binaphthol bridging unit prevents the fast racemization process of the bixanthylidene, which it normally exhibited at ambient temperature. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

E/Z Photochemical switches: syntheses, properties and applications

“…Stereocontrolled coupling of both parts followed by removal of the chiral templates allows the preparation of stable enantiomers as a 4 : 1 mixture (Scheme 10). 47…”

Methyl 7-methoxy-9-oxo-9H-xanthene-2-carboxylate

“…For related structures, see: Evans et al (2004); Shi et al (2004); Macias et al (2001). For synthesis, see: Geertsema et al (2006). For background to the various types of intermolecular interactions, see: Bianchi et al (2004); Steiner (1999) Santos-Contreras et al (2007); Hunter & Sanders (1990).…”

“…First, in a nucleophilic substitution of 4-methoxyphenol and 4-bromoisophthalic acid, to yield 4-(4-methoxyphenoxy)isophthalic acid, by refluxing 45 min in N,N-dimethylformamide with potassium carbonate, sodium iodide and activated Cu-bronze. In the next reaction, called intramolecular Friedel-Crafts acylation was synthesized 7-methoxy-9-oxo-9H-xanthene-2-carboxylatic acid (Geertsema et al, 2006). In last step 7-methoxy-9-oxo-9H-xanthene-2-carboxylatic acid was esterified with methanol by refluxing in thionyl chloride in 45 min and then treated with mixture of methanol and triethylamine in room…”

Fast synthesis and redox switching of di- and tetra-substituted bisthioxanthylidene overcrowded alkenes

“… 4 Our group has a long-vested interest in overcrowded alkenes, for example, towards molecular motors 5 and switches, such as bistricyclic aromatic enes (BAEs). 6–8 These BAEs are a subset of overcrowded alkenes and include widely studied motifs, such as bifluorenylidenes, 9 bis(thio-)xanthylidenes, 10 bianthrones 11 and their derivatives ( Fig. 1a ).…”