Dihydropyrans synthesis

H-pyran synthesis from, 3, 759 bis(trimethylsiloxy) in pyrrole synthesis, 4, 333 chromene synthesis from, 3, 750 cycloaddition reactions with isocyanates, azetidin-2-ones from, 7, 261 dihydropyran synthesis from, 3, 771 fuiyl... 

Butane, 2,3-0-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)-catalyst in homogeneous asymmetric hydrogenation, 6, 781 Butane-1,4-dioic acid, 2,2-di(indolyl)-synthesis, 4, 226 Butanenitrile, 4-hydroxy-dihydropyran synthesis from, 3, 769 Butanoic acid, -y-aryl-y-amino-synthesis, 1, 433 1-Butanol... 

The application of RCM to dihydropyran synthesis includes a route to 2,2-disubstituted derivatives from a-hydroxycarboxylic acids. In a one-pot reaction, the hydroxy esters undergo sequential O-allylation, a Wittig rearrangement and a second O-allylation to form allyl homoallyl ethers 8. A single RCM then yields the 3,6-dihydro-2//-pyran 9. The process is readily adapted not only to variably substituted dihydropyrans but also to 2-dihydrofuranyl and 2-tetrahydrooxepinyl derivatives and to spirocycles e.g. 10 through a double RCM (Scheme 4) <00JCS(P1)2916>. 

Further examples of the use of the hDA reaction in dihydropyran synthesis include the formation of the fused pyrans 18 from vinylallenes 17 and aldehydes (Scheme 8) <00TL6781> and a trans-fused dihydropyran containing a phosphonate group 19 . A total synthesis of the 11-oxa steroid system is based on an intramolecular Diels-Alder reaction involving an orthoquinodimethane derived from a benzocyclobutene (Scheme 9) <00TL1767>. 

A highly enantioselective dihydropyran synthesis results from the use of bis(oxazoline) copper(II) complexes as catalysts <00JA1635 00JA7936> and the value of this approach to carbohydrate synthesis has been noted . 

Dihydropyrane synthesis." A novel synthesis of a dihydropyrane intermediate (2) for a synthesis of pseudomonic acid (3) involves an ene reaction of formaldehyde with a 1,4-diene, followed by a quasi intramolecular Diels-Alder reaction with formaldehyde as dienophile, both reactions being catalyzed by ethylaluininum dichloride. Thus, reaction of 1 with CH20 (3 equiv.) and C2H5A1C12 (4.5 equiv.) gives 2 in 35-40% yield via a number of intermediates. 

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