Alkenes, cyclization coumarins

Rhodium(II)-catalyzed reactions between diazosulfones and aldehydes yielded an entry to carbonyl ylides, which underwent inter- and intra-molecular cyclization reactions with dipolarophiles, such as alkynes and alkenes, to afford tetrasubstituted furans in modest to good yields <01SL646>. The rhodium(ll) acetate catalyzed reaction of 3-diazobenzopyran-2,4(3 -dione with terminal alkynes provided a mixture of 2-substituted furo[3,2-c]coumarin and furo[2,3-f ]coumarin, presumably through a formal [3+2] cycloaddition reaction <01S735>. Furo[3,2-c]coumarins were also produced from 4-hydroxycoumarins and a-haloketones via a tandem 0-alkylation-cyclization procedure <01TL3503>... 

Kaye et al. have found that the DABCO-catalyzed MBH reaction between salicylaldehydes and methyl acrylate gives a mixture of various chromene and coumarin derivatives. Subsequently, they developed a simple one-pot methodology for the synthesis of 27/-l-chromenes 25 via the MBH reaction of 2-hydroxybenzaldehydes and 2-hydroxy-1-naphthaldehydes (23) with various activated alkenes (24) and subsequent regioselective cyclization (Scheme 2.14). In some cases competitive dimerization of the activated alkenes 24 was... 

As illustrated in Scheme 9.9, the proposed mechanism of forming phthalides and iso-coumarins involves orthopalladation of the carboxylic acid, subsequent alkenylation and nucleophihc cyclization or Wacker-type oxidative cyclization. The observed differences in product distributions from the reactions using n-butyl acrylate and styrene suggest that the electronic nature of the alkene plays an important role in product formation, although the exact origin of the difference remains unclear. 

Cheng and co-workers reported a nickel-catalyzed cyclization of oxa-bi-cyclic alkenes with (3-iodo-(Z)-propenoates and o-iodobenzoate to the corresponding coumarins. In the presence of Ni(dppe)Br2 and Zn powder in acetonitrile at 80 °C, the desired benzocoumarin derivatives were obtained in moderate to good yields (Scheme 3.45). 

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