Enynes nitro

An alternative one-step procedure involving treatment of the enediynyl propargylic alcohols 14a and 14b with triphenylphosphine, DEAD and finally o-nitro-benzenesulfonylhydrazine to give the corresponding enyne-allenes 15a and 15b, respectively, under mild thermal conditions has also been reported (Scheme 20.5)... 

Enyne metathesis can also be used to prepare more complex structures. A key step in the synthesis of (+)-viroallosecurinine reported (Tetrahedron Lett. 2004,45,5211) by Toshio Honda of Hoshi University in Tokyo is the selective cyclization of 13 to 15. In this case, the nitro Hoveyda-type catalyst 14 was used. 

Dipolar addition to unsaturated compounds affords routes to unique and interesting structures. The 1,3-dipolar cycloaddition of the 0-methyl ether of dinitromethane to methyl acylate produced N-methoxy-3-nitro-5-carbomethoxy-isoxazolidine in 65% yield. Similar results were obtained with other olefins. Acetonitrile N-oxide adds to enynes to form isoxazolines and isoxazoles. ... 

A branching pathway based on the chemistry of Michael adducts (7) was developed by Porco (Scheme 27.2) [47]. Reduction of the nitro group triggered lactamization to yield y-lactams such as 8. In contrast, with appropriately positioned alkenyl and alkynyl substituents, cyclization via ring-closing metathesis or Pauson-Khand reaction was possible. With =allyl and R =C=CCH20Me, enyne metathesis yielded the cyclic diene 9. In contrast, with R =C=CH and R = allyl, a Pauson-Khand reaction allowed the remarkable bridged cyclopentenone 10 to be obtained. 

Once we determined the best reaction conditions, the scope was explored. Products I-16a-h of [2-I-2-I-2] cycloaddition were isolated from enyne I-15a in 21-85 % yield, along with 1,3-dienes I-17a-h (Table 2.4). The reaction proceeded readily with electron-rich aldehydes. Conversely, in the reaction of I-15a with o-nitro-benzaldehyde, no adduct was formed (Table 2.4, entry 17), which is in contrast with the previously reported results for 1,6-enynes bearing a terminal aUcene moiety 1-9 [Refs. 226, 228 in Chap. 1]. 

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