Annulation benzoin condensation

In order to vary the electronic situation at the carbene carbon atom a number of carbo- and heterocycle-annulated imidazolin-2-ylidenes like the benzobis(imida-zolin-2-ylidenes) [58-60] and the singly or doubly pyrido-annulated A -heterocyclic carbenes [61-63] have been prepared and studied. Additional carbenes derived from a five-membered heterocycle like triazolin-5-ylidenes 10 [36], which reveals properties similar to the imidazolin-2-ylidenes 5 and thiazolin-2-ylidene 11 [37] exhibiting characteristic properties comparable to the saturated imidazolidin-2ylidenes 7 have also been prepared. Bertrand reported the 1,2,4-triazolium dication 12 [64]. Although all attempts to isolate the free dicarbene species from this dication have failed so far, silver complexes [65] as well as homo- and heterobimetallic iridium and rhodium complexes of the triazolin-3,5-diylidene have been prepared [66]. The 1,2,4-triazolium salts and the thiazolium salts have been used successfully as precatalysts for inter- [67] and intramolecular benzoin condensations [68]. 

Modified benzoin condensations in which the acyl acceptor is not an aldehyde constitute a variation of the classical cross-benzoin condensation. Aldehydes and ketones can be coupled in an intramolecular annulation reaction to give tertiary a-hydroxyketones. The selective cross-coupling of... 

A conceptually unique annulation process is available through the intramolecular benzoin condensation of dialdehyde or ketoaldehyde substrates. In one of the few examples of a dialdehyde annulation, Wong and co-workers reported the cyanide-catalyzed intramolecular benzoin reaction of dialdehyde 27 to give benzoxepinone 28 in modest yield. " Very recently, Mennen and Miller described macrocyclization reactions of dialdehydes catalyzed by triazolium carbene catalysts to give 11-14-membered rings in 16-43% yield. ... 

The intramolecular benzoin condensation between an aldehyde and ketone was virtually unknown until 2003, when Suzuki and co-workers established that thiazolylidine catalysts could effect this transformation in good to excellent yield for the synthesis of functionalized preanthraquinones such as 30. Subsequent to this work, reports of annulation reactions of a variety of ketoaldehyde substrates, as well as asymmetric variants, have appeared in the literature. ... 

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