Diels-Alder cyclisation

Even if organocatalysis is a common activation process in biological transformations, this concept has only recently been developed for chemical applications. During the last decade, achiral ureas and thioureas have been used in allylation reactions [146], the Bayhs-Hillman reaction [147] and the Claisen rearrangement [148]. Chiral organocatalysis can be achieved with optically active ureas and thioureas for asymmetric C - C bond-forming reactions such as the Strecker reaction (Sect. 5.1), Mannich reactions (Sect. 5.2), phosphorylation reactions (Sect. 5.3), Michael reactions (Sect. 5.4) and Diels-Alder cyclisations (Sect. 5.6). Finally, deprotonated chiral thioureas were used as chiral bases (Sect. 5.7). 

J. Herscovici, S. Delatre, and K. Antonakis, Enantiospecific naphthopyran synthesis by intramolecular Diels-Alder cyclisation of 4-keto 2,3-unsaturated C-glycosides, Tetrahedron Leu. 32 1183 (1991). 

Fig. 8. The partitioning of a modified More O Ferrall diagram with the corresponding reaction path for the allowed s + s Diels-Alder cyclisation of 1-substituted butadiene with substituted ethene. The extent of electron reorganisation, measured by the value of the functional L, is — 0.34 along this path...

Scheme 6 Putative Diels-Alder cyclised fused ring polyketides...

Feeding studies to Nocardia argentine sis with - C labelled NAC thioesters of di-, tri-, tetra- and pentaketide intermediates demonstrated regiospecific incorporations into nargenicin 37 (Scheme 18) which are consistent with an intramolecular Diels-Alder cyclisation of a linear nonaketide intermediate (Scheme 19) [72,73]. 

Reaction of a 2,6-dimethylphenol (4) with lead tetraacetate in the presence of an excess of acrylic acid leads to ortho-qainol acrylates (5) which then can lead to tricyclic lactones (6) by intramolecular Diels-Alder cyclisation. ... 

E = As, Sb or Bi) react similarly with Grignard reagents, providing routes to cyclic arsines, stibines and bismuthines. Leung s group has reported further applications of asymmetric Diels-Alder cyclisation reactions in phosphine synthesis. A platinum complex chiral auxiliary has been used to promote the asymmetric [4-1-2] Diels-Alder addition of diphenyl(vinyl)-phosphine to 3-diphenylphosphinofuran, giving the eradocycloadduct (70) as the predominant stereoisomer. Related cycloadditions between 3,4-dimethyl-1 -phenylphosphole and ester-functionalised allylic phosphines have provided chemoselective routes to optically-pure P-chiral 1,2- and... 

Nitroso-Diels-Alder cyclisations have also proved to be useful in this area and in the synthesis of non-carbohydrate natural products, e,g. tabtoxin (29). Tabtoxinine B-lactam (30) has also been prepared using this methodology with cyclisation of (31) and... 

A method for the preparation of higher sugars with an aromatic branch has been developed which involves a Michael addition - aldol condensation sequence and uses carbohydrate silylenol ethers, e.g. compound (42), as starting materials. As is demonstrated in Scheme 9, the Michael addition product was obtained as the stable silylenol ether (43). On desilylation, aldol condensation took place to give the cyclic hydroxyketone (44) which aromatised under acetylation conditions. The introduction of an aromatic branch by Diels-Alder cyclisation of a carbohydrate diene is referred to in Chapter 19. 

L-tryptophan, probably forming an amide linkage first,and subsequently closing the lactam ring to form the tetramic acid (123). This is then transformed by reduction and dehydration to (124), which can undergo an internal Diels-Alder cyclisation to form... 

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