Pyrans, dihydro, metathesis

Ru-catalysed enyne metathesis offers a short approach to chiral derivatives of 3-vinyl-5,6-dihydro-2//-pyrans. Some epimerisation can occur at the pyranyl C atom at elevated temperatures (Scheme 3) <02T5627>. The bispropargyloxynorbomene derivative 6 undergoes a cascade of metathesis reactions in the presence of alkenes and Grubbs catalyst incorporating an enyne-RCM that leads to fused cyclic dienes. A dienophile can be added to the reaction mixture, resulting in Diels-Alder reactions and the formation of functionalised polycyclic products <02TL1561>. 

Tandem RCM and cross-metathesis reactions of allyl hexa-l,5-dien-3-yl ether with alkenes leads to 2-alkylidene 3,6-dihydro-27f-pyrans (Scheme 6) <07TL1417>. 

Walters, M.A. et al.. Templates for exploratory library preparation. Derivatization of a functionalized spirocyclic 3,6-dihydro-2H-pyran formed by ring-closing metathesis reaction, J. Comb. Chem., 4,125, 2002. 

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