Michael chiral chromenes

No racemisation is observed during the Pt(IV)-catalysed cyclisation of chiral propargyl ethers to chromenes. The PtCU catalyst appears to activate selectively the triple bond to nucleophilic attack by the arene and enables this well-established route to chromenes to be carried out under mild, neutral conditions and with a variety of substrates (Scheme 12) <03T8859>. A Pt-catalysed 6-endo hydroarylation of an alkynone combined with an intramolecular Michael addition are the key steps in a synthesis of the rotenoid deguelin <030L4053>. 

The tandem Michael addition-cyclization reaaions of ( )-2-(2-nitrovi-nyl)phenols with malononitrile using a chiral thiophosphonodiamide as catalyst deliver 2-amino-3-cyano-4H-chromenes in high yields (14T181). Other derivatives are obtained from a two-step reaction of benzaldehydes... 

Cyclic (hetero- and carbocyclic) vinyl sulfoxides have been prepared by a tandem Michael addition/Homer olefination reaction of a-phosphorylvinyl sulfoxides and carbonyl compounds bearing a nucleophilic center. Using optically active a-phosphorylvinyl sulfoxides a series of enantiomeric cyclic vinyl sulfoxides in which the chiral sulfinyl group is bonded to a chromene, pyrrazolyne, quinoline or cyclopen-tene ring, has been obtained. The H-W-E reaction of aldehydes with sulfinimine-derived 3-oxo pyrrolidine phosphonates (228) represents a new method for the asymmetric synthesis of ring-functionalized cw-2,5-disubstituted 3-oxo pyrrolidines (229) (Scheme 90). ... 

Finally, Sankararaman et al. have reported the synthesis of chiral nitro-chromenes on the basis of an asymmetric domino Michael-Henry reaction followed by dehydration, involving nitrostyrene and orlho-hydio-xybenzaldehyde. This reaction was catalysed by a novel C2-symmetric bis-(pyrrolidine-triazole)-based organocatalyst and provided only moderate yields (22-40%) and poor enantioselectivities (7-24% ee). 

An enantioselective domino oxa-Michael Hemy reaction of substituted salicylaldehydes with nitroalkenes, proceeding through aromatic iminium activation has been developed by Xu et al. by using a chiral secondary amine organocatalyst and salicylic acid as a co-catalyst. This novel domino reaction served as an efficient method for the preparation of chiral 3-nitro-2/f-chromenes with moderate to good enantioseleetivities of up to 92% ee (Scheme 1.78). 

Zhang, X., Zhang, S., Wang, W. (2010). Iminium-aflenamine cascade catalysis one-pot access to chiral 477-chromenes by a highly enantioselective Michael-Michael sequence. Angewandte Chemie International Edition, 49,1481-1484. 

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